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FIGURE 14-49 Absorption of light from an overhead sun by water associated with a 1-km stratus cloud with its top at an altitude of 2 km. The solid line is the absorption due to liquid water, the dashed line water vapor inside the cloud, and the dotted line water vapor in a column in the atmosphere (adapted from Davies et al., 1984).

significantly as well (Fritz, 1951). This has been dubbed the "cloud absorption anomaly" [see reviews by Stephens and Tsay (1990), Liou (1992), and Ramanathan and Vogelmann (1997)]. Although this area might not be considered to fall in the realm of "atmosphere chemistry" per se, it is clearly potentially very important in the relationship between clouds and global climate.

Figure 14.49 shows the absorption of light from an overhead sun by liquid cloud droplets, water vapor inside the cloud, and water vapor in a column in the atmosphere for a f-km stratus cloud whose top is 2 km above the ground (Davies et al., 1984; see also Goldstein and Penner, 1964). There are small amounts of absorption in the tail end of the red region of the visible attributed to water vapor in and outside the cloud. The absorption increases into the near-IR (the region from -780 to 2500 nm or f2,800-4000 cm"1) and mid-IR (2.5-50 yu,m or 4000-200 cm"1) where liquid water in the cloud absorbs (e.g., see Evans and Puckrin, 1996).

Several different approaches have been taken to investigate whether there is more absorption of visible light by clouds than expected based on current models of radiative transfer in the atmosphere. Some of these approaches and results are discussed in Box 14.3.

While there thus appears to be evidence for apparent excess absorption of solar radiation by clouds, there is substantial controversy over whether this is indeed true absorption or whether there is some other explanation for the discrepancies (e.g., see Stephens and

Tsay, f990; Imre et al., 1996; Stephens, 1996; Cess and Zhang, 1996; Pilewskie and Valero, 1996; and Ramanathan and Vogelmann, 1997). For example, Li et al. (1995) also analyzed solar flux surface and satellite data over a 4-year period to obtain values of the ratio of shortwave cloud forcing at the surface to that at the top of the atmosphere. These varied from about 1.4 in the tropics, in agreement with Cess et al. (1995), to values less than 1 in polar regions. They concluded that, within the uncertainties, their analysis does not provide support for excess cloud absorption of solar radiation (Li et al., 1995; Li and Moreau, 1996), although Zhang et al. (1997) suggest there may have been some unrecognized complexities in the analysis of the satellite data.

Similarly, Chou et al. (1998) used measurements of surface radiative fluxes and satellite radiance data in the Pacific warm pool region to conclude that the effect of clouds was similar to that expected, i.e., that the excess absorption, if it exists, is small.

Based on aircraft measurements, Francis et al. (f997) suggested that the excess cloud absorption was insignificant. In comparing satellite and ground-based observations of solar flux to model predictions, Arking (1996) also found no evidence for significant cloud excess absorption, although he reported a discrepancy between models and measurements that is not found over the central equatorial Pacific Ocean under clear skies (Conant et al., 1997). Imre et al. (1996) used collocated satellite and surface observations of short-wavelength fluxes at a site in Oklahoma to probe for the contribution of enhanced absorption by clouds, but found none. They suggested that uncertainties and biases in the analyses, particularly in the clear-sky references used, can give rise to apparent excess cloud absorption that is an artifact of the analysis.

In short, whether excess absorption of solar radiation by clouds even occurs and why there are discrepancies between models and measurements remain controversial. For example, absorption into the visible region can be enhanced by the presence of strongly absorbing species such as soot, either in the cloud droplets themselves or as aerosol particles suspended between the cloud droplets, i.e., as interstitial aerosol particles (e.g., see Stephens and Tsay, 1990; Chylek and Hallett, 1992; and Mel'nikova and Mikhaylov, 1994). However, as discussed shortly, the enhanced absorption of solar radiation by clouds has been reported in many locations globally, including in remote regions, and hence appears less likely to be so directly associated with anthropogenic emissions. It has also been suggested that measurements of cloud drop size distributions have missed the presence of larger "drizzle drops"

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FIGURE 14-49 Absorption of light from an overhead sun by water associated with a 1-km stratus cloud with its top at an altitude of 2 km. The solid line is the absorption due to liquid water, the dashed line water vapor inside the cloud, and the dotted line water vapor in a column in the atmosphere (adapted from Davies et al., 1984).

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