Shortwave and longwave radiative effects of the intense Saharan dust event of 25-26 March 2010 at Lampedusa (Mediterranean Sea)

Aerosol optical properties, shortwave (SW) and longwave (LW) irradiances, were measured at the island of Lampedusa during a very intense Saharan dust event in March 2010. The dust optical depth at 500 nm reached the 10 year record value of 1.9 on 25 March. Outgoing radiative fluxes from the Clouds and the Earth's Radiant Energy System were used to derive the dust radiative effect at the top of the atmosphere (TOA). The SW and LW radiative forcings (RFs) over the sea were derived by combining irradiances measured during the dust event and on a pristine day, and radiative transfer calculations. At the satellite overpass (solar zenith angle of about 35°) the SW instantaneous RF was −209 W m−2 at the surface, −116 W m−2 at TOA, and +93 W m−2 in the atmosphere; the diffuse SW downward irradiance increased by up to 376 W m−2 with respect to the dust-free case. The LW RF was as large as +41.5 W m−2 at the surface, +20 W m−2 at TOA, and −22 W m−2 in the atmosphere. The LW forcing offsets about 20% of the SW instantaneous forcing at the surface, about 17% at TOA, and 24% in the atmosphere. It is estimated that on a daily basis the LW radiative forcing offsets 49% of the SW effect at the surface, 35% at TOA, and about 77% in the atmosphere, thus compensating for a large fraction of the SW heating.