Cloud Microphysical Processes of Tropical Cyclones Over Indian Seas: Impact on Size and Intensity Changes

Abstract

Tropical cyclones (TCs) are natural catastrophic phenomena and the destruction is mostly determined by their movement, intensity, and radial expansion of winds (size), etc. It is essential to have an accurate forecast of these TC traits to lessen the damage to lives and property in coastal regions. In recent decades, a noticeable improvement in the TC track prediction. However, fewer improvements are noted in TC intensity, and little attention was paid to size predictions over the North Indian Ocean (NIO) basin. The TC size and intensity changes are multiscale problems, interacting with both synoptic-scale and vortex-scale processes. Hence, insights into responsible physical and microphysical (MP) processes for size and intensity changes help to improve TC predictions. The thesis started investigating the inter-relationships between size parameters (radius of maximum wind, Rmax; 34-knots wind, R34; and TC-fullness, TCF) and intensity over the NIO. Intensity is found to have a relatively stronger correlation (0.7) with TCF when compared to that with R34 (0.5) and Rmax (0.6). Analysis shows that size changes are weakly correlated with intensity changes (0.37-0.39). Diagnostic analysis has been conducted to address possible reasons for different relationships between size and intensity. For this, TC samples are categorized based on the linear regression coefficient (LRC) between size and intensity evolution. The average LRC of Group–1 to Group–4 are 1.42 km/knots, 2.18 km/knots, 2.7 km/knots, and 3.62 km/knots, respectively. The dry air intrusion in outside the eyewall and low vertical wind shear conditions in Group–1 TCs limit rain-bands development, and support to no/smaller size increases with intensity. In Group 2, Strong surface fluxes in the primary eyewall region support convection and absolute angular momentum (AAM) at upper and lower levels. It leads to increases in size with intensity. Strong and broader surface fluxes and vertical velocities may create rain-bands or secondary-eyewall in Group–3. It supports to enhance the size with limited intensification. In Group–4, larger initial TC vortices maintain wider and more intense surface fluxes, vertical velocities, and AAM in inner-core and outer-core regions of TC. These are favorable to maintaining larger sizes than the remaining groups. In a follow-up study, the impact of MP processes and horizontal grid resolution on TC size is analyzed. TC movement is less sensitive to MP schemes, while the size is more sensitive. The simple-ice scheme produced smaller TCs (R34) due to less MP-heating caused by the evaporation of rainwater and lesser efficiency of freezing. Due to the absence of ice treatment and more rainwater, the warm-rain scheme produced a larger TC size. The size simulated from other schemes is more or less the same. Analyses indicate that higher MP-heating induces intense vertical velocities, and AAM and thus increases the TC size. In addition, finer model resolution results in smaller TC sizes. For any particular resolution, the simulated size differs by 30–50 km among the MP schemes, while the size changes by 5–15 km (2–4 km) between 6 km and 2 km (3 km and 2 km) grid-resolutions for any MP scheme. The study concludes that better TC size can be achieved with appropriate MP schemes at higher/cloud-resolving grid resolution. Results indicate that the inner-core heating is strongly correlated with the precipitated compared to non-precipitated hydrometeors. Furthermore, the vertical distribution of hydrometeors and heating is dependent on inner-core updrafts and relative humidity. A novel composite analysis of microphysical processes indicates that the warmer inner core is close to saturation with excess water vapor, which enhances the latent heat release (LHR) through condensation below the freezing level during the rapid intensification (RI) onset. In addition, during RI, strong updrafts transport the liquid hydrometeors above the freezing level and enhance the LHR because of deposition and freezing respectively. The increased precipitating particles in the saturated inner core also enhance LHR. The symmetric convection structured by the atmospheric moisture causes the formation of prolonged RI episodes, as seen in TC Phailin. During rapid weakening (RW), asymmetric and relatively fewer hydrometeors are evident, along with the presence of weak updrafts and strong shear. The dry-air intrusion into the inner core also causes the cooling processes (evaporation and sublimation). The enhancement or reduction of moist static energy and potential vorticity is associated with increased or reduced LHR in the TC rapid intensity changes. Assimilation of INSAT-3DR thermodynamic profiles provides improved initial conditions for the WRF model. Improved initial conditions help improve intensity in 23 cases (out of 36 cases). The track errors are improved by ∼10%, intensity (10 m max surface winds) by 22%, and MSLP by 28%. The mean errors indicate that the maximum (95th percentile) error in the INSAT run is almost close to the mean error of the CTL run. The quantitative verification indicates that the INSAT detects 42% of rapid intensification and 18% of weakening cases. The INSAT experiment exhibits 14%, 21%, and 12% improvement in mean size (R34, R50, and R64) simulations, respectively. Overall, it can be concluded that the INSAT run improved the distribution of RH around the TC center, adjusting MP-heating/warm-core through precipitable and non-precipitable hydrometeors. Thus, the tangential and radial winds are improved. This study highlights the credibility of INSAT profiles on TC size and intensity, particularly intensity changes. The present thesis provides better insights into cloud microphysical processes controlling changes in TC characteristics, particularly TC size and intensity changes over the NIO basin.