Atmospheric circulation characteristics associated with daytime extreme static instability over Athens, Greece

摘要

The main atmospheric circulation characteristics associated with extreme static instability conditions over the Athens region were examined for the period 1974-2012. The data used consist of daily values of: (1) upper air temperature and dew point, precipitation and cloud cover in Athens and (2) 2.5 degrees x 2.5 degrees 1000 and 500 hPa geopotential heights (GH) and sea level pressure (SLP) over the Mediterranean region, for the period 1974-2012. The stability index K (K-index) was calculated from upper air measurements, and the 5% of the days with the highest K values were selected for both the cold and the warm periods of the year. These days were classified into clusters according to the corresponding GH patterns, using factor analysis and K-means cluster analysis. Results show 9 and 5 clusters for the cold and the warm periods, respectively. The mean 500 hPa GH and SLP patterns were constructed and the cloud cover and precipitation characteristics were investigated for the days classified under each cluster. For the cold period, the main atmospheric circulation factors that are responsible for extreme instability levels are the cyclonic activity in the central Mediterranean, and the warm and humid southeasterly surface flow over Athens; extreme instability is associated with high values of cloud cover, as well as of precipitation amount and frequency of precipitation events. For the warm period, upper air disturbances and the presence of corresponding cold air masses are the main factors leading to extreme instability conditions, while cloud cover and precipitation are relatively lower than during the cold period.