Dynamics and energetics of the cloudy boundary layer in simulations of off‐ice flow in the marginal ice zone

Abstract

Abstract. The case under congideration occurred on March 4, 1993, and was observed as part of the Radiation and Eddy Flux Experiment (REFLEX II) 1993 observational campaign northwest of Spitsbergen. The off-ice flow on this day brought very cold surface air temperatures (-35øC) over a relatively warm ocean surface. The resultant latent and sensible surface heat fluxes produced intense convection and a thermal internal boundary layer (TIBL) which deepened with distance from the ice edge. Two-dimensional cloudresolving model (CRM) simulations were performed to determine the impact of various cloud parameterizations on the structure and evolution of the TIBL. The model was able to reproduce the observed thermal structure of the boundary layer to within the acknowledged limitations of the CRM approach. Sensitivity studies of cloud type showed that inclusion of mixed-phase microphysics had a large impact of BL depth and structure. Radiative heating of the cloud near cloud base and cooling near cloud top along with latent heat release were found to be significant sources of turbulence kinetic energy even in the present case where very strong surface heat fluxes occur. Ice-phase precipitation processes rapidly depleted the BL of condensate, weakening the radiative thermal forcing. A further consequence of condensate depletion in the mixed-phase cloud was a less humid boundary layer that was able to maintain a larger surface latent heat flux and continuously extract heat through condensation and deposition. Not surprisingly, the presence of clouds had a profound impact on the radiative budget at the surface, with the cloudy BL reducing surface radiative losses more that 60% over clear-sky values. Inclusion of the ice phase significantly affected the radiative budget as compared to purely liquid clouds, illustrating the importance of ice-phase-radiative couplings for accurate simulations of arctic clouds and boundary layer dynamics. IntroductionThe marginal ice zone (MIZ) is that region within which, more or less, total sea-ice coverage gives way to a generally ice-free ocean.

A springtime cloud over the Beaufort Sea polynya: Three‐dimensional simulation with explicit spectral microphysics and comparison with observations

“…Rao and Agee [1996] found that inclusion of snow processes in the simulation produced fundamentally different boundary layer turbulence and convective organization. Olsson and Harrington [2000] found that inclusion of mixed-phase microphysics had a large impact on boundary layer depth and structure. Icephase precipitation processes rapidly depleted the boundary layer of condensate, and the less humid boundary layer was able to maintain a larger surface latent heat flux and continuously extract heat through condensation and deposition.…”

“…Numerous simulations of cold air outbreaks have also been conducted [e.g., Chlond, 1992;Bakan et al, 1995;Rao and Agee, 1996;Olsson and Harrington, 2000;Brummer and Pohlmann, 2000;Flamant et al, 2001;Pagowski and Moore, 2001] as well as simulations of cloud plumes from the cooling ponds [e.g., Shannon and Everett, 1978;Leahey et al, 1979;Yamada, 1979;Khvorostyanov, 1991].…”

“…[7] Although some of these models were based on the explicit bin-resolving microphysics [e.g., Khvorostyanov, 1991Khvorostyanov, , 1995Bakan et al, 1995;Olsson et al, 1998;Harrington et al, 1999;Jiang et al, 2000Jiang et al, , 2001Olsson and Harrington, 2000;Khvorostyanov et al, 2001], most of the aforementioned simulation studies used bulk microphysical parameterizations. In bulk microphysical parameterizations, assumptions must be made about the temperature at which ice nucleation occurs, the threshold for the so-called autoconversion of cloud water to rain or snow, the relative humidity threshold for nucleation, and the relaxation times for condensation and sublimation.…”

Vertical Motions in Arctic Mixed-Phase Stratiform Clouds

“…In addition, Herman and Goody (1976) suggest that the stationarity of the synoptic flow in the central Arctic acts to minimize mechanisms of stratus dissipation. Curry (1986) asserts that surface fluxes are not responsible for Arctic stratus formation under the strongly stable conditions present in winter and early spring, but this may not be the case for late spring through October when open ocean is a significant factor (Pinto and Curry 1995;Olsson and Harrington 2000). Although many details of these cloud formation and persistence processes are not well understood, cloud-scale vertical motions are a key feature of mixed-phase clouds that affect their evolution and support their extended lifetimes (e.g., Rauber and Tokay 1991).…”

Mesoscale Modelling of the Arctic Atmospheric Boundary Layer and Its Interaction with Sea Ice

“…They found that it was essential for an accurate modelling of the specifi c humidity fi eld to account for deposition growth of cloud ice. Olsson and Harrington ( 2000 ) conclude from their sensitivity study with a high-resolution 2D model that the amount of cloud condensate strongly infl uences the ABL structure due to radiation effects. …”