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Zhanqing Li and Louis Moreau

Abstract

This study investigated theoretically and experimentally two parameters employed in recent attempts to address cloud absorption anomaly. One is the ratio, R, of shortwave cloud radiative forcing (CRF) at the surface to that at the top of the atmosphere (TOA), and another is the slope, s, of the regressional relationship between TOA albedo and atmospheric transmittance. The physics and sensitivities of the two parameters were first examined by means of radiative transfer modeling. Neither R nor s is a direct measure of cloud absorption. However, R can indicate the effect of clouds on the atmospheric absorption of solar radiation, if clear-sky condition remains the same. A value of R > 1 implies clouds warm the atmosphere, while the converse is true for R < 1. Model simulations suggest that both R and s are sensitive to many factors, especially cloud height and surface condition. Nonetheless, modeled R rarely exceeds 1.25, and modeled s is generally less than −0.7, except for bright surfaces. The slope s can be related to R under certain conditions. Observational values of R and s were then determined using four years worth of global satellite and ground-based monthly mean solar flux data from the Earth Radiation Budget Experiment (ERBE) and the Global Surface Energy Balance Archive (GEBA). The ratio R is highly variable with both location and season and also shows strong interannual variability. Low to moderate values of R, attainable by plane-parallel radiative transfer models, tend to occur over relatively clean regions. Large values of R appear to associate with either heavy pollution in the midlatitudes or frequent occurrence of biomass burning in the Tropics. The large values of R in the Tropics are less reliable than the low and moderate R in the midlatitudes. While this study does not rule out cloud absorption anomaly, it does indicate, however, that its magnitude (if it exists) is not as large, and its occurrence not as widespread, as suggested in some recent reports.

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Zhanqing Li, Louis Moreau, and Albert Arking

Solar energy disposition (SED) concerns the amount of solar radiation reflected to space, absorbed in the atmosphere, and absorbed at the surface. The state of knowledge on SED is examined by comparing eight datasets from surface and satellite observation and modeling by general circulation models. The discrepancies among these contemporary estimates of SED are so large that wisdom on conventional SED is wanting. Thanks to satellite observations, the earth's radiation budget (ERB) at the top of the atmosphere is reasonably well known. Current GCMs manage to reproduce a reasonable global and annual mean ERB, but often fail to simulate the variations in ERB associated with certain cloud regimes such as tropical convection and storm tracks. In comparison to ERB, knowledge of the surface radiation budget (SRB) and the atmospheric radiation budget (ARB) is still rather poor, owing to the inherent problems in both in situ observations and remote sensing. The major shortcoming of in situ observations lies in insufficient sampling, while the remote sensing techniques suffer from lack of information on some variables affecting the radiative transfer process, and dependence, directly or indirectly, on radiative transfer models. Nevertheless, satellite-based SRB products agree fairly well overall with ground-based observations. GCM-simulated SRBs and ARBs are not only subject to large regional uncertainties associated with clouds, but also to systematic errors of the order of 25 W m2, due possibly to the neglect of aerosol and/or inaccurate computation of water vapor absorption. Analyses of various datasets suggest that the SED based on ERBE satellite data appears to be more reliable, indicating 30% reflection to space, 24% absorption in the atmosphere, and 46% absorption at the surface.

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