Incorporating TRMM and other high-quality estimates into the One-Degree Daily (1DD) global precipitation product

The One-Degree Daily (1DD) precipitation dataset was recently developed for the Global Precipitation Climatology Project (GPCP). The 1DD provides a globally-complete, observation-only estimate of precipitation on a daily 1°x1° grid for the period 1997 through late 1999 (by the time of the conference). In the latitude band 40°N-40°S the 1DD uses the Threshold-Matched Precipitation Index (TMPI), a GPI-like IR product with the Tb threshold and (single) conditional rain rate determined locally for each month by the frequency of precipitation in the GPROF SSM/I product and by the precipitation amount in the GPCP satellite-gauge (SG) combination. Outside 40°N-40°S the 1DD uses a scaled TOVS precipitation estimate that has adjustments based on the TMPI and the SG. This first-generation 1DD has been in beta test preparatory to release as an official GPCP product.

In this paper we discuss further development of the 1DD framework to allow the direct incorporation of TRMM and other high-quality precipitation estimates. First, these data are generally sparse (typically from low-orbit satellites), so a fair amount of work was devoted to data boundaries. Second, these data are not the same as the original 1DD estimates, so we had to give careful consideration to the best scheme for forcing the 1DD to sum to the SG for the month. Finally, the non-sun-synchronous, low-inclination orbit occupied by TRMM creates interesting variations against the sun-synchronous, high-inclination orbits occupied by the Defense Meteorological Satellite Program satellites that carry the SSM/I.

Examples will be given of each of the development issues, then comparisons will be made to daily raingauge analyses.