radar data assimilation

[timhoar]

Ed note: This issue was originally an email and is being manually ported over to GitHub.

Dear DART Team;

I have a query related to radar data assimilation. It is found from the DART documentation that only NEXRAD data can be assimilated using create_obs_radar_sequence. But I want to use our own Doppler Weather Radar data. I have converted the binary data to "lat, long, Reflectivity, Radial Velocity, Elevation" format. Now, what do I need to do so that the data can be read by the DART module?

Your suggestions will be highly appreciated.

[timhoar]

Thank you for your interest in DART. We offer limited support for the conversion of radar observations to the DART observation format. Still, there are avenues that can allow you to get there. As an end goal, you'll want to have radar observations that have been quality controlled to remove non-meteorological scatterers and other artifacts, and reduce the native data resolution to something closer to twice the expected horizontal grid spacing in your model (e.g., with 3 km grid spacing, radar observations every 6 km) interpolated along the sweep plane. Reflectivity observations are often partitioned into two types - regular reflectivity observations, and clear air reflectivity observations (where no radar echoes are observed). Quality control is best done with the raw data, so I trust you have an ability to do so on your end. Once quality control is applied, a tool for the interpolation and conversion to DART observation format is provided by the OPAWS utility: https://code.google.com/archive/p/opaws/

Before you can use the OPAWS tool, you'll need to convert your native data format into something that can be read by OPAWS (e.g., DORADE sweep files). You can see if the RADX utility is helpful for this: https://ral.ucar.edu/projects/titan/docs/radial_formats/radx.html

Assuming you are using 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'll need to also 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 (though you would 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 and should be ready to assimilate radar observations. Before assimilating radar observations, you'll also want to use special localization for radar observations, typically 12-24 km. If you leave range-folding in your radar observations, you'll need to build the special version of DART that unfolds the velocity observations on-the-fly.