This study investigates the importance of initial soil wetness in seasonal predictions with dynamical models. Two experiments are performed, each consisting of two ensembles of global climate model integrations initialized from early June observed atmospheric states. In the first experiment, three 1988 observed atmospheric states and 1988, sea surface temperature (SST) are used. One ensemble is initialized with seasonally varying climatological soil wetness and the other is initialized with proxy 1988 soil wetness derived from the ECMWF analysis and forecast system. In the second experiment, observed atmospheric states and observed SST for five different years are used, and each ensemble is initialized with a different climatological soil wetness. After initialization, a coupled atmosphere-biosphere model determines the evolution of the soil wetness fields in all the integrations.
The experiments are analyzed to determine the impact of the initial soil wetness differences. In contrast to several previous studies in which initial soil wetness was prescribed arbitrarily, a somewhat more realistic soil wetness impact is analyzed by comparing integrations initialized with climatological soil wetness to integrations initialized with soil wetness derived from the output of an operational analysis and forecast system. The initial soil wetness impact is found to be largely local and is largest on near surface fields, in agreement with previous results. Significant impacts were found in several tropical and extratropical regions in both experiments. Almost all the regions that experienced significant increases (decreases) in inital soil wetness exhibited significant increases (decreases) in seasonal mean precipitation in response to increased (decreased) initial soil wetness, although the response of the precipitation was more variable and was highly dependent on the response of the moisture flux convergence to the initial soil wetness.
The strength of the impact of initial soil wetness differences, as well as the nature of the impact on precipitation and upper atmospheric fields, depends on several factors. These factors include the areal extent and magnitude of the initial soil wetness difference, the persistance of the soil wetness difference, the strength of the solar forcing, the availability of nearby moisture sources and the strength of the regional dynamical circulation. The results suggest that seasonal atmoshperic prediction could be enhanced by using a realistic inital specification of soil wetness.
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Center for Ocean-Land-Atmosphere Studies
last update: 1 July 1996
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