The regime of desert dust episodes in the Mediterranean based on contemporary satellite observations and ground measurements

Abstract. The regime of desert dust (DD) episodes over the broader Mediterranean basin is studied for the period 2000–2007. The novelty of this work lies in its complete spatial coverage of the region. An objective and dynamic algorithm has been set up, which uses daily measurements of various aerosol optical properties taken by different satellite databases, enabling the identification of DD episodes and their classification into strong and extreme ones. The algorithm's performance was tested against surface based (in situ) Particulate Matter (PM) and (columnar) sun-photometric AERONET measurements from stations distributed across the Mediterranean. The comparisons have shown the reasonable ability of the algorithm to detect the DD episodes taking place within the study region. The largest disagreements with PM data were found in summer and western Mediterranean, when African dust transport has a great vertical extent that cannot be satisfactorily captured by surface measurements. According to our results, DD episodes in the Mediterranean basin are quite frequent (up to 11.4 episodes/year) while there is a significant spatial and temporal variability in their frequency of occurrence and their intensity. Strong episodes occur more frequently in the western Mediterranean basin whilst extreme ones appear more frequently over central Mediterranean Sea areas. Apart from this longitudinal variation, there is a predominant latitudinal variability in both frequency and intensity, with decreasing values from south to north. A significant seasonal variation was also found for the frequency of DD episodes, with both strong and extreme episodes being more frequent during summer in the western Mediterranean basin, but during spring in its central and eastern parts. In most cases (>85%) the Mediterranean dust episodes last a bit longer than a day, although their duration can reach 6 days for strong episodes and 4 days for extreme episodes. A noticeable year by year variability was also found, especially for the frequency of the episodes. The spatial and temporal patterns of Mediterranean DD episodes can be explained based on surface pressure and precipitation spatio-temporal distribution patterns over the study region, as well as by the year by year variability of North Atlantic Oscillation (NAO). In this context, a decreasing frequency of appearance of DD episodes over the Mediterranean basin has been revealed over the period 2000–2007, especially over land surfaces, in line with decreasing NAO Index over the same period. Our findings demonstrate the reasonable ability to detect desert dust outbreaks in the Mediterranean basin from satellites.