Abstract:
Weather simulations are sensitive to subgrid processes that are parameterized in numerical
weather prediction (NWP) models. In this study, we investigated 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 were 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 was also conducted with the convection scheme switched
off. The twenty-four-hour accumulated rainfall during all three simulated days was generally similar
across all six experiments. Larger differences in simulations were 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) dataset, 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 was found that Tiedtke simulates the location to be farther southwest
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
the potential impact of shortcomings on 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.
