[1] The physical and optical properties of Saharan dust aerosol measured by the Met Office C-130 during the Saharan Dust Experiment (SHADE) are presented. Additional radiation measurements enable the determination of the aerosol optical depth, t aerl , and the direct radiative effect (DRE) of the mineral dust. The results suggest that the absorption by Saharan dust is significantly overestimated in the solar spectrum if standard refractive indices are used. Our measurements suggest an imaginary part of the refractive index of 0.0015i is appropriate at a wavelength l of 0.55 mm. Different methods for determining t aerl=0.55 are presented, and the accuracy of each retrieval method is assessed. The value t aerl=0.55 is estimated as 1.48 ± 0.05 during the period of heaviest dust loading, which is derived from an instantaneous DRE of approximately À129 ± 5 Wm À2 or an enhancement of the local planetary albedo over ocean of a factor of 2.7 ± 0.1. A comparison of the DRE derived from the C-130 instrumentation and from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite is presented; the results generally showing agreement to within a factor of 1.2. The results suggest that Saharan dust aerosol exerts the largest local and global DRE of all aerosol species and should be considered explicitly in global radiation budget studies.

[1] The Meteorological Office C-130 aircraft performed a dedicated flight over the Etosha Pan surface-based Aerosol Robotic Network (AERONET) Sun photometer site on 13 September 2000 during the Southern African Aerosol Regional Science Initiative (SAFARI 2000) intensive measurement campaign. Aerosol optical depths at different wavelengths, t aerl , are derived from in situ measurements of the scattering and absorption coefficients and from various radiometric measurements and compared to those derived from the Sun photometer site. The estimates of t aerl from the various measurements are shown to be in good agreement. The exception to this is when t aerl is derived from the Passive Cavity Aerosol Spectrometer Probe (PCASP), as this method is shown to be extremely sensitive to the pitch angle of the aircraft; therefore, t aerl differs for profile ascents and profile descents. However, the aerosol size distribution measured by the PCASP and derived from the AERONET site are in excellent agreement over the 0.05-1.0 mm radius range, which contains the majority of the optically active particles. C-130-derived refractive indices and single scattering albedos are also shown to be in excellent agreement with those derived from the AERONET site. The consistency between in situ and remotely sensed data suggests that, for aerosol well mixed in the vertical, data from AERONET may be used with confidence in validating satellite measurements and modeling studies of the radiative properties and effects of aerosols.

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SUMMARYAircraft and ground-based interferometer measurements are used to investigate the dependence of sea surface emissivity on water temperature and salinity in the infrared spectral region. The effect of dissolved salts is found to be small and in line with previous studies, whereas temperature (often neglected in current emissivity models) has a greater impact. The influence on satellite sea surface temperature (SST) retrievals is found to be significant for high-resolution infrared sounders: neglecting a temperature-dependent emissivity leads to systematic errors in SST of as much as 0.6 K depending on channel frequency.

SUMMARYCalculated irradiances from a new radiation code are compared with in situ observations of short-wave irradiances from the UK Meteorological Office's C-130 aircraft. Three cases of clear skies are studied and four where a liquid-water boundary-layer cloud was present. Under clear-sky conditions the modelled and in situ observations agree to within 3%, which is the estimated accuracy of the observations. In the cloudy-sky cases the albedo and transmittance agree to within fO.l but the absorption in the model is higher than that observed, sometimes by a factor of two; there is no evidence of anomalous absorption in the observations. The observed absorptions do not exceed 6% for the stratocumulus cases considered. The results clearly identify the problems of representing inhomogeneous clouds as plane parallel layers in radiation models. Analysis of the variability of the cloud microphysics provides some insight into the importance of regions of low optical depth within the clouds.