The primary objective of algorithm 3A25 is to compute various rainfall statistics over a month from the level 2 PR products.  The statistics are derived at two spatial resolutions: (1) a standard space scale of 5 degrees by 5 degrees (latitude x longitude) cells and (2) a high resolution subset of 0.5 degree x 0.5 degree cells.

The output variables include rainfall rate (mm/hour) profile at 2, 4, 6, 10, and 15 km, fractional rain, snow ice layer, and surface rain rate (mm/hour). The output statistics include probabilities of occurrence, means and standard deviations, histograms, and correlation coefficients.

All statistics in 3A-25, except near-surface rain rate, are computed only when rain is judged in 1C-21 to be &quot;certain.&quot; When rain is judged in 1C-21 to be &quot;possible,&quot; the observation is treated as a &quot;no-rain&quot; observation. For the near-surface rain rate, the statistics (mean, standard deviation, and histogram) are computed for &quot;rain-possible&quot; as for the usual &quot;rain-certain.&quot; Because the &quot;rain-possible&quot; cases are dominated by noise so that the probability of false-alarm is high, the &quot;rain-certain&quot; statistics should be considered as more representative of the TRMM radar data.

Three types of rain rates are defined in 3A-25: (1) a &quot;near-surface&quot; rain rate that is obtained from the range bin closest to the surface which is not corrupted by the surface clutter, (2) a path-averaged rain rate calculated by summing the values from the storm top (first gate where rain is detected) to the last gate (gate nearest to the surface uncontaminated by surface clutter) and dividing by the number of gates in the interval, and (3) those at fixed heights above the ellipsoid (2, 4, 6, 10, and 15 km).

Spatial coverage is between 40 degrees North and 40 degrees South, owing to the 35 degree inclination of the TRMM satellite.

The data are stored in the Hierarchical Data Format (HDF). The low resolution grids are in the Planetary Grid 1 structure, and the high resolution grids are in the Planetary Grid 2 structure. The file size (one file per month) is about 16 MB (uncompressed). The description of file component objects can be obtained from Volume 4 - Levels 2 and 3 File Specifications provided by the TRMM Science Data and Information System (TSDIS): &quot;http://pps.gsfc.nasa.gov/&quot;.

The Tropical Rainfall Measuring Mission (TRMM) is a joint U.S.-Japan satellite mission to monitor tropical and subtropical precipitation and to estimate its associated latent heating. TRMM was successfully launched on November 27, at 4:27 PM (EST) from the Tanegashima Space Center in Japan.

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.