Global precipitation and thunderstorm frequencies. Part I: Seasonal and interannual variations

Present and past weather reports from similar to 15 000 stations around the globe and from the Comprehensive Ocean-Atmosphere Data Set from 1975 to 19 97 were analyzed for the frequency of occurrence for and the percentage of the days with various types of precipitation (drizzle, nondrizzle, showery, nonshowery, and snow) and thunderstorms. In this paper, the mean geographi cal, seasonal, and interannual variations in the frequencies are documented . Drizzles occur most frequently (similar to5%-15% of the time) over mid- a nd high-latitude oceans. Nonshowery precipitation is the preferred form of precipitation over the storm-track regions at northern mid- and high latitu des in boreal winter and over the Antarctic Circumpolar Current in all seas ons. Showery precipitation occurs similar to5%-20% of the time over the oce ans, as compared with < 10% over land areas except in boreal summer over No rthern Hemisphere land areas, where showery precipitation and thunderstorms occur in over 20% of the days. Inferred mean precipitation intensity is ge nerally < 1.0 mm h(-1) at mid- and high latitudes and similar to1.5- 3.0 mm h(-1) in the Tropics. The intertropical convergence zone and the South Pac ific convergence zone are clearly defined in the frequency maps but not in the intensity maps. Nonshowery precipitation at low latitudes is associated with showery precipitation, consistent with observations of stratiform pre cipitation accompanying mesoscale convective systems in the Tropics. The se asonal cycles of the showery precipitation and thunderstorm frequencies exh ibit a coherent land-ocean pattern in that land areas peak in summer and th e oceans peak in winter. The leading EOFs in the nondrizzle and nonshowery precipitation frequencies are an ENSO-related mode that confirms the ENSO- induced precipitation anomalies over the open oceans previously derived fro m satellite estimates.