Seasonal behavior of Saharan dust events at the Mediterranean island of Lampedusa in the period 1999–2005

Abstract

Multi-filter rotating shadowband radiometer (MFRSR) measurements have been carried out at Lampedusa (35.52°N, 12.63°E) in 1999, and continuously since 2001. This study describes the Saharan dust (SD) events at Lampedusa on the basis of daily average optical depth at 500 nm, τ, and Ångström exponent, α, derived from these observations. Back-trajectories ending at Lampedusa at 2000 and 4000 m altitude were calculated by means of the HYSPLIT model. SD events are identified as those for which the trajectories interact with the mixed layer in places where the surface wind exceeds 7 m s −1, or spend a large fraction of time over the Sahara. The SD days display values of α+Δ α⩽1, with Δ α equal to the standard deviation of the daily α. Out of 911 days with cloud-free intervals, 233 (26%) are classified as SD, and correspond to 111 episodes of various duration, from 1 to 13 consecutive days. The occurrence of SD events is maximum in summer (33%), when also the largest seasonal average of τ (0.40) is measured, and minimum in winter (7%), when the smallest seasonal average of α (0.08) is found. SD days have been identified from the back-trajectories also in days lacking of observations, due to either cloudiness or measurement interruptions. The frequency of occurrence of SD days shows little change with respect to the cloud-free periods (24%). The seasonal distribution shows a peak in May (38%), followed by July (37%). Regions of SD production were derived from the HYSPLIT trajectories and NCEP-reanalysis surface winds. Finally, the MFRSR measurements at the solar zenith angle of 60° have been used to derive the single scattering albedo (SSA) for cases clearly dominated by dust ( τ⩾0.40 and α+Δ α⩽0.5). The average SSA for the whole period is 0.77±0.04 at 415.6 nm and 0.94±0.04 at 868.7 nm.