2A12 provides rainfall rates and the vertical structure of hydrometeors and latent heating based upon the nine channels of the TRMM microwave imager (TMI).  For a complete description of the TMI, the user should refer to Kummerow et al., (1998).

    The algorithm is based upon a Bayesian approach that begins by establishing a large database of potential hydrometeor profiles and their computed brightness temperatures (Tb).  This database is computed from cloud resolving models such as the Goddard Cumulus Ensemble model.  Once the database is established, the retrieval searches the database and in Bayes's formulation, the probability of a particular profile R, given Tb can be written as:
                                           Pr( R | Tb )  =  Pr(R) x Pr(Tb | R)
where Pr(R) is the probability with which a certain profile R will be observed and Pr(Tb | R) is the probability of observing the brightness temperature vector, Tb, given a particular rain profile R.  The probability that a profile R will be observed is taken from the cloud profile database.  The second term is specified in the Bayesian formulation to be the gaussian weight which depends upon the RMS difference between observed and computed Tb.  A more complete description of this portion of the algorithm can be found in Kummerow et al., (1996).  Results on the latent heating retrievals from this algorithm can be found in Olson et al., (1998).

    The algorithm implemented at TSDIS has the further requirement that the Convective/ stratiform fraction of precipitation in the satellite field of view also match that given by the cloud model.  The C/S fraction is computed from the horizontal texture of the Tb field.  This technique is described by Hong et al., (1998).  A manuscript with the complete details of the algorithm is in preparation.

File Format

    The file content description for 2A12 can be obtained from the Volume 4 - Levels 2 and 3 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 that were not addressed in the first release:

1.    Since the algorithm is sensitive to the Tb, any calibration offsets in the TMI data filter directly into the 2A12 algorithm. While the current calibration problems with TMI level 1B data are not thought to have a large impact upon the retrieved rainfall rates, they do have some impact.  The rainfall rates cannot be thought of as final.  (Posted:  9/25/98)

2.  The Latent Heating field is not reliable.  Late in the development process, it was noticed that one portion of the database had "zeroes" in the latent heating column.  These "zeroes" will be averaged with actual latent heating values by the retrieval process and thus will have a smaller dynamic range than should be expected.  It was too late to compute the actual latent heating fields from the models for this first release. (Posted:  9/25/98)

3.  While not an error, 2A12 does depend upon the database of potential profiles to find possible solutions.  When no profile is found in the database that resembles the observed Tb, this results in a large RMS difference which is stored in the "confidence" field.  In particular, confidence values in excess of 100 generally mean that no solution was found.  In this case, the closest profile is selected to provide continuity but it should be viewed with skepticism.  The condition occurs infrequently. (Posted:  9/25/98)
 
Planned Improvements

Work is underway to do careful validation of the 2A12 algorithm by comparing its results both to the space- and ground-based radars.  Any shortcomings of the algorithm identified by this work will be addressed to the best of our ability.  For the next data release, we also expect to add the convective/stratiform separation as determined by the algorithm to the data field.

Problems 1 and 2 above will be fixed for the next data release.  Code development is underway to make the quality flag more physically meaningful.
 
References

Hong, Y., C. Kummerow, and W. S. Olson, 1998:  "Separation of Convective/Stratiform Precipitation Using Microwave Brightness Temperature", J. Appl. Meteorol., (conditionally accepted).

Kummerow, C. , W. Barnes, T. Kozu, J. Shiue, and J. Simpson, 1998:  "The Tropical Rainfall Measuring Mission (TRMM) Sensor Package J. Atmos. and Ocean Tech.., 15, 808-816.

Kummerow, C., W. S. Olson and L. Giglio. 1996:  "A Simplified Scheme for Obtaining Precipitation and Vertical Hydrometeor Profiles from Passive Microwave Sensors," IEEE Trans. on Geosci. and  Remote Sensing, 34, 1213-1232.

Olson, W., C. D. Kummerow and Y. Hong, 1988:  "Atmospheric Latent Heating Distributions in the Tropics Derived from Satellite Passive Microwave Radiometer Measurements", , J. Appl. Meteorol., accepted
 

Last updated: 10/1/98