Maritime precipitation

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:

• The areas west of South America and Africa are relatively dry. This is not surprising, given the persistent atmospheric subsidence and the relatively cold sea surface. Nevertheless the ubiquitous shallow stratus and stratocumulus often yield light drizzle, notably in specific localities, and occasionally a shower of heavy rain is reported. The showers come from storms too small to be distinguished by a satellite, and too shallow for cloud-top temperatures to be low enough for infrared measurements. As regards drizzle, in June - November there is a narrow area where it is relatively frequent, westward from South America, between 0-5° S. This is associated with marine boundary layer air moving south from a warmer to a cooler sea.

• The precipitation frequency and amounts in the southern oceans increases steadily from 45 to 65° S, and this varies little during the year. The increase conflicts with evidence from satellite IR data, which imply less precipitation near the Antarctic continent, perhaps because the distinction between precipitating cloud and sea ice is difficult in both visible and IR imagery.

• Lightning is not as common over the sea as over land, and convective rainfall at sea often is accompanied by little or no lightning. In particular, it now seems that lightning activity at sea occurs mostly within about 2000 km of large tropical and subtropical landmasses, and is quite rare elsewhere, even within the ITCZ. This can be due either to the cloud condensation and ice nuclei concentrations that are typical for the marine environment, or the lack of CAPE (convective available potential energy) and therefore strong updrafts, or the weakness of the background electric field in the troposphere.

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