wrfout_to_cfA Post-Processing Utility For Creating CF Compliant NetCDF Files From WRFOUT Files

Mark W. Seefeldt (mark.seefeldt@colorado.edu)

Department of Engineering - Physics - Systems - Providence CollegeCooperative Institute for Reserach in Environmental Sciences - University of Colorado at Boulder

The Problem: • The native WRFOUT files are not the easiest NetCDF files to use. - The variables are based on WRF development history and not necessarily community acceptance. - Several of the key variables (i.e. u and v wind) are on the staggered WRF horizontal grid. - The vertical levels are on the native eta vertical coordinate. - A large number of variables are included in the WRFOUT files related to the WRF simulation and not the applicable science. - The associated NetCDF attributes may not be helpful. • Modifying the Registry can change the variables included in the WRFOUT files. - It is a tedious and potentially messy process to make significant modifications to the Registry. - Some variables are included in WRFOUT for reasons the user may never know. Removing the variables may create unintended consequences. - The variables are still on the staggered grid.

One Solution: • There are a number of WRF post-processing utilities to create output files and/or graphics for scientific use. (e.g. NCEP UPP, RIP4, ARWpost, VAPOR) • wrfout_to_cf is an NCL based script designed to create CF compliant NetCDF files with user selectable variables, time reference, vertical levels, and spatial and temporal subsetting.

CF Compliant (http://cf-pcmdi.llnl.gov/): • Climate and Forecast (CF) is a metadata convention designed to promote the sharing and utilization of similar NetCDF data files. • CF conventions have been adopted by a number of projects and groups as a primary standard. • Some graphics programs (e.g. IDV) can readily read CF compliant NetCDF files. • The data files are described by specified: units, standard_name, long_name, _FillValue,

missing_value, valid_max, valid_min, and valid_range.

NCL Based (http://www.ncl.ucar.edu/): • NCL is a free interpreted language designed for scientific data processing and visualization. • NCL is available for a variety of operating systems including Linux, Max OS X, and Cygwin/X running on Windows. • wrfout_to_cf heavily leverages the WRF-ARW NCL scripts and functions that are included in the NCL libraries. (http://www.ncl.ucar.edu/Document/Functions/wrf/shtml) • These built-in NCL functions include diagnostics and tools for unstaggering the grids and interpolating to pressure levels.Selectable Variables: • A large collection of variables are user selectable from within the NCL script and will be included in the CF output file. • The user selects as few or as many variables as the user desires. This provides flexibility to control the file sizes of WRF data. • Variables are grouped by category. The entire category can be selected and the selected variables within that category will be included in the CF file. A category can be excluded and all variables from that category will be excluded. • Variables in the vertical can be selected with the WRF eta vertical levels or using pressure levels

Subsetting: • The user can subset the data in the horizontal, vertical, and time to further customize the CF files and the file size.

TODO: • Currently the only CF coordinate system in use is the lat/lon coordinate system. In the future CF compliant projection information will be included. • New output variables are being continually added. • Suggestions for new variables are always welcomed. Not all requested variables will be included in order to maintain a script that has wide user application. Adding variables by individual users is an easy task following the pattern of any of the currently included variables.Conclusion: • The wrfout_to_cf NCL script can be downloaded from: http://foehn.colorado.edu/wrfout_to_cf/ • This is not the most computationally efficient post-processing utility. This utility does not fit all applications. It is designed to be a simple and user flexible post-processing utility.

• Variables contain coordinate information accessed through the coordinates attribute. • Time is encoded with units "time_unit since reference_time".