/**
\page GFS_H2OPHYS GFS Stratospheric H2O Scheme
\section des_h2o Description
To improve the treatment of stratospheric water vapor in the global model, NCEP implemented a parameterization of photochemical production and loss (P-L) of water vapor  through methane oxidation and photolysis of H2O in the upper mesosphere due to solar Lyman alpha absorption is implemented in GFS. 

The Navy Research Laboratory (NRL) linearized parameterization of stratospheric and mesospheric water vapor photochemistry (McCormack at al. (2008)
\cite mccormack_et_al_2008) applies a linearized photochemical tendency to specific humidity q in the form
\f[
\frac{dq}{dt}=(P-L)_{0}+\frac{\partial (P-L)}{\partial q}\mid_{0}(q-q_{0})
\f]
The second term on the right-hand side quantifies the linearized sensitivity to local changes in \f$q\f$, and yields photochemical relaxation to an equilibrium specific humidity \f$q_{0}\f$
\f[
-[\frac{\partial (P-L)}{\partial q}]_{0}=\tau_*^{-1}
\f]
The \f$(P-L)_{0}\f$ and \f$\tau_*\f$ values were computed from perturbation experiments with the NRL two-dimensional photochemical model (CHEM2D). 
The equilibrium profile \f$q_{0}\f$ is specified using a climatology based on a combination of reanalysis and satellite observations.

\section intra_h2o  Intraphysics Communication
- \ref arg_table_h2ophys_run

\section gen_h2o  General Algorithm
- \ref genal_h2ophys


*/