Atmospheric forcing

Theory

Here is some info about the heat exchange module inside SELFE, based on Dr. Mike Zuluaf's slides.

File description

In SELFE, atmoserpic forcings include wind, air pressure and temperature, precipitation, humidity and longwave and shortwave fluxes. These are specified in the netcdf files inside sflux/ dir, and conform to the NetCDF Climate and Forecast (CF) Metadata Convention 1.0.

There are 4 types of files in sflux/dir; see this site for sample files.

1. sflux_inputs.txt:
   $cat sflux_inputs.txt
   &sflux_inputs  ! file name
   start_year = 1998, ! start year
   start_month = 09, ! start month
   start_day = 01, ! start day
   start_hour = 0.0, ! start hour
   utc_start = 0.0, ! offset from UTC in hours, with positive numbers for western hemishpere
   /
   
2. sflux_air_1.0*.nc: netcdf files that have time, wind speed, air temperature and pressure and specific humidity;
3. sflux_prc_1.0*.nc: netcdf files that have precipitation data;
4. sflux_rad_1.0*.nc: netcdf files that have downward long and short (solar) wave radiation fluxes.

Using NARR files for your simulation (North America only)

First, make sure the  NARR grid covers your hgrid.ll. 

In your run directory, mkdir sflux and inside it, create symbolic links to the NARR files. e.g., if you run starts from June 10, 2004 and ends June 20, 2004, then

sflux_air_1.001.nc --> narr_air.2004_06_10.nc
sflux_air_1.002.nc --> narr_air.2004_06_11.nc
...
sflux_air_1.011.nc --> narr_air.2004_06_20.nc
sflux_air_1.012.nc --> narr_air.2004_06_21.nc (extra day to account for time zone difference)

Similarly for sflux_rad_*.nc and sflux_prc_*.nc. The number "1" after "air_" denotes first data set used; you can use up to 2 sets in SELFE (which combines them with some given weights set in sflux_subs.F90); here you only use 1 set.

Don't forget to update sflux_inputs.txt also.

Preparing your own sflux inputs

After familiarize yourself with the NARR files and their format, you may embark on creating your own nc files. The best way is to modify existing matlab scripts (src/Utility/Sflux_nc/readnc*.m) included in the source code bundle, which have extensively in-line comments to guide you along the way.

Conventions for .nc-files in sflux directory and additional files needed for sflux run:

wind: u-component is eastward, v-comp. is northward (normal math convention, not compass convention)

windrot_geo2proj.gr3: rotates winds in case they do not align with coordinate axes, i.e. lat/lon

watertype.gr3: 6 is clear water, 7 is muddiest. Search in selfe_init.F90 for watertype.

A few details

Below is from sflux_9c.F90; search for "Joseph Z" near the beginning.

• Of all attributes in nc file, only 'base_date' is required;
• air, rad and prc each can have up to 2 sources;
• grids for air, rad and prc can be different (but must be the same within

 each type and each source). Additional requirements for the structured grid in .nc:

[lon,lat](nx,ny) give x,y coord., nx is # of pts in x. Suppose a node in the grid is given by (i,j) (1<=i<=nx), then the quad (i,j), (i+1,j), (i+1,j+1,i,j+1) must be along ! counter-clockwise direction; ! (3) search for "relative_weight" (inside netcdf_io) to ! change relative weights of the 2 sources for air, rad and prc if needed. All weights must > 0! ! (4) in case of 2 sources/grids for a variable, use "1" as larger grid (i.e. encompassing hgrid.ll) ! and "2" as smaller grid. The code will calculate weights associated with ! the 2 grids, and if some nodes in hgrid.ll fall outside grid "2" the interpolation ! will be done on grid "1" only (see combine_sflux_data, in particular, bad_node_2 ! based on area coordinates outside [0,1]). Both grids must start from stack ! 1 and have same # of stacks for each variable. However, within each nc file # ! of time steps can vary; ! (5) air_1_max_window_hours (etc) are set in netcdf_io to define the max. time stamp ! (offset from start time in each) within each nc file. Besides those in netcdf_io, ! max_file_times (max. # of time records in each nc file) in routine get_times_etc () ! may need to be adjusted as well.

