University of California, Santa Barbara
Santa Barbara, CA 93066
One cloud radiation issue that has been troublesome for several decades is the absorption of solar radiation by clouds. Many hypotheses have been proposed to explain the discrepancies between observations and modeling results. A good review of these often- competing hypotheses has been provided by Stephens and Tsay (1990). They characterize the available hypotheses as falling into three categories: (1) those linked to cloud microphysical and consequent optical properties; (2) those linked to the geometry and heterogeneity of clouds; and (3) those linked to atmospheric absorption.
Recently, a number of investigators have proposed that current modeling practice is seriously inconsistent with new observational inferences concerning absorption of solar radiation in the atmosphere (regarded as a system including both clouds and clear sky) resulting from the presence of clouds. Using an approach based on a combination of surface and satellite observations of the net broadband shortwave radiation, Cess et al., (1994) have suggested that significant disagreement exists between these observations and radiative transfer computations performed by climate models.
This issue has been investigated, in part, in terms of a ratio (R) of cloud forcing at the surface (measured by upward- and downward- looking pyranometers) to cloud forcing at the top of the atmosphere (Harrison et al., 1992). Combined satellite and pyranometer observations suggest that absorption in the cloud/atmosphere system due to the presence of clouds is drastically underestimated by at least two representative climate models (NCAR CCM2 and ECMWF). Because of the potential importance of such a result, Gautier and her colleagues have investigated this absorption issue with a radiative transfer model for plane parallel conditions.
First, they have addressed the relationship between the absorption taking place in the atmosphere due to cloud (DABSATM = ABSATMcloud - ABSATMclear) and the ratio of surface to TOA cloud forcing (CFSURF/CFTOA). From the definitions used it can be demonstrated that this relationship is:
R = 1 + (DABSATM/CFTOA)
This indicates that if R is larger than 1, the absorption in the atmosphere when clouds are present is larger than that in clear conditions.
Second, they have performed sensitivity studies to a number of parameters: cloud altitude, cloud drop effective radius, cloud particle type (water or ice), atmospheric water vapor content, surface conditions, and sun zenith angle. A summary of their sensitivity results is presented in Figure 1 which presents DABSATM as a function of R. They find a range of values for R from 1 to 1.8, a range that encompasses the value discussed by Cess et al. (i.e., 1 for models and 1.5 from observations). Furthermore, they find that the relationship between DABSATM and R is not unique but depends on atmosphere and surface conditions. For a given set of atmosphere and surface conditions, the highest values of R corresponds to small optical thickness values. These, on the other hand, are associated with small values of absorption. The largest variations in absorption are related to changes in cloud altitude, but the slope of DABSATM/R varies with atmosphere-surface conditions and cloud optical thickness. To summarize, this figure suggests that there is no unique relationship between R and solar radiation absorption in the atmosphere resulting from the presence of clouds. Gautier and her colleagues contend that an emphasis on R may, therefore, not be the optimal way of addressing the cloud solar absorption issue.
They have also looked for evidence of large absorption induced by the presence of clouds in another way, namely through their work on the surface radiation budget with pyranometer and satellite observations (Gautier et al., 1980, Gautier and Landsfeld, 1994). This is an indirect approach, in which they conclude that a discrepancy in R values from 1 to 1.5 would translate into a rather large absorption difference. For a 100% cloud cover they find that, on the contrary, their model (which is extremely close to a Delta-Eddington model in most important respects, including cloud absorption) provides them with lower values of surface insolation than measured by pyranometers. They attribute this discrepancy to satellite calibration uncertainties and poorly represented cloud bidirectional reflectance effects. They conclude, therefore, that their surface and satellite measurements neither support nor refute the hypothesis that there is anomalous solar radiation absorption by clouds.
Finally, the work performed in collaboration with W. O'Hirok, and reported later in this report, suggests that 3-D cloud effects might, in part, be responsible for the large absorption detected in the data analyzed by Cess et al.
References
Cess et al., 1995: Absorption of Solar Radiation by Clouds: Observations Versus Models, Science, 267, 496-499.
Gautier et al., 1980: A simple physical model to estimate incident solar radiation at the surface from GOES satellite data, with G. Diak and S. Masse, Journal of Applied Meteorology, 19, 1005- 1012.
Gautier and Landsfeld, 1994: Surface Radiation Budget during the first year of the Atmospheric Radiation Measurement (ARM) Program: To be submitted to J. Atmos. Sci.
Harrison et al., 1992: Seasonal variation of cloud radiative forcing derived from the Earth Radiation Budget Experiment (ERBE). J. Geophys. Res., 95, 18,687-18,703.
Stephens and Tsay, 1990: On the cloud absorption anomaly. Quart. J. Roy. Meteor. Soc., 116, 671-704.