Abstract
During the Arctic summer season, snow atop the sea ice melts and pools
into low-lying areas on the surface. These melt ponds reduce surface
albedo and increase solar absorption in the Arctic Ocean. Throughout the
summer, melt ponds grow, drain, and connect, through a complex drainage
system. Current melt pond schemes in sea ice models, such as the
level-ice scheme in the Los Alamos Sea Ice Model (CICE), rely on a
linear relationship between pond depth and fraction to predict the
evolution of pond growth as the snow and sea ice melt. Although the
inclusion of melt ponds in models has been shown to improve forecasts of
end-of-summer sea ice extent, observations of melt pond depth and
fraction guiding these models are from SHEBA, a spatially-limited field
campaign which occurred over 20 years ago. Until recently, melt ponds
characteristics have been difficult to resolve from spaceborne platforms
due to their small size (10s - 100s m in diameter), and
indistinguishable radiometric similarity to open water. Here we show
that new, high-resolution laser altimetry measurements from ICESat-2
(IS2), combined with coincident high-resolution satellite imagery,
provides a three-dimensional view of the melting sea ice cover. IS2,
launched in September 2018, has now observed two summer melt seasons in
the Arctic. IS2 operates at 532 nm, a wavelength that penetrates low
turbidity water, and can therefore be used to capture the bathymetry of
shallow water features. Building on previous work, we demonstrate IS2’s
ability to detect and measure melt ponds on multiyear sea ice. We
validate the existence of melt ponds with high resolution (10 m) visible
imagery from the Sentinel-2 (S2) MultiSpectral Instrument. We apply the
“density dimension algorithm – bifurcate” (DDA-bifurcate), an
auto-adaptive algorithm utilizing data aggregation with the ability to
track two surfaces, as well as a second algorithm that tracks melt pond
surface and bottom, to derive melt pond depth for dozens of melt ponds
in 2019 and 2020. Applying a sea ice surface classification algorithm to
S2 imagery, we are able to determine melt pond fraction. We compare our
findings of coincident melt pond fraction and depth with the melt pond
parameterization used in the level-ice scheme in CICE. We discuss our
results in the context of the existing literature on pond depth and
volume.