Hook Echoes and Rear-Flank Downdrafts: A Review

Transcription

852
MONTHLY WEATHER REVIEW
VOLUME 130
Hook Echoes and Rear-Flank Downdrafts: A Review
PAUL M. MARKOWSKI
Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
(Manuscript received 19 March 2001, in final form 24 September 2001)
ABSTRACT
Nearly 50 years of observations of hook echoes and their associated rear-flank downdrafts (RFDs) are reviewed.
Relevant theoretical and numerical simulation results also are discussed. For over 20 years, the hook echo and
RFD have been hypothesized to be critical in the tornadogenesis process. Yet direct observations within hook
echoes and RFDs have been relatively scarce. Furthermore, the role of the hook echo and RFD in tornadogenesis
remains poorly understood. Despite many strong similarities between simulated and observed storms, some
possibly important observations within hook echoes and RFDs have not been reproduced in three-dimensional
numerical models.
1. Introduction
Perhaps the best-recognized radar feature in a horizontal
depiction associated with supercells is the extension of
low-level echo on the right-rear flank of these storms,
called the ‘‘hook echo.’’ Hook echoes are known to be
associated with a commonly observed region of subsiding
air in supercells, called the ‘‘rear-flank downdraft.’’ Rearflank downdrafts have been long surmised to be critical
in the genesis of significant tornadoes within supercell
thunderstorms. In this paper, observations of hook echoes
are reviewed first, beginning with the first documentations
of the 1950s and 1960s. Rear-flank downdrafts are reviewed next, including discussion of pertinent Doppler
radar observations and theoretical and numerical simulation studies. Finally, relatively recent observations made
during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX; Rasmussen et al. 1994)
and smaller subsequent field operations are reviewed. Despite the well-known association among hook echoes, rearflank downdrafts, and tornadogenesis, their dynamical relationship remains poorly understood. Our current confusion serves as a motivation for the collection of new, in
situ observations having unprecedented spatial and temporal detail within hook echoes and rear-flank downdrafts.
These data are the foundation of a companion paper (Markowski et al. 2002).
2. Hook echoes
a. Characteristics
The hook echo first was documented by Stout and
Huff (1953; Fig. 1) in an Illinois tornado outbreak on
Corresponding author address: Dr. Paul Markowski, 503 Walker
Building, University Park, PA 16802.
E-mail: marko@mail.meteo.psu.edu
9 April 1953, although van Tassell (1955) is given credit
for coining the term. The reflectivity appendage usually
is oriented roughly perpendicular to storm motion. Hook
echoes are typically downward extensions of the rear
side of an elevated reflectivity region (Forbes 1981)
called the echo overhang (Browning 1964; Marwitz
1972a; Lemon 1982), with the region beneath the echo
overhang termed a weak echo region (Chisholm 1973;
Lemon 1977) or vault (Browning and Donaldson 1963;
Browning 1964, 1965a). Browning and Donaldson
(1963) and Browning (1965b) noted that the southern
edge of the hook formed a wall of echo ‘‘which was
often very sharp and sometimes rather upright.’’ Hook
echoes are typically several kilometers in length and
several hundred meters in width, at least as viewed by
operational radars (e.g., Garrett and Rockney 1962). A
variety of shapes that the hook echo may take were
presented by Fujita (1973; Fig. 2).
Fujita (1958a) documented hook echoes associated
with other supercells on the same day Stout and Huff
made their observations. He inferred the concept of
thunderstorm rotation from viewing the evolution of the
hook echoes, which he studied in unprecedented (and
since unparalleled) detail (Fig. 3). Brooks (1949) earlier
had referred to these circulations, having radii of approximately 8–16 km, as tornado cyclones.1 Wind velocity data obtained following the installation of Doppler radars in central Oklahoma in the late 1960s confirmed an association between hook echoes and strong
horizontal shear zones associated with storm rotation
and tornadoes (e.g., Donaldson 1970; Brown et al. 1973;
1
The tornado cyclone terminology now typically refers to distinct
circulations having a scale larger than a tornado but smaller than a
mesocyclone (e.g., Agee 1976).
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Fujita (1981) proclaimed ‘‘Mesoscale modelers should
be attracted by such a pair of cyclonic and anticyclonic
torandoes which were evidenced in the Grand Island
storm on 3 June 1980 and in the central Iowa storm on
13 June 1977.’’ However, it was unclear why such attention should be given to the vortex couplet, and the
origin of the couplet was not well understood.
b. Formation
FIG. 1. Radar image from the first documentation of a hook echo.
The hook echo was associated with a tornadic supercell near Champaign, IL, on 9 Apr 1953. [From Stout and Huff (1953).]
Lemon et al. 1975; Ray et al. 1975; Ray 1976; Brandes
1977a; Burgess et al. 1977; Lemon 1977; Barnes
1978a,b).
Garrett and Rockney (1962) were the first to relate a
circular echo on the tip of a hook echo to the tornado
or tornado cyclone. They called this ball-shaped echo
an ‘‘asc’’ (annular section of the cylinder of the vortex),
but the authors did not offer an explanation for the exact
cause of the appearance of the asc. Stout and Huff
(1953) also observed a similar feature, but little was
discussed of it. Donaldson (1970) noted an echo hole
in the tornado he studied, and found that it was collocated with a tornado vortex. Forbes (1981) also observed similar reflectivity features during the tornado
outbreak of 3–4 April 1974, as did Fujita and Wakimoto
(1982) in their study of the Grand Island, Nebraska,
tornadoes of 3 June 1980.
Van Tassell’s (1955) images of a hook echo near
Scottsbluff, Nebraska, on 27 June 1955 (it moved directly over the radar) suggested the presence of a faint
anticyclonic protrusion from the tip of the hook, extending outward in a direction opposite that of the cyclonic protrusion. An anticyclonic reflectivity flare also
has been documented by Brandes (1981), Fujita (1981),
and Fujita and Wakimoto (1982). Multiple Doppler radar wind syntheses almost invariably have revealed a
region of anticyclonic vorticity on the opposite side of
the hook echo as the more prominent (cyclonic) vorticity
region (Brandes 1977b, 1978, 1981, 1984a; Ray 1976;
Ray et al. 1975, 1981; Heymsfield 1978; Klemp et al.
1981; Fig. 4). It is perhaps surprising that the ubiquity
of the vorticity couplet straddling the hook echo largely
has been ignored, with the exception of Fujita and his
collaborators. Fujita and Wakimoto (1982) documented
an anticyclonic tornado within the region of anticyclonic
vertical vorticity (Fig. 5). The cyclonic member of the
vorticity couplet also was associated with a tornado.
Fujita (1958a) originally attributed hook echo formation to the advection of precipitation from the rear
of the main echo around the region of rotation associated
with the tornado cyclone and updraft. Browning (1964,
1965b) also documented hook echoes and attributed
their evolution (Fig. 6) to essentially the same process
described by Fujita (1958a). Fujita (1965) later attributed hook echo formation to the Magnus force. He
explained that this force pulled the spiraling updraft out
of the main echo, resulting in the hook-shaped reflectivity appendage commonly observed on radar displays
(Fig. 7).
Fulks (1962) hypothesized that hook echo formation
was due to a large convective tower extending into the
levels of strong vertical wind shear, which produced
cyclonic and anticyclonic flows at opposite ends of the
tower—the cyclonic flow to the southwest gave rise to
hook echo development. No mention was made of the
possibility of an anticyclonic hook echo forming on the
north side of the tower from the same mechanism.
Probably no one presented as many detailed Doppler
radar analyses of supercells as Brandes [1977a,b, 1978,
1981, 1984a,b; Brandes et al. (1988)]. Brandes (1977a)
looked at a nontornadic supercell on 6 June 1974. Hook
echo formation was attributed to the ‘‘horizontal acceleration of (low-level) droplet-laden air’’ as the downdrafts intensified and the outflow interacted with the
inward-spiraling updraft air. Apparently this hypothesis
was essentially that precipitation advection was responsible for hook echo formation, similar to the Fujita
(1958a) and Browning (1964, 1965b) hypotheses. A
three-dimensional numerical simulation by Klemp et al.
(1981) of the Del City, Oklahoma (20 May 1977) supercell also suggested that the horizontal advection of
precipitation was important for hook echo development.
In some observations of hook echoes associated with
tornadoes in nonsupercell storms, the hook echoes also
have appeared to result largely from the horizontal advection of hydrometeors (e.g., Carbone 1983; Roberts
and Wilson 1995).
Other reliable radar observations have been made that
suggest that hook echo formation, in at least some cases,
appears to result from the descent of a rain curtain in
the rear-flank downdraft (e.g., Forbes 1981; E. N. Rasmussen 2000, personal communication;2 L. Lemon
2
This was an oral presentation at the VORTEX Symposium in
Long Beach, California.
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TABLE 1. Summary of findings pertaining to hook echoes and RFDs.
Observation/Conclusion
RFD originated at or above 7 km
RFD originated below 7 km
Low uw at surface in RFD
Warm air (but not necessarily high uw) at surface in RFD
Hypothesized that the occlusion downdraft is driven by a downward-directed, nonhydrostatic pressure gradient arising from the
intensification of low-level vertical vorticity
Hypothesized the RFD is forced mainly thermodynamically from
aloft (which may result from stagnation)
Hypothesized the RFD is initiated by dynamic pressure excess
aloft but maintained thermodynamically
Reflectivity gradients found on the upshear side of storms
Tornadogenesis observed before hook formation
Tornadogenesis observed at the time of overshooting top collapse
Visual observations of clear slots accompanying tornadoes
Vertical vorticity couplets associated with hook echoes
Anticyclonic reflectivity flares on hook echoes
Hook echo formation attributed to rotation (but not necessarily the
same mechanisms)
Hook echoes associated with strong horizontal shears or tornadoes
(prior to 1980, in the interest of brevity)
Hook echoes associated with downdrafts (prior to 1980, in the interest of brevity)
Hook echoes located in strong vertical velocity and temperature
gradients, somewhat behind the surface windshift associated
with the RFD
Air parcels that enter the tornado pass through the RFD
Hypothesized that the RFD is important for tornadogenesis
References
Nelson (1977), Lemon et al. (1978), Barnes (1978a), Lemon and
Doswell (1979)
Klemp et al. (1981)
van Tassell (1955), Beebe (1959), Ward (1961), Browning and
Ludlam (1962), Browning and Donaldson (1963), Charba and
Sasaki (1971), Lemon (1976a), Nelson (1977), Brandes (1977a),
Barnes (1978a,b), Klemp et al. (1981), Klemp and Rotunno
(1983), Rotunno and Klemp (1985), Wicker and Wilhelmson
(1995), Dowell and Bluestein (1997), Adlerman et al. (1999)
Tepper and Eggert (1956), Garrett and Rockney (1962), Williams
(1963), Fujita et al. (1977), Brown and Knupp (1980), Bluestein
(1983), Brandes (1984a), Johnson et al. (1987), Rasmussen and
Straka (1996)
Klemp and Rotunno (1983), Brandes (1984a,b), Hane and Ray
(1985), Rotunno (1986), Brandes et al. (1988), Trapp and Fiedler (1995), Wicker and Wilhelmson (1995), Wakimoto et al.
