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Soil Wetness and Climate Variability

Delworth, Thomas L.

Titre : Soil Wetness and Climate Variability

Auteur : Delworth, Thomas L.

Université de soutenance : UNIVERSITY OF WISCONSIN - MADISON

Grade  : Doctor of Philosophy (PhD) 1994

The climate system is characterized by substantial amounts of variability at a wide range of spatial and temporal scales. A key issue in climate research is obtaining a better description of this variability and the physical mechanisms responsible for it. In this thesis the importance of land-surface processes for atmospheric variability on seasonal to interannual time scales is examined through detailed analyses of the output of a general circulation model (GCM) of the atmosphere coupled to a land-surface hydrology formulation. The GCM has horizontal resolution of approximately 4.5 ^circ latitude by 7.5^ circ longitude, nine levels in the vertical, and incorporates subgrid scale parameterizations for precipitation and convection. The model has a seasonal cycle of insolation, but no diurnal cycle. A seasonal cycle of sea surface temperatures is prescribed from observations. It is shown that modelled interactions between the land surface and the atmosphere can make a substantial contribution to the spectrum of near-surface atmospheric variability, both increasing the total variability and lengthening the time scales of near-surface atmospheric fluctuations. Such effects are most pronounced in the tropics and summer hemisphere extratropics. The time scales of such variability in the model are shown to depend on both potential evaporation and the water-holding capacity of the soil. Observational evidence is presented supporting this analysis. The relevance of these results for the issue of droughts is also examined. An additional measure of the importance of land surface processes for climate is the degree of precipitation recycling. A water vapor tracer capability was implemented in the GCM such that water vapor could be traced from its evaporative source region to the area where it precipitates. The model is shown to have a high degree of precipitation recycling over continental interiors during summer. The month to month persistence of precipitation originating from continental evaporation is shown to be larger than that from oceanic evaporation. Model recycling estimates compare favorably to recent observed estimates. The sensitivity of the variability results to several key features of the land-surface formulation and of the atmospheric model is also evaluated. While the essential variability results are fairly robust, there is some sensitivity of the results to aspects of the land surface formulation, particularly the total water-holding capacity


Présentation (SAO/NASA)

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