Boundary Layer Mass, Water, and Heat Budgets in Wintertime Cold-Air Outbreaks from the Arctic Sea Ice

Abstract

Eleven cold-air outbreaks from the Arctic sea ice to the open water of the Fram Strait and the Norwegian Sea have been monitored by aircraft during the field campaigns ARKTIS 1991 and ARKTIS 1993. Budgets of mass, water vapor, water, and heat in the atmospheric boundary layer are computed for boxes that are located at different distances from the ice edge ranging from the marginal ice zone to several hundred kilometers downstream. Averaged over all cold-air outbreaks, the large-scale flow is divergent near the ice edge and convergent at larger distances from the ice edge. Regardless of divergence, the large-scale flow exports everywhere water vapor, water, and heat from an atmospheric box within the boundary layer. In the case of the water vapor budget this export and the loss by condensation in clouds are compensated by evaporation from the sea surface. Both the condensation in clouds and surface evaporation increase in downstream direction, as does their ratio from about 0.4 near the ice edge to about 0.8 at distances greater than 300 km. In the water budget, the source by condensation in clouds is compensated by two sinks: large-scale flow export and precipitation. Precipitation increases absolutely from about 1?4 mm day?1 in downstream direction but the ratio of precipitation versus condensation remains approximately constant at a value of 0.75. In contrast to the water vapor and water budgets with only one source, the heat budget has several sources, namely, surface heat flux, entrainment heat flux, condensation in clouds, and possibly radiation, which compensate for the heat export by the large-scale flow. The relative importance of these sources changes with distance from the ice edge. Near the ice edge, the surface heat flux at the first place and the entrainment flux at the second place are the relevant sources, while farther downstream in the region of deep convection?latent heat release by condensation is the dominating heat source. Here, the surface heat flux is of secondary importance and the entrainment flux plays a minor role. Since a systematic transition from roll-like to cellular-like convection patterns is present in Arctic cold-air outbreaks, the differences in the budgets with respect to distance from the ice edge apply as well to the regions of rolls and cells, respectively. It is hypothesized that mesoscale cellular convection with cloud depths of more than 1 km does not occur unless the Bowen ratio is less than about 0.6.