Estimating rain evaporation in convective systems from cold pool surface pressure perturbations

Poster PDF

Description

Because convective cloud systems generally have strong interactions with boundary-layer circulations and thermodynamics, the boundary-layer wind and thermodynamic fields contain a great deal of information about convective cloud systems and their interactions with the boundary layer. We are in the process of “retrieving” this information from 15 years of 5-minute Oklahoma Mesonet data and hourly Arkansas Basin River Forecast Center (ABRFC) gridded precipitation data. We have already demonstrated that estimates of cloud base updraft and downdraft mass fluxes can be retrieved from the surface divergence field. We are currently developing a method to estimate rain evaporation in convective systems from cold pool surface pressure perturbations. In the 1950s, Fujita identified meso-highs in his mesoanalyses and linked them to cold pools produced by rain evaporation (Fujita 1959). We are extending Fujita’s (1959) method for estimating rain evaporation from the hydrostatic surface pressure anomaly and testing it with 3D cloud-resolving model simulations. CRM simulations (Krueger 1988, Tompkins 2001) suggest that the air in cold pools is partly ambient boundary-layer air and partly air from downdrafts. The air from both sources has been cooled by rain evaporation. In order to better understand the relationships between cold pools and downdrafts, we would like to determine the sources of cold pool air and their relative contributions. To do this, we need to estimate the amount of rain evaporation. Information about the sources of cold pool air may be gained by analysis of T, q, and h (moist static energy) of the cold pools, combined with knowledge of the downdraft source level and its thermodynamic properties, and the downdraft mass flux. Unfortunately, it is not easy to determine the downdraft source level(s). However, knowledge of the amount of rain evaporation can be used to close this problem.