Cloud color and ocean radiant heating

It is well recognized that clouds regulate the flux of solar radiation reac hing the sea surface. Clouds also affect the spectral distribution of incid ent irradiance. Observations of spectral and total incident solar irradianc e made from the western equatorial Pacific Ocean are used to investigate th e "color" of clouds and to evaluate its role in upper-ocean radiant heating . Under a cloudy sky, values of the near-ultraviolet to green spectral irra diance are a significantly larger fraction of their clear-sky flux than are corresponding clear-sky fractions calculated for the total solar flux. For example, when the total solar flux is reduced by clouds to one-half of tha t for a clear sky, the near-ultraviolet spectral flux is only reduced simil ar to 35% from its clear-sky value. An empirical parameterization of the sp ectral cloud index is developed from field observations and is verified usi ng a plane-parallel, cloudy-sky radiative transfer model. The implications of cloud color on the determination of ocean radiant heating rates and sola r radiation transmission are assessed using both model results and field de terminations. The radiant heating rate of the upper 10 cm of the ocean (nor malized to the climatological incident solar flux) may be reduced by a fact or of 2 in the presence of clouds. This occurs because the near-infrared wa velengths of solar radiation, which are preferentially attenuated by clouds , are absorbed within the upper 10 cm or so of the ocean while the near-ult raviolet and blue spectral bands propagate farther within the water column. The transmission of the solar radiative flux to depth is found to increase under a cloudy sky. The results of this study strongly indicate that cloud s must be included in the specification of ocean radiant heating rates for air-sea interaction studies.