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Cases of atmospheric circulation evolution favouring the occurrence of desert aerosol episodes (DAEs) over the broader Mediterranean region were investigated using an objective and dynamic algorithm, with daily satellite data for the period 2000-2013. After identifying strong and extreme DAEs, at a 1°×1° geographical-cell level, 255 dust aerosol episode days (DAEDs) and 148 cases of consecutive DAEDs, namely desert aerosol episode cases (DAECs), are defined. For each DAEC, the evolution of the lower tropospheric circulation 1 and 2 days before, during the initiation and after the cessation of the DAEC, is considered. S-mode factor analysis and k-means cluster analysis are applied on mean sea-level pressure and 700 hPa geopotential height fields obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project, classifying the 148 cases of atmospheric-circulation evolution into six homogeneous and discrete clusters. The mean intra-annual variation of the DAECs reveals a primary maximum in May (18.9%), and their mean annual number is equal to 11.4 DAECs. On a seasonal basis, the highest percentage of the DAECs is found in spring (51.4%). Maximum duration of the DAECs is 7 days, with 58.8% lasting 1 day. Annually, the mean monthly number of the DAEs varies from 35.8 (September) to 58.0 (April). The western parts of the Mediterranean are affected by the DAEs when cyclonic conditions prevail in the western Mediterranean and northwestern Africa. In contrast, the central and eastern parts of the study region are affected by dust storms when a low-pressure system in the central Mediterranean or central Europe and an anticyclone in the eastern Mediterranean prevail. As to the mean regional intensity (aerosol optical depth at 550 nm) the strong DAEs vary from 0.67 to 0.77, while the extremes vary from 1.14 to 2.06. Generally, strong DAEs are more frequent than extremes (in five out of six clusters). [ABSTRACT FROM AUTHOR]

The evolution of the atmospheric circulation over Europe and the Mediterranean during fog events in Ioannina, north-western Greece, is examined for the period 1957-2002. A 24-h period starting at 12:00 UTC is defined as a fog day, when at least one of the present and/or past weather codes of the corresponding SYNOPs refers to fog. A sequence of such fog days is defined as a fog event. In total, 1019 fog events are found during the period under study. A methodology scheme using S-mode factor analysis and K-means cluster analysis is applied to the patterns of 850 hPa air temperature and 1000 and 500 hPa geopotential heights for the day before the initiation, the first day of occurrence and the day after the dissipation of fog, resulting in the classification of the 1019 fog events into ten clusters. For each one of the ten clusters, the mean 00:00 UTC patterns of the above parameters are constructed for 3 days. These patterns correspond to the evolution of the atmospheric circulation associated with fog formation and dissipation in the Ioannina region. Most of the patterns are characterized by anticyclonic activity or a weak southerly flow over Greece. The differences among the ten clusters refer mainly to the position and the intensity of the surface and upper air synoptic systems involved. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The article presents a study on the spatial and temporal variability and co-variability and the teleconnection patterns of downward solar radiation (DSR) at the surface of the Earth. The study uses the multivariate statistical method of Factor Analysis (FA) on the DSR data that was computed with a physical deterministic transfer model. Furthermore, the result shows a strong variability of DSR due to the complex interactions of solar radiation with the atmosphere and surface of the Earth.