A HIGH-SPECTRAL-RESOLUTION STUDY OF THE NEAR-INFRARED SOLAR FLUX DISPOSITION IN CLEAR AND OVERCAST ATMOSPHERES

The sensitivity of the near-infrared spectral atmospheric and surface fluxes to the vertical location of clouds is investigated, including a study of factors (drop-size distribution, drop optical depth, solar z enith angle, cloud geometrical thickness, atmospheric profiles) which govern this dependence. Because of the effects of the above-cloud, in- cloud and below-cloud water vapor the atmospheric absorbed flux in eac h spectral band depends critically on the cloud location, with a high cloud resulting in lesser absorption and greater reflection than a low one having the same drop optical depth. The difference between a high and a low cloud forcing of atmospheric absorption increases with drop optical depth. For any optical depth, clouds with larger drops cause a greater forcing of the spectral atmospheric absorption than those wi th smaller ones, so high clouds can even cause an increase rather than a decrease of the atmospheric absorption relative to clear skies. In contrast, the spectral and total surface fluxes are relatively insensi tive to cloud vertical location. Instead, they are determined by the d rop characteristics, notably drop optical depth. This near-invariance characteristic is attributable to the fact that most of the insolation reaching the surface is in the weak water vapor spectral absorption r egions; here drops dominate the radiative interactions and thus there is little dependence on cloud height. In addition, the overlap of the drop spectral features with the moderate-to-strong vapor absorption ba nds ensures that insolation in these regimes fails to reach the surfac e no matter where the cloud is located; instead, these bands contribut e the most to atmospheric absorption. The near-invariant behavior of t he spectral and total surface flux holds separately for a wide variety of conditions studied. As a consequence, the difference in reflection , between two columns containing clouds with the same optical depth bu t located at different altitudes, is approximately balanced in magnitu de by the difference in the atmospheric absorption; this holds for eve ry spectral interval whether it be a weak, moderate, or strong vapor/d rop absorption band. It also follows that the net fluxes at the top an d surface of overcast atmospheres do not have a general, unambiguous r elationship; this is in sharp contrast to a linear relation between th em in clear skies. However, under certain overcast conditions (e.g., s pecific vertical location of clouds and solar zenith angle), a simple linear relationship is plausible.