Cumulonimbus Vertical Velocity Events in GATE. Part II: Synthesis and Model Core Structure

Abstract

The properties of convective drafts and cores are presented in Part I. By our definition a convective updraft must have a positive vertical velocity for 0.5 km, and exceed 0.5 m s?1 for 1 s; a convective updraft core must exceed 1 m s?1 for 0.5 km. Downdrafts and downdraft cores are defined analogously. Here the properties of the drafts and cores are compared to results of previous work. In addition, the implications of the results in Part I are discussed. GATE cores and drafts are comparable in size and intensity to those measured in hurricanes but weaker than those measured in continental thunderstorms. The lesser intensity seems related to the nearly moist adiabatic GATE sounding. The mass flux by GATE cores is consistent with large-scale requirements. It is fairly evenly distributed over a range of core size and intensity. Updraft core vertical velocity and diameter are positively correlated, primarily the result of a few large strong events. The vast majority of GATE convective cores are sufficiently weak, with mean vertical velocities < 3?5 m s?1, that the time scale for air starting at cloud base to reach the upper troposphere can be in excess of 1 h. The microphysical implications of such long time scales are discussed. They include large fractional rainout from the warm part of the cloud, the presence of ice at relatively warm temperatures, and rapid decrease of radar reflectivity with height above the 0°C level. Usually the clouds in GATE were part of a larger, organized mesoscale system. The typical distribution of cumulonimbus clouds, cores and drafts in such a system is synthesized by combining our results with other GATE results. A schematic updraft core and downdraft core in the middle troposphere are presented, emphasizing that these entities were rather narrow and weak in GATE clouds.