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CLARIFI, CLouds and Aerosol Radiative Interaction and Forcing Investigation

M de Graaf, LG Tilstra, P Stammes

The CLouds and Aerosol Radiative Interaction and Forcing Investigation (CLARIFI) project was carried out as part of ESA’s Changing Earth Science Network Initiative. The goal of the project was to quantify the radiative effect of ultraviolet (UV)-absorbing aerosols above clouds, using a new method based on space-based spectral reflectance measurements and radiative transfer modelling. Aerosols play an important role in the global energy balance, by absorbing and scattering solar radiation, and more importantly, by modifying cloud properties and atmospheric column stability. These important secondary effects in the global energy balance are currently far from understood and CLARIFI was designed to help unravel an important part of this problem using existing satellite measurements. The unique broad spectral range of the space-borne spectrometer Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) was used to separate aerosol radiative effects in the UV, and cloud radiative effects in the shortwave infrared (SWIR). In the UV, aerosol absorption from smoke is strong, creating a strong signal in the measured reflectance. In the SWIR, absorbing and scattering effects from smoke are negligible, allowing the retrieval of cloud parameters from the measured spectrum using existing retrieval techniques. The spectral signature of the cloud can be modelled using a radiative transfer model (RTM) and the cloud parameters retrieved in the SWIR. In this way, the aerosol effects can be determined from the measured aerosol-polluted cloud shortwave spectrum and the modelled aerosol-unpolluted cloud shortwave spectrum. The difference between the spectra are directly linked to the aerosol direct radiative effect (DRE), which can be quantified for each SCIAMACHY cloud scene. The aerosol DRE retrieved using this approach is not dependent on retrieved or assumed aerosol properties, which means a huge reduction in the uncertainty of the derived aerosol DRE as compared to existing methods. During project CLARIFI a large LookUp Table (LUT) of precomputed cloud reflectance spectra was created for the fast computation of the aerosol DRE for SCIAMACHY cloud scenes. This LUT contains the shortwave reflectance spectra for a range of aerosol-unpolluted water clouds which are likely to be present under smoke layers. It was shown that this LUT can be used to simulate the reflectance spectrum of a SCIAMACHY cloud scene or other space-borne spectro(radio)meters. Using SCIAMACHY data, the aerosol DRE was quantified for a region over the South-Atlantic Ocean in the boreal summer months of 2006–2009, when smoke from vegetation fires is present over a persistent marine stratiform cloud deck. The aerosol DRE averaged through August 2006 was found to be 23 ± 8 Wm−2 with a mean variation over the region in this month of 22 Wm−2. The largest aerosol DRE over clouds found in that month was 132 ± 8 Wm−2 , which is a significant portion of the incoming solar radiation, heating the atmosphere locally. The results were presented at several workshops and conferences, and a paper was
published in the peer-reviewed literature, describing the method and these results for the first time.

Bibliografische gegevens

M de Graaf, LG Tilstra, P Stammes. CLARIFI, CLouds and Aerosol Radiative Interaction and Forcing Investigation
KNMI number: WR-12-01, Year: 2012, Pages: 26

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