Deep Convection on Day 261 of GATE

Abstract

Aircraft, radar, satellite and ship data, gathered on 18 September 1974 during GATE, have been brought to bear an clouds of the middle and upper troposphere associated with a cloud cluster occurring near the ridge axis of a 700 mb wave. Clouds penetrating above 2.5 km into the middle troposphere were organized in bands about 9 km apart, aligned roughly along the direction of the wind shear in the cloud layer. Radar echoes corresponding to the cumulus congestus were of lifetime roughly 30 min, top height 6 km and peak rainfall rate 1.3 mm h?1. The number density of such echoes increased from one in 15 000 km2 to a value about three times greater, while convergence at ?950 hPa (obtained from satellite cloud tracking) increased from about 1.5 to 3 ? 10?5 s?1. Convergence into a square box of side 150 km, circumnavigated by three aircraft in the moist layer, reached about 3.5 ? 10?5 s?1 near cloud base. The upward flux of water vapor at cloud base was about 0.25 g m?2 s?1, equivalent to rain of intensity 0.9 mm h?1 with 100% conversion of the vapor. During this time mean rainfall rates over the area, and peaks averaged over 18 km2, increased from 0.2 to 0.7, and 23 to 39 mm h?1, respectively. Areas of small rainfall rate merged. High towers became taller and more numerous, but remained the same size, ?50 km2 in area at altitude 7 km, for echo cores of intensity 29 dBZ. A gust front at the ship Oceanographer was associated with one of the cloud bands., it was found that the band propagated discontinuously by new growth in its leading side. It featured a mesoscale pattern of updrafts in front and downdrafts behind, the downdrafts originating near altitude 2.5?3 km. A tentative conclusion is drawn that convective circulations tended to generate horizontal momentum near cloud base. The longitudinal rolls obtained theoretically by Sun (1978), for conditions of relatively strong buoyancy, match the observed bands well. Cumulonimbus clouds reaching 15 km grew only out of an environment already moistened by lesser clouds. Cloud towers a few kilometers wide were photographed. Such towers were linked with groups of echoes identified from a high-resolution display of three-dimensional radar scans. This ?wall chart? revealed that echoes were multicellular, and moved with widely differing velocities. It is deduced that individual echo groups yielded local winds of speed exceeding those in the environment. The lesser echo groups were part of a population having log-normally distributed properties. Those which yielded reflectivities as high as 46 dBZ were a different population; they were elongated norlh-northeast to south-southwest, a direction corresponding to that of a confluence asymptote at ?950 hpa discerned from satellite data.