Work package leaders
Asgeir Sorteberg (BCCR) / Erik Kolstad (UiB)
International partners
K. Hodges, T. Bracegirdle, I. Renfrew, G. W. K. Moore.
Background
Many adverse weather events in the Arctic stem from mesoscale phenomena associated with the location of the ice sheet, the strength of the inversion above it, the persistence and direction of the northerly flow, and the temperature at the sea surface, which can trigger air-sea interaction instabilities on smaller scales (e.g. Emanuel and Rotunno, 1989). But exactly how do different air masses entering the region from elsewhere (the North Atlantic, Siberia or Greenland) modify these conditions? What are the links to large-scale teleconnection patterns and how will the synoptic flow change under a global warming scenario? Under what synoptic conditions should we expect small-scale extreme weather?
Research plan
A major reason for focussing on the larger spatial scales is that the low resolution of reanalysis data and climate simulations prohibits direct reproduction of mesoscale weather events. By investigating the relationship between these scales empirically, we may provide information on how synoptic and planetaryscale conditions at present and under global climate change influence the initiation and life cycle of Arctic mesoscale extreme weather. Before embarking on extensive observational campaigns, it is highly important to know exactly where the extreme weather is most likely to take place. The contribution from this WP is to makedetailed investigations of existing case studies and a database of polar lows in the Nordic Seas (Noer and Ovhed, 2003), and to provide a new fine-scale (5-10 km resolution) dynamical downscaling of the known events. To determine the properties and influence of the entering (and exiting) larger scale air masses, we will use: results from the assimilation work in WP5; information from existing atmospheric reanalyses (e.g. Kolstad, 2006); a finer-scale downscaling of these data using the spectral nudging approach (von Storch et al., 2000) that is already ongoing at the BCCR; new analyses using cyclone tracking (e.g. Hoskins and Hodges, 2002; Sorteberg et al., 2005) and new high-resolution limited-area hindcasts using an NWP model. In addition, numerical simulation using a stretched global atmospheric model (with a resolution of 35-65 km near the focus area) will be performed to simulate future climate scenarios. The motivation for using a global stretched model instead of a regional model is that in a region which is heavily affected by advective processes, such as the Nordic Seas, climate change simulations using regional models will be strongly governed by the (coarse-resolution) boundary conditions. And, even more fundamentally, no feedbacks are possible between the Arctic and the lower latitudes (which are outside or close to the border of the model domain). These are feedbacks that might change the atmospheric heat and moisture transport into the Arctic and induce large-scale circulation changes that are highly important for the mesoscale events. With our approach, the global perspective will be kept intact.
To sum up, the primary tasks of this WP are to: (1) carry out most of the necessary investigations in preparation for the observational campaigns; (2) supply a ‘climatology’ of past and present extreme weather in the Nordic Seas; (3) make predictions about the future of severe weather under the influence of global warming.