Interpretation of an atlas of passive microwave-derived rainfall over the eastern North Atlantic ocean and North Sea

Conventional measurements of rainfall over the ocean are fraught with physi cal and practical problems and instrument difficulties, leading to spatial and temporal inconsistencies in climate datasets. Estimates of rainfall bas ed on the scattering and emission characteristics of passive microwave radi ation, measured by instruments on board orbiting satellites, substantially improve the knowledge of rainfall patterns and processes over open water. A new passive microwave satellite-based Atlas of Rainfall and Wind Speed ove r the Eastern North Atlantic and North Sea has been prepared, including rai nfall totals (mm), rates (mm h(-1)), and percent occurrences (%) for the pe riod 1979-1996 inclusive. Annual, seasonal and monthly maps have been gener ated, as well as histogram and time series products. In this paper, the main rainfall patterns over this region are summarised a nd interpreted in comparison with several other existing datasets. The inte rpretation of passive microwave-derived wind speed over the eastern North A tlantic and North Sea shall be reported in a future paper. Upon the analysi s of above and below normal rainfall periods, significant relationships are revealed with respect to the mean sea level pressure, air and sea surface temperature, and atmospheric precipitable water. It is shown that the locat ion and strength of the belt of prevailing Westerlies, and their interactio n with major land masses, dictate the overall rainfall distribution. Results show that rainfall over this area is well-related to various indice s of the general circulation. Also, patterns of rainfall over the North Sea are significantly related to the direction and strength of the wind. Final ly, it is shown that there is a significant linear relationship between cli mate station rainfall and the estimate over the nearest open water pixel, a lthough the percent of explained variance seldom exceeds 50%. Copyright (C) 1999 Royal Meteorological Society.