The Effects of Clouds on the Diurnal Variation of Underwater Irradiances on Horizontal Surfaces

Abstract

Measurements of underwater downwelling D, underwater upwelling U, both at 5 m, and surface down-welling irradiance I were taken over most of a 5-day period in August and September 1974 south and west of Bermuda. On clear days D/I reached a pronounced maximum at local noon, whereas U/I had a weak minimum at midday. On cloudy days both of the above ratios were larger at all times of the day and did not exhibit the midday maximum. An absorption model for D/I is constructed by decomposing I into components from the sun, clear sky and clouds. The major differences between the components is their spectral and radiance distributions. Atmospheric water vapor and sea surface roughness effects are included. The model agrees with the experimental values of D/I to within 5% of I for A the data, and it reproduces the variation of this ratio with solar zenith angle and cloud cover.