Variational polarimetric radar algorithm for retrieving the properties of rain and hail

From this page you can obtain the code and some test data for a variational algorithm for estimating rain rate and hail intensity from polarimetric radar measurements. Variational retrieval methods involve finding the optimal distribution of variables (e.g. rain rate and mean drop size) that best "forward-model" the observations in a least squares sense. This is particularly useful for the problem of retrieving rain and hail properties from polarimetric measurements of Z, Z_dr and Phi_dp, which provide complementary information yet may be affected by noise. The details of the algorithm are provided in the paper below.

The algorithm has so far been tested on data from the S-band radar (3 GHz) at Chilbolton. This frequency is only weakly affected by attenuation, which makes the retrieval problem easier. An obvious question is whether the algorithm can be applied to higher frequencies, such as C-band (5.6 GHz) or X-band (10 GHz). The answer is "yes" in principle, but probably not without some clever use of constraints and possibly modification to the state variables that are used.

To illustrate the input and output of the algorithm, the top three panels of the figure below show the input fields (radar reflectivity as with a conventional radar, plus two fields from the polarization capability), while the bottom two show the retrieved rain rate and the location of the hail shafts.

Paper

• Hogan, R. J., 2007: A variational scheme for retrieving rainfall rate and hail reflectivity fraction from polarization radar. J. Appl. Meteorol. Climatology, 46, 1544-1564: PDF file

Presentation

• Talk at Department of Meteorology, Reading, January 2008: PPT file

Source code

• The code is in C and is tested on Linux: var_rain_hail-0.7.2.tar.gz. Please read the README file carefully.
• Older versions: var_rain_hail-0.7.tar.gz, var_rain_hail-0.7.1.tar.gz.