Abstract
The point-to-area problem strongly complicates the validation of satellite-based precipitation estimates, using surface-based point measurements. We simulate the limited spatial representation of light-to-moderate oceanic precipitation rates along ship tracks with respect to areal passive microwave satellite estimates using data from a subtropical island-based radar. The radar data serves to estimate the discrepancy between point-like and areal precipitation measurements. From the spatial discrepancy, two statistical adjustments are derived so that along-track precipitation ship data better represent areal precipitation estimates from satellite sensors. The first statistical adjustment uses the average duration of a precipitation event as seen along a ship track, and the second adjustment uses the median-normalized along-track precipitation rate. Both statistical adjustments combined reduce the root mean squared error by 0.24 mm h − 1 (55%) compared to the unadjusted average track of 60 radar pixels in length corresponding to a typical ship speed of 24–34 km h − 1 depending on track orientation. Beyond along-track averaging, the statistical adjustments represent an important step towards a more accurate validation of precipitation derived from passive microwave satellite sensors using point-like along-track surface precipitation reference data.
Highlights
The validation of satellite-based precipitation estimates using surface-based point measurements is substantially hampered by the long-standing point-to-area (p2a) problem [1,2,3]
A meaningful validation of precipitation estimates derived from satellite sensors requires the different spatial representation of precipitation in point-like surface reference data to be properly addressed—a main goal of this study
In contrast to precipitation over land areas for which triple collocation leads to satisfying results [8], the ocean lacks a dense coverage of frequent, high-quality precipitation data [9]
Summary
The validation of satellite-based precipitation estimates using surface-based point measurements is substantially hampered by the long-standing point-to-area (p2a) problem [1,2,3]. A meaningful validation of precipitation estimates derived from satellite sensors requires the different spatial representation of precipitation in point-like surface reference data to be properly addressed—a main goal of this study. In contrast to precipitation over land areas for which triple collocation leads to satisfying results [8], the ocean lacks a dense coverage of frequent, high-quality precipitation data [9]. This study addresses the p2a problem with the aim of simulating the different spatial representation of precipitation along ship tracks compared to areal satellite estimates, and proposes a p2a adjustment. We employ the following method: the S-Pol radar data serves to simulate both along-track ship measurements and PMW satellite pixels. The adjustments need to be solely based on information available from along-track precipitation rates because the sub-pixel variability is usually unknown for satellite-derived precipitation estimates.
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