THE TEMPERATURE-DEPENDENCE OF FROST FLOWER GROWTH ON LABORATORY SEA-ICE AND THE EFFECT OF THE FLOWERS ON INFRARED OBSERVATIONS OF THE SURFACE

This paper describes a laboratory study of frost flower growth on youn g sea ice at different temperatures and the effect of these flowers on the surface temperature observed with an infrared radiometer. The flo wers grew on sea ice which formed in a salt water tank at room tempera tures of -20, -24, and -30 degrees C, with an additional experiment at -16 degrees C, where no flowers appeared. The growth habit and height of the observed crystals depended on the existence of a region of sup ersaturated vapor adjacent to the surface and on the range of temperat ures in the surface boundary layer. The source of the surface brine fr om which the flowers grew was probably brine transport within the ice toward the cold upper surface driven by the thermomolecular pressure g radient. The evaporation of vapor from this liquid into the atmospheri c boundary layer provided the supersaturated region adjacent to the ic e surface. Two kinds of flowers were observed; at -20 and -24 degrees C, dendritic crystals grew approximately between the -12 and -16 degre es C isotherms, and at -30 degrees C, rod-like flowers appeared betwee n -16 and -25 degrees C. These limits correspond to earlier work on cr ystal growth from the vapor. In each case, the maximum flower height a pproximately equaled the height of the isotherm corresponding to the c older temperature limit for each crystal type, -16 degrees C for the d endrites and -25 degrees C for the rods. The effect of the flowers on the radiometer surface temperature was as follows: because the flowers protrude 10-20 mm above the surface into the boundary layer, the infr ared temperature of the flower-covered ice was about 4-6 degrees C col der than that of the same ice cleared of flowers. We also found that t he insulating effect of the flowers caused the ice surface temperature beneath the flowers to be 1-2 degrees C warmer than the surrounding b are ice. The importance of the flower growth is that infrared satellit e observations of thin ice in winter will be colder than the actual su rface temperature, which may account for the absence of warm young ice in infrared satellite images.