(1998), Adlerman et al. (1999), Wakimoto and Cai (2000)
Browning and Ludlam (1962), Browning and Donaldson (1963),
Browning (1964), Nelson (1977), Barnes (1978a), Brandes
(1981), Klemp et al. (1981)
Bonesteele and Lin (1978), Lemon and Doswell (1979)
Nelson (1977), Bonesteele and Lin (1978), Barnes (1978a), Forbes
(1981)
Garrett and Rockney (1962), Sadowski (1969), Forbes (1975)
Fujita (1973), Lemon and Burgess (1976), Burgess et al. (1977)
Beebe (1959), Garrett and Rockney (1962), Moller et al. (1974),
Peterson (1976), Stanford (1977), Burgess et al. (1977), Lemon
and Doswell (1979), Marshall and Rasmussen (1982), Rasmussen et al. (1982), Jensen et al. (1983), Wakimoto and Liu (1998)
Ray (1976), Ray et al. (1975, 1981), Brandes (1977b, 1978, 1981,
1984a), Heymsfield (1978), Klemp et al. (1981), Fujita (1981),
Fujita and Wakimoto (1982), Klemp and Rotunno (1983), Brandes et al. (1988), Wicker and Wilhelmson (1995), Wurman et al.
(1996), Straka et al. (1996), Bluestein et al. (1997), Dowell and
Bluestein (1997), Blanchard and Straka (1998), Gaddy and Bluestein (1998), Wakimoto and Liu (1998), Wakimoto et al. (1998),
Wakimoto and Cai (2000), Wurman and Gill (2000), Ziegler et
al. (2001), Bluestein and Gaddy (2001)
van Tassell (1955), Brandes (1981), Fujita (1981), Fujita and Wakimoto (1982), Wurman et al. (1996), Blanchard and Straka
(1998), Wurman and Gill (2000), Ziegler et al. (2001)
Fujita (1958a, 1965), Fulks (1962), Browning (1964, 1965b), Brandes (1977a)
Stout and Huff (1953), van Tassell (1955), Fujita (1958a, 1965),
Garrett and Rockney (1962), Browning (1964, 1965b), Freund
(1966), Sadowski (1958, 1969), Donaldson (1970), Forbes
(1975), Ray et al. (1975), Lemon et al. (1975), Ray (1976),
Brown et al. (1978), Lemon (1977), Burgess et al. (1977), Brandes (1977a), Barnes (1978a,b)
Browning and Donaldson (1963), Haglund (1969), Fujita (1973,
1975b, 1979), Lemon et al. (1975), Lemon (1977), Brandes
(1977a), Burgess et al. (1977)
Marwitz (1972a,b), Burgess et al. (1977), Lemon and Doswell
(1979), Brandes (1981)
Brandes (1978), Davies-Jones and Brooks (1993), Wicker and Wilhelmson (1995), Dowell and Bluestein (1997), Adlerman et al.
(1999) [and implied by visual observations of Lemon and Doswell (1979), Rasmussen et al. (1982), Jensen et al. (1983)]
Ludlam (1963), Fujita (1975b), Burgess et al. (1977), Barnes
(1978a), Lemon and Doswell (1979), Brandes (1981), DaviesJones (1982a,b)
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FIG. 2. Fujita introduced five variations on the shapes of hook
echoes. [From Fujita (1973).]
2000, personal communication). It may be worth the
effort in future field experiments to use temporally and
spatially high-resolution mobile radar data to systematically investigate the extent to which the hook echo
forms from the above process, versus being due to a
streamer of precipitation that is extruded from the main
echo core by horizontal advection. The relative contribution to hook echo formation from these two processes
is not yet known. It is entirely possible that one might
dominate in one case, while the other dominates in a
different case. Or perhaps the relative contribution from
the two processes could be a function of time within a
single case.
For completeness, it is noted that at least one documentation has been made of hook echoes not associated
with updraft rotation. Houze et al. (1993) showed examples of hook-shaped (in a cyclonic sense, with the
hooks pointing toward the right with respect to storm
motion) reflectivity structures in left-moving severe
storms in Switzerland. These features, termed false
hooks by the authors, apparently were associated with
the cyclonic downdraft regions on the right (southern)
flanks of the anticyclonically rotating storms, in which
the updrafts would have been on the left (northern)
flanks (Klemp and Wilhelmson 1978a; Wilhelmson and
Klemp 1978; Rotunno and Klemp 1982).
c. Tornado forecasting based on hook echo detection
The forecasting potential of hook echo detection began to be explored in the mid-1960s. Sadowski (1958)
documented a tornado that occurred after the hook echo
became visible (in fact, the hook echo was becoming
less discernible on radar at the time of the reported
tornado formation). Sadowski might have been the first
to speculate that if hook echoes generally preceded tornadogenesis, then it might be possible to issue tornado
warnings in advance.
In Stout and Huff’s (1953) report, the evidence had
been inconclusive as to whether the hook echo preceded
tornadogenesis, or vice versa. In van Tassell’s (1955)
summary, it was not mentioned whether the hook echo
FIG. 3. Development of the hook echo associated with the Champaign, IL, tornado from 1724–1738 CST 9 Apr 1953, as analyzed by
Fujita. [From Fujita (1958a).]
developed before or after tornadogenesis. The tornado
studied by Garrett and Rockney (1962) apparently
formed before the hook echo became prominent, unless
a narrow hook echo went undetected by the Weather
Surveillance Radar-3 (WSR-3) (48 beamwidth) prior to
tornadogenesis. The tornado dissipated when the hook
‘‘closed off’’ or merged with the forward-flank echo.
Sadowski (1969) later documented a large amount of
success using hook echoes to detect tornadoes within
thunderstorms. In a 1953–66 study, he computed an
average time of 15 min between hook echo appearance
and tornadogenesis in a sample of 13 cases in which
hook echoes appeared before tornadoes were reported.
Sadowski reported a false alarm rate of only 12%. On
the other hand, Freund (1966) found that only 6 of 13
tornadic storms near the National Severe Storms Laboratory in 1964 were associated with hook echoes, and
Golden (1974) found that only 10% of waterspouts were
associated with hook echoes.
The so-called Super Outbreak of tornadoes on 3–4
April 1974 (Fujita 1975a,b) provided a large sample of
a variety of ‘‘distinctive echoes’’ that were studied by
Forbes (1975, 1981). Forbes (1975) found that 1) a majority of hook echoes were associated with tornadoes,
2) hook echoes often were associated with tornado families, and 3) tornadoes associated with hook echoes tended to be stronger than those from other echoes. Forbes
(1975) also found that, on average, hook echoes appeared 25 min prior to tornadogenesis; however, much
variance was present in his sample—10 of 27 (37%) of
the hook echoes associated with the first tornado produced by a supercell were detected after the reported3
tornado formation times. Forbes (1981) found a false
alarm rate of just 16% when using hook echoes to detect
tornadoes. But because hook echoes were relatively rare
3
The accuracy of the reported tornado times may be questionable
for some of the tornadoes studied.
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FIG. 4. Three-dimensional wind field relative to the Del City, OK, supercell 1845 CST 20 May 1977 at 400 m. Left panel shows horizontal
wind vectors with radar reflectivity superposed. The right panel depicts vertical velocity (m s 21 ), with the 30-dBZ contour accentuated on
both panels. The tornado path is stippled. [From Brandes (1981).]
(as he defined them), a less restrictive shape (a ‘‘distinctive echo’’, e.g., appendages, line-echo wave patterns, etc.) also was considered. Distinctive echoes were
associated with a probability of detection of tornadoes
of 65%. Forbes (1975, 1981) did raise concern about
the generality of his findings, since his statistics were
based on the events of a single day. Also, the statististics
were based on Weather Surveillance Radar-1957 (WSR57) data, which may not have adequately resolved finescale echo structures.
3. Rear-flank downdrafts
a. Association with hook echoes
Rear-flank downdrafts (RFDs) are regions of subsiding air that develop on the rear side of the main updraft
of supercell storms, and these regions of descent have
a well-established association with hook echoes. The
first documentation of an RFD, although not recognized
as such, probably was by van Tassell (1955). In that
case study and in another by Beebe (1959) on the same
storm complex, three ‘‘reliable’’ reports of severe downdrafts on the south side of the Scottsbluff tornado (27
June 1955) were made.
Browning and Ludlam (1962) and Browning and
Donaldson (1963) also were among the first to mention
the presence of a downdraft in the vicinity of the strongest low-level rotation, behind the main storm updraft.
Browning and Donaldson (1963) noted that the hook
echo itself may be associated with this downdraft region.
Haglund (1969), Fujita (1973, 1979), Lemon et al.
(1975), Burgess et al. (1977), Brandes (1977a), Lemon
(1977), and Forbes (1981) also documented an association between hook echoes and downdrafts. According
to Forbes (1981), ‘‘the hook represents a band of precipitation accompanied by downdraft and outflow, surrounding a weak echo region (a region of inflow and
updraft).’’ Brandes (1977a) tentatively concluded that
the hook echo reflected downdraft intensification. Haglund (1969) concluded that the hook echo slightly trails
the surface wind shift associated with the outflow of the
RFD, and that the hook echo is located near the boundary between updraft and downdraft. Surface analyses
and aircraft penetrations have revealed that the hook
echo is located in a region of large vertical velocity and
temperature gradients (Burgess et al. 1977; Marwitz
1972a,b).
b. Visual characteristics
The number of visual and surface observations of
supercells increased during the 1970s, largely because
of organized storm intercept programs at the National
Severe Storms Laboratory (Golden and Morgan 1972;
Davies-Jones 1986; Bluestein and Golden 1993). Many
of these observations have advanced our understanding
of the basic structures associated with tornadoes and
their parent storms.
