The TRMM Precipitation Radar (PR), the first of its kind in space, is an electronically scanning radar, operating at 13.8 GHz that measures the 3-D rainfall distribution over both land and ocean, and defines the layer depth of the precipitation.
                        
The objectives of 2A25 is to correct for the rain attenuation in measured radar reflectivity and to estimate the instantaneous three-dimensional distribution of rain from the TRMM Precipitation Radar (PR) data.  The estimates of attenuation-corrected radar reflectivity factor and rainfall rate are given at each resolution cell of the PR.  The estimated near-surface rainfall rate and average rainfall rate between the two pre-defined altitudes (2 and 4 km) are also calculated for each beam position.
                        
2A25 basically uses a hybrid of the Hitschfeld-Bordan method and the surface reference method to estimate the vertical true radar reflectivity (Z) profile.  (The hybrid method is described in Iguchi and Meneghini (1994)).  The vertical rain profile is then calculated from the estimated true Z profile by using an appropriate Z-R relationship.
                        
The attenuation correction is, in principle, based on the surface reference method. This method assumes that the decrease in the apparent surface cross section (delta sigma-zero) is caused by the propagation loss in rain. The coefficient a in the k-Z relationship, k=a Z**b, is adjusted in such a way that the path-integrated attenuation (PIA) estimated from the measured Zm-profile will match the delta sigma-zero.  The attenuation correction of Z is carried out by the Hitschfeld-Bordan method with the modified a.  Since a is adjusted, this type of surface reference method is called the a-adjustment method.  The a-adjustment method assumes that the discrepancy between the PIA estimate from delta sigma-zero and that from the measured Zm-profile can be attributed to the inappropriate choice of a values, which may vary depending on the raindrop size distribution and other conditions.  It assumes that the radar is properly calibrated and that the measured Zm has no error.
                        
In order to avoid inaccuracies in the attenuation correction when rain is weak, a hybrid of the surface reference method and the Hitschfeld-Bordan method is used (Iguchi and Meneghini, 1994).  The PIA is first estimated from the precipitation echo alone.  The weight given by the hybrid method to the PIA estimate from the surface reference increases as the attenuation estimate increases.  When rain is very weak and the attenuation estimate is small, the PIA estimate from the surface reference is effectively neglected.  With the introduction of the hybrid method, the divergence associated with the Hitschfeld-Bordan method is also prevented.
                        
One major difference from the method described in the above reference is that, in order to deal with the beam-filling problem, a non-uniformity parameter is introduced and is used to correct the bias in the surface reference arising from the horizontal non-uniformity of rain field within the beam. Since radar echoes from near the surface are contaminated by the mainlobe clutter, the rain estimate at the lowest point in the clutter-free region is given as the near-surface rainfall rate for each angle bin.
                        
Spatial coverage is between 38 degrees North and 38 degrees South, owing to the 35 degree inclination of the TRMM satellite. This orbit provides extensive coverage in the tropics and allows each location to be covered at a different local time each day, enabling the analysis of the diurnal cycle of precipitation. There are, in general, 9150 scans along the orbit, with each scan consisting of 49 rays.  The scan width is about 220 km.       
                  
The data are stored in the Hierarchical Data Format (HDF), which includes both core and product specific metadata applicable to the PR measurements.  A fi...