Detection of clear skies using total and diffuse shortwave irradiance: Calculations of shorwave cloud radiaitve forcing and clear-sky diffuse ratio

The effect of clouds on the shortwave (SW) irradiance near the surface is of interest for surface radiative energy budget studies (Long et al. 1994) and investigation of the recently suggested excess SW cloud absorption (Cess et al. 1995; Ramanathan et al. 1995; Pilewski and Valero 1995). One measure of the effect of clouds is cloud forcing: the difference between clear (i.e., cloudless) sky irradiance and measured irradiance. One way of estimating the surface clear sky irradiance is to calculate it using radiative transfer models, which require aerosol, temperature, pressure, and humidity profiles as input, as well as surface albedo. This information is not readily available in many locations where cloud forcing estimates are desirable. Clear sky irradiance can also be estimated by empirical fit to clear sky irradiance measurements (for examples of thi