Proceedings of Coastal Engineering, JSCE, Vol.3, 2012, November

NUMERICAL SIMULATION OF CYCLONE SIDR
USING A WRF-SWAN-SURGE-TIDE
COUPLED MODEL
Khandker Masuma Tasnim1, Koichiro Ohira2 and Tomoya Shibayama3
1Dept. of Civil and Environmental Engineering, Waseda University, tasnim@ruri.waseda.jp
2Chubu Electric Power Company, chonchan.factory@gmail.com
3Dept. of Civil and Environmental Engineering, Waseda University, shibayama@waseda.jp

Computation of weather field on the basis of meteorology has significant importance for theassessment of cyclonic storm surges. The application of mesoscale WRF model for the prediction oftropical cyclones over the Bay of Bengal is quite new and challenging. The present study aims at theapplication of Advanced Research WRF (ARW) model for the investigation of the weather field oftropical cyclone Sidr. This study focused on the numerical simulation of Cyclone SIDR by integrating
WRF (Weather Research and Forecasting), SWAN (Simulating Waves Near shore), storm surge
simulation models and NAO.99b tidal prediction system. The WRF model seems to capture the actual
phenomena of cyclone Sidr well in terms of track and intensity. The performance of this model can be
improved and on the basis of this improved weather data, the integrated meteorology wave hind castingmodel can give more accurate prediction and hind casting of cyclonic storm surges in the Bay of Bengal.
Key Words: Tropical cyclone, prediction, track, intensity, WRF model, forecasting, storm surge.

1. INTRODUCTION
The Bay of Bengal is an ideal breeding groundfor tropical cyclones. Almost 7% of the global
tropical cyclones occur in the North Indian Ocean(Bay of Bengal and Arabian Sea), and they are someof the most deadly. The shallow waters of the Bayof Bengal, the low flat coastal terrain, and thefunneling shape of the coast line lead to devastating
losses of life and property in this region.Due to thehigh population density, the devastation of thecyclones is more pronounced along the coastal
region of Bangladesh. During the last 48 years 18
major cyclones devastated the coastal area. Amongthem the 1970 and 1991 cyclones were the mostsevere. The most recent event was cyclone Sidr inNovember 2007, which caused 3,363 deaths
affecting 8.9 million people and damaged houses,livestock, crops, educational institutions, roads andembankments as reported by Disaster Management
Information Centre, Ministry of Food and Disaster
HistoricallyManagementofBangladesh.
Bangladesh is under persistent threat from
cyclones and flooding. So the timely and accurate
prediction of the track and intensity of tropical
cyclones is essential for Bangladesh in order toprotect against the loss of lives and property.
As the synoptic and statistical methods have
limitations in predicting the track and intensity oftropical cyclones beyond 24 hours, many attempts
have been made to develop numerical weather
prediction (NWP) models for this purpose.
Concerted effort of community model development
at the National Center for Atmospheric Research(NCAR) in association with the Environmental
Modeling Center (EMC) of National Centers forEnvironmental Prediction (NCEP) resulted in
making significant progress in terms of meso-scalemodeling for various weather applications (Rao etal.2011). Much progress has been made in recent
years in addressing real-time tropical cyclone
prediction based on these community models,
especially for the Atlantic and Eastern Pacific basins(Bender et al. 2007; Surgi et al. 2008). Recently afew studies have been performed for tropical
cyclone prediction in the North Indian Oceanregion, particularly for Bay of Bengal storms, byusing NCEP Hurricane Weather Research andForecast (HWRF) model and Advanced Research
WRF (ARW) model. Rao and Tallapragada (2011)
performed a comparative study of the performanceof NCEP operational HWRF, NCAR ARW, and-1-

MM5 models and reported that HWRF model wasbetter at predicting the track and intensity of tropical
cyclone Sidr and Nargis. Kumar et al. (2011)
performed another study to simulate cyclone Sidr byusing The Advanced Hurricane WRF (AHW) modelwith different domain size and boundary conditions.
They reported that model domain resolution has animpact on track and intensity and the westernboundary plays a significant role in controlling thetrack of cyclone Sidr.
The present study focused on the development
and application of a coupled model by integrating
the meteorological model into the wave hind castingmodel. The goal of this research is to investigate theactual phenomena of the cyclone more accurately byusing numerical models so that a more accurate
prediction and evacuation system can be designed.

