Tropical Cyclone Pam waves summary

Tropical Cyclone Pam waves summary
Prepared by: Cyprien Bosserelle – 24/03/2015
Executive summary
Between 10/03/2015 and 15/03/2015 tropical cyclone Pam has caused a lot of damages in the South
Pacific, including wave inundation over an area from Tuvalu to New Zealand. This report attempts to
summarize the height of the swell that caused the damage using the available altimeter data and
other satellite and analysis tools.
The swell that caused inundation in Tuvalu was generated by an area of strong wind sustained over a
long distance that was present as the cyclone quickly gained strength.
In Vanuatu, the waves are believed to have exceeded 10m between the South of Efate to Tanna.
Erromango North East may have experience the worst of the swell with waves as high as 16m, this is
more than twice the expected wave height from a 100 year return period event.
Summary of cyclone path and strength
Tropical Cyclone (hereafter TC) Pam was names after being classified as a tropical cyclone on
09/03/2015 and travelled South until the 15/03/2015 where it was reclassified as a tropical low.
During this time the track of the cyclone covered a distance of 3,200km. During the first half of the
TC life the system was moving relatively slowly (~3m/s) generating a large swell, as it moved south
the cyclone accelerated generating a large swell only on the left of its path (Figure 1).
The cyclone was named on 10th March while located North East of Solomon Islands. The cyclone
quickly gained strength and became a category 4 TC on the 11th. From this stage the track of the
cyclone become more linear and the cyclone accelerated to close to 5m/s. Late on the 12th the
cyclone reached category 5 and continue accelerating south-southwest. After reaching Vanuatu on
the 13th, the cyclone loss some strength and dropped to a category 4 and changed direction to the
south-southeast. The cyclone then continued loosing strength in the cooler waters and continued to
accelerate until reaching a speed of 15m/s.
Figure 1 Tropical Cyclone Pam track and intensity (circles)
Wind
Large swells are the result of strong wind blowing over a large distance. Tropical cyclones are often
too small or moving too fast to generate large waves. In some case however, when particular
conditions are met do large, do cyclone create large and extreme waves. These conditions can be:



Cyclone quickly strengthening (Bombing effect)
Cyclone travels as fast as the wave it generates (Trapped fetch)
Cyclone reaches very large size/strength
In the case of Tropical cyclone Pam all the conditions were met at some stage of the life of the
cyclone.
On the 11th the ASCAT scatterometer satellite measured the wind over TC Pam. The satellite was not
able to measure the wind close to the eye of the cyclone but the maximum winds were above 25m/s
(50 knts). The data also show a wide area North of the cyclone with wind speed reaching 20m/s that
generated the swell that later reached Tuvalu (Figure 2).
Figure 2. Satellite wind speed and direction on the 11th March 2015.
The satellite track did not clearly capture the size or the strength of TC Pam again until the 14th
March, when the cyclone was above Vanuatu. Again, at that stage, the strength of the wing inside
the cyclone is unclear but wind speed reaching 30m/s on the edge of the cyclone suggest that large
waves were generates along the track of the cyclone (Figure 3).
A summary of the extreme winds shows that wind speed exceeding 20m/s occurred as far as 200km
left of the cyclone track potentially creating extremely large waves far outside the cyclone footprint.
(Figure 4)
Figure 3 Satellite wind speed and direction on the 11th March 2015.
Figure 4. Summary of the wind speed above 20m/s generated by the cyclone.
Altimeter Waves
The footprint of the In the South Pacific the only measurements of wave are from satellite altimeters
that correlate the scattering of a radar signal to the wave height.
Unfortunately the satellite track measuring waves missed the cyclone and only one track provides an
insight into the type of wave really close to the cyclone.
Distant swell caused by TC Pam(Tuvalu, Wallis and Futuna)
Cryosat2 and Jason2 tracked over Tuvalu and close to Futuna on the 10th and 11th March. The
satellite detected wave height in excess of 6m and possibly 7m in the West of Tuvalu, the day that
flooding was reported. Further south, the altimeter detected wave as height as 5m west of Futuna
the day before the island reported wave flooding (Figure 5).
This swell was reported to be with long wave period which exacerbated the wave forces and the
inundation. The timing of the swell arrival is likely that the swell was created when TC Pam
expanded and strengthened quickly going from category 1 to 4 almost overnight. The area covered
by gale force wind expanded quickly sending this swell in almost every direction from the cyclone
centre. For Tuvalu these type of swell are very unusual and last occurred with Tropical Cyclone Bebe.
Figure 5 Altimeter track showing the wave height at or near the time of flooding by the swell.
Near field swell: Vanuatu
TC Pam has caused widespread damage in Vanuatu and some of this damage is likely due to the
swell generated by the cyclone. Cryosat2 and Jason2 tracked over Vanuatu and the cyclone on the
13th and the 14th March. The track shows waves in excess of 10m on the left hand of the track. This
swell was formed while the cyclone was moving south west. Wave higher than 8m probably reached
the South tip of Efate and across to the south of Tanna. The largest part of the swell which exceeded
10m is likely to have reached the North East of Erromango (Figure 6).
Figure 6 Altimeter track showing the wave height measured over Vanuatu.
Summary
Wave inundation and damage that occurred on the 12th March in Tuvalu was the result of a 6m swell
that was generated north of TC Pam at the time that the cyclone quickly expanded and
strengthened.
In Vanuatu extreme wave hit the shorelines exposed to the North East when the category 5 cyclone
approached. An estimate of the wave height during the cyclone can be calculated using a parametric
model based on wind strength, cyclone diameter and cyclone forward speed (Young et al. 1998). The
model shows very good correlation with one of the altimeter track (Figure 7). Such model suggest
that wave higher than 10m occurred between Efate and Tanna and waves as height as 16m may
have reached Erromango (Figure 8). Sattelite imagery of the North East of Erromango shows major
coastal erosion and inundation that is expected from such extreme waves. Previously calculated
extreme wave statistics suggested that the 100 year return period wave height was 7.5m in this
area. The waves from TC Pam appear to have reached twice this estimate.
Figure 7. Validation of wave height calculated using the parametric wave model from Young et al.
1998 (shaded box) and altimeter wave height (shaded tracks).
Figure 8. Estimated wave height (m) before the cyclone made landfall in Vanuatu.
Acknowledgements
Time and date in this report are given in UTC reference. For local time in Tuvalu add 12hrs and 11hrs
for Vanuatu.
Data presented in this report was collated from online sources. The altimeter wave data was
collected from the Globwav server; the swell hindcast was from NOAA-NCEP Hurricane model and
the wind data from KNMI. 2010. MetOp-A ASCAT Level 2 12.5km Ocean Surface Wind Vectors.
All the figures in this report were produced using Generic Mapping Tools.