RADIATION EXCHANGE BETWEEN STRATUS CLOUDS AND POLAR MARINE SURFACES

The radiative energy exchange between arctic sea-ice and stratiform cl ouds is studied by means of aircraft measurements and a two-stream rad iation transfer model. The data have been obtained by flights of two i dentically instrumented aircraft during the Radiation and Eddy Flux Ex periments REFLEX I in autumn 1991 and REFLEX II in winter 1993 over th e arctic marginal ice zone of Fram Strait. The instrumental equipment comprised Eppley pyranometers and pyrgeometers, which measure the sola r and terrestrial upwelling and downwelling hemispheric radiation flux densities, and a line-scan-camera on one aircraft to monitor the surf ace structure of the sea-ice. An empirical parametrization of the albe do of partly ice-covered ocean surfaces is obtained from the data, whi ch describes the albedo increasing linearly with the concentration of the snow-covered sea-ice and with the cosine of the sun zenith angle a t sun elevations below 10 degrees. Cloud optical parameters, such as s ingle scattering albedo, asymmetry factor and shortwave and longwave h eight-dependent extinction coefficient are determined by adjusting mod eled radiation flux densities to observations. We found significant in fluence of the multiple reflection of shortwave radiation between the ice surface and the cloud base on the radiation regime. Consistent wit h the data, a radiation transfer model shows that stratus clouds of 40 0 m thickness with common cloud parameters may double the global radia tion at the surface of sea-ice compared to open water values. The tota l cloud-surface-albedo under these circumstances is 30% larger over se a-ice than over water. Parametrizations of the global and reflected ra diation above and below stratus clouds are proposed on the basis of th e measurements and modeling. The upwelling and downwelling longwave em ission of stratus clouds with thicknesses of more than 500 m can be sa tisfactorily estimated by Stefan's law with an emissivity of nearly 1 and when the maximum air temperature within the cloud is used.