Effects of gravity wave drag induced by cumulus convection on the atmospheric general circulation

A parameterization scheme of gravity wave drag induced by cumulus convectio n (GWDC) is implemented in the Yonsei University atmospheric general circul ation model (GCM) and the effects of GWDC on the zonal-mean flow and planet ary waves are investigated through perpetual July simulations. The GWDC par ameterization scheme used in this study includes a momentum gain in the clo ud region to conserve the momentum. The gravity wave stress at the cloud top is concentrated in the intertropic al convergence zone (ITCZ) with its maximum value of 0.14 N m(-2) near the tropopause due to deep cumulus clouds. The wave breaking occurs mainly in t he upper troposphere and lower stratosphere. The maximum westerly accelerat ion in the ITCZ is 0.6 m s(-1) day(-1), which is close to that observed. It is surprising to observe that the zonal wind difference between the simula tions with and without the GWDC parameterization is largest in the Southern Hemisphere (SH) midlatitude stratosphere, where a westerly jet exists, rat her than in the major drag forcing region and that there is an associated w arming in the SH polar stratosphere. The excessive westerly jet in the SH t hat appears in the simulation without the GWDC parameterization is alleviat ed significantly (7 m s(-1)) by its inclusion. This result implies that the nonlinear process through planetary waves rather than by direct drag forci ng might play an important role in changing the zonal-mean flow. The analys is of the geopotential height perturbation reveals that the amplification o f the waves of zonal wavenumbers 1 and 2 in the SH stratosphere is responsi ble for the change in the zonal-mean flow there. In particular, the wave am plitude of zonal wavenumber 2 significantly increases (more than three time s) by the GWDC process in the SH midlatitude upper stratosphere. It is suggested that understanding interactions between the gravity wave dr ag, zonal-mean flow, and planetary waves is necessary to better parameteriz e the gravity wave drag. This study is particularly encouraging in that inc luding the GWDC parameterization can alleviate the excessive westerly bias in the SH midlatitude and its associated cold temperature bias in the SH po lar region reported for many GCMs.