Measuring Rainfall at Sea. Part I. In Situ sensors

1) Introduction Rainfall is an important climatic variable. Extremes in rainfall accumulations over land — either floods or droughts — have major societal implications and are obvious. At sea the effects on human activity are less evident, apart from the inconvenience to deck passengers on cruise liners! However improved knowledge of the rainfall associated with weather systems approaching the UK from the Atlantic would be beneficial to weather forecasting, especially if assimilated into atmospheric models There is an additional, more subtle, effect involving the ocean itself. At sea, the balance between precipitation and evaporation provides a critical feedback in climate change. The present ocean circulation involves both surface and deep currents (see Fig. 1), with the passage of water from the former to the latter occurring in the Labrador and Greenland Seas, where intense cooling by the winds makes the surface waters dense enough to sink to the ocean bottom, a process known as "deep convection". However, where precipitation exceeds evaporation the surface waters become fresher, and thus less dense, making them less susceptible to deep convection. Long-term changes in the freshwater balance (evaporation minus precipitation and ice melt) can thus potentially change the location and extent of such deep mixing, and ultimately disturb the "conveyor belt" illustrated in Fig. 1. Some modelling studies suggest that a change in the freshwater flux could trigger "abrupt climate change", leading to a marked reduction in UK temperatures over a decade, as the country loses the benefit of weather systems heated by their passage over warm Atlantic waters (Ellett 1993). However, before we panic about long-term changes in rainfall and impending climatic catastrophe, we need to know how much rain falls at sea, and what is the natural variability on seasonal and interannual timescales. This is not so simple a measurement task as it is on land, where there exist a large number of rain gauges, and the whole of the British Isles and neighbouring waters are covered by a rain radar network. In this article we look at the various in situ technologies for recording rain rates at sea; a succeeding article will cover techniques using satellite data.