Untitled - Jabatan Meteorologi Malaysia

FOG OCCURRENCE DURING NORTHEAST MONSOON
Md Kamsan Hamdan, Suzalina Kamaruddin, Suhaily Sahat, Mohammad Redzuan
Abdul Moin, Mohd Hafiz Ismail, Mat Kamaruzaman Mat Adam and Mahani Ablah.
ABSTRACT
A statistical approach in order to study fog events occurrence, frequency,
duration, density and its characteristics during northeast monsoon over East
Coast States of Peninsular Malaysia including East Johor is presented in this
work. To achieve this, the fog events are investigated using November till March
(2004–2007) of observations of meteorological parameters such as: visibility, air
temperature, dew point temperature and depression, air relative humidity, wind
vector and cloud base height and coverage at 7 different stations. Half hourly
and/or hourly surface observations of that same period are used to identify fog
events induced by various physical mechanisms at these stations. The temporal
variability of different fog type occurences are examined and the events are
characterized according to their duration and intensity.
The results are somehow affected by regional and local factors of individual
stations. The inland stations, Temerloh (WMBE) and Kuala Krai (WMAL) showed
more numbers of foggy days compared to other stations and fog occurrences over
this area are most dominated by evaporation cooling (frontal fog). Places near
high terrain such as Kuantan airport (WMKD) are usually covered with low lying
clouds in the morning that become heavy fog and have more foggy days
compared to other stations near coastline. Kuantan airport has recorded several
days of fog dominated by radiation cooling (radiation fog) and movement of moist
air along mountain (upslope fog). All fog events occurred mostly in November
between 0400 LT to 0900 LT and is often observed occurs when there have been
instances rain in previous day.
1.0 Introduction
1.1 Fog, mist and haze
Fog and mist technically can be considered are the same things as cloud. They
are all formed when air cools to its dew point. At this point moisture condenses
onto small particles in the air. Haze is different from mist and fog. It is a reduction
in visibility due to dust or smoke in the air and is not related to cooling of moisture.
It can, however, combine with fog to make smog.
The World Meteorological Organization (WMO) reports “fog” (with symbol
FG) is the suspension in the air of very small water droplets or crystals, which
reduces horizontal visibility to less than 1,000 meters and “mist” (with symbol BR),
which is otherwise similarly defined, when visibility is from 1,000 to 5,000 meters.
Weather records (METAR and/or SPECI) show that fog occurs as the surface
temperature approaches the dew point temperature and the relative humidity is
greater than 95 percent but generally near 100 percent. Normally, during the early
morning after rain showers when temperatures are low and humidity is high, mist
or sometimes fog forms in valleys and lowlands and gradually clearing when the
sun reappears. The formation of fog layer occurs when moist air is cooled to its
saturation point (dew point). This cooling can be result of radiation fog, advection
of warm air over cold surfaces (advection fog), evaporation of precipitation
(precipitation or frontal fog), or air being adiabatically cooled while being forced up
a mountain (upslope fog).
Radiation fog is common in the morning after heat has radiated into the
atmosphere during night in calm condition with clear sky. It normally dissipates
just after sunrise. Radiation fog is most common over land in late fall and winter
(Ahrens 1993). Advection fog is formed by humid air moving horizontally over cool
surface by advection (wind), being cooled down from below. Over the land,
advection fog is particularly likely in winter after a cold spell when a supply of
milder air arrives from the sea (E.D. Roberts 1971).
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Widespread fog cause problems of safety for air, sea and land traffic due to
the low visibility. Particularly, when fog occurs over airfield runaways and
taxiways, pilots and air traffic controller are concerned about the risk of accidents.
In Malaysia, fog occurs over airfields in the early morning without rain depends on
synoptic situation, the wind speed and direction, the stability of atmosphere, the
air temperature and the dew point temperature producing low visibility and low
ceiling over the ground.
Recently, fog phenomena often occur in the early morning during northeast
monsoon in Malaysia especially over Kuantan aerodrome (WMKD). Kuantan
Airport (ICAO code is WMKD), latitude 3º 47’ N, longitude 103º 13’ E is known
officially as the Sultan Ahmad Shah Airport, named after the current Sultan of
Pahang located about 15 kilometers from Kuantan, the capital of Pahang Darul
Makmur. Generally during northeast monsoon, East coast states of Peninsular
Malaysia (Kelantan, Terengganu and Pahang), East Johor and state of Sarawak
in East Malaysia will have episode of intermittent heavy rain from November to
March. Eventually, monsoon rain does not occur every day during this period.
