Influence of sea ice lead-width distribution on turbulent heat transfer between the ocean and the atmosphere

Abstract

Abstract. Leads are linear-like structures of open water within the sea ice cover that develop as the result of fracturing due to divergence or shear. Through leads, air and water come into contact and directly exchange latent and sensible heat through convective processes driven by the large temperature and moisture differences between them. In the central Arctic, leads only cover 1 to 2 % of the ocean during winter, but account for more than 70 % of the upward heat fluxes. Furthermore, narrow leads (several meters) are more than twice as efficient at transmitting turbulent heat than larger ones (several hundreds of meters). We show that lead widths are power law distributed, P (X) ∼ X −a with a > 1, down to very small spatial scales (20 m or below). This implies that the open water fraction is by far dominated by very small leads. Using two classical formulations, which provide first order turbulence closure for the fetch-dependence of heat fluxes, we find that the mean heat fluxes (sensible and latent) over open water are up to 55 % larger when considering the lead-width distribution obtained from a SPOT satellite image of the ice cover, compared to the situation where the open water fraction constitutes one unique large lead and the rest of the area is covered by ice, as it is usually considered in climate models at the grid scale. This difference may be even larger if we assume that the power law scaling of lead widths extends down to smaller (∼1 m) scales. Such estimations may be a first step towards a subgrid scale parameterization of the spatial distribution of open water for heat fluxes calculations in ocean/sea ice coupled models.

Inter-comparison and evaluation of sea ice algorithms: towards further identification of challenges and optimal approach using passive microwave observations

“…Variability due to actual ice concentration changes in the order of less than 3 % is below the noise floor of the algorithms. Heat and moisture fluxes between the surface (ocean or ice) and the atmosphere are sensitive to small variations in the near-100 % ice cover (Marcq and Weiss, 2012). This unresolved SIC variability can thus be of significant importance for sea ice models (and consequently coupled climate models) when assimilating these data without proper handling of the uncertainties.…”

Arctic surface temperatures from Metop AVHRR compared to in situ ocean and land data

“…The drifting of Arctic sea ice constantly causes opening and closing of the sea ice cover and changes in ice cover of only a few percent can influence the heat flux between ocean and atmosphere drastically (Maykut, 1978;Marcq and Weiss, 2012). For a model to produce realistic initial surface temperature boundary fields, detailed information of ice concentration and ice drift is needed.…”

Analysis of image vs. position, scale and direction reveals pattern texture anisotropy

“…They differ in nature and origin from the so-called "polynya" which are open water areas inside the ice pack resulting from warm water upwelling or wind-induced drift away from a fixed boundary; there is no polynya in Figure 1A. This albedo difference can be used to convert this grayscale image to a binary image of only fractures or ice, as done in Weiss and Marsan [7] and Marcq and Weiss [8].…”

“…An example is the Gaussian F(x) = exp(−x 2 /σ )/(2πσ ). To achieve the link with the inertia tensor, consider the scalar quantity I and take its transform according to Equation (8). By definition, it has the interesting property:…”