Cloud-resolving and single column model simulations of a Dry Intrusion observed during toga-coare

In this study we seek to evaluate how well cloud resolving models and a single column model represent cloud and radiative properties over the tropical western Pacific.

The event of interest is the recovery of the tropical atmosphere following the arrival of a dry intrusion. These dry intrusions are associated with the advection of extremely dry air from the upper-troposphere or even lower stratosphere in middle latitudes into the middle troposphere over the central and western Pacific. These events are frequent with events observed during TOGA COARE every 10-20 days. The magnitude of the drying from lateral advection can easily exceed that of subsidence. The recovery time of the tropical atmosphere back to moist conditions varies with height and ranges from a few days in the boundary layer to well over a week for higher levels. Unfortunately these advection events are typically poorly replicated in global models.

Using forcing data from the TOGA COARE project we have studied one period (13-20 November 1992) in details using the CNRM-GAME Cloud Resolving Model and the NCAR CCM3 SCM. General observed features are reproduced by the CRM. In particular the model is able to reproduce the succesive observed convective stages: non-precipitating shallow boundary layer clouds, precipitating shallow clouds, and deep convection. Using detailed budgets of simulated moisture, the recovery of the atmosphere after the dry event is analyzed. Consequences for parameterizations in SCM will be given and results of a series of SCM runs discussed. In particular the lack of diurnal cycle and the lack of sustained recovery in the moisture field in the runs of current SCM are discussed.