Spatiotemporal evolution of melt ponds on Arctic sea ice

Abstract

Melt ponds on sea ice play an important role in the Arctic climate system. Their presence alters the partitioning of solar radiation: decreasing reflection, increasing absorption and transmission to the ice and ocean, and enhancing melt. The spatiotemporal properties of melt ponds thus modify ice albedo feedbacks and the mass balance of Arctic sea ice. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition presented a valuable opportunity to investigate the seasonal evolution of melt ponds through a rich array of atmosphere-ice-ocean measurements across spatial and temporal scales. In this study, we characterize the seasonal behavior and variability in the snow, surface scattering layer, and melt ponds from spring melt to autumn freeze-up using in situ surveys and auxiliary observations. We compare the results to satellite retrievals and output from two models: the Community Earth System Model (CESM2) and the Marginal Ice Zone Modeling and Assimilation System (MIZMAS). During the melt season, the maximum pond coverage and depth were 21% and 22 ± 13 cm, respectively, with distribution and depth corresponding to surface roughness and ice thickness. Compared to observations, both models overestimate melt pond coverage in summer, with maximum values of approximately 41% (MIZMAS) and 51% (CESM2). This overestimation has important implications for accurately simulating albedo feedbacks. During the observed freeze-up, weather events, including rain on snow, caused high-frequency variability in snow depth, while pond coverage and depth remained relatively constant until continuous freezing ensued. Both models accurately simulate the abrupt cessation of melt ponds during freeze-up, but the dates of freeze-up differ. MIZMAS accurately simulates the observed date of freeze-up, while CESM2 simulates freeze-up one-to-two weeks earlier. This work demonstrates areas that warrant future observation-model synthesis for improving the representation of sea-ice processes and properties, which can aid accurate simulations of albedo feedbacks in a warming climate.

Quantifying false bottoms and under-ice meltwater layers beneath Arctic summer sea ice with fine-scale observations

“…While both pond growth and drainage can be observed over most areas of the floe, the images show that pond behavior is heterogeneous and the timing and magnitude of meltwater flux varies substantially around the floe Art. 10(1) page 10 of 19 Smith et al: False bottoms and under-ice meltwater layers beneath Arctic summer sea ice (Webster et al, 2022). The initial formation of false bottoms coincides with observations of surface melt pond drainage; the first recorded observations of false bottoms on the MOSAiC floe were at the FYI coring site on July 6, when there was visible reduction in the extent of large ponds nearby (light blue outline in Figure 9).…”

Arctic sea ice albedo: Spectral composition, spatial heterogeneity, and temporal evolution observed during the MOSAiC drift

“…Surface melt ponds appeared in the imagery beginning May 28, in accord with widespread ponding in satellite imagery (Cavalieri et al, 1996, updated yearly). (Webster et al, 2022). Although snowfall and freezing conditions followed for a few days, more widespread ponding at L2 resumed on June 9, and remained approximately continuous for the remainder of the melt season.…”

“…Digital photos were taken at nearly all albedo measurement locations on each sampling date (Smith et al, 2021c). Routine total (snow and ice) thickness measurements were made parallel to Lemon Drop (LD), Root Beer Barrel (RBB), and Kinder lines offset by approximately 1-2 m using a non-destructive electromagnetic induction sounding detector (GEM-2; Hunkeler, 2016;Hunkeler et al, 2016), coincident with snow and pond depth surveys using a Magnaprobe depth detector (Sturm and Holmgren, 2018;Itkin et al, 2021;Webster et al, 2022). While these transects were not precisely colocated with the albedo measurements (to avoid disturbing the surface of the albedo sensor footprint), they are useful for developing an understanding of the surface state and ice thickness distribution and evolution.…”

“…At the beginning of the melt season, liquid water collected at the ice surface, beneath the snow, forming areas with "subnivean" ponds (Webster et al, 2022). Areas where no ponds form developed a nascent SSL.…”

“…Despite the overall trend in decreasing a λ , an increase in a λ is notable at visible wavelengths between 90 and 120 m from June 24 to July 13 (Figure 9a and b). This increase occurred as the result of a pond drainage event between July 11 and 13 (see Webster et al, 2022) that reduced the areal coverage of ponded ice. Subsequent increases in the depth and extent of ponds along with the decreased thickness of pond floors between July 13 and 29 resulted in an overall decrease in spectral albedo, which was most notable at visible wavelengths.…”