Back to basics: Measuring rainfall at sea: Part 1 – In situ sensors

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, 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.