Study of relation between environment and intensity of tropical cyclones formed in the Bay of Bengal during 2000-2011

Abstract

Tropical cyclones (TCs) are one of the most dangerous natural hazards which are responsible for considerable loss of life and do immense damage to property. Due to the position in the tropical region and the funnel shaped structure, Bay of Bengal (BoB) is favorable for the formation of the strongest and deadliest TCs in the world. The BoB , which is the important branch of the North Indian Ocean, is located in the northeastern part of the Indian Ocean between latitudes 5° N to 22° N and longitudes 80° E to 100° E. During the study period from 2000 to 2011, a total number of 44 active TCs were formed in the BoB, which were bimodal in distribution during preand post-monsoon seasons. Within these cyclones, 6 pre-monsoon cyclones named as Cyclone(2004), Mala (2006), Akash (2007), Nargis (2008), Aila (2009) and Laila (2010) and 4 post-monsoon cyclones named as Cyclone (2000), Sidr (2007), Giri (2010) and Thane (2011) were considered for this study because they have sustainable maximum wind speed of 64kt (≥33m/s) or more. The change in intensity of each TC was correlated with the environmental condition. The dynamic and thermodynamic environmental parameters, namely, relative vorticity at low level (1000-850 hPa) and mid level (850-500 hPa), low level vertical wind shear (VWS), low level moisture flux, relative humidity at 500 hPa, convective available potential energy (CAPE), convective inhibition (CIN), precipitable water (PW), sea surface temperature (SST), geopotential thickness and mean sea level pressure (MSLP) were analyzed for this purpose. Joint Typhoon Warning Center best-track data, the National Centers for Environmental Prediction Climate Forecast System Reanalysis 6-hourly data with horizontal resolution of 0.5° x 0.5° and Real-Time, Global, Sea Surface Temperature with a 0.5° x 0.5° resolution data were used in this study. Intensity of TCs is directly related with the decrease of MSLP. In this study, MSLP was found to be decreased with an average value of 2 hPa per intensity level for premonsoon cyclones, whereas, it was approximately 1 hPa for post-monsoon cyclones. The values of CAPE were also noticed as decreasing with the cyclone intensity. The average decreasing values per intensity level of TC were 10% and 5%, respectively for all pre- and post-monsoon cyclones. In case of pre-monsoon cyclones CAPE and MSLP were 4.5% and 0.1% higher than in post-monsoon cyclones. The values of low and mid- level relative vorticites, moisture flux, CIN, and geopotential thickness were found to be increased with the change of cyclone intensity and averaged increasing value per intensity level were approximately 18%, 21%, 16%, 44% and 0.07%, respectively for all pre-monsoon cyclones. For all post-monsoon cyclones the values were increased approximately 9%, 14%, 7%, 42% and 0.01%, respectively per intensity level, however, the values were almost half than that in the pre-monsoon cases. Average value of mid-tropospheric relative humidity for all cyclones was calculated more than 75%, however, no particular value was significant for the intensity of cyclones. Similarly, Precipitable water and SST remained almost constant with the change of cyclone intensity. In all cases Precipitable water values were found greater than 60 kg/m2. Average value of SST was more than 26 °C for all cases and found 3 °C higher in the pre-monsoon period than that in the post-monsoon.