Northern High-Latitude Precipitation as Depicted by Atmospheric Reanalyses and Satellite Retrievals

Abstract

Monthly precipitation based on forecasts from the new 40-yr ECMWF Re-Analysis (ERA-40) is evaluated for the north polar region (the region north of 45°N), the terrestrial Arctic drainage, and its four major watersheds: the Ob, Yenisey, Lena, and Mackenzie basins. Corresponding evaluations are performed for precipitation from the NCEP?NCAR reanalysis, the earlier 15-yr ERA (ERA-15), and satellite-derived estimates from the Global Precipitation Climatology Project (GPCP). Evaluations rely on an improved gridded dataset of precipitation derived from monthly gauge data during the period 1979?93. The available number of gauges has declined since 1993, making it difficult to perform evaluations for later years. ERA-40 depicts monthly precipitation much better than NCEP?NCAR. This is with respect to both lower mean biases and higher squared correlations between modeled and observed grid-cell time series. Squared correlations between monthly time series of ERA-40 and observed precipitation, averaged over the four major Arctic watersheds, typically range from 0.60 to 0.90. Performance over the central Arctic Ocean is poor in winter and spring, but improves in summer and autumn when precipitation amounts are higher. While the overall performance of ERA-40 is better than NCEP?NCAR, it offers no obvious improvement over ERA-15. In some respects, ERA-15 performs slightly better in summer. This lack of improvement may relate to difficulties in assimilating satellite radiances. All of the reanalyses provide better depictions of monthly precipitation than do the GPCP satellite retrievals. This applies to both land areas and the Arctic Ocean. There is no clear improvement in the GPCP estimates after 1987 when the Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) data began to be used. The GPCP estimates are best in summer.