Golden and Purcell (1978) photogrammetrically
documented subsiding air on the south side of the
Union City, Oklahoma, tornado (24 May 1973), apparently a visual manifestation of the RFD and also
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FIG. 5. Radar image and analysis from Grand Island, NE, 0208 UTC 4 Jun 1980 showing a vortex couplet straddling
a hook echo, with tornadic circulations associated with both members of the vortex couplet. [From Fujita (1981) and
Fujita and Wakimoto (1982).]
evidence that the tornado occurred in a strong vertical
velocity gradient. Moreover, a clear slot was seen to
wrap itself at least two-thirds of the way around the
tornado. Other observations of clear slots, which are
probably always visual manifestations of subsiding air
in an RFD,4 have been presented by Beebe (1959; this
was probably the first documentation), Moller et al.
(1974), Peterson (1976), Stanford (1977), Burgess et
al. (1977), Lemon and Doswell (1979), Marshall and
Rasmussen (1982), Rasmussen et al. (1982), and Jensen et al. (1983) (Fig. 8).
Burgess et al. (1977) found that the clear slot could
be associated with a hook echo: ‘‘Perhaps large droplets
are present in the downdraft and are brought down from
the echo overhang, even though the air contains only
ragged clouds or is visibly cloudless at low levels. If
so, since radar reflectivity is more strongly dependent
on the size rather than on the number of droplets, radar
may show substantial echo in the ‘clear’ slot.’’ Analysis
of the 2 June 1995 Dimmitt, Texas, tornadic supercell
also indicated an association between the hook echo and
clear slot, based on photogrammetrically determined
cloud positions (E. N. Rasmussen 2000, personal communication).5
c. Surface characteristics
Direct observations within RFDs have been scarce.
Tepper and Eggert (1956) appear to have been the first
4
The absence of a clear slot does not necessarily indicate an absence of subsiding air.
5
This was an oral presentation at the VORTEX Symposium in
Long Beach, California.
to systematically analyze traces of thermodynamic data
near tornadoes, and consequently, within some RFDs.
Data were obtained within 25 km of tornadoes in more
than 50 cases. Many of the thermograph traces measured
only minor fluctuations during the passage of the tornadoes and associated RFDs, and other traces revealed
cooling and moistening near the tornadoes. Only a few
observations were available within 5 km of the tornadoes, however.
Fujita (1958b) inferred the presence of a surface high
pressure annulus encircling the Fargo, North Dakota,
tornado cyclone (20 June 1957) from pressure traces in
the vicinity of the tornadoes (Fig. 9). Although Fujita
speculated that the high pressure was associated with a
ring of subsiding air around the tornado, he was unable
to verify this speculation. [Ward (1964, 1972) and Snow
et al. (1980) found high pressure rings surrounding laboratory and numerically simulated vortices, but it is not
clear whether these are the same phenomena inferred
by Fujita, which appeared to be of a slightly larger
scale.] Surface pressure excesses within RFDs of up to
a few millibars also have been documented by subsequent investigators (e.g., Charba and Sasaki 1971; Lemon 1976a; Bluestein 1983), although no one else has
documented a high pressure ring as Fujita did.
The studies of the Scottsbluff tornado by van Tassell
(1955) and Beebe (1959) contain some of the first descriptions, albeit qualitative, of surface temperature
within an RFD at close range from a tornado. At least
a couple of observers, located a few hundred meters
south of the tornado, reported that the downdrafts felt
‘‘cold.’’ Browning and Ludlam (1962) and Browning
and Donaldson (1963) also reported cold temperature
858
FIG. 6. Evolution of the hook echo in an Oklahoma supercell on
26 May 1963 studied by Browning. [From Browning (1965b).]
and equivalent potential temperature (u e ) measurements
(with respect to the inflow) in the wakes of the Wokingham, England (9 July 1959) and Geary, Oklahoma
(4 May 1961) supercells, presumably within the RFDs.
Ward (1961) observed ‘‘cooler northwest winds a couple
miles southwest of the (Geary) tornado.’’
In a supercell (25 May 1974) investigated by Nelson
(1977), the lowest wet-bulb potential temperature (u w)
values observed at the surface were within the RFD,
where they were ;6 K lower than the ambient u w values.
Complete separation of the forward-flank downdraft
(FFD) and RFD was evidenced by separate temperature
minima, as Lemon (1974) earlier had found in another
case. Additional observations of relatively low-u e and
low-u w air at the surface within RFDs have been presented by Brandes (1977a; supercell on 6 June 1974;
.3 K u w decrease), Barnes (1978a,b; supercell on 29
April 1970; 2–3 K u w decrease directly behind the RFD
FIG. 7. Fujita once hypothesized that the Magnus force led to the
formation of hook echoes. [From Fujita (1965).]
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FIG. 8. Photograph of a typical clear slot associated with an RFD
(2 Jun 1995 at Dimmitt, TX; photograph by the author).
gust front and $5 K farther behind the gust front), Lemon (1976a; supercell on 25 June 1969; ;6 K u w decrease), and Charba and Sasaki (1971; supercell on 3
April 1964; ;7 K u e decrease). The relatively low u e
and u w values have been shown to be compatible with
environmental air anywhere from 1–5 km above the
ground (e.g., Lemon 1976a; Charba and Sasaki 1971).
In contrast to the findings summarized above, Garrett
and Rockney (1962) reported that a warm downdraft
was observed about 12–15 km south of a tornado near
Topeka, Kansas, on 19 May 1960. The observer described the air as ‘‘suddenly becoming noticeably hot,
similar to a blast of heat from a stove.’’ Williams (1963)
showed that RFD air can arrive at the surface warmer
than the surrounding air. He noted that when such an
event occurs, it may be south of the hook echo or wherever forced descent is less likely to encounter sufficient
liquid water to maintain negative buoyancy.
Fujita et al. (1977) documented a warm RFD (only
temperature data were available) near an F4 tornado in
the Chicago, Illinois, area on 13 June 1976. On the same
day, Brown and Knupp (1980) found nearly constant u w
3 km east of the Jordan, Iowa, F3 and F5 tornado pair,
and those observations probably were in the RFD air
mass, based on the pressure trace, which measured a
pressure excess of a few millibars.
In his summary of Totable Tornado Observatory
(TOTO) observations, Bluestein (1983) documented a
relatively warm RFD and pressure rise of .2 mb in a
nontornadic supercell on 17 May 1981. Bluestein also
presented evidence of a 1.5-K temperature rise in an
RFD approximately 1.3 km south of the Cordell,
Oklahoma, tornado on 22 May 1981.6 Similar to what
Fujita (1958b) first inferred, Bluestein also showed data
that suggested high pressure at least partly encircling a
tornado (his Fig. 7). In the violent Binger, Oklahoma,
tornado on 22 May 1981, only small temperature fluc6
The reported pressure and temperature fluctuations were with respect to the storm inflow environment.
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FIG. 9. Pressure field near the center of the Fargo tornado cyclone (20 Jun 1957); A and B represent barograph
stations. [Adapted from Fujita (1958b).]
tuations (,1 K) were observed as the tornado passed
within a few hundred meters north of TOTO (Fig. 10).
Klemp et al. (1981) referred to ‘‘cold downdraft
(RFD) outflow’’ in the Del City supercell, but no evidence was presented demonstrating that this air actually
was cold—retrieved temperatures by Brandes (1984a)
and observations by Johnson et al. (1987) suggested that
at least parts of the Del City storm’s rear-flank outflow
were warm, although u w values may not have been as
large as in the inflow.
Although there have been surface observations of
warm, high-u e air within RFDs, three-dimensional numerical simulations of supercells almost invariably have
produced cold, low-u e RFDs at the surface (e.g., Klemp
et al. 1981; Rotunno and Klemp 1985; Fig. 11). The
pioneering numerical modeling studies of supercells
conducted in the 1970s (e.g., Schlesinger 1975; Klemp
and Wilhelmson 1978a,b; Wilhelmson and Klemp 1978)
and the parameter space studies of the 1980s (e.g., Weisman and Klemp 1982, 1984), with no known exceptions,
used warm rain microphysics. The relatively simple parameterization was not computationally demanding;
thus, experiments requiring large numbers of simulations were feasible. However, the exclusion of ice may
have promoted unrealistically excessive amounts of latent cooling near updrafts. When ice physics is included,
hydrometeors are distributed over a larger horizontal
region, and the intensity of outflow in close proximity
to the updraft is reduced (Gilmore and Wicker 1998;
Rasmussen and Straka 1998). In some recent, unpublished simulations, relatively warm downdrafts have
been produced at the surface when ice physics and a
relatively fine spatial resolution (,250 m in the horizontal directions) were used (M. Gilmore 2000, personal
communication).
d. Characteristics above the surface
Johnson et al. (1987) presented observations of the
RFD and FFD of the Del City storm collected by a 444m tower as the storm passed overhead. The RFD was
associated with u e values approximately 4 K lower than
the ambient conditions; however, the temperature increased 1.5 K and the dewpoint temperature decreased
2.5 K—if u e was nearly conserved, then air had subsided
from approximately 1 km (all heights are above ground
level). Although the RFD was not sampled well by the
tower, the data that were available suggested the presence of a downward-directed perturbation pressure gradient within the lowest half kilometer.
Dowell and Bluestein (1997) documented the passage
of another tornadic supercell over the same instrumented
860
FIG. 10. Pressure, wind speed, wind direction, and temperature
traces from TOTO near the Binger, OK, tornado on 22 May 1981
from approximately 1922–1931 CST. Only relative wind direction is
known; veering (backing) is indicated by a downward (upward)
change. The tornado passed approximately 600 m north of TOTO.
Note the small (less than ;1 K) temperature fluctuations recorded.
It is inferred that the hook echo and RFD associated with this violent
tornado were relatively warm. [From Bluestein (1983).]
tower on 17 May 1981. They found large u e and u y
(virtual potential temperature) deficits in the portion of
the RFD sampled by the tower (.12 and .5 K, respectively), which was within a region of fairly high
(.40 dBZ) radar reflectivity west of the circulation center. The deficits were prominent over the entire height
of the tower.
With the exception of the direct observations presented by Johnson et al. (1987) and Dowell and Bluestein (1997), thermodynamic quantities above the
ground within RFDs have been obtained only by indirect
means. Brandes (1984a) and Hane and Ray (1985) were
among the first to use the methods proposed by GalChen (1978) and Hane et al. (1981) to retrieve thermodynamic fields in supercells from three-dimensional
wind fields synthesized from multiple Doppler radars.
