A global climatology of extreme precipitation based on satellite data

Two data sets routinely computed by the authors are now long enough, each in excess of 10 years, that it is reasonable to construct a first climatology of global extreme precipitation. The TRMM Multi-satellite Precipitation Analysis (TMPA) provides 0.25°x0.25° 3-hourly estimates of precipitation in the latitude band 50°N-50°S for the years 1998-present, while the GEWEX/Global Precipitation Climatology Project (GPCP) One-Degree Daily (1DD) precipitation product provides 1°x1° daily global estimates of precipitation for 1997-present. The TMPA incorporates all available (intercalibrated) microwave estimates of precipitation in addition to microwave-calibrated infrared (IR) estimates, while the 1DD consists of microwave-calibrated IR estimates in the band 40°N-40°S and TOVS (or AIRS) sounding-based estimates at higher latitudes. Both datasets incorporate monthly raingauge analyses, but it should be emphasized that the day-to-day occurrence of precipitation is entirely based on the satellite data.

Although the 1DD is somewhat more approximate than the TMPA, the 1DD provides an important check on the mean and extreme results computed using the TMPA. In addition, the 1DD provides results over the entire globe, while the TMPA only covers the tropics and mid-latitudes. Finally, the 1DD captures the entire 1997-1998 El Niño, while the TMPA only captures it from the beginning of 1998. The analysis presented here focuses on basic parameters that are stable and well-suited to comparison with station data or model estimates. These include means, frequency of precipitation, 95th percentile values, and the longest spans of consecutive dry and wet days in a year.

Overall, there is fair consistency between the 1DD and TMPA datasets. One result of the comparison is that the longest span of consecutive dry days in a year is sensitive to the details of the retrieval algorithms. Another is confirmation that several of the parameters, including frequency of precipitation and 95th percentile values are sensitive to the spatial scale. In addition to enhancing our confidence in the results previously reported, this comparison allows us to examine issues that are inherent in the two datasets. For example, the 1DD typically shows anomalously high fractional coverage in the latitude bands 40-50°N and 40-50°S due to a smoothing operator that is applied at these latitude bands to accommodate the transition from IR-based to sounding-based estimates. As well, the TMPA tends to have drier estimates than the 1DD at higher latitudes, ~40-50°, particularly in the winter hemisphere, where the microwave algorithms currently lack sensitivity to the reduced precipitation signals.

The characteristic behavior of precipitation in the additional time/space coverage provided by the 1DD will be examined. The 1997 data provide crucial information about the early and middle phases of the significant 1997-1998 El Niño. The high-latitude results could be important for helping assess the conditions that the joint NASA/JAXA Global Precipitation Measurement (GPM) mission will observe.