The Distribution of Convective and Mesoscale Precipitation in GATE Radar Echo Patterns

Abstract

Quantitaive radar data have been used to divide individual radar echoes observed in GATE into convective and mesoscale components. Echoes 2 km² in area were considered, by virtue of their small time and space scales, to be entirely convective. Larger echoes were composed partly of convective regions, characterized by intense fluctuating echo cores, and partly of horizontally uniform mesoscale regions which were less intense and much more slowly varying in structure than the convective regions. The mesoscale regions were apparently associated with widespread precipitating anvil clouds in GATE cloud clusters. About 40% of the total precipitation in GATE fell in these mesoscale regions. The remaining rainfall fell in the convective regions. Only very small amounts of convective rain fell from echoes <5 km in maximum height. Increasing amounts of convective precipitation were associated with echoes of increasing maximum height, from the very small amounts in echoes ≤5 km to a maximum amount from echoes with maximum tops of ∼12 km. A secondary maximum of rain was associated with overshooting echoes reaching 15–16 km. Overshooting was rare in early summer when the level of zero buoyancy was near 11 km, and more common in mid to late, summer when the level of zero buoyancy was near 14 km. When the overshooting echoes were accompanied by weak large-scale upward motion they were relatively isolated, whereas when the overshooting coincided with strong large-scale forcing, the echoes were more widespread. Major events of mesoscale anvil rainfall were always associated with convective echoes which had maximum tops reaching the level of zero buoyancy, and they tended to occur when the large-scale upward motion was enhanced.