Metgrid interpolator

[wiersema1]

Adds a WRF-styled vertical interpolation and quality control to the init_type="metgrid" initialization. This approach closely follows that used in WRF's real.exe (see WRF/dyn_em/module_initialize_real.F). The WRF quality control algorithm prunes data if points are deemed "too close" in pressure, which becomes messy in ERF since we are on a height-based grid and we don't yet have pressure on the ERF grid since it is calculated using the variables that we are interpolating. In an attempt to be apples-to-apples with WRF, the ERF QC checks whether points are problematically close together by calculating a pressure difference assuming a baroclinic atmosphere. This approach seems to work well except for some cases near the surface where ERF QC differs from WRF QC and results in a slightly different selection of points being fed into the vertical interpolator (see attached figures).

There are several new input parameters and the docs have been updated accordingly. Default values are in parenthesis below.

• metgrid_interp_theta (false) calculate theta on origin model vertical levels (see note below).
• metgrid_basic_linear (false) use the old linear interpolation without any quality control.
• metgrid_use_below_sfc (true) QC keeps origin data values below the surface.
• metgrid_use_sfc (true) QC keeps the origin data surface value.
• metgrid_retain_sfc (false) Assign the lowest ERF level as the origin data surface value.
• metgrid_proximity (1000.) Minimum pressure difference for QC algorithm.
• metgrid_order (2) Order of the vertical interpolation scheme.
• metgrid_force_sfc_k (0) QC removes origin data (except the surface value) below this ERF vertical level, forcing use of the origin data surface value in the vertical interpolation.

metgrid_interp_theta controls how potential temperature is to be set when init_type="metgrid". Previously, potential temperature was calculated using the origin data and then vertically interpolated onto the ERF grid. This now corresponds to metgrid_interp_theta=true. When metgrid_interp_theta=false, the origin data pressure and temperature are vertically interpolated onto the ERF grid and then used to calculate the potential temperature. Note that pressure is interpolated not with respect to height but rather p_0exp(-CONST_GRAVz/(t_0*R_d)), i.e., pressure in a baroclinic atmosphere. All other variables are interpolated with respect to height.

Several debugging options are also included, which will likely be useful for future development. The input parameters are as follows.

• metgrid_debug_quiescent (false) initial & boundary conditions are quiescent.
• metgrid_debug_isothermal (false) initial & boundary conditions have potential temperature = 300.
• metgrid_debug_dry (false) initial & boundary conditions have qv = 0.
• metgrid_debug_psfc (false) P & rho are iteratively calculated using a surface pressure of 10**5.
• metgrid_debug_msf (false) map scale factors are set to 1.

These changes have been evaluated using several test cases that all use met_em files generated with GFS origin data. These include,

• "MetGrid" devtest, which is a microscale (dx = 200 m) simulation of a small region entirely over water in the Gulf of Mexico.
• A mesoscale configuration (dx = 2520 m, nx = ny = 400) centered over Nevada. Results from this case are included below.