The Polar Atmospheric Boundary Layer -

Observations and Small Scale Modelling

The atmospheric boundary layer (ABL) is the layer  which is influenced by processes  generated at the surface and which  transport  momentum, heat and moisture. In polar regions the ABL top is characterized by a strong increase of air temperature.

Although  the ABL  is often  very shallow in the inner polar regions  (over sea ice  typically 30 – 400 m)  it is important to understand ABL processes well  because of  their  importance for the  interaction of the atmosphere with sea ice and ocean.

We study ABL processes by aircraft and shipborne observations, by data from automatic weather stations,  and by  smallscale modelling.

Processes influencing the polar ABL structure

...in Bounday Layer Processes

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  the Section Polar Meteorology 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.

Impact of sea ice morphology on energy and momentum transport

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.

Examples of publications related to our parametrizations of momentum transport over sea ice are:

  • Lüpkes, C., and V. M. Gryanik (2015), A stability-dependent parametrization of transfer coefficients for momentum and heat over polar sea ice to be used in climate models, J. Geophys. Res. Atmos., 120, doi:10.1002/2014JD022418.
  • Lüpkes, C., V. M. Gryanik, A. Rösel, G. Birnbaum, and L. Kaleschke (2013), Effect of sea ice morphology during Arctic summer on atmospheric drag coefficients used in climate models, Geophys. Res. Lett., 40, 446–451, doi:10.1002/grl.50081.Lüpkes, C., V. M. Gryanik, J. Hartmann, and E. L Andreas (2012), A parametrization, based on sea ice morphology, of the neutral atmospheric drag coefficients for weather prediction and climate models, J. Geophys. Res., 117, D13112, doi:10.1029/2012JD017630.
  • Birnbaum, G., and C. Lüpkes (2002), A new parametrisation of surface drag in the marginal sea ice zone, Tellus A, 54A(1), 107–

Cold air outbreaks with strong roll convection

The interaction between atmospheric prcesses 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.

Publications

In Section Polar Meteorology we investigate cold-air outbreaks  with aircraft measurements and modelling. More detailed information on our research can be found  in: 

  • Chechin, D.G., C. Lüpkes, I.A. Repina, and V.M. Gryanik (2013), Idealized dry quasi 2-D mesoscale simulations of cold-air outbreaks over the marginal sea ice zone with fine and coarse resolution, J. Geophys. Res. Atmos., 118, 8787–8813, doi:10.1002/jgrd.50679
  • Wacker, U., K. Potty, C. Lüpkes, J. Hartmann, and M. Raschendorfer (2005), A case study on a polar cold air outbreak over Fram Strait using a mesoscale weather prediction model, Boundary Layer Meteorol., 117, 301–336, doi:10.1007/s10546-005-2189-1.
  • Hartmann, J., C. Kottmeier, and S. Raasch (1997), Roll vortices and boundary-layer development during a cold air outbreak, Boundary Layer Meteorol., 84(1), 45–65, doi:10.1023/A:1000392931768

Convection over leads (channels in sea ice) and polynyas

Also in the inner polar ocean regions the sea ice cover is never completely closed and there are always channals (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.

Leads play an important role for air ice interaction processes since the large differences betweeen their surface temperature and the temperature of the near-surface air can amount to 45 K so that convective plumes are generated over leads. Sensible heat fluxes of up to 300 W m-2 are observed in the plumes so that leads can strongly influence the surface and boundary layer energy budgets.

We investigate processes over leads in our group by aircraft measurements (campaign STABLE, Tetzlaff et al., 2015) and modelling on different scales using grid sizes ranging from meters to 100 m and several kilometers (Lüpkes et al., 2008a,b).

Several projects have been funded by DFG in the past (e.g. The physics of turbulence over Antarctic leads and polynyas and its parameterization: a joint study using observations, LES, and a micro-/mesoscale model gepris.dfg.de/gepris/projekt/26825911

and at present there is an ongoing project (Modelling and parameterization of lead generated turbulence in the atmospheric boundary layer over Antarctic sea ice, gepris.dfg.de/gepris/projekt/314651818

References

Lüpkes, C., T. Vihma, G. Birnbaum, and U. Wacker (2008a), Influence of leads in sea ice on the temperature of the atmospheric boundary layer during polar night, Geophys. Res. Lett., 35, L03805, doi:10.1029/2007GL032461

Lüpkes, C., V. M. Gryanik, B. Witha, M. Gryschka, S. Raasch, and T. Gollnik (2008b), Modeling convection over arctic leads with LES and a non-eddy-resolving microscale model, J. Geophys. Res., 113, C09028, doi:10.1029/2007JC004099.

Tetzlaff A., Lüpkes C., Hartmann, J. (2015) Aircraft-based observations of atmospheric boundary-layer modification over Arctic leads, Q. J. R. Meteorol. Soc. 141: 2839–2856, DOI:10.1002/qj.2568

Radiation

We particularly focus on surface albedo, the ratio of reflected to incoming solar radiation.

In the Arctic, we mainly address albedo of sea ice and its seasonal variation, e.g., caused by melting and freezing. Investigations are based on airborne measurements.

In the Antarctic, we focus on the investigation of microscopic and macroscopic snow properties and their influence on snow albedo in the plateau region.

For scientific background information and for results we refer to the  respective projects/campaigns. MELTEX-I and MELTEX-II were performed over Arctic sea ice at the beginning of the melt season and in the peak period of melting.  CoFi-Met is based on atmosphere and snow albedo measurements at Kohnen Station on the plateau of Dronning Maud Land, Antarctica.