Add namelist variable to control strength of turbulent orographic form drag in UGWPv1 suite

[mdtoyNOAA]

Description

The calculation of the turbulent orographic form drag (TOFD) in the UGWPv1 drag suite 'GWD/drag_suite.F90' involves a coefficient 'alpha' as formulated in Beljaars et al. (QJRMS, 2004). Until now, this coefficient has been 'hard wired' to the value 12.0 per the original publication. Through personal communication with Anton Beljaars, the ECMWF has found that varying the alpha coefficient can result in improved forecast skill in the IFS. Experiments with the FV3GFS has confirmed that this is the case for our model. Instead of having this coefficient hardwired in the code, requiring recompilation when changing its value, we would like to convert it to a runtime namelist-controlled variable.

Solution

Add the coefficient 'alpha' to the namelist variable list. (Call it 'alpha_fd' for 'alpha form drag'.)