Evaluating the Performance of a Convective Scale Coupled NWP Model for Atmospheric and Oceanic Hazards Associated with Tropical Cyclones in the Bay of Bengal

Abstract:

Tropical cyclones (TCs) forming over the Bay of Bengal can cause devastation when they make landfall in India and Bangladesh. Despite improvements in track forecasts, accurately forecasting TC intensity and structure remains a challenge. TC intensity is moderated by heat, momentum and moisture exchanges between the atmosphere and ocean. In recent years there have been significant improvements in the skill of TC forecasts due to the implementation of coupled atmosphere-ocean models and high-resolution models capable of explicitly resolving small-scale physical processes influencing storm development.

This study evaluates the impact of atmosphere-ocean coupling on forecasts of six TCs in the Bay of Bengal from 2016 to 2019, with unique track and intensity attributes. A regional convective-scale coupled prediction system is used in both a Met Office Unified Model atmosphere-only configuration, with 4.4 km grid spacing, and coupled to a 2.2 km resolution NEMO ocean model. To determine the impact of coupling on ocean-atmosphere heat exchange, forecast sea surface temperature (SST) is compared to observations. The impact of coupling on track position and storm intensity is evaluated using predictions of minimum sea level pressure (MSLP) and 10 m maximum sustained winds (MSW). Hazard prediction is assessed by calculating error metrics in the predicted fields of 10 m wind speed and rainfall.

Results using a time-lagged ensemble of forecasts of TC Fani (April-May 2019) show that the coupled system produces SST predictions which are consistent with observations. TC track position errors are small, but intensity error metrics for MSLP and MSW show biases relative to observations, with both configurations underestimating peak storm intensity. Rainfall during landfall is overestimated, and there are significant differences in the structure of rain bands in the coupled and uncoupled configurations, with the heaviest rainfall to the west of the track position in the atmosphere-only model and the east in the coupled model. Changes in storm structure are evaluated using satellite and ground-based observations. We use these simulations to investigate how the structure of the storms and the related hazards are changing as they approach and make landfall.