Convective Response to Diurnal Forcing in Weak Temperature Gradient Simulations

The weak temperature gradient (WTG) approximation is an effective parameterization of the large scale tropical environment for limited domain simulations. It is based on the tendency for gravity waves to horizontally homogenize potential temperature. In our cloud resolving model (CRM), the WTG approximation is enforced by generating a vertical velocity that counteracts buoyancy anomalies, and thus relaxes the domain average potential temperature to a reference profile which represents the ambient environment.

The modeled convection in our CRM is sensitive to the WTG reference profile, and evolves in response to prescribed changes in that profile. We use this to investigate the extent to which convection responds to diurnal variations which manifest in the thermodynamic environment.

Using observations obtained during the 2001 East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System (EPIC) field program, we generate diurnal variations which are imposed on the WTG reference profile. The extent to which the convection reproduces the diurnal variations in precipitation depends strongly on the extent to which the WTG approximation is enforced. The closer the domain averaged potential temperature is to the reference value, the more the modeled precipitation rates reflect the diurnal variations observed during EPIC. This suggests that a significant number of the mechanisms which contribute to the diurnal variations manifest in the thermodynamic environment and therefore can be accounted for in WTG simulations.