Frequencies and Characteristics of Global Oceanic Precipitation from Shipboard Present-Weather Reports

Abstract

Ship reports of present weather obtained from the Comprehensive Ocean-AtmosphereData Set are analyzed for the period 1958-91 in order to elucidateregional and seasonal variations in the climatological frequency,phase, intensity, and character of oceanic precipitation. Specificfindings of note include the following: 1) The frequency of thunderstormreports, relative to all precipitation reports, is a strong functionof location, with thunderstorm activity being favored within 1000-3000km of major tropical and subtropical landmasses, while being quiterare at other locations, even within the intertropical convergencezone. 2) The latitudinal frequency of precipitation over the southernoceans increases steadily toward the Antarctic continent and showsrelatively little seasonal variation. The frequency of convectiveactivity, however, shows considerable seasonal variability, withsharp winter maxima occurring near 38 degrees latitude in both hemispheres.3) Drizzle is the preferred form of precipitation in a number ofregions, most of which coincide with known regions of persistentmarine stratus and stratocumulus in the subtropical highs. Less welldocumented is the high relative frequency of drizzle in the vicinityof the equatorial sea surface temperature front in the eastern Pacific.4) Regional differences in the temporal scale of precipitation events(e.g., transient showers versus steady precipitation) are clearlydepicted by way of the ratio of the frequency of precipitation atthe observation time to the frequency of all precipitation reports,including precipitation during the previous hour. The results ofthis study suggest that many current satellite rainfall estimationtechniques may substantially underestimate the fractional coverageor frequency of precipitation poleward of 50 degrees latitude andin the subtropical dry zones. They also draw attention to the needto carefully account for regional differences in the physical andspatial properties of rainfall when developing calibration relationshipsfor satellite algorithms.