Evaluating intense precipitation in high-resolution numerical model over a tropical island: impact of model horizontal resolution

Abstract. A test of sensitivity to the model grid spacing for extreme rainfall simulation is carried out for the tropical island of La Réunion, which holds several world records of precipitation. An extreme rain event occurring during the moist season in 2011 is selected to study the numerical model behavior at four horizontal resolutions: 4 km, 2 km, 1 km and 500 m. The assessment based on raingauge network shows that the performance of daily rain simulation increases as reducing the model grid spacing from 4 km to 1 km. The spatial variability of 24 h rainfall is well captured by the simulation at 1 km and 500 m resolution. However, refining the resolution from 1 km to 500 m has little impact on the model performance compared to the 1 km run. Diagnosis analysis and numerical experiment reveal that only the 1 km and 500 m grid spacings are able to simulate a cold pool located near the coastal area of the island. This cold pool triggers the thermal lifting and creates convergence between the prevailing moist flow and offshore land breeze. The observed precipitation, air temperature and wind get good agreements with these simulated features. However, this cold pool is missed in the 4 km and 2 km simulations. Our study highlights the important role of air mixing with microphysical processes at 1 km scale in simulating such intense precipitations.