This program blends fields from two MPAS files together

Compiling instructions: 

1) Edit build.sh script.  Needed libraries: NETCDF, ESMF, MPI. Might have to edit some other files...

2) run build.sh

Namelist:

&share
   grid_info_file = './grid_info.txt'
   large_scale_file = '/glade/derecho/scratch/harrold/hwt/for_craig/init_2022050100_ens1_hrrrICgfsLBC.nc'
   small_scale_file = '/glade/derecho/scratch/harrold/hwt/for_craig/init_2022050100_ens0_enkfICgefsLBC.nc'
   output_blended_filename = 'blended.nc'
   variables_to_blend = 't2m','theta','qv','uReconstructZonal','uReconstructMeridional'
   average_upscale_before_interp = .true.
   interp_method = 'conserve1'
   extrap_method = 'mpas_nearest'
   extrap_num_levels_creep = 64
   output_intermediate_files_up   = .true.
   output_intermediate_files_down = .true.
   output_blended_edge_normal_wind = .true.
   smooth_going_downscale = .false.
   smoother_dimensionless_coefficient = 0.1
   esmf_log = .true.
/

&latlon_output
   output_latlon_grid = .true.
   is_regional = .false.
   nlat = 721
   nlon = 1440
   lat_ll = -90.
   lon_ll = 0.
   dx_in_degrees = 0.25
   extrap_method_latlon = 'none'
/

Meaning of namelist variables

&share
grid_info_file: Path to text file containing information about the grid hierarchy.  See example/grid_info.txt for more details.

large_scale_file : Path to MPAS file that will be used for the large scales, typically something like a GFS analysis interpolated onto the MPAS mesh.

small_scale_file : Path to MPAS file that will be used for the small scales, typically something like an analysis produced by a data assimilation system on a high-resolution MPAS mesh.

output_blended_filename : Name of the output file with blended fields that will be created by the program.  The file will be created, but the directory where the file will be placed must exist before running the program.

variables_to_blend : List of the MPAS variables (as in the netCDF files) to blend.

average_upscale_before_interp : Logical variable. If true, before interpolating to the next coarsest mesh in the hierarchy, spatially average the field to the scale of the next mesh in the hierarchy.  If false, don't spatially average before interpolating.  Typically set to true.

interp_method : Method for interpolating up and down through the hierarchy. Options: 'bilinear' (bilinear interpolation), 'conserve1' (first-order conservative interpolation), 'conserve2' (second-order conservative interpolation), 'nearest' (nearest-neighbor interpolation), 'patch' (ESMF's homegrown method that is kind of like bilinear interpolation; see the ESMF guide).  Recommended to use either 'bilinear' or 'conserve1'.

extrap_method : If the input mesh is global, this variable is effectively ignored.  If it's a regional mesh (as is typically the case with blending), then an extrapolation option needs to be selected to determine what happens to points that can't be mapped to the next coarsest grid in the heirarchy when interpolating; extrapolation is needed to get reasonable blended fields along the mesh boundary. There may not be too much sensitivity to the option used. Options are "esmf_creep", "esmf_creep_nrst_d", "esmf_nearest_d", "esmf_nearest_stod", and "mpas_nearest".  Those that are prefixed with "esmf" are described here: https://earthsystemmodeling.org/docs/release/latest/ESMF_refdoc/node9.html#opt:extrapmethod (entry 54.17 in the ESMF fortran user's guide).  The "mpas_nearest" option is just a nearest neighbor extrapolation.  All except "esmf_creep" will fill the entire meshes.  The "esmf_creep", and "mpas_nearest" options are the quickest, followed by "esmf_nearest_d". Recommendation is to use "esmf_creep" or "mpas_nearest".

extrap_num_levels_creep : Only used if extrap_method is "esmf_creep" or "esmf_creep_nrst_d"; ignored otherwise.  This sets the number of "levels" for which to do extrapolation at points on the target mesh that can't be mapped to the source mesh.  Think of a "level" as a number of circular rings moving outward from the boundary of a circular MPAS mesh.  The default value is 64. Depending on what meshes are used in the hierarchy, this value may need to be adjusted.  If values along the boundary after blending look questionable, try increasing this value. 

output_intermediate_files_up : Logical variable. If true, output a bunch of files to visualize how the fields are upscaled through the hierarchy.  Doesn't impact results. Useful for diagnostics but can be time consuming, so if not needed, set to false.

output_intermediate_files_down : Logical variable. If true, output a bunch of files to visualize how the fields are downscaled through the hierarchy.  Doesn't impact results. Useful for diagnostics but can be time consuming, so if not needed, set to false.

output_blended_edge_normal_wind : The U and V wind components ('uReconstructZonal','uReconstructMeridional') are the ones that are blended.  If this variable is true, then after the blended U and V fields are determined, use those blended fields to calculated the edge normal wind, which is MPAS's prognostic variable. Typically should be set to true.

smooth_going_downscale = Logical variable.  If true, apply a 2-delta-x smoother when downscaling through the hierarchy.  Not well tested, and a bit slow.

smoother_dimensionless_coefficient : Ignored if smooth_going_downscale = false.  If smooth_going_downscale = true, this is a coefficent between 0 and 1 that can be used for the smoothing.  Not well tested.

esmf_log : Whether to turn on ESMF logging. Typically set to true to output information in the PET files.


&latlon_output

output_latlon_grid : Logical variable.  If true, output data on a lat-lon grid for easy visualization.  Outputting lat-lon files is purely for diagnostic purposes and doesn't impact the blending itself.

is_regional : Logical variable.  If true, the parameters below define a regional lat-lon grid.  If false, parameters below define a global lat-lon grid.

nlat : Number of latitudes in the lat-lon grid.

nlon : Number of longitudes in the lat-lon grid.

lat_ll : Latitude of the lower-left corner of the lat-lon grid.

lon_ll : Longitude of the lower-left corner of the lat-lon grid.

dx_in_degrees : Grid spacing (degrees) of the lat-lon grid.  Applies to both latitude and longitude.

extrap_method_latlon : Extrapolation method for outputting fields on the lat-lon grid that will be used to fill points on the lat-lon grid that don't map to an MPAS mesh.  Options are the same as above for variable 'extrap_method', but can also be set to 'none' for no extrapolation.  If using "esmf_creep" or "esmf_creep_nrst_d", then extrap_num_levels_creep (from &share namelist) will also apply.  Setting to 'none' is probably fine.