Sensitivity Study of Wind Drag Coefficient on Surge Modelling for Tropical Cyclone

Abstract

The Bay of Bengal (BoB) is the breeding ground for the tropical cyclones, accompanied by very strong winds, torrential rains and storm surges with severe intensity during the pre- and post-monsoon seasons. Modelling this storm surge is involved with many uncertainties; among those, the wind stress (associated with wind drag coefficient, Cd) over ocean surface is a crucial surface forcing and a thriving factor. The present study examines the sensitivity of Cd that was used to simulate storm surge in the BoB region using the coupled ADCIRC and SWAN model. In general, ADCIRC uses the formulation of Garratt [1] and SWAN uses that of Wu [2] to evaluate Cd. Both these relations are empirical, and Cd increases linearly with wind speed. Hence, cut-off values (Cd cap) are proposed for Cd, ranging from 2.0 × 10−3 to 3.5 × 10−3 and a no-cap condition in SWAN. The simulations were performed for cyclone ‘Vardah’, and significant wave heights, surge heights and landfall time were computed. Results show that the parameters estimated through the cut-off Cd values reasonably match with the observed values than the no-cap case. The results and analysis interpreted from the current study will be used on a significant scale to understand and determine the sensitive behaviour of Cd for very high wind speeds of a tropical cyclonic condition.