Oceanic precipitation rate climatology

Precipitation is an important source of interference and obscuration to many types of sensors employed for ocean surveillance. Radar attenuation and backscatter due to rain become significant for radar wavelengths less than several centimetres and are sensitive functions of the rain rate. Rain frequency data are also important in the design of supersonic vehicles since rain erosion is related to rain rate. However, accurately assessing oceanic precipitation rates is still a major problem. A three-step method for estimating the climatological frequency distribution of two-minute precipitation rates is described. The method uses observations of weather and air temperature. It has been applied to ship observations taken in the Indian, North Pacific, and North and South Atlantic Oceans to produce seasonal maps. These maps show that precipitation rates of greater than 5 mm/hr are found in about 1% of the observations in extratropical regions and in up to 10% of the observations in some equatorial areas