Cloud-resolving simulations of TOGA-COARE and DYNAMO using parameterized large scale dynamics

Variations in deep convective activity during the 4-month TOGA-COARE field campaign are simulated using a cloud-resolving model (CRM). The model is driven by large-scale vertical velocities that are parameterized using one of two different methods: the damped gravity wave method and the weak temperature gradient (WTG) method. The model is also constrained by imposing sea surface temperature, radiative fluxes, and by relaxing the horizontal mean horizontal wind field to observation. We find that the CRM with parameterized large-scale vertical motion can capture the intraseasonal variations in rainfall to some degree. Experiments in which one of several observation-derived forcings are set to their time mean value suggest that those which influence direct forcings on the moist static energy budget – surface turbulent fluxes and radiative cooling – play important roles in controlling convection. The parameterized large scale vertical velocity has a vertical profile that is too bottom-heavy compared to observations when the wave coupling method is used with vertically uniform Rayleigh damping on horizontal wind, but is too top-heavy when the WTG method is used.

Our ongoing work involves better treatment of horizontal advective tendencies of moisture, which is important for the MJO events observed during the DYNAMO (Dynamics of Madden Julian Oscillation) period. Cloud-resolving simulations indicate that straightforward application of horizontal moisture advection may lead to a persistent dry state. To correct this, we adopt the method of moisture relaxation, in place of horizontal advective tendencies of moisture. Results from cloud-resolving simulations with this new treatment of horizontal advective tendencies of moisture will also be discussed.