Regional climate models
- This page is part of the topic Models of the physical and biological environment of the Antarctic
GCMs have increased in resolution and complexity since the first models were developed in the middle of the 20th Century. However, until recently, little attention has been devoted to modeling of the polar regions. This is partly due to the large amount of computer time needed by the complex atmosphere and ocean models, but also due to a lack of observations and knowledge of the cryospheric components, including sea ice and ice shelves as well as snow, glaciers and permafrost. Compared to the effort devoted to development of parameterizations for mid-latitude processes, the cryosphere is under-represented. Nevertheless, there has been and continues to be improvement in the cryospheric components of GCMs. Even so, because GCMs need by definition to be global, they lack the grid resolution to sufficiently represent or parameterize processes that occur on a subgrid scale – like the scale of the Antarctic Peninsula. While the spatial resolution of GCMs will increase, their capabilities will continue to be limited by computational constraints such as processor speed and disk storage space.
Because of this, there is a niche for Regional Climate Models (RCMs), which can be either atmosphere-only models or coupled atmosphere-ocean models, and which can use the GCMs to supply boundary conditions.
Atmosphere-only regional climate models and stretched-grid global models
There are many aspects of the atmospheric physics and thermodynamics that are very specific to the Antarctic region, including for instance strong and persistent surface inversions, katabatic winds, ‘clear-sky’ precipitation, etc, which pose special challenges to the models. Some, such as clear sky precipitation, still remain a challenge to represent in models. But other aspects, such as the katabatic wind system, are being handled better now that high resolution regional models are being run.
Such models have been applied to regions of the Arctic and Antarctic with some success. For example the model Polar MM5 (mesoscale model 5), based on the Penn State model MM5 has been used to examine a number of problems in Antarctic meteorology, including the katabatic wind system (Bromwich et al., 2001[1]).
The group at the Institute for Marine and Atmospheric Research, University of Utrecht, has also used a regional model to examine many aspects of the Antarctic climate. Their RACMO model is based on the German ECHAM4 model and has been used to look at the impact of the SAM on the atmospheric circulation of the Antarctic (van Lipzig et al., 2006[2]).
Such models obviously have a lateral boundary, and boundary conditions are usually obtained from a global run of a coarse resolution model. In addition, it is also necessary to specify ocean forcing, such as sea ice extent/concentration and sea surface temperatures.
Stretched-grid general circulation models avoid the lateral boundary conditions issue in limited-area models. They are global but the grid may be horizontally stretched to refine resolution over a region of particular interest. For Antarctica, this approach was pioneered at the Glaciology Laboratory in Grenoble, France (Krinner et al., 1997[3]). The finest (~60 km over Antarctica) climate change predictions to date have been obtained with this technique (Krinner et al., 2008[4]; Genthon et al., 2008[5]).
Coupled regional climate models
Implementing a limited area, coupled atmosphere-ocean climate model is much more difficult because of the need to obtain both atmospheric and oceanic data at the lateral boundary and to maintain stability in the atmosphere/ocean fluxes. However, the value of such a system is that important features of the Antarctic climate, such as the under ice shelf cavity, which is not present in the coarse resolution global models, can be included.
Such coupled models are starting to be developed, but there are many challenges in obtaining good coupling between the elements. They will become increasingly important in the future.
References
- ↑ Bromwich, D. H., Cassano, J.J., Klein, T., Heinemann, G., Hines, K.M., Steffen K. and Box, J.E. 2001. Mesoscale modeling of katabatic winds over Greenland with the Polar MM5, Mon. Wea. Rev., 129, 2290-2309.
- ↑ Van Lipzig, N.P.M., Marshall, G.J., Orr, A. and King, J.C. 2006. The relation between the Southern Hemisphere Annular Mode and Antarctic Peninsula summer temperatures: Analysis of a high-resolution model climatology, J. Clim., 21(8): 1649.
- ↑ Krinner, G., Genthon, C. Li, Z.-X. and Le Van, P. 1997. Studies of the Antarctic climate with a stretched grid GCM, J. Geophys. Res., 102, 13731-13745.
- ↑ Krinner, G., Guicherd, G., Ox, K., Genthon, C. and Magand, O. 2008. Simulations of Antarctic climate and surface mass balance change from 1981-2000 to 2081-2100, Journal of Climate, 21, 936-962, DOI: 10.1175/2007/JCLI960.1.
- ↑ Genthon C., Krinner, G. and Castebrunet, H. 2008. Antarctic precipitation and climate change in the IPCC models: Horizontal resolution and margin vs plateau issues, Annals of Glaciology, in press.
