Physical understanding of the tropical cyclone intensity and size relations over the North Indian Ocean

Abstract

Tropical cyclone (TC) size and intensity de ne the potential destructiveness in the land-falling region.This study investigates the inter-relationships between size parameters (radius of maximum wind, R max ; 34-knots wind, R34; and TC-fullness, TCF) and intensity. The best-track (size and intensity) data is obtained from Joint Typhoon Warning center during 2002-2021. The frequently observed R34, R max and TCF are 100-150 km, 20-60 km, and 0.8, respectively for NIO TCs. Intensity and TCF are strongly related (0.7) than R34 (0.5) and R max (0.6). Analysis shows that size changes are weakly related to intensity changes (0.37-0.39).Diagnostic analysis has been conducted to address possible reasons for different TC groups 1) TCs with no size variation with intensity (Group-1), (2) both increase (Group-2), (3) size increases with no intensity change (Group-3), (4) Initial more size (Group-4). The dry air intrusion outside the eyewall in Group-1 TCs in low vertical wind shear condition limit rain-bands development, enabling moisture convergence into the primary eyewall that helps maintaining storm intensity without R34 increase. Strong surface uxes in primary eyewall region supports convection and absolute angular momentum (AAM) at upper and lower levels, which boosts size and intensity in Group-2 TCs. Strong and broader surface uxes and vertical velocities may create rain-bands or secondary-eyewall, causing bigger TCs with limited intensi cation in Group-3 TCs. Larger initial TC vortices maintains wider and intense surface uxes, vertical velocities, and AAM in the TC inner and outer cores, supporting maintenance of larger TC size in Group-4.2. Data and Methodology: Data:This study considers all the TCs formed during 2002-2021 over the NIO basin. There are 82 TCs in this period. Six hourly best track information obtained from Joint Typhoon Waring Center (JTWC). It provides TC track (center latitude and longitude), intensity (minimum sea level pressure (MSLP) and V max ), and size parameters such as R max , R34, R50, R64, and ROCI. Six hourly Infrared (IR) brightness temperature at 0.07° × 0.07° horizontal resolution is obtained from the GridSat under the International Satellite Cloud Climatological Project (ISCCP; Knapp et al. 2011). The identi cation of tropical disturbances and estimation of their characteristics (intensity and wind structure) using this IR brightness temperature has proven to be effective (Knapp et al. 2011). High-resolution (~ 10 km) rainfall is acquired from Global Precipitation Measurement (GPM; Huffman et al. 2019). GPM is well captures the spatial and temporal variation of rainfall in the intense convective regions (Ma et al. 2021;Saikrishna et al. 2021). The atmospheric parameter such as U-wind, V-wind, vertical velocities, surface uxes, relative humidity, potential vorticity, etc are acquired from fth generation European Center for Medium-Range Weather Forecasts data (ERA5; Hersbach et al. 2020). This reasonably estimates the intensity and structure of the TCs over the NIO basin (Malakar et a...