Winter Nighttime Temperature Inversions and Their Relationship with the Synoptic-scale Atmospheric Circulation

The present work aims at examining the association of nighttime temperature inversions and synoptic-scale circulation in the Athens basin. Temperature inversions have a direct impact in the diffusion and transport of pollutants, a fact that highlights the importance of investigating their cause effect relationship. For this purpose, randiosonde measurements from the upper-air station in Hellinikon were used to examine the nighttime temperature inversions and surface circulation maps were analyzed in order to classify the synoptic-scale circulation. The period of study is the wintertime days from 2003 to 2010 and the obtained data provide for each inversion its depth in meters along with the corresponding inversion strength in °C. The classification of atmospheric circulation resulted into 12 circulation patterns (7 cyclonic, 3 anticyclonic and 2 mixed) for a study area covering the greater European region (20° N to 65° N and 20° W to 60° E). The study focuses on the analysis of inversions and circulation patterns frequencies. Inversion days are grouped into categories according to the inversion depth and strength. For each circulation type the frequency distribution of each inversion class is examined and inversion characteristics and mean values are calculated. It is found that for the 57% of the study period days an inversion is observed and the higher frequencies of occurrence are observed during January. The majority of the inversions are characterized as “shallow” and “weak”, while deeper and stronger inversions are observed during February. Regarding atmospheric circulation, the higher frequencies are observed for the smooth pressure field pattern and particular circulation patterns have higher frequencies for specific months. The association of temperature inversions and synoptic scale circulation shows that the inversion strength is strongly influenced by atmospheric synoptic-scale circulation. The above argument is strengthened by the results of the statistical significance tests and the high mean values of inversion strength for specific circulation types. Conversely, inversion’s depth seems to be independent from atmospheric circulation.