Ground validation of oceanic snowfall detection in satellite climatologies during LOFZY

Abstract

A B S T R A C T A thorough knowledge of global ocean precipitation is an indispensable prerequisite for the understanding of the water cycle in the global climate system. However, reliable detection of precipitation over the global oceans, especially of solid precipitation, remains a challenging task. This is true for both, passive microwave remote sensing and reanalysis based model estimates. The optical disdrometer ODM 470 is a ground validation instrument capable of measuring rain and snowfall on ships even under high wind speeds. It was used for the first time over the Nordic Seas during the LOFZY 2005 campaign. A dichotomous verification of precipitation occurrence resulted in a perfect correspondence between the disdrometer, a precipitation detector and a shipboard observer's log. The disdrometer data is further pointto-area collocated against precipitation from the satellite based Hamburg Ocean Atmosphere Parameters and fluxes from Satellite data (HOAPS) climatology. HOAPS precipitation turns out to be overall consistent with the disdrometer data resulting in a detection accuracy of 0.96. The collocated data comprises light precipitation events below 1 mm h -1 . Therefore two LOFZY case studies with high precipitation rates are presented that indicate plausible HOAPS satellite precipitation rates. Overall, this encourages longer term measurements of ship-to-satellite collocated precipitation in the near future.

HOAPS and ERA-Interim precipitation over the sea: validation against shipboard in situ measurements

“…Therefore, for the HOAPS collocation the allowed time difference was set to 45 min and the allowed distance was set to 55 km between the ship's position and the satellite's footprint (radius of the SSM/I pixel size plus decorrelation length). These match-up criteria are comparable to allowed differences of 45 min and 50 km chosen in a study by Klepp et al (2010). According to the satellites' overpass time, collocated data are merged to single events.…”

“…While it is not possible to estimate mean precipitation rates in the tropics and subtropics due to the low number of observed precipitation events for HOAPS, mean ERAInterim precipitation rates are higher than observed ones in the inner tropics. With respect to solid precipitation, although not reaching the high accuracy (96 %) as in a study of Klepp et al (2010) between point-to-area collocations of ship-based ODM 470s and HOAPS data, the detectability of solid precipitation in terms of the success ratio shows a good performance in detecting snowfall by both HOAPS and ERA-Interim.…”

Satellite derived precipitation and freshwater flux variability and its dependence on the North Atlantic Oscillation

“…The reason for the inconsistencies at high latitudes are due to deficiencies of satellite retrievals, that often lack skill to detect frozen precipitation properly and the generally poor data coverage of other input data for the reanalyses (Adler et al, 2001). HOAPS-3 seems to have a high potential for improving this situation as is shown in a companion paper in this special issue by Klepp et al (2010). At North Atlantic mid-latitudes, the negative correlation pattern extending from the central North Atlantic eastward over the Mediterranean region is represented in all data sets.…”

On the Spatial and Temporal Sampling Errors of Remotely Sensed Precipitation Products

“…This could be related to the fact that tropical regions contain a higher fraction of convective and small scale precipitation than higher latitudes [10]. Furthermore, there is a tendency towards less convective and more cellular cloud structures in high latitudes, resulting from cold air advection over warmer ocean surfaces [11], and thus precipitation in high latitudes might be less sensitive to changes in spatial resolution of the products. It should be noted that part of the observed zonal dependence of the results could be related to the use of equal latitude-longitude grids in IMERG, so a grid in lower latitudes covers a larger area than a grid in higher latitudes.…”