Core theme 2: Atmosphere and Air-Sea-Ice interaction
There is a strong need for improvement in the observations and modelling of the Arctic atmosphere. Current climate models, global as well as regional, are more unreliable in the Arctic than for most other regions.
This is demonstrated both by the apparent difficulty in describing current Arctic climate with these models, and by the inter-model spread in scenarios for future climate, which is larger in the Arctic than elsewhere. There are many reasons for this. Some stem from the inability of models to handle mesoscale synoptic disturbances (even the present-day observational network is incapable of detecting all mesoscale cyclones).
Some of the model problems are related to an insufficient understanding of several important local feedback mechanisms that appear to be special to the Arctic. Many of these processes are sub-grid scale in climate models and need to be parameterized. Developing such descriptions has an unavoidable empirical component, and part of the problem in the Arctic is a lack of process-scale data. Many of these processes deal directly with the energy transfer at the surface, and are therefore directly relevant for the melting of the sea ice. To understand these processes, we follow the DAMOCLES objectives to increase the observational coverage of the Arctic marine atmosphere and to improve its modelling. To achieve this overall objective, a focus on the following processes is needed:
- Dynamics and occurrence of mesoscale cyclones. Mesoscale cyclones, such as Polar lows, are common particularly in the marginal ice zone. The strong winds associated with the cyclones have a major influence on sea ice dynamics, and present a serious risk for navigation. In addition, mesoscale cyclones are supposed to make a large, but insufficiently known, contribution to the lateral transport of heat and moisture into the Arctic atmosphere.
- Physical processes in the atmospheric boundary layer (ABL). The Arctic ABL is usually stably stratified, and numerical models typically suffer from the largest errors in such conditions. An additional difficulty is the localized convection over leads and coastal polynyas, which makes the ABL heterogeneous in the grid scale of models.
- Interaction of clouds, radiative fluxes, surface albedo, and snow/ice thermodynamics. Problems evolve around cloud issues, in particular mixed-phase clouds that appear more common in the Arctic than elsewhere. Also in summer, with higher temperatures, clouds appear to have different optical properties compared to their counterparts at lower latitudes. The interaction processes are extremely complex, and involve strong feedback effects, above all the surface albedo feedback. A small reduction in the surface albedo may cause a large increase in the net solar radiation. The snow and ice surface albedo depends on the solar height angle, the partitioning between the direct and diffuse radiation, and the properties of snow and ice, and is therefore addressed in both core themes 1 and 2.
To better understand these processes, we need both in-situ and remote sensing observations, as well as data analyses and modelling.
Objectives:
- To better detect Arctic cyclones.
- To improve modelling of their interaction with sea ice.
- To quantify the contribution of the cyclones to the transport of heat and moisture.
- To better understand and model boundary-layer processes over the Arctic Ocean.
- To develop improved parameterizations of turbulent fluxes in the atmospheric boundary layer (ABL) over the Arctic Ocean.
- To better understand and model the formation and life cycle of Arctic clouds and important interactions of cloud and radiation processes with aerosols and boundary-layer turbulence in particular for mixed-phase clouds.
- To better understand and model radiative transfer through the Arctic atmosphere and its interaction with the snow/ice surface albedo.
