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 1024-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 the sub-grid
CRM. 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.