11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Thursday, 6 June 2002
Tau-Ac-T response functions, cloud-base height skewness and low-cloud radiative feedback
Christopher A. Jeffery, LANL, Los Alamos, NM
Over 25 years ago, Temkin et al (JAS, 1975) compared and contrasted the temperature sensitivity of shortwave optical depth (tau) at fixed cloud fraction (Ac), (dtau/dT)Ac, with the temperature sensitivity of Ac at fixed tau, (dAc/dT)tau, for a single-layer cloud at constant relative humidity. They found (dtau/dT)Ac >0 and (dAc/dT)tau >0 using an idealized non-stochastic model. I expand the work of Temkin et al and consider these response function within the context of a statistical cloud scheme. First, a unified model of low-cloud physical and optical variability is derived that incorporates a new subgrid distribution of cloud-top height fluctuations. Using this statistical scheme and assuming a symmetric distribution of unresolved variability, I find (dAc/dT)tau <0 in contradistinction with Temkin et al. Implications for subgrid-scale low-cloud radiative feedback, in general, and Tselioudis et al (Nature, 1993)'s observation of a negative optical depth sensitivity, in particular, are discussed. The impact of cloud-base height skewness on these two response functions is also examined.

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