Brandes (1984a) retrieved the pressure and buoyancy
fields in the Del City and Harrah, Oklahoma (8 June
1974) tornadic storms. At 3.3 km on the rear side of
the updraft of the Del City storm, relatively cold tem-
VOLUME 130
peratures were retrieved within the RFD—radar reflectivity was a minimum here, possibly implying that evaporation was occurring. Behind the rear-flank gust front
in the eastern mesocyclone quadrants,7 evidence was
found of warm temperatures at low levels during the
tornadic stage. The relatively warm conditions were attributed to subsidence within the RFD. In the Harrah
storm, Brandes (1984a) also retrieved negative buoyancy in the RFD aloft, but no mention was made of the
low-level buoyancy immediately behind the gust front
in the eastern quadrants of the mesocyclone.
Hane and Ray (1985) also completed a thermodynamic retrieval for the Del City storm. In the pretornadic
stage, the pressure distribution included at each level a
high–low couplet across the updraft with the maximum
horizontal pressure gradient generally oriented along the
environmental shear vector at that altitude, in agreement
with linear theory predictions (Rotunno and Klemp
1982). Although the orientation of the horizontal pressure gradient agreed relatively well with linear theory,
its magnitude did not agree as well. The authors stated
that possibly the orientation and magnitude of the environmental shear vector were not known exactly (also,
calculations of the horizontal vertical velocity gradient
may have had significant errors). In the tornadic stage,
the pressure field contained a pronounced minimum at
low levels coincident with the mesocyclone, probably
due to strong low-level vertical vorticity. Hane and Ray
found weak high perturbation pressure (;1 mb) in the
RFD at low levels behind the gust front during the time
of the tornado. Vertical gradients of nonhydrostatic pressure perturbations may be relevant to the formation and
evolution of the RFD, as will be discussed in the next
subsection. The RFD contained significant negative
buoyancy at low levels in Hane and Ray’s analysis (temperature deficits as low as 24.5 K; Fig. 12).
The retrieval results of Brandes (1984a) and Hane
and Ray (1985) in the Del City storm were in relatively
close agreement with the direct measurements within
the inflow and FFD regions reported by Johnson et al.
(1987). But Johnson et al. cautioned that ‘‘noticeable
differences in the RFD region suggested that there was
room for improvement in the retrieval methods.’’ Brandes and Hane and Ray had used considerable smoothing
on the buoyancy field to eliminate noise; the details in
the retrieved low-level buoyancy fields may have been
suspect.
e. Origins and formation
In reviewing previous studies and hypotheses pertaining to RFD formation, it may be helpful to begin
7
The RFD had wrapped cyclonically around the updraft, so that
downdraft air also was found in the eastern (forward) quadrants of
the mesocyclone. This ‘‘occlusion process’’ will be discussed in section 4.
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861
MARKOWSKI
FIG. 11. Contours at z 5 250 m of (a) potential temperature fluctuation (u 5 303 K), in intervals of 1 K; and (b)
equivalent potential temperature fluctuation (u e 5 340 K), in intervals of 2 K, in the simulation conducted by Rotunno
and Klemp (1985). The updraft is represented by the shaded region. The thick circular line encloses the region where
the cloud water is greater than 0.1 g kg 21 (wall cloud). The thick dashed line encloses the region where the cloud
water is greater than 0.1 g kg 21 at z 5 500 m. [Adapted from Rotunno and Klemp (1985).]
with an inspection of the inviscid vertical momentum
equation written as
dw
]w
]p
5
1 v · =w 5 2c p u
1 B,
dt
]t
]z
(1)
where dw/dt is the vertical acceleration following a parcel, ]w/]t is the local vertical acceleration, v 5 (u, y , w)
is the three-dimensional velocity vector, 2v · =w is the
advection of vertical velocity, c p is the heat capacity,
u is the mean potential temperature, p is the perturbation
Exner function, and B is the buoyancy which can be
written as
B5g
FIG. 12. Horizontal distribution of buoyancy and vector horizontal
wind at 1 km at 1847 CST 20 May 1977 in the Del City, OK, storm.
Buoyancy (potential temperature fluctuation) has been filtered to remove noise, and is contoured at 1.58C intervals (negative values
dashed). Updraft maxima are denoted by J and vorticity maxima
are denoted by !. [From Hane and Ray (1985).]
1 u 1 0.61q9 2 q 2 q 2 ,
u9
y
l
i
(2)
where u9 and q9y are potential temperature and water
vapor mixing ratio fluctuations from the base state, respectively, and q l and q i are liquid water (includes cloud
water and rain water) and ice mixing ratios, respectively.
By taking the divergence of (1) and assuming that
=p ; 2p (a reasonable assumption for ‘‘well-behaved’’ fields away from the boundaries), it can be
shown (Rotunno and Klemp 1982) that
862
p}
[1 2
]u
]x
1
1 2 1 2
]y
]y
2
1
2
1
]w
]z
2
]B
,
]z
]
1
where | D | and | h | are the magnitudes of the total deformation and vorticity, respectively. The [(]u/]x) 2 1 (]y /
]y) 2 1 (]w/]z) 2 ] terms are referred to as the fluid extension terms. If (3) is linearized about a base state containing vertical wind shear (primed velocity components
represent fluctuations from the base state, which is given
by v (z) 5 [u (z), y (z), 0]), it may be rewritten as
p}
[1
]y 9
1
]y
2
]w9
1
]z
]
2
]v
]B
· =w9 2
]z
]z
12
1
1 2c p u
5 2c p u
2
]p b
1B
]z
1
2
(11)
where 2c pu ]p dn /]z sometimes is referred to as the dynamic forcing and (2c pu ]p b /]z 1 B) sometimes is referred to as the buoyancy forcing. If the vertical gradients of the fluid extension terms are neglected, along
with the vertical gradients of the deformation and horizontal vorticity, then it can be shown that
1
(4)
(10)
]p dn
]p
1 2c p u b 1 B ,
]z
]z
]p nl
]z 2
}2
]z
]z
2 1 2 1 2
]y 9 ]u9
]w9 ]u9
]w9 ]y 9
1 21
1
1
]x ]y
]x ]z
]y ]z 2
2
2
]w
]p
]p
1 v · =w 5 2c p u nl 2 c p u l
]t
]z
]z
1
1 [|D| 2 2 |h| 2 ]
2
(3)
]u9
]x
VOLUME 130
(12)
2
]p l
] ]v
}
· =w
]z
]z ]z
(13)
]p b
]2B
} 2 2,
]z
]z
(14)
(9)
where z is the vertical vorticity.
From (11) it is evident that descent can arise owing
to negative buoyancy, which can be generated according
to (2) by cold anomalies produced by evaporative cooling or hail melting, or by precipitation loading, and by
vertical perturbation pressure gradients that can arise
from, according to (12)–(14), vertical gradients of vertical vorticity, ‘‘stagnation’’ of environmental flow at
an updraft,8 and pressure perturbations due to vertical
buoyancy variations (which are partially due to hydrostatic effects), respectively.9 Research presented in the
past 40 years has found that all of the terms in (11) can
be significant.
Browning and Ludlam (1962) and Browning and
Donaldson (1963) suggested that the RFDs in the Wokingham and Geary supercells might have been driven
by negative buoyancy (i.e., ‘‘thermodynamically’’
forced) due to evaporation. Browning (1964) surmised
that the rightward propagation of supercells increased
and p dn collectively refers to the nonlinear and linear
effects as ‘‘dynamic’’ effects on p. Equation (7) indicates that nonlinear dynamic high (low) pressure perturbations are associated with convergence and divergence and deformation (rotation). Equation (8) indicates
that linear dynamic high (low) pressure perturbations
are located upshear (downshear) of an updraft. Equation
(9) indicates that high (low) pressure perturbations due
to buoyancy are located above (below) the level of maximum buoyancy.
Using (5) and (6), the vertical momentum equation
can be written as
8
Note that the use of the term stagnation here does not imply that
supercell updraft are solid obstacles, as as been suggested by several
investigators in the past (e.g., Newton and Newton 1959; Fujita 1965;
Fujita and Grandoso 1968; Alberty 1969; Fankhauser 1971; Charba
and Sasaki 1971; Brown 1992). Theoretical studies have exposed
serious weaknesses in the obstacle analogy (e.g., Rotunno 1981; Rotunno and Klemp 1982; Davies-Jones et al. 1994), and these studies
also have shown that the pressure distribution around an updraft is
not what would be expected if the updraft was behaving as an obstacle, except at the storm top (Davies-Jones 1985). Some studies
have shown that updrafts occasionally can display behavior that appears similar to how a solid obstacle might be expected to behave
(e.g., Lemon 1976b; Klemp et al. 1981).
9
For additional discussion of downdraft forcings in terms of the
vertical momentum equation, the reader is referred to the review by
Knupp and Cotton (1985).
5 p nl 1 p l 1 p b
(5)
5 p dn 1 p b ,
(6)
where p nl , p l , and p b are the contributions to p from
nonlinear, linear, and buoyancy effects, respectively,
p nl }
[1
]
2 1 2 1 2
]y 9 ]u9
]w9 ]u9
]w9 ]y 9
1 21
1
1
]x ]y
]x ]z
]y ]z 2
]u9
]x
2
1
]v
· =w9
]z
pl } 2
pb } 2
]B
,
]z
]y 9
]y
2
1
]w9
]z
2
(7)
(8)
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MARKOWSKI
their midlevel storm-relative flow so as to increase evaporative cooling, and ultimately aid in the genesis of
downdrafts (both on the rear and forward storm flanks).
These hypotheses were proposed at least partly because
of findings by Browning and Ludlam (1962) and Browning and Donaldson (1963) of low u w air in the wakes
of the Wokingham and Geary storms, which apparently
had midlevel origins.
Brandes (1981) also concluded that RFDs are initiated
by the production of negative buoyancy aloft: ‘‘presumably the initiating downdraft (associated with the rearflank gust front) is formed by precipitation falling from
the sloping updraft . . . we suppose the intruding flow
has low u w , and when chilled by evaporation, becomes
negatively buoyant . . . because the entrained air penetrates well into the storm, evaporative cooling rather
than perturbation pressure forces may initiate the downdraft.’’ Brandes (1984a) made a similar claim, based on
retrieved buoyancy: ‘‘at 3.3 km, cool temperatures on
the southern fringe of the storm were suggestive of evaporative cooling as environmental air mixed with storm
air.’’
