The objective of the present study is to investigate in detail the sensitivity of cumulus parameterization (CP), planetary boundary layer (PBL) parameterization, microphysics parameterization (MP) on the numerical simulation of severe cyclone LAILA over Bay of Bengal using Weather Research & Forecasting (WRF) model. The initial and boundary conditions are supplied from GFS data of 1 o × 1 o resolution and the model is integrated in three 'twoway' interactive nested domains at resolutions of 60 km, 20 km and 6.6 km. Total three sets of experiments are performed. First set of experiments include sensitivity of Cumulus Parameterization (CP) schemes, while second and third set of experiments is carried out to check the sensitivity of different PBL and Microphysics Parameterization (MP) schemes. The fourth set contains initial condition sensitivity experiments. For first three sets of experiments, 0000 UTC 17 May 2010 is used as initial condition. In CP sensitivity experiments, the track and intensity is well simulated by BettsMiller-Janjic (BMJ) schemes. The track and intensity of LAILA is very sensitive to the representation of large scale environmental flow in CP scheme as well as to the initial vertical wind shear values. The intensity of the cyclone is well simulated by YSU scheme and it depends upon the mixing treatment in and above PBL. Concentration of frozen hydrometeors, such as graupel in WSM6 MP scheme and latent heat released during auto conversion of hydrometeors may be responsible for storm intensity. An additional set of experiments with different initial vortex intensity shows that, small differences in the initial wind fields have profound impact on both track and intensity of the cyclone. The representation of the mid-tropospheric heating in WSM6 is mainly controlled by amount of graupel hydrometeor and thus might be one of the possible causes in modulating the storm's intensity.

The objective of the present study is to investigate in detail the impact of different physical parameterization schemes on track and intensity of two severe cyclonic storms, AILA and JAL, which formed over Bay of Bengal, using a WRF mesoscale model. Three 2-way interactive nested domains with horizontal resolutions of 60, 20 and 6.6 km are used with initial and boundary conditions from NCEP-FNL data. Three sets of experiments include sensitivity to cumulus, microphysics and planetary boundary layer parameterization schemes, respectively. From cumulus parameterization experiments, Betts-Miller-Janjic is found to be better in the group. The strength of mid-latitude trough and presence of southward wind surge for cyclone AILA, the strength of the crossequatorial flow as well as stronger easterly wind fields in the mid-tropospheric levels for cyclone JAL, and the amount of potential vorticity for both cyclones are some of the factors, which affect the large-scale flow and, hence, the track of both storms. WSM6 Microphysics scheme is able to produce a realistic feature of the cyclones as compared to the other schemes. The realistic representation of mid-tropospheric heating contributed by snow and graupel hydrometeors may be one of the reasons for better intensity simulation by WSM6. The higher values of relative humidity in and above the boundary layer favor the deep vertical mixing in YSU and thus contribute towards the better intensity simulation.

During August 2018 and 2019 the southern state of India, Kerala received unprecedented heavy rainfall which led to widespread flooding. We aim to characterize the convective nature of these events and the large-scale atmospheric forcing, while exploring their predictability by three state of the art global prediction systems, the National Centre for Environmental Prediction (NCEP) based India Meteorological Department (IMD) operational Global Forecast System (GFS), the European Centre for Medium Range Weather Forecast (ECMWF) integrated forecast system (IFS) and the Unified Model based NCUM being run at the National Centre for Medium Range Weather Forecasting (NCMRWF).Satellite, radar and lightning observations suggest that these rain events were dominated by cumulus congestus and shallow convection with strong zonal flow leading to orographically enhanced rainfall over the Ghats mountain range, sporadic deep convection was also present during the 2019 event. A moisture budget analyses using the ERA5 (ECMWF Reanalyses version 5) reanalyses and forecast output revealed significantly increased moisture convergence below 800 hPa during the main rain events compared to August climatology. The total column integrated precipitable water tendency, however is found to be small throughout the month of August, indicating a balance between moisture convergence and drying by precipitation. By applying a Rossby wave filter to the rainfall anomalies it is shown that the large-scale moisture convergence is associated with westward propagating barotropic Rossby waves over Kerala, leading to increased predictability of these events, especially for 2019.Evaluation of the deterministic and ensemble rainfall predictions revealed systematic rainfall differences over the Ghats mountains and the coastline. The ensemble predictions were more skilful than the deterministic forecasts, as they were able to predict rainfall anomalies (>3 standard deviations from climatology) beyond day 5 for August 2019 and up to day 3 for 2018.

This study addresses the problem of incorporating moist processes (resolving the grid scale and parameterizing the subgrid scale) at resolutions of 9 and 3 km with double-and triplenested domains, respectively, in predicting the track and intensity of four cases of weaker tropical cyclones over the North Indian Ocean. The sensitivity experiments are carried out with three convective parameterization schemes, and the results are evaluated based on the track and intensity of cyclones. The Betts-MillerJanjic scheme shows the most reasonable representation of track and intensity and therefore is used for all sensitivity experiments related to microphysical schemes in two and three domains. Three sets of microphysics sensitivity experiments are carried out: The first set includes experiments with parameterized moist convection (referred to as the 9-km experiment) in two domains (27 and 9 km). The second and third sets of simulation experiments are carried out at 9 km in two domains (27 and 9 km; referred to as the 9-km-noCP) and at 3 km in three domains (27 km, 9 km and 3 km; referred to as the 3-km experiment), respectively, by resolving the grid-scale convection explicitly with the four bulk microphysical schemes. The explicit moist convection treatment at 9-and 3-km resolution produces a better cyclone simulation than the parameterized convection at 9-km resolution. The latent heat released in the generation of hydrometeors such as snow and graupel in the mid-tropospheric levels appears to influence the heating within the inner core of the cyclone. The comparable and more realistic representation of mid-tropospheric heating is possibly one of the main reasons behind the improvement at 9-km-noCP and 3 km. The stronger vertical advection of moist static energy gives wellorganized mesoscale convection within the cyclone environment at 9-km-noCP and 3-km resolution. This study therefore demonstrates the importance of microphysical processes in the simulation of weaker TC over the North Indian Ocean.