Aircraft data from Winter MONEX have been combined with other data to study mesoscale features, and organization of cumulus clouds, on 10–12 December 1978. A moderate cold surge in the northeasterly monsoon flow, toward cloudiness in an equatorial trough off Borneo, peaked on 11 December. Clouds in the northeasterly monsoon flow were similar to those in the trades, with variations in convective regime on length scales on the order of 100 km. Marked mid-tropospheric subsidence was accompanied by low-level divergence near 20°N. During 10 December, anvil clouds near Borneo expanded; cumulus congestus and cumulonimbus formed on the periphery of this area. The approach of the low-level northeasterlies to the area of anvils was marked by a diminution of subsidence, conditional instability, and a weak field of low-level convergence, with randomly organized cumulus of increasing height. A low-level easterly jet was found in this transition zone, downstream from cloudiness over the Philippines. South of Vietnam, a clear area was associated with low air temperatures, and not subsidence. Congestus and cumulonimbus clouds formed near the eastern coast of the Malay Peninsula. Cloud streets were seen from latitude 19°N to the Malaysian coast (with a break south of Vietnam). These clouds were confined below the level of an inflection point in the profile of winds normal to the street direction. Greatest spacings of streets occurred with greatest vertical shears of the cross-winds. Cloud number densities were more closely related to the instability of the vertical stratification than to any other parameter. Cross-wind organization of clouds occurred in circumstances of unstable, stratification and apparently of net ascent. Alignment of clouds was at an angle to the directions of both winds and vertical wind shears. It is inferred that when convergence was strong, deep clouds occurred along lines of convergence in the surface streamlines.

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.

On 18 September 1974, a cloud cluster growing in the GATE ship array was examined using aircraft flying close to one another at different heights, the geostationary satellite SMS-1, and radar, rawinsonde and ship data, with a view to elucidating mechanisms of convection. In this paper we concentrate analysis on cloudy convection in the moist layer. In and above southerly surface monsoon flow approaching the cluster, clouds indigenous to the moist layer took the form of rows of tiny cumulus, and of arcs of cumulus mediocris, with patterns different from those of deeper clouds. From satellite visible images, arcs were traced for periods exceeding 2 h. Airborne photography showed that the arcs were composed of many small clouds. Radar data showed that they originated after precipitation. Apparently, throughout their life cycle, they perpetuated the pattern of an initiating dense downdraft. Eventually they yielded isolated cumulus congestus, again bearing precipitation. Aircraft recorded the distribution of thermodynamic quantities and winds at altitudes within the mixed layer, and at 537 and 1067 m. These data indicated that the arcs persisted as mesoscale circulations driven by release of latent heat in the clouds, rather than being driven by the original density current at the surface. The cloudy circulations were vigorous near and above cloud base, becoming weaker upward through altitude 1 km. The entire mesoscale circulation systems were of horizontal scale roughly 40 km. The mesoscale cloud patterns of the moist layer appeared to play a primary role in heat transfer upward within this layer, and contributed to the forcing of showering midtropospheric clouds.

Statistics of echoes observed on day 261 of GATE, using the radar aboard the Canadian ship Quadra, are described. This is a case in which detailed observations have been obtained from many sensors during the growth of a cloud cluster. Dividing the radar field of view into many “bins,” it was found that rainfall rates and echo top heights were log-normally distributed UP to a certain limit, with a few additional high values. After identifying echo cores, bounded by a reflectivity threshold 24 dB above 1 mm6 m−3, their area were found to be distributed in a similar manner. Upper limits of log-normality before (and during) the growth of the cluster were about 5 (and 30) mm h−1 for rainfall rate, 7 (and 7) km for echo top height (at the 29 dB level of intensity) and 100 (and 400) km2 for area (within contours of intensity 24 dBZ). To echoes organized on a length scale of about 100 km for more, one attributes maximum rainfall rates reaching roughly 50 mm h−1, peak heights reaching 14 km, and maximum areas of elongated cores reaching 5000 km2.