Retrieval of Cloud Parameters by Multiple High Spectral Resolution Observations in the Near-Infrared Under Conditions of Varying Solar Illumination

Abstract

The near-infrared region of the upwelling earth’s spectrum is important because of the combination of thermal and solar radiation. Clouds modulate both the incoming solar radiation by scattering, and the outgoing thermal radiation by emission. Unfortunately, this combination of thermal and solar radiation has made it difficult to use observations in this region, and numerous investigators have avoided using daytime 3.7 μm observations (e.g. Strong and McClain, 1984, d’Entremont, 1986, Saunders and Kriebel, 1989). Clouds have the effect of cooling the atmosphere above their tops, and warming of the atmosphere below their bases (c.f. Liou, 1980). Since stratus and stratocumulus (S,SC) are generally low in the atmosphere, their net effect is to cool the atmosphere as a whole. The annual frequency of occurrence of S,SC over the world’s oceans is about 45%, while the annual S,SC cloud amount is about 35% (Warren, et al., 1988). The corresponding figures over land are about 25% and 18% (Warren et al., 1986). These low level water clouds have an effect on both the thermal and solar radiation due to a significant departure from unity of their effective emissivity (Hunt, 1973, Twomey and Cocks, 1982). This is because the cloud droplet size approaches the wavelength of the interacting light. For example, Hunt (1973) computes the emissivity at 3.7 gm for optically thick water clouds to be 0.74 at a mode radius of 4 μm, but 0.90 at a mode radius of 10 μm. In comparison, the emissivity at 11μm was computed to be 0.997 and 0.998 for the respective mode radii.