WRF ARW User's Guide - MMM - UCAR

WPS
/
15000 , 10000 ,
5000 , 1000
Within the &mod_levs namelist record, the variable press_pa is used to specify a list of
levels to keep; the specified levels should match values of xlvl in the intermediate
format files (see the discussion of the WPS intermediate format for more information on
the fields of the intermediate files). The mod_levs program takes two command-line
arguments as its input. The first argument is the name of the intermediate file to operate
on, and the second argument is the name of the output file to be written.
Removing all but a specified subset of levels from meteorological data sets is particularly
useful, for example, when one data set is to be used for the model initial conditions and a
second data set is to be used for the lateral boundary conditions. This can be done by
providing the initial conditions data set at the first time period to be interpolated by
metgrid, and the boundary conditions data set for all other times. If the both data sets
have the same number of vertical levels, then no work needs to be done; however, when
these two data sets have a different number of levels, it will be necessary, at a minimum,
to remove (m – n) levels, where m > n and m and n are the number of levels in each of the
two data sets, from the data set with m levels. The necessity of having the same number
of vertical levels in all files is due to a limitation in real.exe, which requires a constant
number of vertical levels to interpolate from.
The mod_levs utility is something of a temporary solution to the problem of
accommodating two or more data sets with differing numbers of vertical levels. Should a
user choose to use mod_levs, it should be noted that, although the vertical locations of the
levels need not match between data sets, all data sets should have a surface level of data,
and, when running real.exe and wrf.exe, the value of p_top must be chosen to be below
the lowest top among the data sets.
C. calc_ecmwf_p.exe
In the course of vertically interpolating meteorological fields, the real program requires
3-d pressure and geopotential height fields on the same levels as the other atmospheric
fields. The calc_ecmwf_p.exe utility may be used to create such these fields for use with
ECMWF sigma-level data sets. Given a surface pressure field (or log of surface pressure
field) and a list of coefficients A and B, calc_ecmwf_p.exe computes the pressure at an
ECMWF sigma level k at grid point (i,j) as Pijk = Ak + Bk*Psfcij. The list of coefficients
used in the pressure computation can be copied from a table appropriate to the number of
sigma levels in the data set from
http://www.ecmwf.int/products/data/technical/model_levels/index.html. This table should
be written in plain text to a file, ecmwf_coeffs, in the current working directory; for
example, with 16 sigma levels, the file emcwf_coeffs would contain something like:
0 0.000000 0.000000000
1 5000.000000 0.000000000
2 9890.519531 0.001720764
3 14166.304688 0.013197623
4 17346.066406 0.042217135
WRF-ARW V3: User’s Guide 3-28