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Abstract. A novel approach to measuring rainfall in the ocean is presented. The method is based on the remote sensing of rain by navigational radar and subsequent calibration of the radar signal against the upper ocean salinity changes. The estimate of the area-average precipitation is obtained as the product of the calibration factor and the fraction of area covered by rainfall. We describe an application of this technique in the field during a recent voyage of the Australian R/V Franklin to the eastern tropical Indian Ocean. It is suggested that with further refinement of radar signal processing, navigational radar can be used to routinely measure open ocean precipitation from research vessels and ships of opportunity. IntroductionAir-sea moisture exchange affects both the ocean and the atmosphere. The most conspicuous influence of the hydrological cycle on the coupled ocean-atmosphere system is related to phase transitions of water, which lead to evaporative' cooling of the ocean and heating of the atmosphere due to condensation and eventual precipitation of moisture. Also important are other processes related to the hydrological cycle, such as radiative forcing of the rain clouds and ocean mixed layer changes due to salinity variations. In short, moisture exchange is one of the key links in the oceanatmosphere system. Accurate knowledge of the global precipitation, its regional distribution and interannual, seasonal, and synoptic variability is necessary to assess the performance of coupled oceanatmosphere models and the reliability of climate change predictions. To meet this urgent demand, a substantial international effort is now going into world-wide rainfall measurements. A highlight of this activity has been the launch of the Tropical Rainfall Measuring Mission (TRMM) satellite on November 27, 1997, which provides remote sensing of precipitation over the tropical oceans. Aside from technical matters, the success of this mission depends to a large extent on the quantity and quality of ground rainfall measurements required for validation of satellite data and preparation of precipitation analyses. Recent developments in radar technology have led to the emergence of new approaches to precipitation measurements. 13,697