Jm. Haywood et al., Optical properties and direct radiative effect of Saharan dust: A case study of two Saharan dust outbreaks using aircraft data, J GEO RES-A, 106(D16), 2001, pp. 18417-18430
The radiative effects of Saharan dust are measured during two flights by th
e Met Office C-130 aircraft off the west coast of Africa. Data from the bro
adband radiometers suggests that the perturbation to the top of the atmosph
ere net solar irradiance is as strong as -60 W m(-2) +/-5 W m(-2) during th
e dust events. In situ measurements with the nephelometer and particle soot
absorption photometer suggest that the single scattering albedo is approxi
mately 0.87 at a wavelength of 0.55 mum. This is in agreement with the opti
cal parameters calculated from independent measurements of the particle siz
e distributions combined with suitable refractive indices and Mie-scatterin
g theory. The wavelength dependence of the extinction coefficient derived f
rom measurements of the scattering coefficient by the nephelometer is also
in excellent agreement with the calculations. Independent surface-based mea
surements from Cape Verde suggest that the wavelength dependence of the aer
osol optical depth appears reasonable. Calculations of the downward solar i
rradiances within the aerosol layer are generally in good agreement with th
e measurements demonstrating consistency between the measurements and the m
odeling efforts. The terrestrial radiative effect is not detectable by the
current instrumentation, though it cannot be considered negligible. These m
easurements suggest that satellite retrieval algorithms may misclassify the
aerosol outbreak as cloud because the aerosol optical depth at 0.55 mum is
as high as 1.15, which is in excess of the thresholds used in some cloud d
etection algorithms. The measurements demonstrate that this method could be
used to provide an accurate benchmark for satellite-based estimates of the
radiative effect of aerosols.