Simulation of tropical precipitation using the weak temperature gradient approximation to the Goddard cumulus ensemble model

Abstract

Tropical precipitation due to deep convection is typically forced in limited area models by specifying the large-scale vertical velocity or vertical advection terms. Since the dominant thermodynamic balance in the tropics is between adiabatic cooling and diabatic heating (or vice versa), and the latter is due mostly to precipitating convection, the standard approach determines the model precipitation rate almost independently of model physics. The model thus cannot be used to address the question: what causes deep convection to occur or not occur? We apply an alternate approach, based on the Weak Temperature Gradient (WTG) approximation, to the Goddard Cumulus Ensemble Model (GCEM), in order to use the model to address this question. In the WTG approach, the horizontal mean temperature profile is specified, and the large-scale vertical velocity is determined by model physics and the assumption that adiabatic cooling balances diabatic heating. In this approach, the precipitation cannot be determined from the forcings independently of the model physics, and so we can use the model to understand what controls tropical deep convection.

The GCEM is a full-physics cloud resolving model, run here in 2D on a domain 512 km in the horizontal, with 1km horizontal resolution and 41 vertical levels. The horizontal boundary conditions are periodic, and the lower boundary is an ocean surface with uniform sea surface temperature (SST). Artificial momentum forcing is applied to the horizontal mean tropospheric wind to maintain a chosen vertical profile. Keeping the horizontal mean temperature profile fixed, we vary the SST, surface wind speed, and vertical shear, and examine the resulting statistically steady states simulated by the model, focusing on the precipitation and relative humidity profiles. As might be expected, the precipitation increases strongly and nonlinearly with SST. Interesting changes are also observed in the large-scale vertical velocity, mean vertical profiles of cloud water and ice, and radiative cooling profiles.