ITCZ structure as determined by parameterized versus explicit convection in aquachannel and aquapatch simulations

Abstract

Numerous studies using both global and regional models of the atmosphere have found daunting sensitivities of the structure and dynamics of the intertropical convergence zone (ITCZ) to the representations of unresolved processes, particularly the convective parameterization (CP). Evaluations of these results by comparison to high-resolution simulations with explicit convection have been rather limited, due to the large computational burden of using grid spacings less than 10 km over large domains representative of the Earth's tropics. This study introduces a framework that allows the use of cloud-resolving grid spacings over the tropics and larger spacings over remainder of the domain. The Weather Research and Forecasting (WRF) model is used in an "aquachannel" beta-plane configuration, zonally periodic with length equal to that of the real equator. This model reproduces the general circulation and eddy statistics of similarly configured aquaplanet models. A channel shortened to one third the length of the equator (the "aquapatch") also reproduces the zonal-mean circulations and eddies. Finally, nested grids embedded in the aquapatch are used to simulate tropical convection with 5.15 km resolution. The nested 5.15 km simulations produce broader and lighter rainfall distributions, making single ITCZs wider and smoothing out double ITCZ structures. They also show quite different rainfall production rates for atmospheric parameters such as convective available potential energy (CAPE) and column relative humidity (CRH). The apparent reason for these differences is that the higher resolution allows for the representation of squall lines and associated cold pools that propagate meridionally, redistributing rainfall away from the ITCZ.