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Climatic Implications of the Observed Temperature Dependence of the Liquid Water Path of Low Clouds in the Southern Great PlainsSatellite observations of low-level clouds have challenged the assumption that adiabatic liquid water content combined with constant physical thickness will lead to a negative cloud optics feedback in a decadal climate change. We explore the reasons for the satellite results using four years of surface remote sensing data from the Atmospheric Radiation Measurement Program Cloud and Radiation Testbed site in the Southern Great Plains of the United States. We find that low cloud liquid water path is approximately invariant with temperature in winter but decreases strongly with temperature in summer, consistent with the satellite inferences at this latitude. This behavior occurs because liquid water content shows no detectable temperature dependence while cloud physical thickness decreases with warming. Thinning of clouds with warming is observed on seasonal, synoptic, and diurnal time scales; it is most obvious in the warm sectors of baroclinic waves. Although cloud top is observed to slightly descend with warming, the primary cause of thinning is the ascent of cloud base due to the reduction in surface relative humidity and the concomitant increase in the lifting condensation level of surface air. Low cloud liquid water path is not observed to be a continuous function of temperature. Rather, the behavior we observe is best explained as a transition in the frequency of occurrence of different boundary layer types: At cold temperatures, a mixture of stratified and convective boundary layers is observed, leading to a broad distribution of liquid water path values, while at warm temperatures, only convective boundary layers with small liquid water paths, some of them decoupled, are observed. Our results, combined with the earlier satellite inferences, imply that the commonly quoted 1.50 C lower limit for the equilibrium global climate sensitivity to a doubling of CO2, which is based on models with near-adiabatic liquid water behavior and constant physical thickness, should be revised upward.
Document ID
19990070927
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
DelGenio, Anthony D.
(NASA Goddard Inst. for Space Studies New York, NY United States)
Wolf, Audrey B.
(Science Systems and Applications, Inc. New York, NY United States)
Date Acquired
August 19, 2013
Publication Date
May 1, 1999
Subject Category
Environment Pollution
Report/Patent Number
GNC-99-41
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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