Abstract. We present a new model to compute turbulent surface heat and momentum fluxes over leads in the Arctic sea ice. The momentum roughness length uses a sea state parameterization which is fully consistent with the surface turbulent flux parameterization. The flux parameterization accounts for the fetch limitation of the airflow over a lead. The surface roughness length for heat is determined from an application of surface renewal theory to the air-sea interface. The modeled fluxes are compared with in situ observations of lead fluxes. We also compare our model results with a bulk flux algorithm which has been commonly used to evaluate surface heat fluxes from leads. We perform sensitivity studies to examine the role of the surface renewal timescale, the importance of the cool skin, and impact of wave age dependence on the momentum roughness length. We have computed integral heat fluxes as a function of lead width/fetch for various atmospheric states to determine the magnitude of heat flux in a mesoscale model grid in which a lead is present.

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Leads provide a significant source of heat and moisture to the Arctic winter atmosphere, and plumes from wide leads have been observed to penetrate the Arctic inversion. We have developed a two‐dimensional, high‐resolution, deep anelastic numerical model to investigate the atmospheric convection from leads with widths ranging from 100 m to 10 km. A second‐order turbulence closure scheme is used to parameterize the atmospheric turbulence in the horizontally inhomogeneous system. This study describes how the lead‐induced circulations can enhance the vertical transport of heat into the atmospheric boundary layer. This model is compared with large‐eddy simulation results and with lidar observations of a lead‐induced ice crystal plume. The model is used to study the effect of varying lead widths and ambient atmospheric conditions on the resultant convection from leads, and some preliminary results are described.

[1] Basin-scale sea ice models are often run uncoupled to either an atmosphere or ocean model to evaluate the sea ice model, to compare different models, and to test changes in physical parameterizations. Such simulations require that the boundary forcing be specified. The specification of atmospheric forcing associated with the surface heat and freshwater fluxes has been done in various sea ice simulations using climatology, numerical weather prediction analyses, or and satellite data. However, the errors in the boundary forcing may be so large that it is difficult to determine whether discrepancies between simulated and observed properties of sea ice should be attributed to deficiencies in the sea ice model or to the boundary forcing. To assess the errors in boundary forcing, we use data from the Surface Heat Budget of the Arctic Ocean (SHEBA) to evaluate various data sets that have been used to provide boundary forcing for sea ice models that are associated with the surface heat and freshwater fluxes. The impact of errors in these data sets on a sea ice model is assessed by using a single-column ice thickness distribution model, which is alternately forced with in situ measurements from SHEBA and output from large-scale analyses. Substantial discrepancies are found among the data sets. The response of the sea ice model to the different forcing data sets was considerable.

[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.