/**
\page GFS_GWDC GFS Convective Gravity Wave Drag Scheme
\section des_gwdc  Description
 GFS physics includes parameterizations of gravity waves from two
 important sources: mountains and convection. This parameterization
 addresses the latter. Non-orographic gravity waves are forced by dynamical motions such as convection, frontogenesis,
 and jet stream activity (Fritts and Nastrom (1980) \cite fritts_and_nastrom_1980 ). They have 
 vertical wavelengths which vary from less than one to many tens of kilometers and horizontal wavelengths 
 which vary from tens to thousands of kilometers (Ern et al. 2004 \cite ern_et_al_2004). Thus these
 waves are generally unresolved or under-resolved by the model as the generating process is often poorly 
 represented, and therefore have to be parametrized.

 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, Fritts (1984) \cite fritts_1984 
 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) \cite palmer_et_al_1986). 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.

 \image html  GFS_gwdc.png "Figure 1: Gravity waves generated by penetrative convection in the presence of background wind. (Based on Hooke(1986); and Kim et al. (2003))" width=10cm

 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 
 Kim et al. (2003) \cite kim_et_al_2003). Amongst, Chun and Baik (1998) \cite chun_and_baik_1998 
  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. 

\section intra_gwdc Intraphysics Communication
\ref arg_table_gwdc_run

\section gena_gwdc General Algorithm
\ref gen_gwdc



*/