Title: Synoptic-scale circulation features associated with weather extremes in the extratropics


Abstract: Extreme weather events have a major impact on society, but assessments of their future changes are still associated with substantial uncertainties. For better characterizing the dynamical conditions related to weather extremes and their representation in climate models, we investigate the linkage between such extreme events and associated atmospheric flow features. In particular, we apply simple metrics for quantifying the relevance of cyclones and atmospheric blocking for the occurrence of precipitation and temperature extremes.
Based on the ERA-Interim reanalysis data set, it is shown that a huge percentage of precipitation extremes, widely exceeding 70-80%, is directly associated with cyclones in the main extratropical storm track regions, but also in areas where cyclones are less abundant, e.g., around the Mediterranean and in several regions affected by tropical cyclones. In addition to their direct impact (through ascending motions and moisture convergence), cyclones can influence precipitation extremes at remote locations, e.g., by directing moist air flow towards the Alpine ridge in Central Europe. Warm temperature extremes are strongly affected by atmospheric blocking at the same location (which leads to clear sky conditions and strong radiative forcing), in particular over the high latitude continents. Blocking can also influence cold extremes at remote locations via cold air advection. A trajectory approach is used for further exploring this fundamental difference between cold extremes, which are mainly due to the import of cold air masses, and hot extremes that are strongly driven by local warming.
The quantitative results on the effects of cyclones and blocking on weather extremes are used for the evaluation of a present-day climate simulation from CCSM4. Regions are identified where the GCM properly represents the major importance of the atmospheric flow features, and others where it has larger deficiencies. These results may be used as a starting point for model improvement, and for assessing the reliability of future projections of weather extremes on regional scales.

Speaker's website: http://www.iac.ethz.ch/people/pfahls