Evaluation of the ERA-40 Surface Water Budget and Surface Temperature for the Mackenzie River Basin

Abstract

The systematic biases in temperature and precipitation, and the surface water budget of European Centre for Medium-Range Weather Forecasts (ECMWF) 40-yr reanalysis (ERA-40) for the Mackenzie River basin are assessed by comparing monthly averages from ERA-40 with basin averages of surface observations of temperature, precipitation, evaporation, and streamflow from the Mackenzie Global Energy and Water Cycle Experiment (GEWEX) Study (MAGS). The bias and spinup of precipitation in ERA-40 changes significantly over the analysis period. On an annual basis, both precipitation bias and spinup are correlated with the analysis increment of atmospheric total column water vapor. ERA-40 has, in addition, a high bias of precipitation in spring and a low bias in fall. The monthly precipitation analysis is best for the most recent decade, when the bias of the 0?12-h forecast precipitation is only a few percent higher than the MAGS observations, and ERA-40 represents rather well the variability of monthly precipitation. Annual evapotranspiration from ERA-40 is higher than a MAGS estimate by 30%. The annual runoff in ERA-40 is comparable to the annual streamflow, but the interannual variability is poorly correlated. ERA-40 has two runoff peaks: in April, when snowmelt runs off quickly over the frozen ground, and in August, when the lowest model layer melts and reaches a soil moisture threshold, when deep drainage increases rapidly. In the model liquid water budget, the soil water analysis increment contributes only 17 mm of water to the annual liquid budget (primarily in summer), which is small compared with the mean rainfall (323 mm) and snowmelt (194 mm). However, in the frozen budget, the analysis increment of snow water equivalent, with an annual mean total of 97 mm, is not much smaller than the mean annual snowfall (140 mm). Improvements to the model snow treatment are needed: snow melts too soon in the model, and is replaced by the snow analysis increments. For the Mackenzie River, ERA-40 has a distinct seasonal temperature bias, with a 2?3-K warm bias from December to April, and a cool bias in summer, reaching ?1.5 K in July. This signal is larger for the heavily forested southern basins. The warm winter bias may be related to a too-low albedo for snow under tall vegetation, while the cool summer bias may indicate excess evaporation. In a comparison of the subbasins with the MAGS estimates, ERA-40 has more precipitation than the MAGS observations for the northern and western mountainous basins, but for those basins the data are sparse. For evaporation, ERA-40 has less variation across the basins than the MAGS estimate. ERA-40 appears to represent well the climatological gradient of deep soil temperature across the Mackenzie basin, from continuous permafrost in the north to no permafrost in the south.