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

Tropical cyclone (TC) size and intensity define the potential destructiveness in the land-falling region. This study investigates the inter-relationships between size parameters (radius of maximum wind, Rmax; 34-knots wind, R34; and TC-fullness, TCF) and intensity. The best-track (size and intensity) data is obtained from the Joint Typhoon Warning Center during 2002–2021. The frequently observed R34, Rmax, and TCF are 100–150 km, 20–60 km, and 0.8, respectively for NIO TCs. Intensity and TCF are more strongly related (0.7) than R34 (0.5) and Rmax (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 maintain storm intensity without R34 increase. Strong surface fluxes in the primary eyewall region support convection and absolute angular momentum (AAM) at upper and lower levels, which boosts size and intensity in Group-2 TCs. Strong and broader surface fluxes and vertical velocities may create rain-bands or secondary-eyewall, causing bigger TCs with limited intensification in Group-3 TCs. Larger initial TC vortices maintain wider and intense surface fluxes, vertical velocities, and AAM in the TC inner and outer cores, supporting the maintenance of larger TC size in Group-4.