Description

GFS physics includes parameterizations of gravity waves from two important sources: mountains and convection. This parameterization addresses the latter.

The importance of convectively-generated tropical waves in driving the equatorial stratospheric semi-annual oscillation (SAO) and quasi-biennial oscillation (QBO) has been appreciated for many years. In a review paper on gravity waves in the middle atmosphere, [24] showed that a large portion of observed gravity wave momentum flux has higher frequencies than those of stationary mountain waves. This phenomenon was explained by cumulus convection, which is an additional source of tropospheric gravity waves, and is particularly important in summertime. When the surface wind and stability are weak, the magnitude of the surface drag and the resultant influence of orographically-induced gravity wave drag on the large-scale flow are relatively small compared with those in wintertime ([65]). In this situation, the relative importance of cumulus convection as a source of gravity waves is larger. In addition, in the tropical regions where persistent convection exists, deep cumulus clouds impinging on the stable stratosphere can generate gravity waves that influence the large-scale flow.

Figure 1: Gravity waves generated by penetrative convection in the presence of background wind. (Based on Hooke(1986); and Kim et al. (2003))

Compared with orographic gravitity waves, it has proven more difficult to model the way in which gravity waves are generated by various convective sources; The simplest situation is depicted in Figure 1. There are several proposed generation mechanisms in the literature (see section 3b in [48]). Amongst, [15] proposed a way for parameterizing convection-induced subgrid-scale gravity wave momentum flux in large-scale models. For the momentum flux profile up to the cloud-top height, use of the linear analytical form was suggested. In GFS operaional physics suite, the effect of convection-induced gravity wave momentum flux is included only above the cloud-top height because subgrid-scale cumulus parameterization is activated in a conditionally unstable atmosphere.

Intraphysics Communication

Argument Table

General Algorithm

GFS Convective GWD Scheme General Algorithm