air-files (header displayed using the ncdump utility)

netcdf sflux_air_1.001 {
dimensions:
        nx_grid = 349 ;
        ny_grid = 277 ;
        time = UNLIMITED ; // (8 currently)
variables:
        float time(time) ;
                time:long_name = "Time" ;
                time:standard_name = "time" ;
                time:units = "days since 2001-01-01" ;
                time:base_date = 2001, 1, 1, 0 ;
        float lon(ny_grid, nx_grid) ;
                lon:long_name = "Longitude" ;
                lon:standard_name = "longitude" ;
                lon:units = "degrees_east" ;
        float lat(ny_grid, nx_grid) ;
                lat:long_name = "Latitude" ;
                lat:standard_name = "latitude" ;
                lat:units = "degrees_north" ;
        float uwind(time, ny_grid, nx_grid) ;
                uwind:long_name = "Surface Eastward Air Velocity (10m AGL)" ;
                uwind:standard_name = "eastward_wind" ;
                uwind:units = "m/s" ;
        float vwind(time, ny_grid, nx_grid) ;
                vwind:long_name = "Surface Northward Air Velocity (10m AGL)" ;
                vwind:standard_name = "northward_wind" ;
                vwind:units = "m/s" ;
        float prmsl(time, ny_grid, nx_grid) ;
                prmsl:long_name = "Pressure reduced to MSL" ;
                prmsl:standard_name = "air_pressure_at_sea_level" ;
                prmsl:units = "Pa" ;
        float stmp(time, ny_grid, nx_grid) ;
                stmp:long_name = "Surface Air Temperature (2m AGL)" ;
                stmp:standard_name = "air_temperature" ;
                stmp:units = "K" ;
        float spfh(time, ny_grid, nx_grid) ;
                spfh:long_name = "Surface Specific Humidity (2m AGL)" ;
                spfh:standard_name = "specific_humidity" ;
                spfh:units = "1" ;

// global attributes:
                :Conventions = "CF-1.0" ;
}

rad-files (header displayed using the ncdump utility)

netcdf sflux_rad_1.001 {
dimensions:
        nx_grid = 349 ;
        ny_grid = 277 ;
        time = UNLIMITED ; // (8 currently)
variables:
        float time(time) ;
                time:long_name = "Time" ;
                time:standard_name = "time" ;
                time:units = "days since 2001-01-01" ;
                time:base_date = 2001, 1, 1, 0 ;
        float lon(ny_grid, nx_grid) ;
                lon:long_name = "Longitude" ;
                lon:standard_name = "longitude" ;
                lon:units = "degrees_east" ;
        float lat(ny_grid, nx_grid) ;
                lat:long_name = "Latitude" ;
                lat:standard_name = "latitude" ;
                lat:units = "degrees_north" ;
        float dlwrf(time, ny_grid, nx_grid) ;
                dlwrf:long_name = "Downward Long Wave Radiation Flux" ;
                dlwrf:standard_name = "surface_downwelling_longwave_flux_in_air" ;
                dlwrf:units = "W/m^2" ;
        float dswrf(time, ny_grid, nx_grid) ;
                dswrf:long_name = "Downward Short Wave Radiation Flux" ;
                dswrf:standard_name = "surface_downwelling_shortwave_flux_in_air" ;
                dswrf:units = "W/m^2" ;

// global attributes:
                :Conventions = "CF-1.0" ;
}

prc-files (header displayed using the ncdump utility)

netcdf sflux_prc_1.001 {
dimensions:
        nx_grid = 349 ;
        ny_grid = 277 ;
        time = UNLIMITED ; // (8 currently)
variables:
        float time(time) ;
                time:long_name = "Time" ;
                time:standard_name = "time" ;
                time:units = "days since 2001-01-01" ;
                time:base_date = 2001, 1, 1, 0 ;
        float lon(ny_grid, nx_grid) ;
                lon:long_name = "Longitude" ;
                lon:standard_name = "longitude" ;
                lon:units = "degrees_east" ;
        float lat(ny_grid, nx_grid) ;
                lat:long_name = "Latitude" ;
                lat:standard_name = "latitude" ;
                lat:units = "degrees_north" ;
        float prate(time, ny_grid, nx_grid) ;
                prate:long_name = "Surface Precipitation Rate" ;
                prate:standard_name = "precipitation_flux" ;
                prate:units = "kg/m^2/s" ;

// global attributes:
                :Conventions = "CF-1.0" ;
}