Gkikas A., Houssos E.E., Lolis C.J., Bartzokas A., Mihalopoulos N. and Hatzianastassiou N. (2014)
Quarterly Journal of the Royal Meteorological Society (published online). DOI: 10.1002/qj.2466
Abstract
In the present study, the cases of atmospheric circulation evolution favouring the occurrence of Desert Aerosol Episodes (DAEs) over the broader Mediterranean region are investigated using an objective and dynamic algorithm, with daily satellite data for the period 2000-2007. After identifying strong and extreme DAEs, at 1ox1o geographical cell level. 98 Dust Aerosol Episode Days (DAEDs) and 62 cases of consecutive DAEDs (Desert Aerosol Episode Cases – DAECs) are defined. For each DAEC, the lower tropospheric circulation evolution one and two days before, during the initiation and after the cease of the DAEC is considered. S-mode Factor Analysis and K-Means Cluster Analysis are applied on NCEP/NCAR Mean Sea Level Pressure and 700 hPa geopotential height fields, classifying the 62 cases of atmospheric circulation evolution into 6 homogenous and discrete clusters. The mean intra-annual variation of the DAECs reveals a primary maximum in July (22.6%), while their mean annual number is equal to 8.9 DAECs. On a seasonal basis, the highest percentage of the DAECs is found in spring (54.9%). The 64.5% of DAECs last one day while their maximum duration is 5 days. Annually, the mean monthly number of DAEs varies from 35.0 (September) to 61.5 (October). The western parts of the Mediterranean are affected by DAEs when cyclonic conditions prevail in the western Mediterranean and NW Africa. On the contrary, 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 intensity (aerosol optical depth at 550nm) the strong DAEs vary from 0.74 to 0.90, while the extreme ones from 1.25 to 2.21. Generally, strong DAEs are more frequent than extreme ones (in five out of six clusters).