Klemp et al. (1981) attributed the RFD in the Del
City storm to water loading and evaporation based on
precipitation trajectories crudely approximated using estimated terminal fall speeds. Moreover, midlevel flow
approaching the storm from the east flowed through the
FFD—not through the RFD as Browning (1964) had
conceptualized. RFD air at the surface appeared to have
come from 1–2 km above the ground, directly behind
the gust front, based on trajectory analyses in their numerical simulation and observations of the storm. Air
from higher levels reached the surface further behind
the storm.
Nelson (1977) found an erosion of the hydrometeor
field at and below 7 km, as well as a sharp reflectivity
gradient on the west flank of an Oklahoma multicell
storm that evolved into a supercell on 25 May 1974—
these radar observations were believed to have been a
manifestation of RFD formation that apparently occurred at the start of the transition from multicell to
supercell. Nelson noted two mechanisms suggestive of
RFD formation—evaporative cooling and/or dynamic
pressure perturbations (presumably he was referring to
those related to linear effects, i.e., stagnation). Nelson
believed that the evaporation-driven effect was more
likely because of the echo erosion aloft; he also cited
strong storm-relative winds (;16 m s 21 in the 7–9 km
layer) and a large dewpoint depression (;21 K) at the
level of apparent RFD formation. Forbes (1981) found
similar radar signatures suggesting echo erosion and
RFD formation during the 3–4 April 1974 tornado outbreak.
Lemon et al. (1978) and Barnes (1978a) also concluded that the RFD forms at middle to upper levels.
Lemon et al. based their findings on an analysis of the
Union City tornadic supercell. They analyzed a persistent difluent flow region in the 7–10 km layer northwest
863
(upshear) of the mesocyclone that they believed was
associated with a downdraft. Barnes’ conclusion that
the RFD formed between 6.0–7.5 km was based on his
study of tornadic storms in Oklahoma on 29–30 April
1970. He surmised that the storm-relative midlevel flow
(20–25 m s 21 ) approaching the cyclonically rotating
updraft was decelerated and deflected on the upwind
(south) side while the relative upwind stagnation point
shifted to the left of the intercepting wind vector; that
is, toward the southwest flank. Here ‘‘stagnating’’ air
experienced the longest contact with the adjacent updraft while mixing only slightly with it—both cloud and
small precipitation drops chilled this air by evaporation
and began its downward acceleration before saturation
could occur. Barnes added ‘‘We emphasize that the high
horizontal momentum and proximity to the updraft
make the RFD a potentially important interactant with
the gust front and updraft’s surface roots . . . We also
note that the location and extent of such a downdraft
probably depends upon the ambient flow relative to the
storm, which very likely requires a specific vertical
shear profile to place it on the rear flank of a storm
where it attains an influential position.’’ Barnes interpreted the large reflectivity gradient on the midlevel
upwind (southwest) flank as indicating dry ambient air
adjacent to a precipitation-laden updraft. Bonesteele and
Lin (1978) made a similar inference.
Lemon and Doswell (1979) developed a conceptual
model of a supercell from an extensive compilation of
surface, visual, and radar observations (Fig. 13). This
model included an FFD and RFD, a surface gust front
structure resembling a midlatitude cyclone, a hookshaped reflectivity region surrounding a cyclonically rotating updraft, and a tornado, if present, that resided
within the vertical velocity gradient between the updraft
and RFD. This model has undergone little modification
since its presentation over 20 years ago. Based largely
on the work by Barnes (1978a,b), Lemon and Doswell
inferred that the RFD typically originated between 7–
10 km on the relative upwind side of the updraft [note
that they did not say upshear side; refer to (8); Rotunno
and Klemp (1982) showed that the linear forcing for
pressure fluctuations depends on the vertical shear, and
numerical results confirmed this theoretical prediction,
as did some later dual-Doppler radar findings (e.g., Hane
and Ray 1985)]. The authors cited the observation of
an echo-free hole at 7.5 km, directly above a notch
behind the low-level hook echo—they believed this to
be the signature of the RFD. Lemon and Doswell proposed that storm-relative inflow impingement was the
RFD source, because Darkow and McCann (1977)
showed that the relative flow at these levels is much
stronger than the storm-relative flow minimum they
found at 4 km, and because of the Barnes (1978a,b) and
Nelson (1977) observations. Lemon and Doswell also
hypothesized that the RFD initially is dynamically
forced, and then enhanced and maintained by precipitation drag and evaporative cooling.
864
VOLUME 130
behind the gust front, air from higher levels reached the
surface.
For the sake of completeness, it might be worth mentioning that Shapiro and Markowski (1999) recently investigated the formation of downdrafts in simple twolayer vortices using an analytic model.10 The applicability of the idealized model to real atmospheric vortices, in which buoyancy, buoyancy gradients,
precipitation, and asymmetries probably are important,
is questionable. Their results demonstrated how the
‘‘vortex valve’’ effect (Lemon et al. 1975; Davies-Jones
1986) can transport vorticity from the top of a homogeneous, axisymmetric, rotating fluid to low levels via
an annular downdraft and secondary circulation, when
the top layer of fluid rotates with an angular velocity
larger than that of the bottom layer of fluid.
Prior to 1983, investigators sought forcing for the
RFD from middle and upper levels, as has been reviewed in this section. But downdraft forcing also can
arise at low levels. This is the subject of the next section.
FIG. 13. Conceptual model of a tornadic supercell at the surface
based on observations and radar studies. Thick line encompasses
radar echo. The thunderstorm gust front structure and occluded wave
also are depicted using a solid line and frontal symbols. Surface
positions of the updraft (UD) are finely stippled, forward-flank downdraft (FFD) and rear-flank downdraft (RFD) are coarsely stippled.
Associated streamlines are also shown. Tornado location is shown by
an encircled T. [From Lemon and Doswell (1979).]
Brandes (1984a) retrieved excess pressure aloft on
the rear of the Del City storm, which may have suggested that the RFD was partly forced by a downwarddirected dynamic pressure gradient, as Lemon and Doswell had proposed, but the vertical pressure gradients
in the stagnation region could not be examined due to
a paucity of scatterers and corresponding lack of radar
velocity data. (A lack of data due to the pristine air
common in RFD regions probably still is one of the
biggest obstacles in understanding the formation mechanisms and the role of the RFD today.)
Klemp et al. (1981) simulated a supercell with a composite sounding derived from three ‘‘proximity’’ soundings on 20 May 1977, and compared the simulated storm
characteristics to those observed in the Del City storm.
‘‘Trajectory’’ analysis (these were not true trajectories,
but rather streamlines—if the storm was assumed to be
quasi-steady, then the streamlines would be similar to
trajectories) in the simulated supercell showed obstaclelike flow at 7–10 km. Parcels at 7 km that impinged
upon the upshear side of the updraft did not appear to
sink [in contrast to observations made in different cases
by Barnes (1978a), Nelson (1977), and Lemon and Doswell (1979)], but those at 4 km did; that is, the RFD
apparently was 4–7 km deep (parcels from 4–7 km did
not reach the surface, but negative vertical velocities
extended to 4–7 km). Directly behind the gust front,
RFD air appeared to come from 1–2 km aloft; farther
4. Occlusion downdrafts
a. Evolution as simulated in numerical models
Klemp and Rotunno (1983) investigated the transition
of a supercell into its tornadic phase through use of a
high-resolution (250-m horizontal grid spacing) model
initiated within the interior of the domain of the Del
City supercell simulation performed by Klemp et al.
(1981). With the enhanced resolution, Klemp and Rotunno found that the low-level cyclonic vorticity increased dramatically and the gust front rapidly occluded
as small-scale downdrafts developed in the vicinity of
the low-level circulation center. They concluded that the
intensification of the RFD during the occlusion process
was dynamically driven by the strong low-level circulation, that is, by way of a dynamic perturbation pressure
gradient such as that given by (12). This was the first
study to propose such a mechanism for downdraft genesis and intensification. Later, Brandes (1984a,b), Hane
and Ray (1985), and Brandes et al. (1988; see section
3) made the same conclusion based on Doppler radar
analyses of tornadic storms, as did Trapp and Fiedler
(1995), Wicker and Wilhelmson (1995), and Adlerman
et al. (1999) based on numerical simulations.
Klemp and Rotunno (1983) defined the RFD as the
downdraft ‘‘which supports the storm outflow behind
the convergence line on the right flank.’’ They stated
that since nontornadic storms often were observed to
persist for long periods of time with a well-defined gust
front, these storm-scale downdrafts were not uniquely
linked to tornadogenesis within a storm. On the other
hand, noted Klemp and Rotunno, if a storm progressed
into a tornadic phase, the gust front became occluded
10
Idealized three-layer vortices also were investigated using a numerical model.
APRIL 2002
MARKOWSKI
FIG. 14. Low-level flow field (z 5 1 km) in Klemp and Rotunno’s
(1983) nested, 250-m horizontal resolution simulation of the Del City,
OK, supercell (20 May 1977) at the time that an occlusion downdraft
was observed. The region in which the cloud water exceeds 0.4 g kg 21
is shaded. The heavy black line encloses the region where the rain
water mixing ratio exceeds 0.5 g kg 21 . The circulation center is
indicated with the black dot. Vertical velocity is contoured at 5 m s 21
intervals. What has been referred to as the occlusion downdraft is
located within a broader region of negative vertical velocities, which
has been referred to as the rear-flank downdraft. [Adapted from Klemp
and Rotunno (1983).]
and a strong downdraft formed directly behind the gust
front at low levels and also might divide the updraft at
midlevels. Klemp and Rotunno referred to this smallerscale downdraft as the occlusion downdraft. The occlusion downdraft, which was associated with a local vertical velocity minimum, was situated within a broader
region of negative vertical velocity, which was referred
to as the RFD; that is, the downdraft region on the rear
side of the storm, which partially wrapped around the
circulation center at the time when low-level vertical
vorticity reached its peak, was a single, contiguous entity (Fig. 14). The occlusion downdraft also was located
within a larger, contiguous downdraft region in the highresolution simulations of Wicker and Wilhelmson
(1995; their Figs. 5–9).
