Radiative properties and direct effect of Saharan dust measured by the C‐130 aircraft during Saharan Dust Experiment (SHADE): 2. Terrestrial spectrum

Abstract

In situ measurements of terrestrial radiation from the C‐130 aircraft during the Saharan Dust Experiment (SHADE) are used to quantify the effect of a strong dust outbreak on radiance and brightness temperatures. The dust gives a distinct spectral signature in upwelling and downwelling terrestrial radiation when high spectral resolution data for a dusty day is compared to data from a clear day. A radiative transfer model is used together with a size distribution retrieved from Sun photometers and atmospheric profiles from dropsondes to simulate the radiance data and provide a constraint on the refractive indices of Saharan dust in the terrestrial part of the spectrum. The degree of agreement between observed and simulated brightness temperatures is dominated by the choice of refractive index, the mass loading, and the altitude of the dust layer. The uncertainties in size distribution appear to have less of an effect so long as large particles (radius greater than 1 μm) are included. In the terrestrial spectrum the dust produced a relative warming rate of up to 0.5 K/day below the dust and a relative cooling of up to 0.5 K/day within the dust layer itself. The effect on irradiance due to this dust outbreak was a decrease in upwelling terrestrial radiation at the top of the atmosphere of 6.5 Wm−2 and an increase in downwelling terrestrial radiation at the surface of 11.5 Wm−2. The dust led to decreases in brightness temperature of 2–4 K in the window region, consistent with apparent features in the sea surface temperature retrieved from the advanced very high resolution radiometer.