GPM/DPR Level-3 Algorithm Theoretical Basis Document (ATBD)
The Level-3 radar products provide daily and monthly global statistics of the Level-2 Ku, Ka and DPR products on a latitude-longitude grid. In version 7 (V07), the organization of the products has changed with the highest-level categorization into FS (full swath), MS (matched or inner swath) and HS (high sensitivity) swath products. The next level of division is into high and low spatial resolution grids that are defined such that the low-resolution grid (G1) is 50×50 (lat×lon) while the high-resolution grid (G2) is 0.250× 0.250. For variables defined on the low-resolution grid, the statistics include mean, standard deviation, counts and histogram. For variables defined on the high-resolution grid, the same statistics are computed except for a histogram, which is omitted. Figure 1 shows the monthly mean of the precipitation rate near the surface over the G1 and G2 grids for June 2018. The version history of L3 is given in section 3.
The level 3 code is written so that the 15 or 16 orbits of Level-2 DPR data produced daily can be processed in two runs, where one output file contains statistics from the ascending orbital passes while the other file contains statistics from the descending passes. Since all orbits for the day are processed in each run, there is no need for intermediate files. What is produced are two daily level 3 HDF files. Nominally, the standard Level 3 product is obtained by processing the twice-daily HDF files over a calendar month; however, this is not required. Output products can be generated from any set of daily HDF files. It should be noted that the daily files contain a mean square statistic rather than the standard deviation. For the monthly (or multi-day) file, however, the mean square statistic is replaced by the standard deviation.