The Use of Cloud-Resolving Simulations of Mesoscale Convective Systems to Build a Mesoscale Parameterization Scheme

A method is described for parameterizing thermodynamic forcing by the mesoscale updrafts and downdrafts of mesoscale convective systems (MCSs) in models with resolution too coarse to resolve these drafts. The parameterization contains improvements over previous schemes, including a more sophisticated convective driver and inclusion of the vertical distribution of various physical processes obtained through conditional sampling of two cloud-resolving MCS simulations. The mesoscale parameterization is tied to a version of the Arakawa– Schubert convective parameterization scheme that is modified to employ a prognostic closure. The parameterized Arakawa–Schubert cumulus convection provides condensed water, ice, and water vapor, which drives the parameterization for the large-scale effects of mesoscale circulations associated with the convection. In the mesoscale parameterization, determining thermodynamic forcing of the large scale depends on knowing the vertically integrated values and the vertical distributions of phase transformation rates and mesoscale eddy fluxes of entropy and water vapor in mesoscale updrafts and downdrafts. The relative magnitudes of these quantities are constrained by assumptions made about the relationships between various quantities in an MCS’s water budget deduced from the cloud-resolving MCS simulations. The MCS simulations include one of a tropical MCS observed during the 1987 Australian monsoon season (EMEX9) and one of a midlatitude MCS observed during a 1985 field experiment in the Central Plains of the United States (PRE-STORM 23–24 June).