Momentum Flux by Lines of Cumulonimbus over the Tropical Oceans

Abstract

Examination of aircraft and rawinsonde data gathered in nine tropical mesoscale convective line cases indicates that all but two lines systematically increased front-to-rear momentum at heights greater than about 4 km, and rear-to-front momentum at lower levels, where “front” is defined as the direction toward which the line is moving. The convective lines were characterized by a leading 10–20 km wide band of convective clouds, and a trailing region of stratiform cloudiness. Most wore “propagating” lines, moving into the wind at all levels. Consistent with mixing-length theory, the vertical transport of the horizontal wind component parallel to the lines was down the vertical gradient of the component, resulting in a decrease of its vertical shear. Smaller, more random cloud groups and the upper portions of a convective line with isolated towers transported both components of horizontal momentum downgradient. Normalization of the vertical flux of horizontal momentum normal to the line (u′¯w′¯) suggests that it is achieved mainly by updraft cores which could be traced to the undisturbed mixed layer ahead of the line. The air in the cores is accelerated upward and backward into a mesoscale area of low pressure located in the rear portion of the line's leading convective region. The low pressure is primarily hydrostatic, its intensity proportional to the depth and average buoyancy of the cloudy air overhead. However, dynamic pressure effects are important where convective cores are particularly concentrated. From the aircraft data, the momentum transport by the trailing, stratiform region appears small, but this conclusion needs confirmation by sensing platforms more suited to gathering mesoscale wind field data. The failure to account for the momentum transport properties of two-dimensional convective lines might explain the lack of success in parameterizing the effects of cumulus clouds on the mean wind profile.