Cloud-resolving simulations of the northern DYNAMO sounding array with either imposed or parameterized large scale dynamics

The northern sounding array deployed in the central Indian Ocean captured two MJO events from October 1 to December 15, 2011 during the CINDY/DYNAMO period. Using the large scale forcing dataset from the northern sounding array, we have performed a set of cloud-resolving simulations (CRM) with the WRF model. Similar to many other studies, we impose large scale vertical velocity, sea surface temperature, and relaxation of horizontal winds to the CRM. While such forcing tightly constrains the moist budget, hence surface rainfall, cloud population and radiative fluxes are still free to evolve to a large extent. To bracket this uncertainty, sensitivity experiments to the microphysics and radiation schemes are conducted, and a systematic comparison between model simulations and various observations is also performed. Our results indicate that cloud—radiation interaction may play a major role in the bias of the simulated moist static energy budget.

We will also present some preliminary results using CRM with parameterized large scale dynamics in which large scale vertical velocity is derived instead of being specified. Two methods are used here: the weak temperature gradient method and the damped gravity wave method. Results using these two methods indicate that our model can capture the intraseasonal variations in surface rainfall to some degree using the radiative fluxes derived from the standard run. We will also discuss the complications related to the treatment of horizontal advection of moisture using parameterized large scale dynamics.