Cluster analysis of five years of back trajectories arriving in Athens, Greece

Cluster analysis was applied to back trajectories reaching the Greater Athens area, Greece. The database contains five years of four-day kinematic back trajectories arriving at three heights, 750 m, 1500 m and 3000 m above mean sea level, computed with version 4.7 of the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT). In this study, the analysis consists of two steps. The first step involves the clustering of trajectories at each height level and the separation of the retained clusters based on the length of their centroids (cluster-mean trajectories) using the Haversine formula. In order to obtain this separation, three categories of back trajectories are defined based on the above mentioned length: short-slow-moving (SSM) with length up to 1500 m, medium-moving (MM) with length from 1500 m to 3000 m and long-fast-moving (LFM) with over 3000 m. Cluster analysis was applied again on each of these three categories of back trajectories at each of the above heights (second step). The results show that as height increases the number of back trajectories in the SSM/LFM categories decreases/increases due to the weaker influence of the ground and the stronger winds. In addition, airflows categorized in SSM come almost from all points of the compass, in contrast to the MM and LFM categories where the flows originate mostly from the west and north sectors. Specifically, at 750 m the north sector prevails for all categories, at 1500 m the north sector dominates for SSM and LFM, while for MM the west sector dominates. The situation is different at 3000 m, where domination of the south sector for SSM and of the west sector for MM and LFM is observed. Moreover, patterns of local circulation with very short trajectories are observed in all heights, presenting a seasonal distribution with higher frequency of occurrence at 750 m and 1500 m in the winter, and at 3000 m in the summer.