Abstract
Abstract. Measurements carried out on the island of Lampedusa, in the central Mediterranean, on 7 September 2005, show the occurrence of a quasi-periodic oscillation of aerosol optical depth, column water vapour, and surface irradiance in different spectral bands. The oscillation has a period of about 13 min and is attributed to the propagation of a gravity wave able to modify the vertical structure of the planetary boundary layer, as also confirmed by satellite images. The wave occurred during a Saharan dust event. The oscillation amplitude is about 0.1 for the aerosol optical depth, and about 0.4 cm for the column water vapour. The modulation of the downward surface irradiances is in opposition of phase with respect to aerosol optical depth and water vapour column variations. The perturbation of the downward irradiance produced by the aerosols is determined by comparing the measured irradiances with estimated irradiances at a fixed value of the aerosol optical depth, and by correcting for the effect of the water vapour in the shortwave spectral range. The direct radiative forcing efficiency, i.e., the radiative perturbation of the net surface irradiance produced by a unit of optical depth aerosol layer, is determined at different solar zenith angles as the slope of the irradiance perturbation versus the aerosol optical depth. The estimated direct surface forcing efficiency at about 60° solar zenith angle is −(181 ± 17) W m−2 in the shortwave, and −(83 ± 7) W m−2 in the photosynthetic spectral range. The estimated daily average forcing efficiencies are of about −79 and −46 W m−2 for the shortwave and photosynthetic spectral range, respectively.
Highlights
M ing the measured irradiances with estimated irradiances at a with different aerosol components and conditions
Observations collected at Lampedusa during the morning of 7 September 2005 display a quasi-periodic modulation of aerosol optical depth, column water vapour, and downward shortwave irradiance
The value of the tio is found for large solar zenith angles, while the highest albedo a is calculated following Jin et al (2004) taking into is found for θ of about 35◦
Summary
Observations collected at Lampedusa during the morning of 7 September 2005 display a quasi-periodic modulation of aerosol optical depth, column water vapour, and downward shortwave irradiance. Of the gravity wave displays only one maximum, i.e., it is confined within half vertical wavelength of the wave structure Such condition depends critically on the structure of relative maxima and minima The critical level, where the phase velocity of that the observed oscillations in the aerosol optical depth and the wave matches the horizon1ta8l velocity of the wind and the column water vapour are inde1e9d produced by a gravity wave Richardson number reaches a minimum value of 0.05 is at propagating through the atmosphere at that time
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