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An analysis of the synoptic and climatological applicability of circulation type classifications for Ireland



Circulation type classifications compiled as part of the COST733 Action, ‘Harmonisation and Application of Weather Type Classifications for European Regions’, were evaluated based on their ability to describe variations in surface temperature (maximum and minimum) and precipitation across the Irish landmass. In all 16 different classification schemes, representative of four general approaches in synoptic typing (leader algorithm, optimization scheme, predefined types, eigenvector analysis) were considered. Several statistical measures variously quantifying performance in arranging daily observations into clearly defined homogenous groups were employed. Based on the results it was not possible to identify a single optimum classification or general approach in synoptic typing. This is related to inconsistencies in performance with respect to the specific target variable and statistical measures used; the results were also shown to be conditional on the number of circulation types (CTs) as well as spatiotemporal dependencies in performance. However, the study did indicate that those typing schemes based on predefined thresholds (Litynski, GrossWetterTypes, Lamb Weather Type) – along with the Kruizinga and Lund classifications – were better able to resolve surface temperature. With respect to precipitation those classifications derived using some optimization procedure (simulated annealing, Self Organizing Maps, k-means clustering) were consistently among the best-performing schemes. In capturing the relationship between synoptic-scale circulation and precipitation the importance of incorporating some measure of vorticity was highlighted; in contrast the inclusion of discrete directional patterns was shown to be important for resolving variations in local temperature. The classifications generally performed best for winter, reflecting the closer coupling between circulation and surface conditions during this period. Spatial patterns in the synoptic–climatological relationship were more apparent for precipitation. In this case those more westerly/south-westerly stations open to zonal airflow exhibited a stronger response to circulation variability.