Blocking Detection Based on Synoptic Filters

The Tibaldi-Molteni blocking index is supplemented by additional filter criteria to eliminate cut-off lows and subsynoptic structures. We introduce three blocking filters and analyse their sensitivities: (i) a quantile filter requiring a minimum geopotential height anomaly to reject cut-off lows, (ii) an extent filter to extract scales above a minimum zonal width, and (iii) a persistence filter to extract events with a minimum duration. Practical filter application is analysed in two case studies and the blocking climatologies for the Northern and the Southern Hemisphere. 1. Introduction Blocking is one of the most prominent flow patterns and has received attention during the last decades due to its influence on seasonal weather characteristics. The first qualitative criteria for the blocked atmospheric flow was introduced by Rex [1] in 1950 suggesting the atmospheric jet becomes split in two seperated parts with a considerable associated mass transport, the flow to be meridional at the splitting point and downstream, and the state to persist for at least ten days with a zonal width of at least 45 degrees. The original ideas formulated by Rex became the basis for several blocking indices. Most of the indices detect typical atmospheric patterns (e.g., [2, 3]). For shorter timeseries subjective criteria have been used (e.g., [2, 4]). While the majority of the analyses consider the 500？hPa geopotential height (e.g., [5, 6]), Pelly and Hoskins [7], for example, detected blocking by a negative meridional potential temperature gradient on a potential vorticity surface (2？PVU level). Despite the differences between the index definitions, they draw similar conclusions regarding the patterns of the Northern Hemisphere blocking climatology. However, the blocking frequency, which is the percentage of timesteps a certain longitude, is blocked, can be as low as 5% [8], or higher than 20% [5]. The main reasons for this wide range of frequencies in automatic detection are modifications of the Rex criteria and the use of different parameters. Due to its simplicity, the index of Tibaldi and Molteni (henceforth denoted as TM-Index) [5], which is based on the original idea of Lejen？s and ？kland [9], has become a standard in automated blocking detection analyses. An important problem of the TM-Index is that it cannot effectively distinguish between blocking and cut-off low patterns, since both fulfill the TM-Index criteria. The modified TM-Index MTM [8] has a higher rejection rate for cut-off lows. Restrictions in the Rex criteria can be considered as implementations of