Abstract
Abstract. Sea ice leads are an important feature in pack ice in the Arctic. Even covered by thin ice, leads can still serve as prime windows for heat exchange between the atmosphere and the ocean, especially in the winter. Lead geometry and distribution in the Arctic have been studied using optical and microwave remote sensing data, but turbulent heat flux over leads has only been measured on-site during a few special expeditions. In this study, we derive turbulent heat flux through leads at different scales using a combination of surface temperature and lead distribution from remote sensing images and meteorological parameters from a reanalysis dataset. First, ice surface temperature (IST) was calculated from Landsat-8 Thermal Infrared Sensor (TIRS) and Moderate Resolution Imaging Spectroradiometer (MODIS) thermal images using a split-window algorithm; then, lead pixels were segmented from colder ice. Heat flux over leads was estimated using two empirical models: bulk aerodynamic formulae and a fetch-limited model with lead width from Landsat-8. Results show that even though the lead area from MODIS is a little larger, the length of leads is underestimated by 72.9 % in MODIS data compared to TIRS data due to the inability to resolve small leads. Heat flux estimated from Landsat-8 TIRS data using bulk formulae is 56.70 % larger than that from MODIS data. When the fetch-limited model was applied, turbulent heat flux calculated from TIRS data is 32.34 % higher than that from bulk formulae. In both cases, small leads accounted for more than a quarter of total heat flux over leads, mainly due to the large area, though the heat flux estimated using the fetch-limited model is 41.39 % larger. A greater contribution from small leads can be expected with larger air–ocean temperature differences and stronger winds.
Highlights
Leads are linear structures of the ocean surface within pack ice that are exposed to the atmosphere during an opening event caused by various forces, such as wind and water stresses
The temperature signature of small leads in the northern part of the image area is largely reduced in the Moderate Resolution Imaging Spectroradiometer (MODIS) ice surface temperature (IST) map, due to its coarse resolution and heterogeneous pixels, compared to that from Thermal Infrared Sensor (TIRS)
The correlation of IST from two sensors estimated by interpolating MODIS IST to the TIRS scale (30 m) is quite good, with a Pearson coefficient of approximately 0.9 (0.902 and 0.896 before and after correction for stray light, respectively)
Summary
Leads are linear structures of the ocean surface within pack ice that are exposed to the atmosphere during an opening event caused by various forces, such as wind and water stresses. Thin ice forms quickly in newly opened leads due to the large temperature difference between the ocean and the atmosphere (Kwok, 2001). Opened leads are the main source of ice production, brine rejection, and heat transfer from the ocean to the atmosphere (Alam and Curry, 1998). Turbulent heat flux over open water could be 2 orders of magnitude larger than that through mature ice (Maykut, 1978). In the central Arctic, open water usually comprises no more than 1 % of the ice pack area during the winter. A model study shows that an increased lead fraction by 1 % can lead to local air temperature warming up to 3.5 K in winter (Lüpkes et al, 2008)
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