Monday, 15 January 2007: 4:30 PM
A Tropospheric Assessment of Contemporary Global Reanalyses in the Polar Regions
212B (Henry B. Gonzalez Convention Center)
The reliability of the global reanalyses in the polar regions is evaluated. The overview stems from an April 2006 Scientific Committee on Antarctic Research (SCAR) workshop on the performance of global reanalyses in high latitudes held at the British Antarctic Survey. Overall, the skill is much higher in the Arctic than the Antarctic, where the reanalyses are only reliable in the summer months prior to the modern satellite era. In the Antarctic, large circulation differences between the reanalyses are found prior to and even after 1979, when vast quantities of satellite sounding data started to be assimilated. Specifically for ERA-40, this data discontinuity creates a marked jump in Antarctic snow accumulation, especially at high elevations. In the Arctic, the largest differences are related to the reanalyses' depiction of clouds and their associated radiation impacts; ERA-40 captures the cloud variability much better than NCEP1, but the ERA-40 clouds are too optically thin for shortwave radiation. To further contrast the reanalyses skill, cyclone tracking results are presented. In the Southern Hemisphere, cyclonic activity is markedly different between the reanalyses, where there are few matched cyclones prior to 1979. In comparison, only some of the weaker cyclones are not matched in the Northern Hemisphere from 1958-2001, again indicating the superior skill in this hemisphere. Although this presentation focuses on deficiencies in the reanalyses, it is important to note that they are a valuable tool for climate studies in both polar regions when used with recognition of their limitations.