CLouds and Aerosol Radiative Interaction and Forcing Investigation (2010-2011)

The Clarifi project aims at quantifying the absorption of solar radiation by tropospheric aerosols in the presence of underlying clouds, using space-borne passive imagery. Clarifi is part of ESA’s Changing Earth Science Network

Aerosols absorb and scatter solar energy in the atmosphere and modify solar radiation transport indirectly by modifying cloud properties. These effects of aerosols could potentially partially offset or even amplify the global warming due to greenhouse gases, but the relative contribution of aerosols to climate change is still largely unquantified (IPCC, 2007). Since many sources of tropospheric aerosols are heterogeneously distributed and often located in remote areas, the observation of aerosol distributions in the atmosphere can be challenging. Space-based observations from Earth-orbiting satellites have proved to be essential for the continuous monitoring of aerosols in the Earth’s atmosphere (Kaufman, 2002). However, most space-based aerosol retrieval schemes rely on cloud screening algorithms before retrieving aerosol properties from an observed scene, considering cloud contaminated scenes too bright to allow aerosol retrievals (e.g. Veefkind, 2000; King, 2003). Therefore, direct studies of cloud and aerosol interactions are often not possible from these observations. Furthermore, most aerosol retrieval schemes rely on aerosol microphysical property assumptions, which often introduce large uncertainties in the retrieved aerosol parameters.

Both these problems can be avoided using the Absorbing Aerosol Index (AAI), which uses radiation measurements in the UltraViolet (UV) in order to detect absorbing aerosols (Torres, 1998, De Graaf, 2005) . The AAI from the space-based spectrometer ENVISAT/SCIAMACHY (Bovensmann, 1999) can be used to detect absorbing aerosols that absorb solar radiation over land and ocean surfaces, both in the presence of clouds (if the absorbing aerosol layer overlies the cloud) and cloud-free scenes (De Graaf, 2007). An example is shown in Figure 1, where smoke from the African continent is advected over a permanent cloud deck over the South Atlantic ocean. This can be detected using the AAI.

SCIAMACHY has the unique feature of measuring over a large spectral range in the UV, visible and InfraRed (IR), a range that contains almost all of the solar energy. Figure 2 shows the reflectance spectra of the rectangle areas in Figure 1 as measured by SCIAMACHY. An indication of the amount of energy that is absorbed by the aerosols in the UV can be obtained by comparing the ‘clean’ cloud reflectance and the ‘polluted’ cloud reflectance, as indicated in Figure 2. The reflectance of the polluted cloud scene is much lower in the UV than the clean cloud scene reflectance due to the absorption by the aerosols.

The two cloud scene reflectance shown in Figure 2 are rather arbitrary, CLARIFI aims at accurately quantifying the amount of aerosol absorption using reflectance measurements of SCIAMACHY, and a Radiative Transfer Model (RTM) to simulate the clean cloud reflectance. The aerosols in the scene can be detected using the AAI, the cloud parameters can be retrieved from the IR, where the optical thickness of the aerosols is negligible. Using the RTM and the measured cloud parameters, the reflectance spectrum in the UV of the scene without aerosols can be simulated. The effect of the UV-absorbing aerosols on the reflectance spectrum can then be determined directly, by comparing the measured and the simulated scene. From this the direct effect of aerosols may be quantified, which is the amount of absorbed radiation by aerosols in a scene. Currently, this quantity can be determined when aerosol microphysical properties are known, which, using passive satellite imagery, can be done in cloud-free scenes only, as explained above.

SCIAMACHY data that is available since 2002 will be screened for the aerosol direct effect above clouds, to investigate the semi-direct effect of aerosols: the absorption of the solar radiation by the aerosols heats the atmosphere locally, which can influence the vertical stability of the atmosphere and the cloud layer and can evaporate cloud droplets.

The semi-direct effect is known to be dependent on the vertical distribution of the aerosols and clouds. Active space-based sensors, like the space-based lidar CALIPSO/CALIOP and the planned EarthCare instrument, provide a vertical profile of the scattering and absorbing properties of the atmosphere and aerosols above a cloud, or down to the surface in the absence of clouds. Therefore, co-located measurements from SCIAMACHY and CALIPSO may give detailed measurements of the aerosol direct effect, and the relative location of the clouds and the aerosols.