The spectral effects of clouds on solar irradiance

Knowledge of the spectral attenuation associated with clouds is important f or accurate estimates of natural irradiance at the Earth's surface. We comp are spectral measurements of visible downwelling irradiance, under varying sky conditions at Halifax, Nova Scotia, Canada, with results from a clear-s ky model. The spectral effect of clouds is estimated by taking the ratio of the measurements to the modeled irradiances and removing-spectrally consis tent instrumental effects and errors in the model. Empirical relationships derived between the spectral cloud effect and both CF, the cloud factor (th e ratio of measured to modeled irradiances at 490 nm), and f, the fraction of sky covered by cloud, were found to follow a wavelength (lambda) depende nce of the form a(CF or f) + b(CF or f)(lambda/490)(-4) in the 412-700 nm w avelength range. Both this relationship and a previously published linear r elationship were found to be inadequate for describing cloudy irradiance da ta from the Bering Sea, indicating that the spectral effect of clouds can v ary with cloud type and location. We show here that the spectral cloud effe ct can be mimicked by using a clear-sky model and changing the magnitude of the sky reflectivity or the spectral shape and magnitude of the ground alb edo within the model. An investigation of the effects of cloud-dependent ch anges in irradiance spectra on calculations of bio-optical properties is al so presented. Estimates of chlorophyll concentration from near-surface radi ances are found to vary by up to 30%, whereas the effects on estimates of p hotosynthetically available and usable radiation at the sea surface are neg ligible.