"Linking the Paths of Extratropical Cyclones to Hazards and Blocking"

Jimmy Booth
City University of New York, City College

Extratropical cyclones can create hazardous weather conditions due to their winds and precipitation. The spatial distribution of the hazards tends to be anisotropic and, in the case of extreme impacts, relatively small in scale. This makes it difficult for general circulation models (GCMs), with horizontal resolutions around 80 km, to accurately simulate the hazards. On the other hand, the spatial pattern of the sea level pressure field for these cyclones is synoptic in scale and more likely to be captured by GCMs. With this as motivation, the first part of the talk determines the links between hazards along the east coast of North America and the Lagrangian tracks of the cyclones’ central pressure minima. For windstorms, cyclones tend to approach the region from the southwest. Additional analysis shows that this result is mainly due to the orientation of the cyclone cold front relative to the cyclone center. For storm surge extremes, the associated extratropical cyclone tracks pass very close to the region of impact, and often have a stronger northward component to the track path than in windstorms. Finally, cyclones that cause precipitation extremes are considered. In this case, the preferred path of the cyclones varies with season, but northward tracks are again identified as a key contributor to the high impact events. The second part of the talk characterizes the climatological relationship between cyclone track path and speed, and blocking and the North Atlantic Oscillation (NAO). An analysis of Lagrangian cyclone track propagation shows that the percentage of cyclones with blocks is larger for cyclones that propagate northward or southeastward, as is the size of the blocked region near the cyclone. Cyclone-centered composites show propagation of cyclones relative to blocks is consistent with steering by the block: northward tracks more often have a block east/northeast of the cyclone; slow tracks tend to have blocks due north of the cyclone. Comparison with the NAO shows that to first order blocking and the NAO steer cyclones in a similar manner. However, blocked cyclones are more likely to propagate northward, increasing the likelihood of cyclone related impacts.