Rainfall Measurements on the R/V Ronald H. Brown During EPIC2001

During the EPIC2001 cruise of the R/V Ron Brown, nearly 700 mm of rain was measured on the ship. Of this, about 70 mm occurred in the three days preceding our arrival at the survey site near 10N, 95W due to the ship's proximity to tropical storm "Ivo". In the two days following departure from the survey site we encountered about another 120 mm. The remainder, around 470 mm, fell during the 18 days on site. This average daily rainfall of about 26 mm/day far exceeds climatology for this time of the year. The imprecision of the above rainfall figures reflects the well-known instrumental uncertainty in shipboard rainfall estimates. Wind flow over the ship deflects raindrop trajectories so that raingauges can lose (or acquire) catch depending on their location. The Ron Brown is festooned with raingauges in an effort to overcome this uncertainty, and we independently installed four optical raingauges (ORGs) on the foredeck tower. We find that the ORGs invariably measured higher rainfall than the siphon gauges, independent of location, including a siphon gauge specifically designed to overcome the wind deflection problem. Accurate rainfall estimates are needed to determine such questions as surface layer stratification, ocean freshwater budgets and for models of ocean mixing. Large rainfall amounts can also be a significant component of the ocean surface energy budget. During one of the heaviest storms during EPIC, an average rainrate over 100 mm/hr was maintained for almost half-an-hour, representing heat loss from the ocean of 800 Wm-2 during this period, well in excess of typical latent heat flux. In this analysis we evaluate rainfall measurements from the various shipboard sensors, assign best values for the EPIC in situ rainfall time series and provide accuracy estimates.