Sensitivity of tropical cyclone Idai simulations to cumulus parametrization schemes

Bopape, M.-J. M., Cardoso, H., Plant, R. S. ORCID: https://orcid.org/0000-0001-8808-0022, Phaduli, E., Chikoore, H., Ndarana, T., Khalau, L. and Rakate, E. (2021) Sensitivity of tropical cyclone Idai simulations to cumulus parametrization schemes. Atmosphere, 12 (8). 932. ISSN 1598-3560 doi: 10.3390/atmos12080932

Abstract/Summary

Weather simulations are sensitive to subgrid processes, that are parameterized in numerical weather prediction (NWP) models. In this study, we investigate the response of tropical cyclone Idai simulations to different cumulus parameterization schemes, using the Weather Research and Forecasting (WRF) model with a 6 km grid length. Seventy-two hour (00 UTC 13 March to 00 UTC 16 March) simulations are conducted with the New Tiedtke (Tiedtke), New Simplified Arakawa Schubert (NewSAS), Multi-scale Kain Fritsch (MSKF), Grell-Freitas, and the Betts-Miller-Janjic (BMJ) schemes. A simulation for the same event is also conducted with the convection scheme switched off. The twenty-four hour accumulated rainfall during all three simulated days is generally similar across all six experiments. Larger differences in simulations are found for rainfall events away from the tropical cyclone. When the resolved and convective rainfall are partitioned, it is found that the scale-aware schemes (i.e. Grell-Freitas and MSKF) allow the model to resolve most of the rainfall, while they are less active. Regarding the maximum wind speed, and minimum sea level pressure (MSLP), the scale aware schemes simulate a higher intensity that is similar to the Joint Typhoon Warning Center (JTWC) data set, however the timing is more aligned with the Global Forecast System (GFS), which is the model providing initial conditions and time dependent lateral boundary conditions. Simulations with the convection scheme off were found to be similar to those with the scale-aware schemes. It is found that Tiedtke simulates the location to be farther south west compared to other schemes, while BMJ simulates the path to be more to the north after landfall. All of the schemes as well as GFS failed to simulate the movement of Idai into Zimbabwe, showing potential impact of shortcomings in the forcing model. Our study shows that the use of scale aware schemes allows the model to resolve most of the dynamics, resulting in higher weather system intensity in the grey zone. The wrong timing of the peak shows a need to use better performing global models to provide lateral boundary conditions for downscalers.