Rains come in the forms of spells. Between two rain spells, a few days of fine
weather or no rain can be expected.
There is a report about the fog occurrence during northeast monsoon over
Donmuang Airport, Thailand by Vinai Thongphasuk (2002). The report describes
the formation of fog over Donmuang Airport in the northeast monsoon season
during the periods 9 to 11 February 1999 and 13 to 16 February 2000. The
conditions favorable for fog at sunrise were found to be as follows:
(i)
An easterly wing above the surface with speed 5 knots
(ii)
A clear sky at night
(iii)
Dew point at the surface was almost equal to the air temperature
giving a relative humidity almost 100%.
However, from November till end of March are the dry winter season and
also the end of rainy season in Thailand. The climate of Malaysia and Thailand
are different during this period, as Malaysian weather is rainy especially over east
coast states of Peninsular Malaysia.
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1.2 Visibility
Visibility is related to the concentrations of water droplets or solid particles, such
as smoke. When water droplets are present in suitable concentration and sizes,
mist or droplet fog may be formed. The occurrence of fog is usually associated
with radiation cooling or advection cooling, resulting in air temperatures to fall
below dew point. Visibility means the distance at which the outlines of a building
can be seen in daytime, or at night, at which ordinary house lights can be
discerned.
2.0 Methodology
One of the first classifications of fog into types, was made by Willett (1928), and
later modified by Byers (1959), who identified 11 types of fog. Both researchers
based their classifications on the formation mechanisms and weather scenarios
that were associated with fog. In between, George (1951) proposed a
classification of fog into six (6) types. The classification of fog events presented in
this study, is based on the classification methodology followed by Tardif and
Rasmussen (2007). Many physical factors interacting and counteracting are
involved with fog formation processes. Since the classification is made with the
use of half hourly surface observations solely, we restricted our classification to
broadly defined fog types.
The classification methodology of Tardif and Rasmussen (2007) was
applied to hourly observations of visibility, air temperature, wind speed and
direction, for a 15-year period (1991–2005). During this period, 282 fog events
were recorded and a fog type was assigned for each of them. The fog events
classification is based on the use of criteria that applied to the hourly values of the
above meteorological variables for a period of 5 to 8 hours before the onset of fog.
The criteria and thresholds for some of the variables were derived based on
references in Tardif and Rasmussen (2007), so that the primary physical
mechanisms responsible for fog formation of each type are taken into account.
These criteria are presented in detail as shown in Table 1.
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Table 1: Criteria applied in the classification of fog events into the four types
identified and associated primary mechanisms responsible for fog formation of
each type.
Fog Type
Criteria
Advection
-
Wind speed ≥2ms-1
-
Clear sky one hour or height of cloud base < 200m
-
Sudden decrease in visibility
-
Onset like a wall approaching the station
-
Wind speed < 2ms-1
-
Clear sky one hour before or height of cloud base 400m if
Radiation
followed by fog onset at the surface
-
Cooling before onset or slight warming at the hour before
fog onset if preceeded by cooling
-
Formation during the night
Cloud base
Gradual lowering of the height of base with initial height of
lowering
cloud base lower than 1km
Precipitation
Precipitation recorded at fog onset or 1 hour before
3.0 Objectives
The objectives of the research reported in this paper were to find the occurrence
of fog and its frequency, duration and density during northeast monsoon over East
Coast States of Peninsular Malaysia that include state of Kelantan, Terengganu,
Pahang and also East Johor. Then, understand the characteristics and the
general behavior of them into specific types. An analysis of historical data on fog
occurrence and it’s meteorological and environment could provide useful
information in understanding the physical processes and character of specific
region. However, there has been no proper forecasting of fog occurrence
especially in Malaysia. A research on fog forecasting will play a part so that the
reliable warnings of low visibility due to fog can be issued to pilots.
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4.0 Data and Procedure
This study examines the half-hourly and/or hourly surface meteorological data
(METAR) and Special Meteorological aviation reports (SPECI) from 7 stations
which are situated at East Coast States of Peninsular Malaysia. METAR is a
routine weather report issued at half-hourly and/or hourly intervals.
It is a
description of the meteorological elements observed at a station at a specific time.
