2B31 provides the vertical structure of rainfall (rates and drop-size-distribution parameters) based upon the TRMM microwave imager (TMI) and the TRMM radar (PR), within the PR swath. The day-1 version uses only the 10.7 GHz TMI channel, while version 2X31 uses all five vertically polarized passive channels.

    The algorithm is based upon a Bayesian approach that begins by inverting the radar measurements for every likely value of the drop-size-distribution (DSD) shape parameters (see Haddad et al, 1997b, for a description of the DSD parametrization). The resulting rainfall estimates are used to produce the corresponding expected brightness temperatures, which are then compared to the actual passive measurements to decide which DSD shape parameter value was most likely. A more complete description of this portion of the algorithm can be found in Haddad et al., 1997a.

File Format

    The file content description for 2B31 can be obtained from the Volume 4 - Level 2 File Specifications provided by the TRMM Data and Information System (TSDIS). It
is available at: http://tsdis02.nascom.nasa.gov/tsdis/Documents/ICSVol4.pdf.

Known Deficiencies

    The algorithm is quite complicated and therefore has a number of known deficiencies, some of which are still being addressed:

1.    In the first "radar-only" inversion step, the algorithm does not take into account the possible presence of ice particles and their effect on the measured radar reflectivity.

2.    Because the algorithm tries to select between various DSD's, it uses many different radar-rain relations (one for each assumption about the shape of the DSD). There is therefore an inherent mismatch with the radar-rain relations used by the radar-only retrieval algorithm 2A25.

3.    The algorithm uses average relations to infer the expected 10.7, 19, 21, and 37 GHz brightness temperatures, based on the estimated rain rate (rarther than the estimated radar attenuation). It uses the 85GHz passive channel measurement and the radar-qualitative algorithm 2A23 results to select the appropriate rain-radiance relation. There is therefore an inherent mismatch with the radiometer-only algorithm 2A12.

References

Haddad, Z. S., E. A. Smith, C. D. Kummerow, T. Iguchi, M. R. Farrar, S. L. Durden, M. Alves, and W. S. Olson, 1997a: "The TRMM `Day-1' Radar/Radiometer Combined Rain-Profiling Algorithm", J. Meteor. Soc. Japan, 75, 799-809.

Haddad, Z. S., D. A. Short, S. L. Durden, E. Im, S. Hensley, M. B. Grable and R.A. Black, 1997b: "A New Parametrization of the Rain Drop Size Distribution", IEEE Trans. Geosci. Rem. Sens., 35, 532-539.