/******************************************************************************

   README file for the algorithm 3B31, "Rainfall Combined", Version 7.0.
   
   L. Woltz	8/25/2011

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1.  Overview of 3B31

Algorithm 3B31, "Rainfall Combined", is to produce monthly surface rainfall 
totals for 0.5x0.5-degree boxes and monthly means of hydrometeor contents (cloud 
liquid water, rain water, cloud ice, and precipitating ice) for 0.5x0.5-degree 
boxes and 28 vertical layers.  This algorithm uses instantaneous rain retrievals 
generated in 2B31 to calibrate retrievals from 2A12.  The 2B31-derived vertical 
hydrometeor profiles and surface rainfall using data from the PR and 10 GHz 
channels of the TMI and is supposed to provide high quality retrievals for the 
narrow swath.  The 2A12 provides surface rainfall and hydrometeor profiles using 
TMI data in a wide swath.  The ratios of monthly products from 2B31 to those 
from 2A12 in 0.5x0.5-degree boxes will also be calculated.


2.  Process of 3B31

The algorithm 3B31 is executed once per granule.  For each execution, output 
data of 2A12 and 2B31 are read and then gridded into 0.5-degree boxes.  All 2A12 
pixels and 2B31 pixels that have scanStatus.dataQuality = 0 and 
scanData.pixelStatus = 0 are accumulated and counted for averaging if the pixel 
value is greater than or equal to 0.  Accumulations of both products are 
performed and saved in an intermediate data file that is read and updated for 
each execution.  Since the two products are output at different vertical 
resolutions, averaging the outputs of 2B31 over several radar range gates to 
match the 28 TMI layers is performed.  Also, the rain rate (mm/hr) at each radar 
gate in 2B31 is converted into rain water content (g/m^3) to match that in 2A12. 

While reading data of 2A12 and 2B31, the month period flag (begin/middle/end) is 
checked.  For the granule in the beginning (or end) of one month, data from the 
previous (or next) month are skipped.  For the granule in the middle of one 
month, all data are processed.  If the flag indicates the beginning of a month, 
the 3B31 will perform the initialization.  If the flag indicates the end of a 
month, monthly means are computed for the 0.5x0.5-degree boxes.  Monthly surface 
rainfall totals are derived from multiplying monthly mean by the number of hours 
of that month.  Then the 3B31 products are written into its output file.


3.  Inputs of 3B31

The following files are input to 3B31:

2A12 file:          name of 2A12 HDF file.

2B31 file:          name of 2B31 HDF file.

intermediate file:  name of 3B31 intermediate binary data file.  This file 
                    is used (read and update) for each execution.


4.  C Struct for 3B31

typedef struct {
   float surfacePrecipTMI[720][160];
   float convectPrecipTMI[720][160];
   float rainWaterTMI[28][720][160];
   float snowTMI[28][720][160];
   float graupelTMI[28][720][160];
   int npixTotalTMI[720][160];
   float surfacePrecipCOMB[720][160];
   float rainWaterCOMB[28][720][160];
   float snowCOMB[28][720][160];
   float graupelCOMB[28][720][160];
   int npixTotalCOMB[720][160];
   float surfAdjRatio[720][160];
   float surfAdjRatioOverlap[720][160];
} L3B31_GRID;