Study on vertical structure of tropical cyclones formed in the Bay of Bengal during 2007-2016

Abstract

Tropical cyclone (TC) is a rotating cylindrical warm core system with a height from surface to almost 100 hPa pressure levels. In this study, the vertical characteristics of TCs formed over the Bay of Bengal (BoB) during 2007-2016 is analyzed using National Centers for Environmental Prediction (NCEP) and Climate Forecast System Reanalysis(CFSR) data. These data have a horizontal grid resolution of 0.5° × 0.5° and temporal resolution of 6 hours. The BoB experiences 34 cyclones during the study period, among them 13 cyclones are found to be mature cyclones (maximum wind speed ≥65kt). Within the mature TCs, 4 cyclones are developed in the pre-monsoon (March-May) and 9 cyclones are found during the post-monsoon (October-December). The different atmospheric variables and their anomalies around the centre, left quadrant and right quadrant of all mature TCs at maximum wind speed of 65 kts are analyzed for 37 vertical pressure levels. In this study, the vertical pressure levels are classified into three levels named lower level (1000-850 hPa), middle level (700-500 hPa) and upper level (>500 hPa). For detail analysis areas were selected to be an square area enclosed 2° x 2° centering the eye, an area 2° x 2° at the left side of the eye and 2° x 2° at the right side of the eye of TCs. In this study, the vertical cross-section of meridional wind distribution suggests that most of the cyclones i.e. 88 % cyclones in the post-monsoon are symmetric,whereas the symmetric cyclone occurrence rate is 50% in the pre-monsoon. In the centre of TCs, vertical distributions of relative humidity (rh), temperature (T) and equivalent potential temperature (Өe ) show higher values in the middle level but relative vorticiy (rvor) is more intense (> 17x10-5 s-1) at the lower level in all cases. The average values of rh, T, Өe , rvor along the centre core of TCs are 9%, 20%, 47%, 30% higher in the post-monsoon than that in the pre-monsoon season respectively. The values of temperature, horizontal wind, rh, Өe , rvor, zonal and meridional components of wind are almost same in the left and right sides of TCs in the post-monsoon season but in the pre-monsoon season these parameters differ significantly in both quadrants. The average rh and Өe are respectively 8% and 8 °C larger in the right side than that of left side for pre-monsoon TCs and rvor is ~3 × 10-5 s-1 more intense in the right than the left side. In both seasons, left side of TCs is strongly affected by zonal component of wind, whereas, meridional component of wind is higher in the right side. The average values of vertical wind shear (VWS) between 1000 and 850 hPa is 6.4 ms-1 larger (~1.3 ms-1 smaller) in the left side than that in the right side in the pre-monsoon (post-monsoon). The VWS between 850 and 200 hPa is 3ms-1 and 2ms-1 higher in the left side than the right side during the pre-monsoon and post-monsoon seasons, respectively. Therefore, on average, the vertical distributions of atmospheric parameters show the symmetric characteristics from the eye of post-monsoon cyclones and the intensity of parameters are high in the center core than that in the pre-monsoon TCs.