The color of melt ponds on Arctic sea ice

Abstract

Abstract.Pond color, which creates the visual appearance of melt ponds on Arctic sea ice in summer, is quantitatively investigated in this study. A two-stream radiative transfer model is used for ponded sea ice: the upwelling irradiance from the pond surface is determined, and then the upwelling spectrum is transformed into the RGB color space through a 10 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 components of pond color, whereas the red component 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 F 0 with wavelength affects the pond color rather than its level. The pond color changes from dark blue to brighter blue with 15 increasing scattering in ice, but the influence of absorption in ice on pond color is limited. The pond color reproduced by the model agrees well with field observations on Arctic sea ice in summer, which supports the validity of this study. More importantly, 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 . The results show that retrievals of H i for thin ice agree better with field measurements than retrievals for thick ice, but that retrievals of H p are not good. Color has been shown to be a new potential 20 method to obtain ice thickness information, especially for melting sea ice in summer, although more validation data and improvements to the radiative transfer model will be needed in future.

Impact of a Surface Ice Lid on the Optical Properties of Melt Ponds

“…The mean incident solar irradiance F 0 ( λ ) under overcast sky conditions at noon in August, where the solar disk was not visible (Grenfell & Perovich, ), is employed to represent Arctic summer. A sensitivity study on F 0 is ignored here because model simulations showed that variations in melt‐pond albedo and transmittance due to different values of F 0 are less than 3%, similar with the results in Lu et al () and Lu, Leppäranta, et al (). However, the spectral distribution rather than the absolute value of F 0 ( λ ) is needed when calculating the AOPs.…”

“…Pond color, as sensed by human eyes, is another visual characterization of melt ponds in addition to their albedo. A colorimetric method to determine the color of a melt pond has been developed by Lu, Leppäranta, et al (), which transforms the upwelling spectral irradiance from the pond surface into red, green, and blue intensities in the RGB color space using color matching functions (Hunt, ), so that the melt‐pond color can be quantitatively evaluated. In this part of the analysis, the thickness and scattering coefficient of the ice lid are altered to examine the RGB color of refreezing melt ponds.…”

“…More validations are desired, but formal investigations on melt‐pond color are rare. A detailed comparison between simulated and observed melt‐pond color refers to Lu, Leppäranta, et al (), where quantitative measurements on the color of melt ponds covered with a newly formed ice layer (1–3 cm) by Istomina et al () were employed. Such thin ice lids pose negligible impacts on the melt‐pond albedo (Malinka et al, ) as well as on the pond color, so the comparison was not repeated here with illustrations.…”

“…Such thin ice lids pose negligible impacts on the melt‐pond albedo (Malinka et al, ) as well as on the pond color, so the comparison was not repeated here with illustrations. A high correlation coefficient (0.744) with a significance level less than 0.01 argued for the feasibility of the RTM and the method of melt‐pond color estimation (Lu, Leppäranta, et al, ).…”

“…Our focus was first on the model construction (Lu et al, ) and then on a better understanding of how solar radiation is partitioned in melting sea ice (Lu, Cheng, et al, ). Thereafter, we looked for any dependence of melt‐pond color on ice thickness (Lu, Leppäranta, et al, ).…”