Optical properties and direct radiative effect of Saharan dust: A case study of two Saharan dust outbreaks using aircraft data

Citation
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
Citations number
47
Categorie Soggetti
Earth Sciences
Volume
106
Issue
D16
Year of publication
2001
Pages
18417 - 18430
Database
ISI
SICI code
Abstract
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.