Maritime precipitation

B. Geerts and E. Linacre


A knowledge of precipitation patterns over the oceans is important for weather forecasting, especially for medium range and interseasonal predictions. More basically, the oceans comprise 71% of the Earth surface and a knowledge of the longterm mean rainfall climatology is essential to improve our understanding of the general circulation of the atmosphere.

Very few marine observations of rainfall amounts are available, and those from islands and coastal stations are not representative for the surrounding waters because of orographically or topographically produced effects. There are several satellite-based rainfall estimation techniques, and while these remote data provide superb coverage, their accuracy still depends on a calibration against in situ measurements.

One source of in situ data is the weather reports in ships’ logs. Commercial vessels rarely have standard rain gauges, but the weather logs contain elements of relevance to rainfall. An analysis of over 13 million marine weather logs during 1958 - 1991 has yielded some surprising results (1). These are summarized below:



(1) Petty, G. W. 1995. Frequencies and characteristics of global oceanic precipitation from shipboard present-weather reports. Bull. Amer. Meteor. Soc. 76, 1593-616.