GFS Operational Physics Documentation  Revision: 81451
Orographic and Convective Gravity Wave Drag

Parameterization developed specifically for orographic and convective source of gravity waves are documented separately. More...

Detailed Description

Parameterization developed specifically for orographic and convective source of gravity waves are documented separately.

At present, global models must be run with horizontal resolutions that cannot typically resolve atmospheric phenomena shorter than ~10-100 km or greater for weather prediction and ~100-1000 km or greater for climate predicition. Many atmospheric processes have shorter horizontal scales than these "subgrid-scale" processes interact with and affect the larger-scale atmosphere in important ways.

Atmospheric gravity waves are one such unresolved processes. These waves are generated by lower atmospheric sources. e.g., flow over irregularities at the Earth's surface such as mountains and valleys, uneven distribution of diabatic heat sources asscociated with convective systems, and highly dynamic atmospheric processes such as jet streams and fronts. The dissipation of these waves produces synoptic-scale body forces on the atmospheric flow, known as "gravity wave drag"(GWD), which affects both short-term evolution of weather systems and long-term climate. However, the spatial scales of these waves (in the range of ~5-500 km horizontally) are too short to be fully captured in models, and so GWD must be parameterized. In addition, the role of GWD in driving the global middle atmosphere circulation and thus global mean wind/temperature structures is well established. Thus, GWD parametrizations are now critical components of virtually all large-scale atmospheric models. GFS physics includes parameterizations of gravity waves from two important sources: mountains and convection.

Atmospheric flow is significantly influenced by orography creating lift and frictional forces. The representation of orography and its influence in numerical weather prediction models are necessarily divided into the resolvable scales of motion and treated by primitive equations, the remaining sub-grid scales to be treated by parameterization. In terms of large scale NWP models, mountain blocking of wind flow around sub-grid scale orograph is a process that retards motion at various model vertical levels near or in the boundary layer. Flow around the mountain encounters larger frictional forces by being in contact with the mountain surfaces for longer time as well as the interaction of the atmospheric environment with vortex shedding which occurs in numerous observations. Lott and Miller (1997) [39], incorporated the dividing streamline and mountain blocking in conjunction with sub-grid scale vertically propagating gravity wave parameterization in the context of NWP. The dividing streamline is seen as a source of gravity waves to the atmosphere above and nonlinear subgrid low-level mountain drag effect below.

In a review paper on gravity waves in the middle atmosphere, Fritts (1984) [15] 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 (Palmer et al. 1986 [44]). 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.

GWD parameterization in GFS

Intraphysics Communication

Collaboration diagram for Orographic and Convective Gravity Wave Drag:

Modules

 Convective Gravity Wave Drag
 This subroutine is the parameterization of convective gravity wave drag based on the theory given by Chun and Baik (1998) [8] modified for implementation into the GFS/CFS by Ake Johansson(Aug 2005).
 
 Orographic Gravity Wave Drag and Mountain Blocking
 This subroutine includes orographic gravity wave drag and mountain blocking.