2. OVERVIEW OF CYCLONE SIDR
Super Cyclone "SIDR" was one of the 10
strongest cyclones to hit Bangladesh between 1876and 2007. It was first observed on 9 November 2007
near the southeast of the Andaman Islands with aweak low-level circulation near the Nicobar Islands.
An indication that it had formed into a tropical
cyclone arrived on 11 November, while located ashort distance south of the Andaman Islands. On 13November, the depression had turned into a cyclonicstorm with a core of hurricane force winds. CycloneSidr moved northwards and was centered at 9 p.m.
on 14 November 2007 about 725 km
South-southwest of Chittagong port, 645 km
South-southwest of Cox’s Bazar port and 670 km
South of Mongla port. Sidr intensified quickly,
moved slowly northwestward into the Bay ofBengal, headed northward, and finally, madelandfall at the Bangladesh coast as a very severe
cyclone (category 4 equivalent) at approximately
1700 UTC 15 November. After the landfall, it
weakened rapidly and dissipated the next day.
Cyclone Sidr hit Bangladesh’s offshore islands
at approximately 18:30 hours on the evening of 15
November and made landfall across the Barisal
coast at 21:00 hours local time during ebb tide. The
maximum wind speed of Cyclone Sidr was 69 m/s,and its lowest central pressure before landfall was944hPa (Nasrin et al.2012, Hasegawa etal.2008).Across 30 districts of Bangladesh, 8.7
million people were affected by this cyclone. A totalof 3,295 people were reported dead andapproximately 53,000 people were reported missing.
Approximately 1.5 million households weredamaged, leaving millions of people without shelter.
The estimated economic loss due to SIDR was morethan US$3.1 billion (Nasrin et al.2012). Some local



officials have described the damage as being evenworse than that from the 1991 cyclone. The coastal
cities of Patuakhali, Barguna and Jhalokati District
were hit hard by a storm surge of over 5 meters.
About a quarter of the world heritage site ofSunderbans were damaged. Researchers said themangrove forest of Sunderban will take at least 40
years to recover from this catastrophe.
The Japan Society of Civil Engineering (JSCE)conducted a field survey in Bangladesh from 26 to28 December 2007 to investigate and assess thestate and mechanisms of disaster from the storm
surge. The third author of the present paper served
as the team leader of survey team. According to thesurvey report, the maximum inundation height ofthe surge was 9.6m at a steel tower (the inundation
depth 6.5m) in the coastal area of Kuakata, which ismuch higher than other observation heights. The
inundation height observed at the crest of theembankment in West Kuakata was 5.6m. Storm
surge overtopped the embankment and inundated
the area behind. However, the inundation heightbehind the embankment was relatively low, around2.3m (JSCE Investigation report, 2008).
Hind casting of the storm surge of cyclone Sidr
had been made using an available storm surge
model by the Institute of Water Modeling (IWM) toassess the temporal variation of storm surge heightand coastal flooding during the passage of the Sidralong the Bay of Bengal and across the Barisal
coast. This hind casting showed surge level of 5.5 to
6m PWD, at the outfall of Baleswar River, 5m at
Sarankhola and 3.5m at Hiron point. The surge levelexceeds polder embankments, built to a heightof+5m, PWD. But surge level does not exceed thesea facing embankment, which are at more than 6min the Barguna, Barisal and Bagerhat districts(Emergency Response and Action Plans Interim
Report 2007, Ministry of Food and Disaster
Management Bangladesh)

3. DESCRIPTIONMODELOFNUMERICAL

In this study an integrated WRF (WeatherResearch and Forecasting)-SWAN (Simulating
Waves Near shore)-Storm Surge-Tide model is used
for the numerical simulation of Cyclone Sidr. The
Weather Research and Forecasting (WRF) model isused to compute the weather field of Cyclone Sidr.
The WRF model is being developed as acollaborative effort between the NCAR Mesoscaleand Microscale Meteorology (MMM) Division, theNational Oceanic and Atmospheric Administration’s(NOAA), National Centers for Environmental
Prediction (NCEP) and more than 150 other-2-organizations and universities. The Advanced
Research WRF (ARW) is designed to be a flexible,
state-of-the-art atmospheric simulation system thatis portable and efficient on available parallel
computing platforms. A detailed description ofWRF (ARW) was provided by Skamarock et al.
(2005). SWAN is a third-generation wave model
developed by Delft University of Technology thatcomputes random, short-crested wind-generated
waves in coastal regions and inland waters.
The NCAR version 3.0 of Advanced Research
WRF (ARW) model with 2 way nesting was used toinvestigate the weather field of cyclone Sidr. The
initial and boundary conditions for the large-scaleatmospheric fields were derived from the 1×1
degree NCEP global final analysis (FNL) using theWPS (WRF Pre-processing System) software
package. The extracted wind and pressure fields
from the WRF model were applied in both SWAN
and the Storm Surge model as inputs to compute thewave and surge (both wind and pressure drivensurge). The storm surge model developed by Ohiraand Shibayama (2012) was used, where the surge iscalculated by differentiating the nonlinear longwave equation using the Leap-frog method.
NAO.99b tidal prediction system is a global ocean
tide model developed by Matsumoto et al (2000).
The flowchart of the integrated model is shown inFig. 1 (Ohira and Shibayama, 2012). Finally wave,wind & pressure driven surge, and tide obtainedfrom coupled model are combined to get the storm
surge level.
For our simulation the outermost domain wasfixed with 12.95 km grid spacing (230 × 250 gridpoints) and covers an area of 2978 km × 3237 km
from 75.9°E to 102.5°E and 3.75°N to 31°N. One
fixed nested domain was used with 1.85km grid
spacing (365×267 grid points) that covered an areaof 675 km × 494 km. Fig. 2 shows the model
domain. The model was configured with a two-way
nesting option and run for 96 hours with a time stepof 60 s for the parent and 10 s for the nested
domain. Both parent and nest had 28 vertical
layers. The model run started at 00UTC of 13
November 2007 and ended at 00UTC of 17
November 2007.
In order to see how WRF ARW works forweather field simulation another model run wasperformed with a relatively small outmost domain
covering 79.32°E to 99.13°E and 5.06°N to23.74°N. For this simulation the parent and thenested domain had a horizontal resolution of 9.25
km and 1.85 km respectively. The time step was 45s for the parent and 9 s for the nested domain. Allother conditions were kept the same.

