DART provides limited support for the conversion of radar observations to
obs_seq format. As an end goal, you want to assimilate radar observations
Have been quality controlled to remove non-meteorological scatterers and other artifacts
Have horizontal resolution that has been reduced to approximately twice the expected horizontal grid spacing of your model. For example, if your model has 3 km grid spacing, you should reduce your radar observations to every 6 km, interpolated along the sweep plane.
Reflectivity observations are often partitioned into two types:
Regular reflectivity observations
Clear-air reflectivity observations where no radar echoes are observed.
Quality control is best done with raw data. You should have an ability to perform quality control before converting your observations to obs_seq format.
Synthetic radar observations
create_obs_radar_sequence program generates one or more sets of
synthetic WSR-88D (NEXRAD) radar
observations. It can generate reflectivity and/or doppler radial velocity
observations with clear-air or storm sweep patterns. These synthetic
observations can be used for testing your assimilation setup or for conducting
Observing System Simulation Experiments (OSSEs).
create_obs_radar_sequence, change directory into the
input.nml is configured properly and run the build
cd work ./quickbuild.csh
Real radar observations
Once you have ensured that your data are quality controlled, use the Observation Processing And Wind Synthesis (OPAWS) utility convert your data to obs_seq format. The OPAWS utility reads specific types of files as input, such as DORADE sweep files and NCAR EOL Foray data.
OPAWS analyzes and grids data in either:
two-dimensions (on the conical surface of each sweep), or
If your raw data are not in such a format, additional utilities are available for conversion such as the RADX library which is part of the LIDAR/RADAR Open Software Environment.
Guidance for Weather Research and Forecasting (WRF) users
If you intend to assimilate radar observations into WRF, you’ll need to make some code modifications to allow for forward operator calculations. For reflectivity, most of the available microphysics schemes have built-in capability to output reflectivity, assuming a 10 cm wavelength. If you are not using an S-band radar, be aware that attenuation is not accounted for in the built-in reflectivity operator.
For radial velocity, you will also need to generate a new diagnostic field: terminal fall velocity. There is very limited support for fall velocity in WRF, although it is partially supported in the Thompson microphysics scheme.
You will still need to modify WRF code to get this diagnostic output to history files.
With these two fields available in your WRF history files, you can add them to your DART wrf_state_variables list.
You should also use a special localization radius for radar observations, typically 12-24 km. If you leave range-folding in your radar observations, you will need to build the special version of DART that unfolds the velocity observations on-the-fly.
With all of those configurations in place, you will be ready to assimilate radar observations using WRF and DART.
For more information, see the WRF tests directory in
DART/models/wrf/regression/Radar/ for pointers to data to run a radar test