Full year cycle of desert dust spectral optical thickness and precipitable water vapor over Alexandria, Egypt

We study the annual cycle of dust loading in Alexandria, Egypt. Observations were performed from December 1997 to November 1998, including during the Kamaseen storms of March 1998. A ground-based, manual Sun photometer was used to measure aerosol optical thickness (τa) at four spectral bands in the 340-870 nm wavelength range (namely 340, 440, 675, 870). Total precipitable water vapor (W) was also measured simultaneously, based on the 936 nm channel measurements and extrapolations of the aerosol optical thickness from the neighboring 675 and 870 nm channels. Aerosol optical thickness τa at all the four spectral bands were very high (averaging 2.0-4.0) during the Kamaseen storms of late March 1998. Correspondingly, the Ångström wavelength exponent α of the optical thickness fell close to zero during that time, implying a substantial increase in dust. Overall, the monthly mean and median aerosol optical thickness were highest during January-May and lowest in June-October. During the January-May period total precipitable water vapor and Ångström exponent were lower than during the June-October period. There is a high correlation between the Ångström exponent and the optical thickness (r=0.63), with α ranging from 0.0 to 0.5 for τa > 1.0, indicating high dust concentration. Trajectory analysis shows that the presence of dust was associated with air masses arriving predominantly from the Sahara or North Africa. No significant correlation was found between the optical thickness and the precipitable water vapor. These basic systematic observations are vital for assessing dust climatology in this important part of the world and also for validating satellite observations and dust transport models. Copyright 2001 by the American Geophysical Union.