Numerical modeling of tropical cyclone size over the Bay of Bengal: influence of microphysical processes and horizontal resolution

The destructiveness of tropical cyclones (TCs) is associated with uneven distribution of winds, TC-size, rainfall and storm-surge. The TCs in the Bay of Bengal (BoB) have shown a steady increase in size, as measured by the 34-knot wind radius (R34) over the past two-decades. TC-size information is essential in estimating areas to be evacuated to minimize the damage and loss of life. The study addresses the significance of microphysical (MP) processes and horizontal grid-resolution for improved TC-size. The Weather Research and Forecasting model is run at different grid-resolutions and various MP-schemes. Results show that TC movement is less sensitive to MP-schemes, while the size is more sensitive. The simple-ice (WSM3) scheme produced smaller TCs in R34 (228-km) due to less MP-heating caused by the evaporation of rainwater and lesser efficiency of freezing. Due to absence of ice-treatment and more rainwater, the warm-rain (Kessler) scheme produced larger TC-size (295-km). The size simulated from other schemes is more or less the same (266–284 km). Analyses indicate that higher MP-heating induces intense vertical-velocities, absolute angular momentum and thus increases the TC-size. Finer model resolution results in smaller TC-size. Though WSM3 performed better for size simulation, it somewhat underestimates at finer resolutions. For any particular resolution, the simulated size differs by 30–50 km among the MP schemes, while the size changes 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.