Klemp and Rotunno proposed that the occlusion process and its associated occlusion downdraft were dynamically induced by the strong near-ground rotation
that evolved along the convergence line. The rotation
induced low pressure coincident with the center of circulation and dynamically forced air down from above—
the downdraft formed first at low levels and then extended upward as the flow adjusted to the nonhydrostatic
vertical pressure gradient force. Rotunno (1986) also
hypothesized that the occlusion downdraft was initiated
by the explosive growth of vertical vorticity at low levels.
865
The finding of Klemp and Rotunno that the occlusion
downdraft is driven by low-level rotation sometimes has
been implied as being in conflict (e.g., Carbone 1983;
Brandes 1984a; Klemp 1987) with their predecessors’
early hypotheses that the RFD is driven from aloft thermodynamically or dynamically and is responsible for
increasing low-level rotation (e.g., Fujita 1975b; Burgess et al. 1977; Barnes 1978a; Lemon and Doswell
1979). However, it is the author’s opinion that the apparent conflict may be one of semantics. If the occlusion
downdraft and RFD are to be viewed as two distinct
entities, as proposed by Klemp and Rotunno, then the
formation mechanisms and roles of the occlusion downdraft and RFD should not be anticipated to be necessarily identical. Observations of RFD formation preceding the increase of vorticity near the ground are plentiful (e.g., Barnes 1978a; Lemon and Doswell 1979;
Brandes 1984a,b). Once a downdraft forms, the distribution of vortex lines invariably must be affected according to Helmholtz’s theorem, and feedbacks on the
downdraft by the new vorticity distribution would be
probable (e.g., when rotation near the ground becomes
substantial, a downward-directed vertical pressure gradient could become established, accelerating air toward
the ground). Early hypotheses that the RFD is responsible for bringing rotation to low levels never asserted
that once low-level rotation began to intensify that dynamic effects could not feed back on the downdraft.
Therefore, it is believed that the occlusion downdraft
can be viewed as a rapid, small-scale intensification of
the RFD that occurs after the RFD transports larger
angular momentum air toward the ground. Stated another way, the occlusion downdraft may be viewed as
a by-product of the near-ground vorticity increase, and
vorticity cannot become large next to the ground without
the RFD (Davies-Jones 1982a,b; Davies-Jones and
Brooks 1993; Brooks et al. 1993, 1994; Wicker and
Wilhelmson 1995); that is, the RFD is ultimately necessary for occlusion downdraft formation. Perhaps not
surprisingly, observations have not been made in supercell storms of an occlusion downdraft preceding or
occurring in the absence of an RFD, or even at a location
not within an RFD.11
It might be tempting to argue that the RFD and occlusion downdraft should be considered separate down11
In some nonsupercell tornadoes in which preexisting vertical
vorticity is present at the surface, an RFD is not needed to transport
circulation to low levels in order for tornadogenesis to occur. It might
be possible for the low-level vorticity amplification associated with
this tornadogenesis process to dynamically induce a downdraft similar
to that which Klemp and Rotunno (1983) called an occlusion downdraft in a supercell storm; thus, it might be possible for an occlusion
downdraft to occur in the absence of an RFD in such a case. Carbone
(1983) compared a downdraft he observed in a nonsupercell tornado
event to the occlusion downdraft defined by Klemp and Rotunno.
However, the dominant forcing for the downdraft was uncertain; thus,
it is not known whether the downdraft documented by Carbone (1983)
should be regarded as an occlusion downdraft, at least as defined by
Klemp and Rotunno.
866
drafts because the dominant forcings are different. It is
the author’s opinion that a contiguity criterion should
be considered when debating whether two phenomena
having different forcings are regarded as ‘‘separate.’’
Otherwise, the updraft of a supercell should be viewed
as two separate updrafts, with one updraft being driven
by nonhydrostatic pressure gradient forces below the
level of free convection, and another updraft being driven largely by buoyancy forces above the level of free
convection. The evolution put forth in the paragraph
above is not at odds with early proposals that the RFD
is initiated at middle to upper levels and is responsible
for initiating rotation near the ground, nor is it in conflict
with contentions that strong subsidence develops near
the tornado during or after its formation.
It is speculated that the clear slot may be a visual
manifestation of an intensifying RFD or occlusion
downdraft. Updrafts also have been shown to weaken
during the stage when low-level rotation rapidly increases, probably also due to the formation of a downward-directed dynamic pressure gradient induced by the
rotation (e.g., Brandes 1984a,b). Fujita (1973), Lemon
and Burgess (1976), and Burgess et al. (1977) have
documented the collapse of overshooting storm tops
near the time of tornadogenesis, which presumably is a
manifestation of updraft weakening.
It also should be noted that although the occlusion
downdraft in the Klemp and Rotunno (1983) simulation
was found to be driven by low-level vertical vorticity
amplification, the occlusion downdraft did not descend
along the axis of low-level rotation. An explanation was
not offered. One might expect that the vertical pressure
gradient associated with the vertical gradient of vertical
vorticity would lead to a maximum acceleration along
the rotation axis. Two reasons might account for the
asymmetry: 1) the dynamic vertical perturbation pressure gradient associated with the vertical gradient of
vertical vorticity squared (]z 2 /]z) does not contribute to
vertical velocity directly, but rather to vertical accelerations—thus, dw/dt might be a minimum in the vorticity maximum center, but if this occurs within the updraft (where w k 0), then a downdraft (w , 0) may
first appear on the periphery of the updraft, away from
the center of rotation, where w is less positive; and 2)
other terms in the vertical momentum equation, when
combined with the dynamic vertical perturbation pressure gradient force, may force the strongest downward
acceleration away from the axis of largest vertical vorticity—for example, the buoyancy forcing may favor
ascent in the updraft center, so that the net effect of the
buoyancy forcing and dynamic vertical perturbation
pressure gradient may lead to the strongest downward
acceleration on the updraft periphery. Superposition of
the fields of the vertical momentum equation forcing
terms in Klemp and Rotunno’s (1983) simulation leads
to the strongest downward acceleration being to the
southeast of the maximum low-level rotation (Fig. 15).
Thus, it is entirely possible for an occlusion downdraft
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to be ‘‘driven’’ by low-level rotation even if the occlusion downdraft is not collocated with the low-level rotation.12
b. Observations
Brandes (1978; the Harrah storm) appears to have
made observations prior to the Klemp and Rotunno
(1983) simulation of a downdraft not becoming prominent until after low-level rotation became substantial.
Brandes (1981) also stated, following his analysis of the
Del City storm, that ‘‘the sudden appearance of strong
rear downdrafts in storms persisting for hours may also
relate to the intensity and distribution of updrafts and
vorticity.’’ Brandes (1984a) attributed sudden occlusion
downdraft formation in the Del City and Harrah storms
to the vertical pressure gradient owing to the explosive
growth of low-level vorticity as Klemp and Rotunno
(1983) found. Furthermore, Brandes’s data also showed
that the occlusion downdraft did not descend along the
axis of the strong low-level vorticity. Brandes (1984b)
claimed that the occlusion downdraft formed after the
incipient tornado had been detected, and roughly coincided with tornado formation. Hane and Ray (1985)
also documented occlusion downdraft formation in the
Del City storm.
Based on their analyses of the Lahoma and Orienta,
Oklahoma, supercells (2 May 1979), Brandes et al.
(1988) hypothesized that because RFDs possess weak
positive or negative helicity (because couplets of vertical vorticity straddle RFDs), the decline of storm circulation might be hastened by turbulent dissipation
when the downdraft air eventually mixes into supercell
updrafts. As did Brandes (1984a,b) and Klemp and Rotunno (1983), Brandes et al. claimed that ‘‘the updraft
minimum in the Lahoma storm and RFD in the Orienta
storm apparently owed their existence to the build-up
of low-level vorticity and related downward vertical
pressure gradients.’’ Large downward pressure forces
existed within the RFD and left-hand portions of the
persistent updraft region in the Orienta storm, and to
the rear of the persistent updraft in the Lahoma storm.
Brandes et al. (1988) probably presented the most comprehensive analyses, discussion, and insight into the
pressure distribution in supercells to date.
5. Role of RFDs in tornadogenesis
a. Observations-based hypotheses
Ludlam (1963) was one of the first to write that downdrafts, especially those located on the rear flank of supercells, actually may be important to tornadogenesis:
‘‘It is tempting to look for the spin of the tornado in
12
Wakimoto and Cai (2000) proposed that it also may be possible
for an occlusion downdraft to reach the surface away from the center
of strongest near-ground vorticity if a mesocyclone is vertically tilted.
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FIG. 15. (a)–(c) Cross sections of the forcing terms in the vertical momentum equation at t 5 2 min and
z 5 250 m in the nested, 250-m horizontal resolution simulation by Klemp and Rotunno (1983). Contours
are drawn at 10 22 m s 22 intervals: (a) dynamically induced pressure gradient, (b) advection terms, and (c)
buoyancy forcing. The black dot indicates the location of the circulation center. [From Klemp and Rotunno
(1983).] (d) A composite of the forcing terms in the vertical momentum equation. The hook echo and regions
where w exceeds 0 and 3 m s 21 are shaded according to the legend. The region where downward local
vertical velocity accelerations (]w/]t) are largest also is shaded. This region is located where the superposition
of the buoyancy forcing, advection, and dynamic pressure forcing leads to the strongest downward local
vertical velocity changes (]w/]t K 0). The regions where the buoyancy forcing and advection of vertical
velocity are positive, and where the dynamic vertical pressure gradient is most negative (directed downward),
also are indicated in the legend. Note that the largest downward acceleration occurs southeast of the lowlevel circulation center; a downdraft driven by increasing low-level rotation need not be collocated with the
axis of strongest rotation. [Adapted from Klemp and Rotunno (1983).]
the vorticity present in the general air stream as shear
and tilted appropriately in the vicinity of the interface
between the up- and down-motions.’’ Fujita (1975b)
also proposed that the downdrafts associated with hook
echoes may be fundamentally critical to tornado formation, in terms of his ‘‘recycling hypothesis’’: 1)
downdraft air is recirculated into the (developing) tor-
nado, 2) this process results in an appreciable convergence on the back side of the (developing) tornado, and
3) the downward transport of the angular momentum
by precipitation and the recycling of air into the tornado
will create a tangential acceleration required for the intensification of the tornado. Research conducted with
the aid of coherent radars in the ensuing years led others
868
(e.g., Burgess et al. 1977; Barnes 1978a; Lemon and
Doswell 1979; Brandes 1981) to make the same general
speculation. Burgess et al. (1977) believed that the RFD,
hook echo, and tornadogenesis were intimately connected: ‘‘The formation and evolution of the RFD is
judged extremely important to tornado formation. . .the
severe tornado [the Oklahoma City tornado of 8 June
1974] appears related to the increased vorticity source
provided by presumed downdraft intensification and
gust front acceleration along the right flank.’’ Forbes
(1981) also discovered signatures (e.g., a sharp reflectivity gradient along the upshear side of the updraft, and
occasionally a small echo mass several kilometers to
the right of the right-rear edge of the main echo) suggesting RFD formation (1–10 min) prior to tornadogenesis.
