The color of melt ponds on Arctic sea ice

Abstract. Pond color, which creates the visual appearance of melt ponds on Arctic sea ice in summer, is quantitatively investigated using a two-stream radiative transfer model for ponded sea ice. The upwelling irradiance from the pond surface is determined and then its spectrum is transformed into RGB (red, green, blue) color space using a colorimetric method. The dependence of pond color on various factors such as water and ice properties and incident solar radiation is investigated. The results reveal that increasing underlying ice thickness H_i enhances both the green and blue intensities of pond color, whereas the red intensity is mostly sensitive to H_i for thin ice (H_i  <  1.5 m) and to pond depth H_p for thick ice (H_i  >  1.5 m), similar to the behavior of melt-pond albedo. The distribution of the incident solar spectrum F₀ with wavelength affects the pond color rather than its intensity. The pond color changes from dark blue to brighter blue with increasing scattering in ice, and the influence of absorption in ice on pond color is limited. The pond color reproduced by the model agrees with field observations for Arctic sea ice in summer, which supports the validity of this study. More importantly, the pond color has been confirmed to contain information about meltwater and underlying ice, and therefore it can be used as an index to retrieve H_i and H_p. Retrievals of H_i for thin ice (H_i  <  1 m) agree better with field measurements than retrievals for thick ice, but those of H_p are not good. The analysis of pond color is a new potential method to obtain thin ice thickness in summer, although more validation data and improvements to the radiative transfer model will be needed in future.