Short‐wave and long‐wave radiative properties of Saharan dust aerosol

Abstract

The simulated irradiances are less sensitive to particle shape than radiances. However, it is shown through modelling of the surface and top-of-atmosphere (TOA) DRE over all daylight hours that significant differences exist at TOA due to variation in the asymmetry parameter. The TOA short-wave diurnally averaged DRE was modelled as between 0 and -20 W m −2 depending on particle shape. A long-wave interferometer measured downwelling and upwelling radiances to derive surface emissivity across the window region. Measured nadir brightness temperatures from high level show signature of dust. A drop in brightness temperature of 14K was determined using modelled pristine-sky spectra. The modelled outgoing long-wave DRE due to dust from this case was +14 W m −2 averaged over 24 h, or +17 W m −2 per unit AOD. Modelling studies illustrate the sensitivity to aerosol refractive index and size distribution for both short-wave and long-wave DREs. Considering the full spectrum, a refractive index dataset from the literature has been selected that best represents the Saharan dust encountered during GERBILS.

Motivation, rationale and key results from the GERBILS Saharan dust measurement campaign

“…Clearly, there is a need for further research and field measurements in this area, particularly at backscattering angles, where differences in scattering are manifest between smooth and irregular particles. Osborne et al (2011) also show that while the SW spectral signature of mineral dust is clearly detectable in low-altitude zenith measurements over land, there are significant difficulties in detection in highaltitude nadir measurements owing to the variability in the underlying surface reflectance. The aircraft measurements and radiative transfer modelling suggest that daytime mean reductions of downwelling SW radiation at the surface are estimated to be 137-153 W m −2 /AOD 550 .…”

“…The Balkanski et al (2007) refractive indices appear to perform reasonably in obtaining radiative closure in the LW region of the spectrum Haywood et al, 2011). Osborne et al (2011) suggest a diurnal average LW radiative impact of around 18 W m −2 /AOD 550 .…”

“…Upon approaching an area of heavy dust loading as determined from satellite data and from model forecasts, the aircraft performed a profile down into the mineral dust, a low-level straight and level run, and a series of banked orbits to make measurements analogous to the almucantar scans of AERONET (see Osborne et al, 2011), before profiling back up and continuing at FL200-FL250 (7-8 km) to the destination airport. Nevertheless, these flight patterns proved very effective at sampling peak concentrations of mineral dust aerosol.…”

“…Ground-based measurements during SAMUM ) estimate the single scattering albedo at Tindouf in northern Morocco to be 0.96 ± 0.02 (at 540 nm) when high concentrations of mineral dust are present, which is similar to our estimates. Osborne et al (2011) used aircraft orbits and SW spectral radiance measurements in close proximity to the Dakar AERONET site (Holben et al, 1998) to show that the aircraft SWS reproduced the AERONET sun photometer radiances as a function of the scattering angle during almucantar scans (e.g. Dubovik et al, 2006).…”

“…Kokhanovsky (2003) performed laboratory studies showing that irregular polycrystals can represent scattered radiances from mineral dust to high accuracy. However, Osborne et al (2011) are the first to show that use of polycrystals definitively leads to better agreement between measured and modelled atmospheric radiances than the use of spheres or spheroid models (i.e. models with smooth surfaces) for high optical depth mineral dust aerosol.…”