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 physical and practical problems and instrument difficulties, leading to spatial and temporal inconsistencies in climate datasets. Estimates of rainfall based on the scattering and emission characteristics of passive microwave radiation, 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 over the Eastern North Atlantic and North Sea has been prepared, including rainfall totals (mm), rates (mm h(-1)), and percent occurrences (%) for the period 1979-1996 inclusive. Annual, seasonal and monthly maps have been generated, as well as histogram and time series products. In this paper, the main rainfall patterns over this region are summarised and interpreted in comparison with several other existing datasets. The interpretation of passive microwave-derived wind speed over the eastern North Atlantic and North Sea shall be reported in a future paper. Upon the analysis 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 location and strength of the belt of prevailing Westerlies, and their interaction with major land masses, dictate the overall rainfall distribution. Results show that rainfall over this area is well-related to various indices of the general circulation. Also, patterns of rainfall over the North Sea are significantly related to the direction and strength of the wind. Finally, it is shown that there is a significant linear relationship between climate station rainfall and the estimate over the nearest open water pixel, although the percent of explained variance seldom exceeds 50%. Copyright (C) 1999 Royal Meteorological Society.