Impact of the ECMWF reanalysis soil water on forecasts of the July 1993 Mississippi flood


  • Pedro Viterbo,

  • Alan K. Betts


We reexamine the impact of soil water on the precipitation for the United States for July 1993 (the time of the Mississippi flood), previously discussed by Beljaars et al. [1996], using soil moisture from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA). Ensembles of three precipitation forecasts for the month of July, initialized on July 1, 2, and 3, using different initial soil water fields, are compared with the 12 to 24 hour ERA precipitation forecasts for the month and the observed precipitation. Both the 12 to 24 hour forecasts and the July integrations depict the July mean anomaly field well, although the mid-West precipitation maximum is displaced northward in both the ECMWF short and long-term forecasts. The July 1993 ERA soil water anomaly does not account for the anomalous July precipitation, but replacing the July 1, 1993, soil water with the much drier soil water from June 1988 reduces the July 1993 ensemble forecast precipitation by about 40%. It is probable that soil water nudging has reduced the variability of soil water in the ERA fields.