Lemon and Doswell (1979) noted that just before
tornadogenesis, the mesocyclone center shifted from
near the updraft center to the zone of high vertical velocity gradient. The early mesocyclone apparently was
a rotating updraft, whereas the transformed mesocyclone had a divided structure, with strong cyclonically
curved updrafts to the east in the ‘‘warm inflow sector’’
and strong cyclonically curved downdrafts to the west
in the ‘‘cold outflow sector.’’ And while the tornado was
apparently found in a strong vertical velocity gradient,
Lemon and Doswell noted that it probably was located
on the updraft side of that gradient.
Lemon and Doswell explained the evolution of the
RFD and tornadogenesis as follows: 1) air decelerates
at the upwind stagnation point, is forced downward, and
mixes with air below, which then reaches the surface
through evaporative cooling and precipitation drag; 2)
the initially rotating updraft is then transformed into a
new mesocyclone with a divided structure, in which the
circulation center lies along the zone separating the RFD
from the updraft (this process appears to result, in part,
from tilting of horizontal vorticity); and 3) ‘‘descent of
the mesocyclone circulation occurs simultaneously
(within the limits of temporal resolution) with the descent of the RFD.’’
Observations of low-level vorticity couplets within
RFDs that seem to straddle the hook echoes (introduced
in section 2a) may be indications that tilting of vorticity
by the RFD is important in the formation of tornadoes
within supercell storms, as hypothesized by DaviesJones (1982a,b) and Davies-Jones and Brooks (1993).
Furthermore, it may be worth noting that during the
tornadogenesis phase in supercells, the parcels of air
that enter the tornado or incipient tornado regularly
seem to pass through the hook echo and RFD (Brandes
1978; Klemp et al. 1981; Dowell and Bluestein 1997;
J. Wurman et al. 2000, personal communication;13 Fig.
16), which may have been the basis for Fujita’s (1975b)
recycling hypothesis. Visual observations of mesocy13
This was a poster presented at the 20th Severe Local Storms
Conference in Orlando, Florida.
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FIG. 16. Brandes’s (1978) conceptual model of low-level mesocyclone characteristics during the tornadic phase included an occluded mesocyclone with air parcels from the RFD feeding the tornado. [Adapted from Brandes (1978).]
clones being nearly totally occluded by the RFD, as
evidenced by observations of the clear slot during and
just prior to the tornadic stage, such as those made by
Lemon and Doswell (1979), Rasmussen et al. (1982),
and Jensen et al. (1983), also may imply that the air
entering the tornado comes from the RFD. The possible
importance of the clear slot also did not escape the
attention of Ludlam (1963): ‘‘. . . often the funnel is
photographed spectacularly against a segment of bright
and practically cloud-free sky beyond the edge of the
arch cloud.’’14 In addition to the observational evidence,
simulations also have indicated that air parcel trajectories pass through the RFD en route to intensifying
near-ground circulations (Davies-Jones and Brooks
1993; Wicker and Wilhelmson 1995; Adlerman et al.
1999).
Davies-Jones (1998) recently has questioned whether
the hook echo is really a passive indicator of a tornado,
since close-range airborne and mobile radar observations during recent field experiments have revealed hook
echo formation prior to tornadogenesis in every case.15
Davies-Jones hypothesized that the hook echo may actually instigate tornadogenesis, either baroclinically, by
way of buoyancy gradients within the hook echo (Davies-Jones 2000a), or barotropically, by redistributing
angular momentum (Davies-Jones 2000b).
14
The ‘‘arch cloud’’ Ludlam refers to was that ‘‘on the forward
right flank of severe storms, outside of the precipitation region;’’ that
is, that which is on the leading edge of the trailing gust front.
15
Previous studies documenting cases in which tornadoes were
reported prior to hook echo detection (e.g., Garrett and Rockney 1962;
Forbes 1975) relied on temporally and spatially coarser radar data
compared to what was available in field experiments such as VORTEX, and reported tornado times may not always have been accurate
to within a few minutes.
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b. Theoretical considerations
Most of the theoretical and numerical modeling studies pertaining to supercell storms have investigated the
development of midlevel and low-level rotation by way
of tilting of horizontal vorticity (either associated with
the large-scale mean vertical wind shear or generated
solenoidally by a baroclinic zone) by an updraft (e.g.,
Rotunno 1981; Rotunno and Klemp 1982, 1985; Lilly
1982, 1986a,b; Davies-Jones 1984). However, DaviesJones (1982a,b) noted that in order to obtain large vertical vorticity at the ground in an environment in which
vortex lines are initially quasi-horizontal, a downdraft
would be necessary. Tornadoes may arise in the absence
of a downdraft in environments containing preexisting
vertical vorticity at the surface, such as in some cases
of ‘‘nonsupercell tornadogenesis’’ (e.g., Wilson 1986;
Wakimoto and Wilson 1989; Roberts and Wilson 1995;
Lee and Wilhelmson 1997a,b, 2000).
Davies-Jones (1982a,b) concluded that in a sheared
environment with negligible background vertical vorticity, an ‘‘in, up, and out’’ circulation driven by forces
primarily aloft would fail to produce vertical vorticity
close to the ground [this conclusion depends on eddies
being too weak to transport vertical vorticity downward
against the flow; this was verified by Rotunno and
Klemp (1985) and Walko (1993)]. If a Beltrami model
is crudely assumed to represent the flow in a supercell
(Davies-Jones and Brooks 1993), then vortex lines are
coincident with streamlines and parcels flowing into the
updraft at very low levels do not have significant vertical
vorticity until they have ascended a few kilometers. Otherwise, argued Davies-Jones, abrupt upward turning of
streamlines, strong pressure gradients, and large vertical
velocities would be required next to the ground.
Davies-Jones (1982a,b) neglected baroclinic vorticity
and suggested that the downdraft had the following roles
in near-ground mesocyclogenesis: 1) tilting of horizontal vorticity by a downdraft produces vertical vorticity,
2) subsidence transports air containing vertical vorticity
closer to the surface, 3) this air flows out from the
downdraft and enters the updraft where it is stretched
vertically, and 4) convergence beneath the updraft is
enhanced by the outflow. Davies-Jones also showed kinematically that the flow responsible for tilting and concentrating vortex lines also tilts and packs isentropic
surfaces, thus explaining observations of strong entropy
gradients across mesocyclones near the ground.
c. Potentially relevant simulation results
Davies-Jones and Brooks (1993) showed that the vertical vorticity of air parcels descending in an RFD can
be reversed during descent, from anticyclonic initially,
to less anticyclonic, then to cyclonic in the lowest 50–
125 m of their descent. As air subsided in the downdraft,
vortex lines turned downward due to the barotropic
‘‘frozen fluid lines’’ effect (Helmholtz’s theorem), but
FIG. 17. Schematic diagram showing how cyclonic vorticity may
be generated from tilting of baroclinic horizontal vorticity in a downdraft. In the case of streamwise vorticity with flow to the right of the
horizontal buoyancy gradient and a southerly shear component, a
combination of tilting and baroclinic generation causes the vorticity
of parcels to change from anticyclonic (denoted by a) to cyclonic
(denoted by c) while still descending. [Adapted from Davies-Jones
and Brooks (1993).]
with less inclination than the trajectories because horizontal southward vorticity was being generated continuously by baroclinity within the hook echo. Because of
the geometry, vortex lines crossed the streamlines from
lower to higher ones (with respect to the ground), and
the barotropic effect served to turn the vortex lines upward even during descent. The baroclinic effect acted
to increase horizontal vorticity further but did not control the sign of the vertical vorticity; thus, air with cyclonic vertical vorticity appeared close to the ground.
As this air passed from the downdraft into the updraft,
its cyclonic spin was amplified substantially by vertical
stretching (Fig. 17).
Brooks et al. (1993, 1994) found that the formation
of persistent near-ground rotation was sensitive to the
strength of the storm-relative midlevel winds. When
storm-relative midlevel flow was weak, RFD outflow
undercut the updrafts and associated mesocyclones.
When storm-relative midlevel flow was too strong, the
cold pool was not oriented suitably for vorticity generation within the baroclinic zone immediately behind
the updraft, which was found to be needed for the development of near-ground rotation in their simulations.
Wicker and Wilhelmson (1995) used a two-way interactive grid to study tornadogenesis. During a 40-min
period, two tornadoes grew and decayed within the mesocyclone. Wicker and Wilhelmson’s Fig. 9 depicted a
spiraling, asymmetric RFD associated with tornadogenesis. Their figure also indicated anticyclonic vertical
vorticity on the opposite side of the RFD as the cyclonic
vertical vorticity. Furthermore, the RFD contained low
u e values (u9e was as small as 215 K; u9e was approximately 25 to 28 K in the hook echo).
Wicker and Wilhelmson found that parcels entered
the mesocyclone from the RFD (Fig. 18) and descended
870
FIG. 18. Vertical velocity and trajectories at 100 min (tornadolike
vortex present in simulated storm at this time) for z 5 100 m in the
120-m resolution simulation by Wicker and Wilhelmson (1995). The
trajectories entering the vortex have come from the hook echo and
RFD region. [Adapted from Wicker and Wilhelmson (1995).]
from ;500 m, as Davies-Jones and Brooks (1993) had
found. Furthermore, these parcels initially contained
negative vertical vorticity; however, vertical vorticity
increased to only weakly negative values, not to large
positive values as in the simulations of Davies-Jones
and Brooks. Trajectories into the tornado from the RFD
revealed that positive vertical vorticity was acquired
only after parcels began to ascend, not while they were
still descending (Davies-Jones and Brooks 1993; Trapp
and Fiedler 1995; Adlerman et al. 1999). It is believed
that these differences should be regarded as minor, for
there is a time tendency for increasing positive vertical
vorticity in the parcels passing through the downdraft
in all of the studies.
