Graduate School of Creative Science and Engineering Waseda University

Doctoral Thesis Synopsis

Thesis Theme Meteorology Based Numerical Simulation of Past and Future Storm Surges in the Bay of Bengal

(Applicant Name) Khandker Masuma TASNIM

Department of Civil and Environmental Engineering Research on Coastal Engineering and Management

October, 2014 This dissertation work analyzes tropical cyclone behavior over the Bay of Bengal and numerically simulated storm surges by using a Meteorology-Wave-Coastal Ocean Tide coupled model. The reason to study this topic is due to the fact that the destruction due to storm surge flooding is a serious concern along the coast of the Bay of Bengal. Amongst other natural disasters, storm surges stand out, so far, as the most damaging natural disaster which causes large scale death and destruction affecting the coast of India, Bangladesh and recently Myanmar too. According to the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC 4AR) it is likely that future tropical cyclones will become more intense, with larger peak wind speeds and heavy precipitation associated with on-going increases in sea surface temperatures (SSTs) and sea level rise. Recent research works using higher resolution models predict greater increases in cyclone intensity than those with lower resolutions, and thus it is possible that the conventional models might be underestimating potential future problems. In the Bay of Bengal region, the simulations of storm surges have often involved using results from a small set of historical storms with simple adjustments, which often lead to a biased distribution of water levels, with an unrealistic count of extreme events. Therefore there are large uncertainties in estimating future coastal flooding associated with tropical cyclones in this region. After the 2013 typhoon Yolanda event it is almost certain that the world has started to experience the climate change induced super cyclonic storms. Therefore, in order to prepare for the upcoming challenges related to climate change, the Indian Ocean and especially the Bay of Bengal should be of particular concern because of the high population density along its coastlines which is highly vulnerable under storm condition.

The present thesis consists of seven chapters. Chapter 1 presents a general introduction as well as overview of the possible future threats due to climate change over the Bay of Bengal and recent research works related to storm surge simulation over this region. The objectives and scope of present research work is also stated. The main objective of this research was to investigate the weather field of tropical cyclones more accurately by using Numerical Weather Prediction models. In addition, possible future change in tropical cyclone behavior due to climate change and Sea Surface Temperature (SST) rise was also investigated. Chapter 2 includes a description of the history of tropical cyclones over the Bay of Bengal together with a brief description of three recent strongest cyclone events cyclone Sidr 2007 in Bangladesh, Nargis 2008 in Myanmar and Phailin 2013 in India. A review of the existing cyclone preparedness activities in the countries around the Bay of Bengal and associated problems and challenges were also presented. In addition a brief description of the previous storm surge models was also included in this chapter. Chapter 3 includes the summary of the fields surveys conducted in Bangladesh and Myanmar after cyclone Sidr and Nargis respectively. The observed water levels at different locations, local conditions and the experience of local people during those events are discussed in this chapter.

No.1 Chapter 4 and 5 contains the main body of the present research. Chapter 4 includes the methodology of the numerical simulation with a detail description of the numerical model (WRF- SWAN- FVCOM- Nao.99b coupled model). Brief description of each component of this coupled model with the governing equations is stated in this chapter. Numerical simulation was carried out for the hind casting of historical cyclone events over the Bay of Bengal and as case studies three recent cyclone events (Sidr, Nargis and Phailin) were chosen. The weather field of the historical cyclones was investigated by using NCEP Final Analysis Data (FNL) as the initial and boundary condition in WRF ARW model with 3 nests. In order to simulate the weather field of the real time cyclones more accurately, two different approaches were considered. The first one was using NCEP FNL data for model initialization. The second approach was to introduce artificial vortex data at the time of model initialization. This second approach was applied only when the simulation result based on FNL data resulted in large deviation with the observations. For cyclone Nargis and Phailin cases, the first method could forecast weather field with reasonable accuracy. For cyclone Sidr case, the second approach showed significant improvement in the performance of the model. The sensitivity of WRF ARW model with different initial conditions was also investigated and found that the performance of the model in terms of track and intensity greatly varies with different initial conditions. At the same time, in order to have an understanding of the characteristics and damage of future tropical cyclones, especially under climate change conditions, future potential weather field and storm surge were simulated for the year 2100. For the prediction of future cyclones, Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenario A1B was considered. According to scenario A1B, the Sea Surface Temperature (SST) around the Bay of Bengal will rise 2.2 C and sea level rise will be 0.35m by the year 2100.The future cyclone for the year 2100 was simulated first by creating the weather field of the cyclone by using historical weather data at the time of the passage of the past cyclone but increasing SST across the whole domain by 2.2 degrees. WRF simulated wind and pressure fields of the historical as well as the future cyclones were applied as external forcing into the SWAN and FVCOM models to compute the wave and surge (both wind and pressure driven surge) for present and future conditions. Nao.99b model was used for computing astronomical tide. The total water level resulting from the passage of a cyclone consists of wave, tide and surge elements (both wind and pressure driven surge), which were simultaneously calculated by using this coupled model in order to simulate the real time water levels under storm condition. Brief description of the calculation conditions using the WRF ARW and FVCOM model are included in Chapter 4.

