Surface characteristics and atmospheric footprint of springtime Arctic leads at SHEBA

Abstract

[1] Observations of several freezing leads that occurred in spring near the Surface Heat Budget of the Arctic Ocean (SHEBA) ice station were made. The leads that formed during this study were between 3 and 400 m wide. Ice production in the leads less than 20 m wide was predominantly through congelation growth, while both frazil ice production and congelation ice growth was observed in the wider leads. The production of frazil ice and its advection downwind allowed open water to persist in the wider leads for between 5 and 24 hours, depending on the crossing angle of the wind. The surface energy budget of a wide freezing lead was estimated from observations and with a model that resolves the coupling between surface turbulent fluxes and ice growth across the lead. Both estimates of the net heat flux agreed with the increases in ice thickness observed throughout the 24-hour period, though the modeled net heat flux deficit was 50% larger. The larger net heat flux deficit obtained with the model can be attributed to the simulation of larger turbulent heat fluxes. It was found that the surface roughness length for nilas given by Guest and Davidson [1991] was too large resulting in excessive surface cooling at night. Using a smaller roughness length improved the nighttime bias but resulted in a warm bias during the day. The daytime warm bias was due, in part, to neglecting the impact of frost flowers on the surface albedo. Additional uncertainty in the treatment of solar absorption by nilas also likely contributed. The modeled ice thickness and skin temperature were also affected by the treatment of the oceanic heat flux, which acted to warm the surface. The length of time that a lead affects the atmosphere is determined by lead surface conditions, atmospheric stability, wind speed, fetch, and upwind temperature. Under leadperpendicular winds the atmospheric influence of a 400 m wide lead extended more than 2.5 km downwind. Sensible heat fluxes observed 70 m downwind of the lead were a function of across lead fetch, upwind stability, and open water fraction. The sensible heat fluxes measured at this site were elevated above background values for nearly two days despite 11.5 cm of ice growth in the lead.

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

“…This is due to the fact that there are different states of refreezing over leads. Pinto et al (2003) have studied the refreezing of leads and noticed that the surface of the openings wider than 20 m was refrozen after 5-24 h. The refreezing process has different stages and the heat fluxes over a lead decrease as newly-formed ice gets thicker. Ultimately, the heat fluxes over a refrozen lead are equal to their minimal values once the ice is covered by snow.…”

“…Note that the surface roughness parameterizations assume that the surface is open water. However, at very low air temperatures (−20 to −30 • C), open water in leads will be covered very quickly by a thin layer of ice (Pinto et al, 2003) causing a change in surface roughness. This is a source of uncertainty in this model.…”

“…There seems to be a clear distinction between the peak associated to ice and the ones associated to leads. By converting the image to a binary one, this method does not differentiate open water from frazil or snow-free ice, which may have an impact on the value of the heat fluxes (Pinto et al, 2003). Ridges due to convergence may appear dark grey as the result of shade and therefore could be assimilated to open water although they are regions of thick ice (Weiss and Marsan, 2004).…”

Sea-Ice Wintertime Lead Frequencies and Regional Characteristics in the Arctic, 2003–2015

“…As such, a narrow region of open water can cause the same temperature signal within a MODIS pixel as a broader region covered with thin ice. However, the significance of the bulk temperature anomaly can be expected to promote the ocean-atmosphere fluxes of sensible and latent heat (e.g., [40,41]) within a pixel where a lead is recognized in the presented product. This feature makes the quasi-daily lead maps a valuable source for a verification of ice-ocean models, heat-flux parametrization and an indicator for sea-ice divergence.…”

Effects of Arctic Sea Ice Decline on Weather and Climate: A Review

“…Several observational studies have addressed the effects of leads and polynyas on the Arctic atmosphere, but these studies have mostly focused on small-and mesoscale process understanding (e.g., Andreas et al 1979;Pinto et al 2003). With a climatological perspective, Serrezze et al (2009) and Screen and Simmonds (2010a, b) have shown that the recent anomalously large open water areas in September have resulted in a strong transfer of heat from the ocean mixed layer to the atmosphere, causing a large increase in air temperature.…”