Fig.1 Model Flow (Ohira and Shibayama, 2012)

Nest Domain

Parent Domain

Fig.2 Model Domain for WRF ARW

The WRF ARW forecasted wind data for thenested domain (grid spacing 1.85 km) was used inthe SWAN model to estimate the wave field in thecoastal region of Bangladesh. For this SWAN run
GEBCO 30 s bathymetry data with 1 km grid
spacing was used. SWAN was run for 96 hours
starting from 13 November 00UTC with a time stepof 3s. For storm surge simulation, the wind andpressure data of the nest domain was used. Themodel was run for 96 hours with a time step of 3s.
Finally the water level is computed in thefollowing way:
Water Level = Wind and pressure surge + Wave+Tide

4. RESULTS AND DISCUSSIONSThe 96 hour forecast of cyclone Sidr by WRF
ARW shows reasonable accuracy in terms ofcyclone track and intensity. Fig. 3 shows the-3-observed Joint Typhoon Warning Centre (JTWC)
track and model simulated tracks. For the small
domain model simulated initial and landfall positionof the cyclone looks very close to the observed one,but the model track deflected slightly to the leftfrom 14 November 00UTC to 15 November 06UTC.
After that the simulated track shows reasonableaccuracy with the observed track till landfall(landfall near 89.64°E and 21.44°N, 35 km awayfrom the observed landfall position).
(a)(b)

Fig.4 Wind and pressure fields at Initial time, 00 UTC on
13, November, forecasted by WRF in: (a) large domain;and (b) small domain.

(a)(b)

Fig.3 Observed and simulated track of cyclone

For the large domain run, the simulated trackshows reasonable accuracy for the first 24 hour ofsimulation. However, the simulated track started todiverge toward the northwest from the actual trackat 00 UTC 14 November and continued simulatingslightly deflected track after that. It finally madelandfall near 88.53°E and 21.4°N which is 90 km
away (to the west) from the observed landfall
position. Both models simulated a relatively slower
moving cyclone. The simulated landfall time wasaround 00UTC on 16 November for both domains.
In terms of cyclone intensity, the large domain run
gives better forecast than the small domain. 96 hour
forecasting by WRF model for the large domain
gives low pressure of 970hPa near 88.06°E and20.54°N, whereas small domain shows a low
pressure of 978hPa close to 88.6°E and 19.3°N. The
highest wind speed obtained is 46m/s for both
domains. Fig. 4 and Fig. 5 show the model
simulated wind and pressure field at both the initialtime and after 60 hour of computation, respectively.

Fig.5 Wind and pressure fields after 60 hour of simulation,
12 UTC on 15, November, forecasted by WRF in: (a) large
domain; and (b) small domain

The weather field simulated for the large domain
was used for the simulation of wave height andsurge (both wind and pressure driven surge).Fig. 6shows the water level measured and simulated by
Institute of Water Modeling (IWM) and the waterlevel simulated by the WRF-SWAN-Storm
Surge-Tide coupled model at Hiron point. At Hironpoint, IWM measured a water level of 2.3 m (IWMFinal Report Volume-I, April 2009) whereas thecoupled model used in the present paper simulated awater level of 4m.
For storm surge computation a small scale
bathymetry is necessary. Due to the lack of fine gridbathymetry in the shallower region of theBangladesh coast, the storm surge computed by thecoupled model shows some deviation from theobserved water level.

-4-

(a)

(b) Fig.6 Time history of water level simulated by: (a) Institute ofwater Modeling (IWM); and (b) WRF-SWAN-Surge-Tide
coupled model

5.
CONCLUSIONSRESEARCH

ANDFUTURE

This study showcased a coupled meteorologicalwave hind casting model for the numericalsimulation of storm surge over the Bay of Bengaland examined the accuracy of the results for thesuper cyclonic storm cyclone Sidr. WRF ARWmodel could forecast the weather field and track ofSidr well. Since the application of WRF model for
the Bay of Bengal cyclones is new, further
modification of this model is possible, which is
expected to give a more accurate prediction andhind casting of tropical cyclones. More experimentswith domain size, initial and boundary conditions aswell as application of moving nest and vortex
initialization technique can be helpful for the further
improvement of the model.
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