The Polar Atmospheric Boundary Layer -

The structure of  the polar ABL is influenced by large scale advection of heat and moisture and by  local processes. These are turbulent and radiation processes which both depend on the sea ice surface characteristics, but also on low level clouds.

Cold-air outbreaks represent an example for the strong impact of turbulence. During such events cold air masses which are advected from the inner polar regions are strongly heated over the open ocean. The generated turbulence can lead to an increase of the ABL thickness from e.g. 100 m over sea ice to 2 km in a downstream distance of 200 km to the marginal sea ice zone. On the contrary, during on-ice flow regimes radiative cooling of air over sea ice can lead to very shallow ABLs caused by radiative cooling. The resulting stable stratification suppresses strongly the turbulent motions.

Turbulent processes depend on the difference between air and surface temperature and thus also on sea ice characteristics as concentration (lead cover) and ice thickness. But also the surface topography (sea ice pressure ridges, edges of floes and ponds) plays an important role for the generation of turbulent eddies.  Snow and melt pond cover and especially cloud cover have a large impact on radiation processes. All the relevant parameters show a strong spatial variability, which forms a large challenge for the derivation of turbulence parametrizations. This holds especially for climate models with grid sizes being much larger than the spatial scale of sea ice inhomogeneity.

AWI researchers of  our Section have investigated these regimes in the past years by many case studies (modeling and measurements). Based on the results, turbulence parameterizations have been developed for strong convective flow, for near-surface fluxes over sea ice in the inner polar ocean regions and over the marginal sea ice zone with specific conditions. Also the strong impact of convection over leads (channels in sea ice) has been investigated.

The interaction of the lower atmosphere with sea ice and ocean is strongly influenced by processes caused by the surface morphology, which is characterized in the Polar ocean regions first of all by the sea ice cover and by the surface temperature distribution. This holds for all seasons. But especially during winter the differences between surface temperatures of open water and sea ice and their difference to the near-surface air temperatures are large enough to generate convective turbulence in the lower atmosphere when open water is present. Such open water patches exist in form of small polynyas and so-called leads in the inner polar ocean regions. Much open water is present in the marginal sea ice zones between drifting ice floes. We have been studying the related processes in the different regions by modeling and airborne observations (see e.g. campaign STABLE).

An important feature of sea ice is also its aerodynamic roughness. Initially young sea ice (nilas) can be smooth but due to break-up of floes and rafting caused by divergent forcing by the atmospheric and oceanic flow it becomes more and more rough with a topography dominated by pressure ridges. They have a large variability of geometric shapes but obstacles become even more variable when blowing snow forms additional roughness elements. The dynamic pressure at such obstacles and also at the edges of ice floes and at melt ponds during summer can be related to the turbulent atmospheric transport of momentum which in turn can influence the sea ice drift.

We study in our group both the energy and momentum transport in much detail by aircraft observations and modeling and develop parametrizations of these processes for climate models.

The interaction between atmospheric processes in polar and mid-latitude regions is strongly affected by cold-air outbreaks (CAOs). During such events cold air originating from (sea)ice covered regions is transported over large distances towards lower latitudes, while over open water a shallow but strongly heated convective boundary layer develops.  In this layer the flow is  organized in so-called convection rolls. These are visible in satellite images as ‘cloud streets’ developing in the roll type atmospheric circulation. The typical width of rolls amounts to 1-5 km  while they reach  lenghts of several hundreds of kilometers. During cold-air outbreaks  near-surface fluxes of heat can amount to  hundreds of W/m² over large regions which means that these flow regimes have a large importance for the interaction between atmosphere and ocean. This concerns both hemispheres since cold-air outbreaks occur in the Arctic and Antarctic as well. Typical regions for cold-air outbreaks in the Arctic are the Fram Strait and Barents Sea. The Figure shows an example of a cold-air outbreak with convection rolls over Fram Strait caused by a low pressure system over Svalbard.

Also in the inner polar ocean regions the sea ice cover is never completely closed and there are always channels (leads) with open water or with thin new ice so that the concentration is often larger than 98 % but smaller than 100 % if an area of 200 km x 200 km is considered. These leads and small, lake-like polynyas are caused by the divergent sea ice drift. The typical width of leads ranges from meters (cracks) to several kilometers, but their length can extend up to hundreds of kilometers.