Influence of Lead width on the Turbulent Flow Over Sea Ice Leads: Modeling and Parametrization

Abstract

A new turbulence parametrization is developed for a non‐eddy‐resolving microscale model to study the effects of leads (elongated open‐water channels in sea ice) of different width on the polar atmospheric boundary layer (ABL). Lead‐dominated sea ice regions are characterized by large horizontal inhomogeneities of the surface temperature causing strong convection. Therefore, the new parametrization is based on a previous formulation where inhomogeneous conditions of dry convection over leads and nonlocal effects on heat fluxes had already been taken into account for a fixed lead width. A nonlocal lead width dependent approach is applied now for both heat fluxes and momentum fluxes in the convective region. Microscale model results obtained with the new, the previous nonlocal, and a local parametrization are shown, where 10 idealized cases of a lead‐perpendicular, near‐neutral ABL‐flow below a strong capping inversion are considered. Furthermore, time‐averaged large eddy simulation (LES) results of those cases are considered for analyzing the integrated effects of the dry convection on ABL characteristics. Microscale model results obtained with the new nonlocal parametrization agree well with the LES for variable lead widths and different atmospheric forcing although there is a room for further improvement. Furthermore, several features obtained with a local closure clearly disagree with LES. Thus, the microscale study also points to difficulties that might occur in mesoscale studies over regions where leads dominate the flow regime when local closures are applied.

Modelling and parametrization of the convective flow over leads in sea ice and comparison with airborne observations

“…As described, for example, in the above-mentioned studies and most recently by Michaelis et al (2020, henceforth abbreviated by M20), leads are formed mainly by divergent sea ice drift driven by ocean currents and wind (see also Smith et al, 1990). Their length ranges between hundreds of meters and hundreds of kilometres and their width between a few meters to a few kilometres (e.g., Andreas et al, 1979;Smith et al, 1990;Lindsay and Rothrock, 1995;Marcq and Weiss, 2012;Tetzlaff et al, 2015, henceforth abbreviated by T15).…”

The Impact of Lead Patterns on Mean Profiles of Wind, Temperature, and Turbulent Fluxes in the Atmospheric Boundary Layer over Sea Ice

“…With their approach, they obtained a good agreement of their model results with LES [12] and with the observations from STABLE [8]. Michaelis et al [8,12] applied horizontal grid sizes ∆ x = O(10 2 ) m in their simulations so that the entire convective plumes but not the single turbulent eddies were resolved. Thus, compared to Andreas and Cash [31], Michaelis et al [8,12] considered much wider leads (500 m to 10 km width).…”

“…Entrainment, counter-gradient heat fluxes, and the enhanced heat transport over the lead surface in general contribute to the warming and to a stabilization of the downwind ABL including the formation of a stable IBL over the downwind sea ice surface (e.g., [2,3,8]). Potential sensitivities of such local ABL modifications on the meteorological forcing or on the lead geometry were investigated in numerous studies mostly using LES, microscale modeling, or both, e.g., [5,[9][10][11][12].…”

“…Michaelis et al [8,12] considered leads with a width L larger than at least 500 m. They showed that, for such leads, L is an important parameter that affects the fluxes of heat and momentum in the entire turbulent ABL above and downwind of leads. With their approach, they obtained a good agreement of their model results with LES [12] and with the observations from STABLE [8]. Michaelis et al [8,12] applied horizontal grid sizes ∆ x = O(10 2 ) m in their simulations so that the entire convective plumes but not the single turbulent eddies were resolved.…”

“…Michaelis et al [8,12] applied horizontal grid sizes ∆ x = O(10 2 ) m in their simulations so that the entire convective plumes but not the single turbulent eddies were resolved. Thus, compared to Andreas and Cash [31], Michaelis et al [8,12] considered much wider leads (500 m to 10 km width). Michaelis et al [12] developed a lead-width-dependent turbulence parameterization, which was based on a non-local approach to account for the above-mentioned counter-gradient heat transport.…”