The atmospheric conditions over Europe and the Mediterranean, favoring snow events in Athens, Greece

Abstract

Abstract. The 3-dimensional structure and the evolution of atmospheric circulation favoring snowfall in Athens are examined. The study refers to 61 snow events, which occurred during the period 1958-2001. For each one of the events, the patterns of MSL pressure, 850 hPa and 500 hPa air temperatures, 500 hPa geopotential height and 1000-500 hPa thickness are constructed for the European region, for the day before (D-1), the first day (D) and the day after the end of the event (END). A statistical methodology involving Factor Analysis and Cluster Analysis is applied to the above data sets and the 61 cases are finally classified into five clusters. These clusters are generally characterized by a north-easterly flow in the lower troposphere over the Athens area. This flow is associated with the presence of a low pressure system around Cyprus and an anticyclone over Europe. The position, the intensity and the trajectories of the surface and the upper air systems during D-1, D and END days are generally different among the five clusters.

The atmospheric circulation characteristics favouring snowfall in an area with complex relief in Northwestern Greece

“…Indeed, Lagouvardos and Kotroni () examined a heavy snowfall event over central Greece (east of the Pindus mountain range) from the 4th to the 6th of January 2002, describing a synoptic situation with a low pressure system centred over the southeastern Aegean Sea, moving to the east, inducing a northeasterly flow and a consequent cold advection. Moreover, Houssos et al () studied the main atmospheric circulation types related to snowfall in Athens, eastern Greece, finding out that a northeasterly airflow near the surface, associated with a low pressure system over Cyprus and an anticyclone over Europe, is the main atmospheric circulation characteristic related to snowfall in eastern Greece. On the other hand, the results of this study are in agreement with the findings of Houssos et al (), showing that Ioannina experience snowfall when a cold advection is induced by a northwesterly flow, related to the presence of a low pressure system over the Ionian Sea.…”

“…In this study, following the main steps of the methodology used by Houssos et al () for Athens, the 3‐h SYNOP observations in Ioannina are utilized for the definition of the following terms: snow day is defined as the day with at least one of the eight SYNOPs reporting snowfall, snow event is a sequence of snow days, D day is the starting day of a snow event, D‐1 day is its previous one and END day is the day after its cessation (Figure ). From the 56‐year data set, 160 snow events are extracted.…”

“…Recently, large scientific teams focused their efforts on studying the effectiveness of such methodologies in the framework of the COST Action 733 (http://www.cost.eu/COST_Actions/essem/Actions/733) entitled 'Harmonization and Applications of Weather Types Classifications for European Regions' and the STARDEX Project (http://www.cru.uea.ac.uk/projects/ stardex/) standing for 'statistical and regional dynamical downscaling of extremes for European regions'. In this study, following the main steps of the methodology used by Houssos et al (2007) for Athens, the 3-h SYNOP observations in Ioannina are utilized for the definition of the following terms: snow day is defined as the day with at least one of the eight SYNOPs reporting snowfall, snow event is a sequence of snow days, D day is the starting day of a snow event, D-1 day is its previous one and END day is the day after its cessation ( Figure 4). From the 56-year data set, 160 snow events are extracted.…”

“…Lagouvardos et al (), studying the same event, reached similar to the aforementioned results and also denoted the influence of orography in cold air advection towards southern Greek Peninsula. Houssos et al () examined the atmospheric circulation characteristics before, during and at the end of snow events in Athens using objective multivariate statistical methodology. In a more recent study of Louka et al () concerning a heavy snowfall event in the greater Athens area during 16 and 18 February 2008, it was concluded that the advection of cold air masses from northeastern continental Europe towards Greece, passing over the Aegean Sea and being enriched with water vapour and heat cause severe snowy weather in parts of the windward eastern continental Greece.…”

Atmospheric circulation patterns associated with extreme precipitation amounts in Greece

“…Next, K-Means Cluster Analysis is applied to the factor scores matrix (369 rows x 7 columns) in order to classify the 369 cases into homogeneous distinct clusters that correspond to the main atmospheric circulation structures associated with extreme precipitation amounts over Greece. A similar methodology has been used in other previous studies (Plaut and Simonnet, 2001;Houssos et al, 2007;Lana et al, 2007). The number of clusters is decided by taking into account the results of a procedure called "jump method", which is based on distortion, a measure of within-cluster dispersion, proposed by Sugar and James (2003).…”