Effects of Vertical Wind Shear on the Cumulus Transport of Momentum: Observations and Parameterization

Abstract

Dynamical effects of organized cumulus convection on its environment with vertical wind shear are studied. Analyses of the wind field and momentum budget residual for mesoscale convective systems observed during SESAME and PRE-STORM reveal systematic differences in the vertical transport of horizontal momentum between mesoscale convective complex (MCC) and squall line cases. In the MCC, a distinct minimum in wind speed appears over the area of intense convection and the momentum budget residual acts to decelerate the environmental wind and to reduce the upper-level vertical shear. Therefore, the inferred vertical transport of momentum in the MCC is downgradient in the upper layer. On the other hand, in the squall line, there is no wind speed minimum and the upper-level vertical shear of the line-normall component of the environmental wind increases as convection develops. Thus, the vertical transport of momentum normal to the squall line is upgradient in the upper layer, although the transport of momentum parallel to the line is downgradient. The effects of cumulus convection on the environmental flow are through the subsidence of environmental air that compensates the cloud mass flux, the detrainment of momentum from clouds, and the convective-scale horizontal pressure gradient force acting on the environment. A cumulus momentum parameterization including the convective-scale pressure gradient force is formulated. The pressure gradient force is related to the vertical wind shear, cloud mass flux, and orientation of organized convection. The parameterization is capable of reproducing both the upgradient and downgradient transports of horizontal momentum, provided the mode of organization of cumulus convection is known.