WRF ARW User's Guide - MMM - UCAR

WRF-VAR
Namelist parameters are used to control the reading of corresponding BUFR files into
WRF-Var. For instance, USE_AMSUAOBS, USE_AMSUBOBS, USE_HIRS3OBS,
USE_HIRS4OBS, USE_MHSOBS, USE_AIRSOBS, USE_EOS_AMSUAOBS and
USE_SSMISOBS control whether or not the respective file is read. These are logical
parameters that are assigned to FALSE by default; therefore they must be set to true to
read the respective observation file. Also note that these parameters only control whether
the data is read, not whether the data included in the files is to be assimilated. This is controlled
by other namelist parameters explained in the next section.
NCEP BUFR files downloaded from NCEP’s public ftp server
ftp://ftp.ncep.noaa.gov/pub/data/nccf/com/gfs/prod/gdas.${yyyymmddhh} are Fortranblocked
on big-endian machine and can be directly used on big-endian machines (for example,
IBM). For most Linux clusters with Intel platforms, users need to first unblock the
BUFR files, and then reblock them. The utility for blocking/unblocking is available from
http://www.nco.ncep.noaa.gov/sib/decoders/BUFRLIB/toc/cwordsh
c. Radiative Transfer Model
The core component for direct radiance assimilation is to incorporate a radiative transfer
model (RTM, should be accurate enough yet fast) into the WRF-Var system as one part
of observation operators. Two widely used RTMs in NWP community, RTTOV8 * (developed
by EUMETSAT in Europe), and CRTM (developed by the Joint Center for Satellite
Data Assimilation (JCSDA) in US), are already implemented in WRF-Var system
with a flexible and consistent user interface. Selecting which RTM to be used is controlled
by a simple namelist parameter RTM_OPTION (1 for RTTOV, the default, and 2
for CRTM). WRF-Var is designed to be able to compile with only one of two RTM libraries
or without RTM libraries (for those not interested in radiance assimilation) by the
definition of environment variables “CRTM” and “RTTOV” (see Installing WRF-Var
section).
Both RTMs can calculate radiances for almost all available instruments aboard various
satellite platforms in orbit. An important feature of WRF-Var design is that all data structures
related to radiance assimilation are dynamically allocated during running time according
to simple namelist setup. The instruments to be assimilated are controlled at run
time by four integer namelist parameters: RTMINIT_NSENSOR (the total number of sensors
to be assimilated), RTMINIT_PLATFORM (the platforms IDs array to be assimilated
with dimension RTMINIT_NSENSOR, e.g., 1 for NOAA, 9 for EOS, 10 for METOP and
2 for DMSP), RTMINIT_SATID (satellite IDs array) and RTMINIT_SENSOR (sensor
IDs array, e.g., 0 for HIRS, 3 for AMSU-A, 4 for AMSU-B, 15 for MHS, 10 for SSMIS,
11 for AIRS). For instance, the configuration for assimilating 12 sensors from 7 satellites
(what WRF-Var can assimilated currently) will be
*
Current release is RTTOV9, while there is no plan to incorporate RTTOV9 into WRF-
Var.
WRF-ARW V3: User’s Guide 6-22