Abstract
A major source of uncertainty in both climate projections and seasonal forecasting of sea ice is inadequate representation of surface–atmosphere exchange processes. The observations needed to improve understanding and reduce uncertainty in surface exchange parameterizations are challenging to make and rare. Here we present a large dataset of ship-based measurements of surface momentum exchange (surface drag) in the vicinity of sea ice from the Arctic Clouds in Summer Experiment (ACSE) in July–October 2014, and the Arctic Ocean 2016 experiment (AO2016) in August–September 2016. The combined dataset provides an extensive record of momentum flux over a wide range of surface conditions spanning the late summer melt and early autumn freeze-up periods, and a wide range of atmospheric stabilities. Surface exchange coefficients are estimated from in situ eddy covariance measurements. The local sea-ice fraction is determined via automated processing of imagery from ship-mounted cameras. The surface drag coefficient, CD10n, peaks at local ice fractions of 0.6–0.8, consistent with both recent aircraft-based observations and theory. Two state-of-the-art parameterizations have been tuned to our observations with both providing excellent fits to the measurements.
Highlights
Surface temperatures are rising at a rate more than twice the planetary average, a process known as Arctic Amplification (Serreze and Barry, 2011; Cohen et al, 2014; Stuecker et al, 2018; Dai et al, 2019)
A major source of uncertainty in models is the representation of turbulence-driven surface exchanges (Bourassa et al, 2013; Vihma et al, 2014; Tsamados et al, 2014; LeMone et al, 2018)
We investigate the relationship between surface drag and seaice concentration within the existing framework suggested by L2012, E2016, and its recent extension by Lüpkes and Gryanik (2015; L2015) using over 500 new estimates of surface drag and local sea-ice concentration measurements derived from on-board imagery, over varying sea-ice conditions and a range of near-surface atmospheric stabilities
Summary
Elvidge et al (2016; E2016) used aircraft measurements over the Arctic MIZ to develop a data set of 195 independent estimates of CD10n over the MIZ, more than doubling the number of observations previously available Their observations were consistent with the theory of L2012; they found a large variation in CD10ni (CD10n for 100% ice cover) demonstrating that this depends strongly on ice morphology – as found by Castellani et al (2014) who applied bulk parameterizations to ice morphology data based on laser altimetry. CD10nw and CD10ni are, respectively, the neutral transfer coefficients for momentum over water and ice surfaces, and A is the fraction of the surface covered by ice. Over sea ice, an additional drag contribution, the form drag, CD10nf, is generated due to air-flow pressure against the edges of floes, leads, and melt ponds (Andreas et al, 2010; L2012; L2015; E2016).
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