Surface characteristics and atmospheric footprint of springtime Arctic leads at SHEBA

[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 lead-perpendicular 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.