Gkikas, A., Houssos, E.E., Bartzokas, A., Mihalopoulos, N. and Hatzianastassiou, N. (2011)
The 3rd Urbino Symposium on Air Quality and Climate Change: Interactions and Feedbacks
Abstract
Desert dust (DD) is the predominant aerosol type of the Mediterranean basin and its burden is strongly dependent by the prevailing synoptic conditions. Due to the proximity with the Sahara and the Middle East deserts, intense dust outbreaks (episodes) are reported, primarily, in spring and secondarily in summer. In the present study, a description of the prevailing synoptic conditions associated with dust outbreaks over the Mediterranean is attempted, for the period 1 March 2000 – 28 February 2007. This was achieved based on a synergistic use of daily satellite measurements from the MODerate Resolution Imaging Spectroradiometer (MODIS-Terra), Earth Probe and Ozone Monitoring Instrument (OMI). An objective and dynamic algorithm has been developed, which, in its initialization stage, asseses appropriate aerosol optical properties for the identification of desert dust episodes. Each day in which the number of pixels, over the Mediterranean, with available AOD550nm values is greater than 300 and also the total (strong+extreme) number of DD episodes is greater than 30, is named as Desert Aerosol Episode Day (DAED). Their total number is 98. In order to study the synoptic conditions, prevailing during these DAEDs, the fields of geopotential height at 700mb (Z700) and mean sea level pressure (MSLP) provided by the NCEP/NCAR Reanalysis Project, were used. After the application of the multivariate statistical methods of Factor Analysis(S-mode) and Cluster Analysis the atmospheric circulations, during the DD episodes, have been classified into 6 representative clusters. The intra-annual variation of the DAEDs reveals maximum frequency in July (23%), while the maximum seasonal percentage is observed in spring (54%). Among the six clusters, the most frequent is Cluster 3 (48%), while the rarest is Cluster 6 (2%). The central parts of the Mediterranean basin are affected under the atmospheric circulation of Cluster 1 in spring. On the contrary, the western parts of the study region are affected by dust episodes, during the synoptic conditions of Cluster 2 and 3. This atmospheric circulation, at the low troposphere, favours the occurrence of DD episodes, which their intensity can reach up to 4.3. According to the seasonality analysis, Cluster 2 is reported only in March (7 DAEDs) while Cluster 3 is more frequent in July (23 DAEDs). The eastern parts of the Mediterranean basin are affected by dust outbreaks, under the prevailing synoptic conditions of Cluster 4 and 5. The intensity of these DD episodes can reach up to 5 for extreme cases. Both clusters are reported more frequently in April with 5 and 9 DAEDs, respectively. The Cluster 6 is the less frequent, since only 2 DAEDs out of 98 are classified into it. The intensity of these DD outbreaks can reach up to 4.