Effects of the Horizontal Scales of the Cloud-Resolving Model on Tropical Cyclones in the Superparameterized Community Atmosphere Model

In this study, the Superparameterized Community Atmosphere Model (SPCAM) is used to simulate tropical cyclones (TCs) using the hindcast approach. Three hindcast experiments are conducted with 32-km (D32), 128-km (D128), and 1,024-km (D1024) horizontal scales in the sub-grid cloud-resolving models (CRMs). The results show that D1024 produces reasonable TCs compared with the reanalysis data. It is 3.42 TCs per 10 days for the reanalysis data, while there are 8.07, 4.88, and 3.73 for D32, D128, and D1024. The bias of overestimating TC numbers grows with decreasing CRM scale. The D32 experiment also produces stronger TCs with a higher precipitation rate and wind speed. The bias is highly related to the efficiency of adjusting convective instability in CRMs. The D32 exhibits higher column-integrated water vapor under warm conditions compared with D1024, indicating its inefficiency in removing water vapor by the weaker convective mass fluxes in the small CRM scale. That is, the vertical transport of convection in a smaller horizontal scale will be restricted by stronger subsidence because CRM columns for compensating are limited. The distribution of accumulated convective instability is broader and more frequent in D32. As a result, large-scale precipitating systems tend to develop in D32, leading to a higher probability of TC genesis. This study highlights the importance of sub-grid configuration when estimating TC activities using SPCAM.