Using observations of deep convective systems to constrain atmospheric column absorption in the optically thick limit

The absorption of solar energy in the atmosphere is a fundamental part of the earth's energy cycle but is an extremely difficult quantity to measure. To investigate the atmospheric column absorption by clouds, we have collected five years of collocated satellite-surface observations for the optically thick Deep Convective Systems (DCS) at the Department of Energy Atmosphere Radiation Measurement (ARM) Tropical Western Pacific (TWP) and Southern Great Plains (SGP) sites during the March 2000-December 2004 period. The surface data were averaged over a 2-hour interval centered at the time of the satellite overpass, and the satellite data were averaged within a 1 x 1 area centered on the ARM sites. The temporally averaged surface observations should be equivalent to the spatially averaged satellite results in this study. In the optically thick DCS, the water vapor and cloud particle phase/shape, as well as 3-D cloud structure are not important to the radiation budgets, also the satellite-retrieved cloud optical depth can be constrained by surface-measured solar insolation. There is more column absorption (~2.5%) in the tropics than in the middle latitudes, but this difference disappears, i.e, the column absorptions of solar energy at both regions converge to the same value (~0.275) in the optically thick limit (τ > 50). Comparing the observations with the state-of-art radiative transfer model calculations under more restricted conditions, the differences between the observations and the model calculations in top-of-atmosphere (TOA) albedo, surface absorption and atmospheric column absorption are 0.0%, 0.3%, and 0.5%, respectively. The model-calculated TOA albedo and atmospheric column absorption are sensitive to, but the surface absorption is independent of, the cloud particle size in the optically thick limit. Therefore we conclude that there is an excellent agreement between the collocated satellite-surface observations and the state-of-art model calculations in the optically thick DCS.