HR: 16:50h
AN: NG54A-03 INVITED     [Abstracts]
TI: The Deep Impact of Cloud Turbulence (and Microphysics) on Solar Radiation Transport: An Asymptotic Scaling Analysis
AU: * Davis, A B
EM: adavis@lanl.gov
AF: Los Alamos National Laboratory, Space and Remote Sensing Group, LANL/ISR-2, P.O. Box 1663 (Mail Stop B-244), Los Alamos, NM 87545, United States
AB: Clouds have a first-order role in the balance of the Earth's climate system both in the hydrological and energy (radiation) budget. Yet their representation in climate models, as well as in remote sensing retrievals of their condensed water content, is beyond simplistic: uniform plane-parallel slabs with infinite horizontal extent, irrespective of the scale of computational or observation resolution. At best, a nominal cloud fraction is used to account for unresolved variability, and sometimes 1-point cloud optical depth variability is accounted for; very rarely are spatial correlations even considered. Notwithstanding, the turbulence of Earth's cloudy atmosphere exhibits even to the most casual observer spatial correlations across broad ranges of scales, and these can often be quantified by power-law scaling relations (viz. wavenumber spectra typically in 1/k^5/3). A straightforward corollary of such scaling is stochastic continuity (almost everywhere) of the cloud field, as sampled in situ by aircraft as well as remotely with air- and space-borne remote sensing instruments. I will show that, as long as the scaling range contains the scales that matter optically (most importantly, the photon mean- free-path), this quasi-universal behavior has nontrivial consequences for the mean flow of radiation in the cloudy atmosphere. Indeed, the basic propagation kernel in integral radiative transfer equation is no longer exponential but, given the nature of the observed variability, a step distribution with a power-law tail. From there, new scaling laws follow for domain-average radiation properties at wavelengths dominated by multiple scattering using (1) Lévy's generalizations of the central limit theorem to infinite variance steps, (2) the Sparre-Anderson/Frisch law for first passages of random walks in a half-space, and (3) an ansatz that accounts for the finite thickness of the cloudy atmosphere. Recent observations of solar photon path length statistics using O₂transmission spectroscopy and numerical simulations are used to assess the relevance of the new asymptotic scaling relations to the real atmosphere. An interesting evolution of the mean-field radiation transport model ensues. The analytic theory remains qualitatively correct and makes important predictions for the impact of spatially variable cloudiness on path length statistics for reflected sunlight. Such observations will become available none to soon in 2008 when NASA's Orbiting Carbon Observatory (OCO) carries first high-resolution oxygen A-band spectrometer into space.
DE: 0321 Cloud/radiation interaction
DE: 0360 Radiation: transmission and scattering
DE: 3265 Stochastic processes (3235, 4468, 4475, 7857)
DE: 3359 Radiative processes
DE: 4468 Probability distributions, heavy and fat-tailed (3265)
SC: Nonlinear Geophysics [NG]
MN: 2007 Joint Assembly