Title: Cluster Analysis of 4-day Back Trajectories Arriving in the Barcelona Area (spain) from 1997 to 2002

A cluster algorithm was implemented to analyze the backward trajectories arriving in the Barcelona area (BCN), located at the eastern coast of the Iberian Peninsula. Five years of 4-day kinematic back trajectories, computed with the HYbrid Single-Particle Lagrangian Integrated Trajectory model version 4 (HYSPLIT), were clustered and classified in groups of similar length and curvature. To better describe the tropospheric circulations at BCN, backward trajectories arriving at 5500, 3000, and 1500 m above sea level were analyzed. The main transport patterns are identified at 5500 m: westerly flows (49% of the total situations), northwesterly flows (17%), southwesterly flows (20%), and regional recirculations over Europe and the Mediterranean Sea (15%). An annual distribution of the transport patterns is described. During summertime, moderate westerlies (19%) and southwesterlies (13%) in the middle troposphere, slow westerlies (11%) and southwesterlies (10%) at 3000 m, and regional recirculations (29 %) at 1500 m characterize the BCN long-range transport. This general pattern varies during wintertime with more westerlies at 5500 m, and an increase of northern and northwestern situations at 1500 m. Large number of situations with decoupling between lower from middle troposphere are observed when combining 5500 m and 1500 m cluster results. Inter-annual variability is discussed, and the influence of the North Atlantic Oscillation phase is captured by winter average regime patterns.