2012-05-04: Retrieval of the aerosol direct radiative effect over clouds from space-borne spectrometry.

Aerosols play an important role in the Earth's radiation balance, by scattering and absorbing solar radiation. More importantly, aerosols can change cloud dynamics as cloud condensation nuclei and by absorbing solar radiation, thereby heating the atmosphere. This will influence the atmospheric column stability and cloud forming processes. In this paper the absorption of solar radiation by aerosols is quantified using satellite spectrometry, for aerosol layers that are present over clouds.

Space-based observations of aerosols in the presence of clouds is normally difficult, because of the brightness of the clouds. The scattering of radiation by clouds is generally an order of magnitude larger than the scattering by aerosols. Therefore, aerosol parameters are normally retrieved only after cloud screening. In constrast, the absorption of radiation by small aerosols, like smoke, is strong in the ultraviolet (UV), and can be detected even above clouds using UV-measuring instruments. In the shortwave infrared (SWIR) on the other hand, the influence of the aerosol disappear and unperturbed cloud parameters can be retrieved. This separation of clouds and aerosols was used to quantify the direct radiative effect (DRE) of aerosols above clouds, which is the amount of absorbed radiation by the aerosols.

The DRE was computed from reflectances measured by SCIAMACHY, a space-borne spectrometer covering 92% of the shortwave spectrum (red curve in figure 1). From cloud information retrieved from this reflectance spectrum in the SWIR, the shortwave reflectance spectrum can be simulated for clouds that are not perturbed by aerosol absorption (blue curve in figure 1). The difference between the two curves is caused by aerosol absorption and the integral over the diffence is proportional to the aerosol DRE, which can be retrieved in this way. The SCIAMACHY aerosol DRE was retrieved over the South Atlantic Ocean near the west coast of Africa on 13 August 2006 (figure 2). On this day a large plume of smoke from biomass burning events in Angola was found over a semi-permanent marine boundary layer cloud deck. This cloud deck is visualised by the MERIS RGB colour composite. White cloud fields are smoke-free, while the grey-brownish cloud fields are areas where the light reflected from the clouds is attenuated by aerosol absorption. This aerosol absorption is quantified by the overlaid SCIAMACHY aerosol DRE. Aerosol DRE values of up to 128 +/- 7 Wm^-2 can be observed over the clouds on this day, which is about 10% of the incoming solar radiation. This constitutes a considerable local climate effect.

M. de Graaf, L.G. Tilstra, P. Wang and P. Stammes (2012), Retrieval of the aerosol direct radiative effect over clouds from space-borne spectrometry.
Figure 1. SCIAMACHY measured reflectance spectrum (red) on 13 August 2006, 09:19:43 UTC, and the modelled equivalent aerosol-free reflectance spectrum (blue) for this scene. The arrows indicate the parts of the spectrum where information about the atmosphere of this scene is retrieved.
Figure 2. MERIS RGB composite overlaid with SCIAMACHY aerosol DRE on 13 August 2006, from 09:13:27 - 09:22:48 UTC, near the West coast of Africa, showing the horizontal distribution of clouds, and the radiation absorbed by aerosols overlying the clouds. The aerosol DRE is retrieved only in nadir states (shown by the grey boxes) and only over marine water clouds.