Representation of mean Arctic precipitation from NCEP-NCAR and ERA reanalyses

An improved monthly precipitation climatology for the Arctic is developed b y blending the Legates and Willmott gridded product with measurements from Russian "North Pole" drifting stations and gauge corrected station data for Eurasia and Canada. The improved climatology is used to examine the accura cy of mean precipitation forecasts from the National Centers for Environmen tal Prediction (NCEP) and European Reanalysis Agency (ERA) reanalysis model s, based on data for the period 1979-88. Bath models capture the major spat ial features of annual mean precipitation and general aspects of the season al cycle but with some notable errors. Both underestimate precipitation ove r the Atlantic side of the Arctic. NCEP overestimates annual totals over la nd areas and to a somewhat lesser extent over the central Arctic Ocean. Exc ept for the North Atlantic-Scandinavia sector, the NCEP model also depicts the seasonal precipitation maximum consistently one month early in July. Ov erall, the ERA predictions are better. Both models perform best during wint er and worst during summer. The most significant problem with the NCEP model is a severe oversimulation of summer precipitation over land areas, due to excessive convective preci pitation. Further investigation far July reveals that both the NCEP analyse s and 12-h forecasts are too wet below about 850 mb and have more negative low-level temperature gradients as compared to available rawinsonde profile s. This suggests that low-level observations are not being effectively inco rporated in the analyses. Given this finding, the high humidities are consi stent with excessive surface evaporation rates. This problem may in turn re late to soil moisture, which NCEP updates by the modeled precipitation. If soil moisture is too high, this would favor excessive Evaporation and high low-level humidities, fostering excessive precipitation, in turn keeping so il moisture and evaporation rates high. The NCEP down-welling shortwave flu xes are also much too high, contributing to excessive evaporation and possi bly influencing the low-lever temperature gradients. By comparison, soil mo isture in the ERA model is adjusted using the difference between the model first guess and analysis value (the analysis increment) of low-level humidi ty, which prevents model drift. The ERA downwelling shortwave fluxes are al so closer to observations. These attributes are consistent with the superio r ERA precipitation forecasts in summer and suggest avenues for improving t he performance of the NCEP model.