The temperature dependence of the liquid water path of low clouds in the southern Great Plains

Satellite observations of low-level clouds have challenged the idea that in creasing liquid water content with temperature combined with constant physi cal thickness will lead to a negative cloud optics feedback in a decadal cl imate change. The reasons for the satellite results are explored using 4 yr of surface remote sensing data from the Atmospheric Radiation Measurement Program Cloud and Radiation Testbed site in the southern Great Plains of th e United States. It is found that low-cloud liquid water path is approximat ely invariant with temperature in winter but decreases strongly with temper ature in summer, consistent with satellite inferences at this latitude. Thi s behavior occurs because liquid water content shows no detectable temperat ure dependence while cloud physical thickness decreases with warming. Thinn ing of clouds with warming is observed on seasonal, synoptic, and diurnal t imescales; it is most obvious in the warm sectors of baroclinic waves. Alth ough cloud top is observed to slightly descend with warming, the primary ca use 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 observed is best e xplained as a transition in the frequency of occurrence of different bounda ry layer types. At cold temperatures, a mixture of stratified and convectiv e boundary layers is observed, leading to a broad distribution of liquid wa ter path values, while at warm temperatures, only convective boundary layer s with small liquid water paths, some of them decoupled, are observed. Our results, combined with the earlier satellite inferences, suggest a reexamin ation of the commonly quoted 1.5 degreesC lower limit for the equilibrium g lobal climate sensitivity to a doubling of CO2, which is based on models in which liquid water increases with temperature and cloud physical thickness is constant.