SPECI is a special weather report issued when there is a significant change
(deterioration or improvement) of one or more meteorological elements at the
stations, such as surface wind, visibility, cloud and weather. All the stations
include coastal stations; Kuantan (WMKD), Kuala Terengganu (WMKN), Kota
Bahru (WMKC), Mersing (WMAU) and inland stations; Temerloh (WMBE),
Muadzam Shah (WMP9), Kuala Krai (WMAL).
These datasets during northeast monsoon from November till March (20042007) was acquired from Malaysian Meteorological Department (MMD) and the
independent variables were values at 02:00am to 09:00am of the weather
elements observation as follows:
(i)
T, Dry Bulb Temperature (ºC)
(ii)
Td, Dew point temperature (ºC)
(iii)
T-Td, Dew point depression (ºC)
(iv)
W, Surface wind speed (knots)
(v)
X, The direction of surface wind
(vi)
RH, Relative humidity (percent)
(vii)
V, Horizontal Visibility (meter)
Datasets for November to March (2004-2007) on the 7 stations were used
for examine the frequency of fog occurrence during northeast monsoon. While,
data of Kuantan station for November 2007 and 2008 were used for correlation
analysis (Table 2) and develop 3D Scattered Graph. These procedures were used
by SSPS program version 17.0, Edition August 2008.
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Generally, fog and mist occurs when the visibility is 1 km or less and 1 to 5
km respectively. However, for this study, foggy day was defined as the day when
there was a visibility of 5 km and less observed from the station as the standard
visibility over the study area. Figure 1 shows a map of the location and topography
of the airport. The existence of the nearby Bukit Berkelah (21 km from west of
Kuantan station) and Bukit Tapis (56 km from northwest of Kuantan station)
should be noticed.
X
Kuantan airport
(WMKD)
Figure 1: Relief map of the region around Kuantan airport
(Source: http://maps.google.com)
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Bukit Hangus
(27km)
Hangar TUDM
Bukit
Bangkong
(10km)
Bukit
Tapis
(56km)
Hangar TUDM
Bukit Kecil
(4800m)
900m
Sultan Ahmad Shah Airport
460m
Bukit
Berserah
(20km)
Bukit
Berkelah
(21km)
Hill
Hill
700m
350m
TUDM’s buildings
320m
Power antenna
700m
Bukit Sulai (6400m)
800m
Golf’s field
Kuantan Meteorological Station
(WMKD)
Figure 2: The visibility chart that identifies each daytime and nighttime visibility
marker with direction and distance to the marker of Kuantan Meteorological
Station (3° 46' 31" N, 103° 12' 33" E).
5.0 Findings
The following are the detailed descriptions of the findings:
5.1 Frequency of fog over East Coast State of Peninsular Malaysia during
northeast monsoon (2004-2007)
Figure 3a showed the comparison of 7 stations where number of foggy days
increased from 2004 and more dominant in the month of November (Figure 3b).
At the end of northeast monsoon (March), no fog occurred over east coast states.
However, the inland stations, Temerloh (WMBE) and Kuala Krai (WMAL) showed
more numbers of foggy days compared to other stations. Foggy days are less for
stations near coastline in the east coast states except for Kuantan (WMKD), as
shown in Table 2.
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Table 2: Frequencies of fog from November till March (2004-2007) recorded at
selected stations.
Station
Kuala
Kota
Kuala
Terengganu
Bharu
Krai
WMKN
WMKC
WMAL
0
0
0
7
3
0
0
0
2
0
0
0
0
7
Kuantan
Temerloh
Mersing
B.Embun
WMKD
WMBE
WMAU
WMP8
1
3
1
3
6
4
11
20042005
20052006
20062007
12
Frequency of fog occurence (Day)
10
8
04/05
6
05/06
06/07
4
2
0
WMKD
WMBE
WMP8
WMKC
WMAU
WMKN
WMAL
Stations
Figure 3a
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Frequency of fog occurrence (Day)
18
16
14
12
04/05
10
05/06
06/07
8
6
4
2
0
Nov
Dec
Jan
Month
Figure 3b
8
Feb
Mar
4.5
Frequency of fog Occurence (Day)
4
3.5
3
Nov
Dec
2.5
Jan
2
Feb
Mar
1.5
1
0.5
0
04/05
05/06
06/07
07/08
08/09
Year
Figure 4 (WMKD)
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Frequency of fog occurence (Day)
10
8
Nov
Dec
6
Jan
Feb
Mar
4
2
0
04/05
05/06
06/07
Year
Figure 5 (WMBE)
3.5
Frequency of fog occurence (Day)
3
2.5
Nov
2
Dec
Jan
Feb
1.5
Mar
1
0.5
0
04/05
05/06
Year
Figure 6 (WMAU)
9
06/07
3.5
Frequency of fog occurence (Day)
3
2.5
Nov
2
Dec
Jan
Feb
1.5
Mar
1
0.5
0
04/05
05/06
06/07
Year
Figure 7 (WMAL)
6.0 Analysis
All weather elements data from Meteorological Stations over east coast states of
Peninsular Malaysia comprised of 4 years (2004-2008) data have been analyzed
to look for any correlation between the elements. This included the weather
reports during rainy days and days with no rain. Consequently, this dependent
data covered 24 hours per day starting 0000 LT to 2300 LT.
