Relationship between Precipitation Rates at the Ground and Aloft—A Modeling Study

Abstract

Two cloud-resolving mesoscale models, the Karlsruhe Atmospheric Mesoscale Model (KAMM) and the fifth-generation Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model (MM5), were used to study reflectivity factor?rain-rate (Z?R) relationships and instantaneous and horizontally averaged profiles of precipitation rate for convective storms of varying intensity. Simulations were conducted for idealized terrain. KAMM modeled a single shower cloud; MM5 was used to study split-storm supercell development. Both models consistently confirm analytical results from earlier studies: convective drafts and stratification of air density significantly alter the local rain rate and, therefore, also any Z?R relation relying on conditions of stagnant air and sea level air density. Air density effects can be almost completely corrected for by a recently proposed algorithm, but effects of convective drafts remain. They can lead to upward precipitation mass fluxes of significant magnitude and subsequent horizontal displacement of precipitation. Applicability of simple Z?R relations over complex terrain with distinct watershed boundaries will be strongly degraded by such convection effects on precipitation mass fluxes.