Abstract
The satellite-derived HOAPS (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data) precipitation estimates have been validated against in-situ precipitation measurements from optical disdrometers, available from OceanRAIN (Ocean Rainfall And Ice-phase precipitation measurement Network) over the open-ocean by applying a statistical analysis for binary estimates. In addition to using directly collocated pairs of data, collocated data were merged within a certain temporal and spatial threshold into single events, according to the observation times. Although binary statistics do not show perfect agreement, simulations of areal estimates from the observations themselves indicate a reasonable performance of HOAPS to detect rain. However, there are deficits at low and mid-latitudes. Weaknesses also occur when analyzing the mean precipitation rates; HOAPS underperforms in the area of the intertropical convergence zone, where OceanRAIN observations show the highest mean precipitation rates. Histograms indicate that this is due to an underestimation of the frequency of moderate to high precipitation rates by HOAPS, which cannot be explained by areal averaging.
Highlights
Detailed knowledge of precipitation over the ocean is essential for understanding the global water cycle since it contributes to more than 75% of the global annual precipitation [1]
The difficulties encountered by comparing in-situ point or along-track measurements with areal averages, the latter impacted by a lower satellite retrieval-based threshold, are crucial in averages, the latter impacted by a lower satellite retrieval-based threshold, are crucial in the statistical analysis
One might expect that HOAPS should detect precipitation more often the statistical analysis
Summary
Detailed knowledge of precipitation over the ocean is essential for understanding the global water cycle since it contributes to more than 75% of the global annual precipitation [1]. Due to a lack of available precipitation in-situ data over sea [14], only a few validation studies of satellite-derived precipitation data against measurements over the ocean exist. Reference [15] validated the TRMM Multisatellite Precipitation Analysis (TMPA, [5,16]) against buoys, showing an overestimation of TMPA over most of the buoy locations combined with an underestimation of high and light precipitation events. This underlines the need to perform more in-situ precipitation measurements over sea. We use binary statistics similar to the procedure of the German Weather Service validating their local model output against rain gauge measurements [23]
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