From the study, fog occurred mostly in November compared to other
months (Figure 3b) in Northeast Monsoon. In Figure 3a, days with fog in year
2007 were recorded much higher compared to the previous years (2004-2006) as
shown by the high peak for total number of days with fog for some of the stations.
Furthermore, the findings from the data also shown that all fog events occurred
between 0400 LT to 0900 LT as illustrated in Figure 8 and Figure 11. Thus, no fog
was formed after 0900 LT. Nevertheless, the fog is often observed occurs when
there have been instances rain in previous day. The wind speed was also the
important element in fog forming. From the data, fog will formed if the wind speed
is less than 2 knots. Low level cloud amount also associated with fog occurrence
whilst places near high terrain such as Kuantan airport are usually covered with
low lying clouds in the morning that become heavy fog. However, from the
findings, the surface pressure did not show any relation to fog occurrence.
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7.0 Results and Discussion
Figure 8: Relationship between the weather elements during Northeast monsoon
season in November 2008 by scattering plot.
Figure 9: The 3 axis graph has shown the relationship between time, RH an
visibility during Northeast monsoon season in November 2008.
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Figure 10: Relationship between the weather elements during Northeast
monsoon season in November 2007 by scattering plot.
Figure 11: The 3 axis graph has shown the relationship between Time, RH and
Visibility during Northeast monsoon season in November 2007
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Figure 12: The Regression analysis between RH vs Vis during Northeast
monsoon season in November 2007.
Figure 8 until 11: Illustrated a graphic of scattered plot and 3 axis graph of the
weather elements observed and recorded at Kuantan Meteorological station for
the whole day in November 2007 and 2008. It also recorded that low level cloud
was observed each time fog occur. Figure 8 and 10 showed the relationship
between the weather elements for the whole day in November 2007 and 2008 and
there was no correlation between surface pressure and the fog forming as the
daily pressure changed gradually. In contrast with other countries in mid latitude,
surface pressure becomes one of many factors that contributed to the worst
condition of fog. Whereas in Figure 12 showed that fog will occur when the
surface air humidity is approaching more than 97%, it means that there are
inverse between RH and Vis. The rise in RH was primarily a moisture factor also
due to the accompanied low level cloud (Stratus). All of these factors resulted in a
dense of moisture in air and make it fog become thick.
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8.0 Conclusion
At the end of the research, the occurrence of fog and its frequency, duration and
density during northeast monsoon over East Coast States of Peninsular Malaysia
including East Johor have been studied. Then, the characteristics and the
behavior of them into specific types in Kuantan airport and other stations also
indentified. Two regions fog over East Coast States of Peninsular Malaysia and
East Johor have been identified: the East Coast state of Peninsular Malaysia, the
inland area of East Coast State of Peninsular Malaysia. During northeast
monsoon, fog events most likely to occur in inland areas than near the coastline
areas. However, Kuantan airport has recorded several days of fog dominated by
radiation cooling (radiation fog) and movement of moist air along mountain
(upslope fog). Fog occurrences over inland area (Temerloh and Kuala Krai) are
most dominated by evaporation cooling (frontal fog). As the result, extensive
measurements and collecting data of fog under various conditions and technique
are needed to make the data relevant and can be used for major purpose in
aviation. Along with the careful statistical analysis of climatological data, could
further enhance the understanding of issues related and improve the forecasting.
The high-resolution satellite imageries can also play a role in improving now
casting.
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