The verification of the performance of the model is showcased in Chapter 5. WRF model showed good accuracy in hind casting track and intensity of all the three historical events. FVCOM simulated storm surge and SWAN simulated significant wave height near shore showed good agreement with the observations at different locations. The change in sea surface temperature (SST) seems not to cause No.2 any significant change on the track of tropical cyclones in future, but the landfall locations can be significantly different. For all the three case studies, future cyclones are found to be much stronger in intensity with higher wind speed and larger pressure drops. As a result the countries around the Bay of Bengal might have the risk of large scale inundation resulting from higher levels of storm surge and wind waves than the present time. For all the three case studies, FVCOM simulated surge level for the future events lies within the range of 4 to 5 meter. SWAN simulated wave height near shore for all the three cases are significantly higher than the present time which will eventually result in higher wave run up in future.

Discussion on the performance of the numerical model is presented in Chapter 6. A general discussion on how the countries around the Bay of Bengal should prepare for the climate change induced future cyclones on the basis of their existing cyclone preparedness programs is also included in this chapter. At the same time recommendations for the future disaster management policy on the basis of the finding of numerical simulation are made.

Finally, Chapter 7 presents the conclusions of the present research work as well as some of the limitations which require further improvement by future research efforts in this field.

No.3 No.1

(List of research achievements for application of doctorate (Dr. of Engineering), Waseda University)

(Khandker Masuma Tasnim) () As of February, 2015 (theme, (By Type) journal name, date & year of publication, name of authors inc. yourself) Journal Paper

1) Field observation and numerical simulation of past and future storm surges in the Bay of Bengal: Case study of cyclone Nargis, Natural Hazards, January 2015, Volume 75, Issue 2, pp 1619-1647 (DOI 10.1007/s11069-014-1387-x), Khandker Masuma Tasnim, Tomoya Shibayama, Miguel Esteban, Hiroshi Takagi, Koichiro Ohira, Ryota Nakamura.

Book Chapters Observation and numerical simulation of storm surge due to cyclone Sidr 2007 in Bangladesh. In 1) Coastal Disasters: Lessons Learnt for Engineers and Planners. Esteban M., Takagi, H., and Nguyen D.T. (eds). Elsevier (in print)

2) Storm surge due to cyclone Nargis in Myanmar and post cyclone disaster preparedness activities. In Coastal Disasters: Lessons Learnt for Engineers and Planners. Esteban M., Takagi, H., and Nguyen D.T. (eds). Elsevier (in print)

Conference Paper

1) Numerical simulation of cyclonic storm surges over the Bay of Bengal using a meteorology-wave-surge-tide coupled model, 34th International Conference on Coastal Engineering (ICCE 2014), Seoul, Korea, June 15-20, 2014, (DOI: http://dx.doi.org/10.9753/icce.v34.currents.26), Khandker Masuma Tasnim, Koichiro Ohira, Tomoya Shibayama and Miguel Esteban

2) Numerical simulation of tropical cyclone Nargis by using OSIS model: Hind casting of historical cyclone as well as prediction of future storm surges, Proceedings of International Session in Coastal engineering, JSCE, vol.4, 2013, Khandker Masuma Tasnim, Koichiro Ohira and Tomoya Shibayama.

3) Numerical simulation of cyclone Sidr using a WRF-Swan-Surge-Tide coupled model, Proceedings of International Session in Coastal engineering, JSCE, vol.3, pp 1-5, November, 2012, Khandker Masuma Tasnim, Koichiro Ohira and Tomoya Shibayama.

4) Investigation of the physical characteristics of dynamic topography change due to Cyclone SIDR around Kuakata , Proceedings of International Conference on Water and flood Management, ICWFM 2011 in Dhaka, January 2011, Tasnim Khandker Masuma, Tajima Yoshimitsu.