Observational Investigations of Entrainment Within the Weak Echo Region

Abstract

Abstract Data were obtained for three High Plains thunderstorms in which penetrations were made of the weak echo region by an instrumented aircraft. The data from one of the cases are presented in detail. Two of the storms were steady state, as revealed by chaff analysis and subsequent subcloud passes. The third storm dissipated during the penetrations. The three storms were each characterized by negatively buoyant air at cloud base. Chaff released into the updrafts of the storms did not decelerate below the level of free convection (LFC). A vertical pressure perturbation gradient, therefore, existed below the LFC and within the weak echo region which acted to accelerate the air parcels in the presence of negative buoyancy. The analysis of the equivalent potential temperature fields for the two steady storm cases revealed considerable entrainment of environment air into the weak echo region. The mixing of the entrained parcels probably caused the observed increase of turbulence with height.