Some Investigations Of The Cloud Absorption Anomaly

One approach to the cloud absorption anomaly has been to examine the energy budget of the so-called "warm pool" area in the western Pacific Ocean, which is the region from approximately 140°E to 170°E and 10°N to 10°S (Ramanathan et al., 1995). This area has relatively high annual mean sea-surface temperatures (SST), up to 302.5 K, resulting in a humid and cloudy atmosphere with which is associated frequent deep convection. The annual mean heat transport (D) both horizontally and vertically out of the ocean surface mixed layer is known to be small, D < 20 W m~2. To give a constant temperature of the mixed layer, this small heat transport, D, out of the layer must be balanced by the net energy flux (H) at the ocean surface. As shown in Fig. f4.50, H is a balance between positive contributions from incoming solar radiation, S, and negative contributions from net outgoing thermal radiation, F (i.e., up minus down), and evaporative (£) and turbulent sensible (h) heat fluxes, i.e., H = S - F - (E + h). Using available data to estimate values for all of the energy terms except S leads to a calculated value for the solar input of 175 W nT2. However, the solar radiation input under clear skies is well known to be 275 W m~2. If the total solar input is treated as the sum of incoming solar radiation and a contribution due to clouds, then the latter must be -100 W nT2.

The outgoing solar radiation at the top of the atmosphere in this location based on 5 years of data from the Earth Radiation Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS) is estimated to be — 66 W m~2 (the negative sign indicating cooling by reflection of solar radiation by clouds). The ratio of the shortwave cloud energy flux at the surface to that at the top of the atmosphere must be 100/66 = 1.5, representing a long-term average for this effect.

A similar conclusion is reached using direct measurements of solar fluxes at the top of the atmosphere (TOA) and at surface sites under clear compared to cloudy conditions (e.g., Cess et al., 1995, 1996b; Evans et al., 1995). Figure 14.5fa shows the absorptance, defined as the fraction of the down-

Clear sky solar radiation

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