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| Titel |
Estimate of the Saharan dust shortwave and photosynthetic radiative forcing efficiency at the surface during the propagation of a gravity wave in the central Mediterranean |
| VerfasserIn |
Alcide di Sarra, Daniele Fuà, Daniela Meloni |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250080172
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| Zusammenfassung |
| This study is based on measurements made at ENEA Station for Climate Observations
(35.52Ë N, 12.63Ë E, 50 m asl) on the island of Lampedusa, in the Southern part of the
Central Mediterranean. A quasi periodic oscillation of aerosol optical depth, column water
vapour, shortwave (SW) and photosynthetic active radiation (PAR) is observed to occur
during the morning of 7 September 2005. The quasi-periodic wave is present from about 6 to
10 UT, with solar zenith angles (SZA) varying between 77.5Ë and 37.2Ë . In this period the
aerosol optical depth at 500 nm, Ï, varies between 0.29 and 0.41; the column water
vapour, cwv, varies between 2.4 and 2.8 cm. The oscillations of Ï and cwv are in
phase, while the modulation of the downward surface irradiances is in opposition of
phase with respect to Ï and cwv. The period of the oscillation is about 13 min. The
oscillation is attributed to the propagation of a gravity wave which modulates the
structure of the planetary boundary layer. The measured aerosol optical properties are
typical of cases dominated by Saharan dust, with the Ångström exponent comprised
between 0.5 and 0.6. The backtrajectory analysis for that day shows that airmasses
overpass Northern Libya (trajectories arriving below 2000 m), Tunisia and Northern
Algeria (trajectories arriving above 2000 m), carrying Saharan dust particles to
Lampedusa. The combined modulation of downward irradiance, water vapour column, and
aerosol optical depth is used to estimate the aerosol effect on the irradiance. From
the irradiance-optical depth relation, the aerosol surface direct forcing efficiency
(FE) is derived, under the assumption that during the measurement interval the
aerosol microphysical properties do not appreciably change. As a first step, all
SW irradiances are reported to the same cwv content (2.6 cm), by using radiative
transfer model calculations. Reference curves describing the downward SW and PAR
irradiances are constructed by using measurements obtained at a fixed Ï values. The
radiative perturbation produced by dust is thus obtained as the difference between
the measured irradiances, reported at cwv = 2.6 cm, and the reference curve at
the same SZA. The radiative forcing efficiencies are then determined in different
intervals of SZA as the slope of the linear fit of the radiative perturbation versus the
measured aerosol optical depth. The estimated values of the dust FE are -188±18
Wm-2 for the SW, and -93±7 Wm-2 for PAR at about 70Ë SZA, -163±16 Wm-2
for the SW and -112±3 Wm-2 for PAR at about 37Ë SZA. The estimated daily
average FE is -79 Wm-2 for the SW range, and -46 Wm-2 for PAR. The obtained
values are in good agreement with previous estimates for desert dust over the ocean. |
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