92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Wednesday, 25 January 2012
Atmospheric Blocking in the Community Earth System Model (CESM)
Hall E (New Orleans Convention Center )
Richard Neale, NCAR, Boulder, CO

Persistent atmospheric blocking patterns often lead to extremes in surface temperature around the globe. Winter-time blocking can result in excessively cold temperatures in North-Western Europe and summer-time blocking can lead to heat-waves in the Central and Southern US. Unlike the more predictable progression of mid-latitude weather systems high-pressure block pattern evolution, in terms of their initiation, persistence and break-down, is extremely difficult to predict in forecast models and to represent in climate models. The association with extreme events underpins the importance of simulating these large-scale features accurately. Blocking characteristics have been evaluated in a suite of simulations using a range of resolutions and model configurations of the Community Earth System Model. Horizontal resolutions vary from 4 degrees to 0.25 degrees and configurations encompass dynamical core, physics and coupling differences. Winter-time blocking location is generally well captured in the Eastern Atlantic and Western Pacific sectors in all model version, but with an underestimated frequency particularly in the Eastern Atlantic and especially in the presence of coupling to the ocean. Although observed blocking occurrence is much less in Summer, CESM does a very good job at capturing the peak locations and frequency of blocking. Capturing summer-time blocking well would seem to be a pre-requisite for capturing the statistics of the heat-wave events associated with these phenomena as well as for their changes in the future. This presentation will further demonstrate the need for a minimal horizontal resolution to capture realistic blocking in addition to the role that changes to the physics representations (CAM4 versus CAM5) and dynamical core methods (finite volume versus spectral element) play in determining blocking statistics.

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