On the Morphology of Cloud Absorption: Preferential Attenuation of Near Infrared Solar Radiation

Ground based measurements of solar flux based on visible and near infrared pyranometers have yielded important information concerning the morphology of the anomalous cloud absorption. Comparisons of this flux for clear and overcast skies from April to August, 1996, support mounting experimental evidence that some clouds preferentially attenuate the near infrared (NIR) region of the solar spectrum. Under cloudy conditions, for the same value of visible flux reaching the ground, the NIR flux can be as much as 60 W/m2 less than under clear conditions. Histograms of the ten day average distribution of the percentage of NIR radiation to total solar radiation show a peak at 41% for the clear sky plot, and a much lower peak, at 25%, for the overcast sky plot. The cloud types most frequently associated with this preferential NIR absorption are altocumulus, stratocumulus and especially stratus fractus.