Basin-scale surface water and energy budgets for the Mississippi from the ECMWF reanalysis

This paper compares with observations the energy and water budgets for the subbasins of the Mississippi (the Arkansas-Red, the upper Missouri, the upp er Mississippi, the Ohio, and the lower Mississippi and Tennessee Rivers), which were computed on-line with an hourly time scale from the European Cen tre for Medium-Range Weather Forecasts (ECMWF) reanalysis from 1985 to 1993 . The model has a significant precipitation spin-up between the analysis cy cle and the 12-24 hour forecast, ranging from 24% to about 40% for the drie r Missouri basin. The spin-up of the model "large-scale" precipitation rang es from 30 to 50%, roughly double that of the spin-up of the model "convect ive" precipitation. The model has an erroneous peak in convective precipita tion near local noon, but on 2 day and monthly timescales, the 12-24 hour f orecast precipitation is only 10 to 20% higher than the observed precipitat ion for most of the subbasins. The model runoff, which is all deep runoff f rom the base soil layer, is low on an annual basis, primarily because the m odel has very little Spring runoff. The nudging of soil water in the analys is cycle, based on 0-6 hour forecast errors in low-level humidity, plays a major role in the model liquid hydrology, The nudging term has a large annu al cycle, positive in summer and negative in winter. Although nudging preve nts the downward interannual drift of soil water, associated with a shortfa ll of precipitation in the analysis cycle, it also attempts to compensate f or other errors in the model, such as errors in the seasonal cycle of evapo ration and runoff, and may damp the variability of soil water. The model fr ozen hydrology in winter is not conservative and snowmelt is probably too s mall. Overall, the ECMWF reanalysis gives a valuable description of the sur face energy and water balance of the Mississippi River subbasins on timesca les longer than the diurnal, and at the same time, it is clear that improve ments in the model physics are needed.