Managing the fields in your WRF EMS GRIB files

Users of the WRF EMS have the ability to modify the fields and levels contained in the GRIB 1 and 2 files generated when running ems_post. This data management is accomplished through the use of a control file that is read by the wrfpost routine during execution.  It is possible to specify a different control file for each WRF domain, thus allowing users to have different fields and levels in the GRIB files of the parent than those of child domains files.

Users specify which control file to use for a given domain with the GRBCNTRL parameter in the conf/ems_post/post_grib.conf configuration file. This parameter defines the list of control files, separated by a comma (,) to be used by wrfpost. Each entry in the list corresponds to the control file that is used when processing each domain. For example:

GRBCNTRL= wrfpost_cntrl_d01.parm, wrfpost_cntrl_d02.parm, ..., wrfpost_cntrl_d0N.parm

Wherein “wrfpost_cntrl_d01.parm” will be used for domain 1 (primary domain), “wrfpost_cntrl_d02.parm” will be used for domain 2 (1st nested domain), etc. The actual name you give the control file is not important, as long as that file can be found under the “static” directory. If ems_post fails to locate a control file it will automatically use the default wrfpost_cntrl.parm file located in wrfems/data/tables/post.

You do not have to specify a unique GRIB control file for each domain. In the absence of multiple files, the last file listed in GRBCNTRL will be used.

GRBCNTRL = wrfpost_cntrl.parm

In the above example if you were running 3 total domains, one primary and two nests, all three domains would use the same wrfpost_cntrl.parm file to define the contents of the GRIB files.

Deciphering the wrfpost_cntrl.parm file

The initial look at a wrfpost control file can be very confusing.  You will find many semi-cryptic field names followed by incomprehensible string of 1s and 0s.  Each 1/0 represents a level that is turned on/off for output to the GRIB file. The wrfpost routine is expecting to find a string of 80 1s and 0s organized into 16 groups of 5. Having fewer than the 80 levels represented, e.g., if you copied this file from the WRF EMS V2 and didn't add additional values, will cause the post processor to crash.

Here is an example of a line showing the distribution of levels to be output for a particular field to the GRIB file:

L=(11111 11111 11111 11100 10010 01001 00100 10010 01001 00100 10010 01001 00100 10010 01001 00101)

For the sake of this discussion we will assume these represent pressure levels but they could also be model or isentropic levels.

The DEFAULT pattern of 1s and 0s, as depicted above, outputs 3D fields every 25mb between the surface and 25mb, similar to EMS V2. Additional pressure levels have been added in V3 that includes every 10mb from the surface to 500mb for those users needing higher resolution within the lower troposphere. Remember that the vertical resolution of the 3D fields in the GRIB files is no better than the vertical resolution of the model!

The complete list of available pressure surfaces in groups of 5, as represented by the 1s and 0s from left to right is:

10,  25, 50, 100, 150 mb
200, 225, 250, 275, 300 mb
325, 350, 375, 400, 425 mb
450, 475, 500, 510, 520 mb
525, 530, 540, 550, 560 mb
570, 575, 580, 590, 600 mb
610, 620, 625, 630, 640 mb
650, 660, 670, 675, 680 mb
690, 700, 710, 720, 725 mb
730, 740, 750, 760, 770 mb
775, 780, 790, 800, 810 mb
820, 825, 830, 840, 850 mb
860, 870, 875, 880, 890 mb
900, 910, 920, 925, 930 mb
940, 950, 960, 970, 975 mb
980, 990, 1000, 1010, 1013.20 mb

Each 1 and 0 in the line above corresponds to a pressure level in the list. For example, the first line in the list, 10,  25, 50,100,150 mb, corresponds to the first block of 5 1s and 0s on the left (11111 ).

Again, a value of "1" turns ON that pressure level while a value of "0" turns it OFF.

The last line of pressure levels, 980, 990, 1000, 1010, 1013.20 mb, corresponds to the last block of 1/0 values (00101) and indicates that the 1000 and 1013.20 pressure levels will be turned ON in the GRIB output while the 980, 990, and 1010 mb levels are OFF.

 

Some fields are only valid on a single level, such as the surface. In this case the output is controlled by the first 1/0 listed in the string. So, if your want total precipitation turned ON, the entry in the wrf_postcntrl.parm would look like:

(ACM TOTAL PRECIP ) SCAL=( 4.0)
L=(10000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000)

 

Finally, fields are available over a variety of depths, such as sub-surface and boundary layer based fields. An example of this would be storm relative helicity and convective available potential energy. The storm relative helicity field is available from the surface to 1000 and 3000 meters above the ground. For these fields the output layers are controlled by the string of 1s and 0s beginning from the left:

(STORM REL HELICITY ) SCAL=( 3.0)
L=(11000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000)