Effects of a Convectively Forced Gravity Wave Drag Parameterization on a Mesoscale Convective System Simulated by a Mesoscale Model (MM5)

A parameterization of gravity wave drag induced by cumulus convection (GWDC) proposed by Chun and Baik is implemented in the PSU/NCAR Mesoscale Model (MM5) and its effect on simulations of a convective system that developed in August 1998 over the Korean peninsula is investigated. The cloud-top momentum flux is concentrated in major convective regions, and its magnitude is sensitive to wind and stability conditions. The drag forcing by the GWDC parameterization changes the wind field above convection, but its effect extends down to the lower troposphere. This is because negative drag forcing due to breaking of convectively generated gravity waves above clouds induces convergence (divergence) on the upstream (downstream) of the convection, and it results in lower level divergence (convergence) on the upstream (downstream) of the convection. Throughout this process, convection is enhanced on the downstream and the maximum updraft region moves to the downstream side. Impact of the GWDC parameterization on mesoscale circulation is studied analytically by solving the two-dimensional gravity waves induced by localized momentum forcing that represents convective gravity wave drag. Gravity-wave response to the momentum forcing explains the convergent/divergent patterns above clouds and in the lower troposphere obtained in MM5 simulation. This suggests that the enhancement of the convection on the downstream by the GWDC process is due to the grid-resolvable gravity waves forced by the parameterized gravity wave drag. Therefore, the GWDC parameterization affects mesoscale circulation not only directly in the local regions where wave breaking occurs but also indirectly in the lower troposphere through the non-local grid-resolvable gravity waves. This unique feature appears only in the mesoscale model. Sensitivity experiments of the GWDC parameterization to cumulus parameterization schemes show that convection is enhanced by the GWDC process in general, but the degree of enhancement differs significantly for each cumulus parameterization scheme.