Cumulus Ensembles in Shear: Implications for Parameterization

Abstract

A systematic numerical investigation is conducted into the role of ambient shear on the macrophysical properties of tropical cumulus ensembles maintained by convective available potential energy generated by constant surface fluxes of temperature and moisture and large-scale advective cooling and moistening. The effects of five distinct idealized wind profiles on the organization of convection, and quantities relevant to the parameterization of convection and convectively generated clouds, are examined in a series of 6-day two-dimensional cloud-resolving simulations. Lower-tropospheric shear affects the mesoscale organization of convection through interaction with evaporatively driven downdraft outflows (convective triggering), while shear in mid-to-upper levels determines the amount of stratiform cloud and whether the convective transport of momentum is upgradient or downgradient. Shear significantly affects the convective heating and drying, momentum transport, mass fluxes, and cloud fraction. Sensitivity is strongest in weaker forcing. Cloud-interactive radiation has little direct effect on a 6-day timescale. In particular, the effects of shear on convective momentum transport and cloud fraction are large enough to be potentially significant when included in parameterizations for climate models.