Interpretation of FRESCO cloud retrievals in case of absorbing aerosol events

Abstract

Abstract. Cloud and aerosol information is needed in trace gas retrievals from satellite measurements. The Fast REtrieval Scheme for Clouds from the Oxygen A band (FRESCO) cloud algorithm employs reflectance spectra of the O 2 A band around 760 nm to derive cloud pressure and effective cloud fraction. In general, clouds contribute more to the O 2 A band reflectance than aerosols. Therefore, the FRESCO algorithm does not correct for aerosol effects in the retrievals and attributes the retrieved cloud information entirely to the presence of clouds, and not to aerosols. For events with high aerosol loading, aerosols may have a dominant effect, especially for almost cloud free scenes. We have analysed FRESCO cloud data and Absorbing Aerosol Index (AAI) data from the Global Ozone Monitoring Experiment (GOME-2) instrument on the Metop-A satellite for events with typical absorbing aerosol types, such as volcanic ash, desert dust and smoke. We find that the FRESCO effective cloud fractions are correlated with the AAI data for these absorbing aerosol events and that the FRESCO cloud pressure contains information on aerosol layer pressure. For cloud free scenes, the derived FRESCO cloud pressure is close to the aerosol layer pressure, especially for optically thick aerosol layers. For cloudy scenes, if the strongly absorbing aerosols are located above the clouds, then the retrieved FRESCO cloud pressure may represent the height of the aerosol layer rather than the height of the clouds. Combining FRESCO and AAI data, an estimate for the aerosol layer pressure can be given.

Utilization of O<sub>4</sub> slant column density to derive aerosol layer height from a space-borne UV–visible hyperspectral sensor: sensitivity and case study

“…Kokhanovsky and Rozanov (2010) estimated dust altitudes using the O 2 A band between 760 and 765 nm after the determination of the dust optical depth. In addition, several previous studies also investigated estimation methods for aerosol height information by using hyperspectral measurement in the visible spectrum (e.g., Dubuisson et al, 2009;Koppers and Murtagh, 1997;Sanders and de Haan, 2013;Sanghavi et al, 2012;Wang et al, 2012). Because the surface signal is significantly smaller than the aerosol signal in the near-UV spectrum, these wavelength regions are useful to derive aerosol height information from space-borne measurements.…”

Retrieval of aerosol parameters from the oxygen A band in the presence of chlorophyll fluorescence

“…Furthermore, there are indications that the FRESCO cloud retrieval algorithm also provides information on aerosols when there are optically thick aerosol layers (Wang et al, 2012). Dubuisson et al (2009) have presented a method to retrieve the altitude of aerosol plumes over ocean from the ratio of reflectances in the two O 2 A band channels of the MEdium Resolution Imaging Spectrometer (MERIS: Rast et al, 1999) instrument and the POLarization and Directionality of the Earth's Reflectance (POLDER: Deschamps et al, 1994) instrument.…”

Overview of the O3M SAF GOME-2 operational atmospheric composition and UV radiation data products and data availability

“…18, where the ash plume originating from the Puyehue volcano can be seen to travel from its source eastward over the south Atlantic Ocean. This event has been analysed in Wang et al (2012).…”