Though numerical models have been instrumental in
advancing our understanding of supercell dynamics,
models may have limited utility in exploring the thermodynamic characteristics of RFDs, and the potential
sensitivity of low-level vorticity intensification to these
characteristics, owing to the unavoidable parameterization of microphysical processes. For example, threedimensional simulations have not been able to produce
the warm and moist RFDs that often have been observed
near some strong tornadoes—the RFDs of simulated
supercells almost invariably have large potential temperature and equivalent potential temperature deficits,
as discussed in section 3c. However, some idealized
simulations have suggested that tornadogenesis may be
favorable if downdrafts are not too cold. Eskridge and
Das (1976) proposed that a warm, unsaturated downdraft could be important for tornadogenesis; however,
VOLUME 130
they did not specify whether the downdraft also could
be cold, nor what the advantages of a warm downdraft
over a cold downdraft were. Davies-Jones (2000b) recently has shown that an annular rain curtain can transport sufficient angular momentum from aloft to the
ground to result in tornadogenesis in an idealized axisymmetric numerical model.16 No evaporation was permitted in the model; thus, no cold downdraft air was
present. Furthermore, Leslie and Smith (1978) found
that some vortices could not establish contact with the
ground when low-level stable air was present, even if
very shallow. Remarkably, Ludlam (1963) many years
earlier had argued that ‘‘at least a proportion of the air
that ascends in the tornado must be derived from the
cold outflow; if this contains the potentially cold air
from middle levels its ascent might be expected soon
to impede if not destroy the tornado . . . it may be particularly important for the intensification and persistence
of a tornado that some of the downdraft air may be
derived from potentially warm air which enters the left
flank of the storm at low-levels.’’
Significant advances in our understanding of supercells no doubt have been made by numerical models. It
is probable that some conclusions drawn from simulation results never could have been made from observations or theory alone. However, the author shares the
view expressed by Doswell (1985): ‘‘The RFD’s role
remains confusing with respect to tornadogenesis. Truly
confirming evidence about the various aspects of numerical simulations awaits better observations, despite
the compelling similarities between simulations and real
storms.’’
6. Recent observations from VORTEX
New radar and surface observations having unprecedented spatial and temporal resolution have been acquired by VORTEX and smaller, post-VORTEX field
experiments. The findings of these recent operations that
are pertinent to this review are highlighted below, with
the exception of analyses of new surface data within
hook echoes and RFDs, which will be presented by
Markowski et al. (2002).
Recent radar observations, which include data obtained from both airborne and ground-based mobile
Doppler radars (Jorgensen et al. 1983; Ray et al. 1985;
Bluestein and Unruh 1989; Bluestein et al. 1995; Daugherty et al. 1996; Wurman et al. 1997), have resolved
structures within hook echoes that were barely resolvable or unresolvable in the early radar studies of supercell storms. For example, in high-resolution (,100
m spatially) data of tornadoes presented by Wurman et
16
This simulation had some similarities with those conducted by
Das (1983, unpublished manuscript), in which a precipitation-driven
downdraft was imposed upon a wind field in which angular momentum increased with height. The downdraft was found to be able to
transport sufficient vorticity from aloft to result in tornadogenesis.
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871
FIG. 19. Radar (left) reflectivity (dBZ ) and (right) velocity fields associated with a hook echo and tornado on 2 Jun 1995 near Dimmitt,
TX. In the left image, the arrow labeled N points toward the north. In the right image, the arrow labeled R points toward the radar position.
[From Wurman and Gill (2000).]
al. (1996), Wurman and Gill (2000), and Bluestein and
Pazmany (2000), the echo-free holes that were only marginally resolved by Garrett and Rockney (1962) were
well resolved (Figs. 19 and 20). The images presented
by Bluestein and Pazmany (2000) even begin to marginally resolve structures, possibly subvortices, within
the echo-free hole itself. Moreover, the radar reflectivity
depictions ‘‘looked like a tropical cyclone, with concentric inner bands and outer spiral bands’’ (Bluestein
and Pazmany 2000). Hook echoes as narrow as 100 m
or less have been detected (Wurman et al. 1996; Bluestein and Pazmany 2000), perhaps implying that precipitation loading and evaporative cooling within the
hook echoes of some storms may not be the most significant effects in driving the associated RFDs, at least
at low levels.
Just as the early multiple-Doppler radar analyses of
the 1970s and 1980s revealed, radar observations obtained in the last decade also have detected vorticity
couplets straddling the hook echoes of tornadic storms
(Rasmussen and Straka 1996; Wurman et al. 1996;
Straka et al. 1996; Bluestein et al. 1997; Dowell and
Bluestein 1997; Dowell et al. 1997; Wakimoto and Liu
1998; Wakimoto et al. 1998; Wurman and Gill 2000;
Ziegler et al. 2001) and nontornadic storms (Gaddy and
Bluestein 1998; Blanchard and Straka 1998; Wakimoto
and Cai 2000; Bluestein and Gaddy 2001). These vorticity doublets could be evidence that RFDs are involved
in a downward displacement of initially quasi-horizontal
vortex lines, perhaps necessarily transporting rotation
to the surface during tornadogenesis, as many previous
investigators (e.g., Ludlam, Fujita) have conjectured.
Using surface observations obtained from automobile-borne sensors (Straka et al. 1996), Rasmussen and
Straka (1996) documented a relatively warm RFD south
of the Dimmitt, Texas, tornado. In the same case, which
was during VORTEX, the hook echo was collocated
with the surface divergence maximum, implying an association between the hook echo and (at least) a lowlevel downdraft, as also had been suggested by numerous predecessors.
In two other VORTEX storms, Wakimoto et al. (1998)
and Wakimoto and Cai (2000) concluded that the occlusion downdraft was driven largely by the reversal of
the vertical gradient of dynamic pressure, owing to increasing vorticity at low levels. In the supercell documented by Wakimoto et al. (1998; the 16 May 1995
VORTEX storm), it was found that the precipitationloading forcing of the occlusion downdraft was an order
of magnitude less than the forcing provided by the nonhydrostatic vertical pressure gradient. [In a different
case, Carbone (1983) previously had suggested that precipitation loading may contribute to occlusion downdraft genesis.] In the 12 May 1995 VORTEX storm
studied by Wakimoto and Cai (2000), a thermodynamic
retrieval indicated that the occlusion downdraft was associated with a warm core.
Perhaps the most remarkable observational finding
during the last 10 years is that the differences between
tornadic and nontornadic supercells may be subtle, if
872
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FIG. 20. Radar reflectivity fields (dBZ) associated with a hook echo on 15 May 1999 (left) early in the life of a tornado, (center) during
the mature stage, and (right) during the dissipation stage. [From Bluestein and Pazmany (2000).]
even distinguishable, even in dual-Doppler radar analyses of the wind fields just prior to tornadogenesis. Blanchard and Straka (1998) documented a mobile radar
signature of a spiraling hook echo in a nontornadic supercell having an appearance similar to those that have
been associated with tornadic supercells (e.g., similar
to the radar image in Fig. 20). Perhaps such near-ground
circulations are more common in nontornadic supercells
than previously believed. Trapp (1999) and Wakimoto
and Cai (2000) also documented circulations in nontornadic supercells at levels close to the ground. Trapp
found that the low-level mesocyclones associated with
tornadogenesis had smaller core radii and were associated with more substantial vorticity stretching than
those associated with tornadogenesis ‘‘failure.’’ Based
on a comparison of pseudo-dual-Doppler analyses, Wakimoto and Cai concluded that the ‘‘only difference between the Garden City storm and Hays storm (the 16
May 1995 and 12 May 1995 VORTEX storms) was the
more extensive precipitation echoes behind the rearflank gust front for the Hays storm.’’
7. Concluding remarks
The association between hook echoes, RFDs, and tornadogenesis has been well documented for nearly 50
years; however, the precise dynamical relationship still
is not known today. The analysis of the three-dimensional wind structure of supercells afforded by Doppler
radar, along with speedy increases in the feasibility of
numerical cloud modeling, led to relatively rapid gains
in knowledge of the recurrent storm structures and evolution associated with supercells. Within 30 years of the
first radar image of a hook echo, we knew that the most
damaging tornadoes were associated with supercells, we
knew about the existence of the parent circulations of
tornadoes (mesocyclones), we developed an understanding of the dynamics of midlevel storm rotation and storm
propagation, radar and visual features common to supercells were well documented, and downdrafts were
recognized as being important in tornadogenesis. Yet in
the decades that followed the period of rapid advances,
no breakthroughs emerged with respect to the role of
the RFD in tornadogenesis. In fact, it is debatable whether we can better anticipate tornadogenesis within supercell storms today than we could 20 years ago.
If the hook echo and its associated RFD truly are
critical to tornadogenesis, as hypothesized for many
years, then perhaps significant gains in understanding
will not be possible until more spatially and temporally
detailed observations of this region can be made, in
addition to numerical simulations with more realistic
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MARKOWSKI
representations of entrainment and microphysical processes. It is believed that some of the important outstanding questions include
R What are the dominant forcings for RFDs, as a function of location within the RFD and stage in storm
evolution?
R How do the dominant RFD forcings vary across the
spectrum of supercell types (e.g., nontornadic vs tornadic, low-precipation vs heavy-precipitation
storms)?
R How do the thermodynamic and microphysical characteristics of hook echoes and RFDs vary across the
supercell spectrum, and why?
R How does the large-scale environment affect RFD
characteristics?
R Is the tornadogenesis process sensitive to the thermodynamic and microphysical properties of RFDs?
R What is the role of the RFD in tornadogenesis, and
does the hook echo have an active role?
A number of direct observations have been reviewed
herein; however, these observations have been relatively
scarce and often have been simply fortuitous. A new
mobile surface observing system (Straka et al. 1996),
introduced in 1994 for VORTEX, recently has collected
the largest number of in situ measurements within supercell storms to date. Analyses of these spatially and
temporally dense ‘‘mobile mesonet’’ observations within hook echoes and RFDs will be presented in a companion paper (Markowski et al. 2002), and these data
may begin to shed some light on at least a couple of
the questions posed above.
Acknowledgments. I am indebted to Drs. Jerry Straka
and Erik Rasmussen for their encouragement and perspectives during the course of this work. I also extend
thanks to Dr. Chuck Doswell and two anonymous reviewers, who noticeably improved the clarity and organization of the paper. Finally, I also am grateful to
Drs. Robert Davies-Jones, Fred Carr, and Brian Fiedler,
who reviewed earlier versions of the presentation.
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Hook Echoes and Rear-Flank Downdrafts: A Review

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