Stronger Local Overturning in Convective-Permitting Regional Climate Model Improves Simulation of the Subtropical Annual Cycle

Abstract

Global climate models fail to represent the annual cycle of tropical-extratropical cloud bands and produce too much summer rainfall over subtropical southern Africa. This study demonstrates that running a regional convective-permitting climate simulation alleviates these biases, counteracting biases that are present in the parent model. The improvement emerges from stronger vertical mass flux in the tropics, which forces a stronger local Hadley overturning into the summer hemisphere. This enhanced overturning increases upper-level subsidence in the subtropics and amplifies the forcing of the local subtropical jet. Together, these improvements halve the wet subtropical rainfall bias and are associated with a 50% increase to an 80% match between the simulated and observed annual cycle of crucial tropical-extratropical cloud band rainfall systems. The results advocate for the increased use of convective-permitting climate models with domains that include regional tropical convection hot spots, in order to fully benefit from the explicit representation of deep convection.