AMS Forum: Climate Change Manifested by Changes in Weather


Coupling of water vapor convergence, clouds, precipitation and land-surface processes in the climate over land

Alan K. Betts, Atmospheric Research, Pittsford, VT

On timescales of a day and space scales of order 800km, the climate over land is a complex balance of many highly coupled processes. In the atmosphere, water vapor convergence is linked to precipitation and clouds, which in turn modify the radiation fields. Over land therefore the surface energy budget is strongly influenced by the cloud field, and the availability of water for evaporation. ERA-40 reanalysis data for sub-basins of the Mississippi in summer will be used to explore the links between these processes in a fully coupled model system. For more than a decade cloud feedbacks in models have been regarded as a major source of uncertainty in climate modeling. This paper proposes that the effective cloud albedo at the surface is one missing link which connects the cloud fields to both surface and large-scale processes. Moisture convergence leads to clouds and precipitation, but the relation of the diabatic precipitation heating and the surface cloud radiative forcing, while a function of cloud albedo, is largely independent of moisture convergence. The surface cloud radiative forcing largely determines the surface net radiation, while evaporative fraction is largely determined by temperature, soil water and vegetation parameters. Near-surface relative humidity, the lifting condensation level, soil moisture and precipitation are also closely linked. The proposed methodology provides both a diagnostic framework for model evaluation in terms of observables; and suggests how combinations of satellite data and surface variables may give better estimates of precipitation and the surface energy budget.

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Session 2, Linking Weather and Climate
Wednesday, 17 January 2007, 8:30 AM-12:00 PM, 214D

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