Abstract
Abstract. The FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band) algorithm is a simple, fast and robust algorithm used to retrieve cloud information in operational satellite data processing. It has been applied to GOME-1 (Global Ozone Monitoring Experiment), SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography), GOME-2 and more recently to TROPOMI (Tropospheric Monitoring Instrument). FRESCO retrieves effective cloud fraction and cloud pressure from measurements in the oxygen A band around 761 nm. In this paper, we propose a new version of the algorithm, called FRESCO-B, which is based on measurements in the oxygen B band around 687 nm. Such a method is interesting for vegetated surfaces where the surface albedo is much lower in the B band than in the A band, which limits the ground contribution to the top-of-atmosphere reflectances. In this study we first perform retrieval simulations. These show that the retrieved cloud pressures from FRESCO-B and FRESCO differ only between −10 and +10 hPa, except for high, thin clouds over vegetation where the difference is larger (about +15 to +30 hPa), with FRESCO-B yielding higher pressure. Next, inter-comparison between FRESCO-B and FRESCO retrievals over 1 month of GOME-2B data reveals that the effective cloud fractions retrieved in the O2 A and B bands are very similar (mean difference of 0.003), while the cloud pressures show a mean difference of 11.5 hPa, with FRESCO-B retrieving higher pressures than FRESCO. This agrees with the simulations and is partly due to deeper photon penetrations of the O2 B band in clouds compared to the O2 A-band photons and partly due to the surface albedo bias in FRESCO. Finally, validation with ground-based measurements shows that the FRESCO-B cloud pressure represents an altitude within the cloud boundaries for clouds that are not too far from the Lambertian reflector model, which occurs in about 50 % of the cases.
Highlights
The Global Ozone Monitoring Experiment-2 (GOME-2) is a spectrometer flying on the MetOp series of satellites: on MetOp-A since 2006, on MetOp-B since 2012 and on MetOp-C, which was launched in November 2018
We present a new cloud retrieval algorithm called FRESCO-B for GOME-2 measurements in the oxygen B band
We simulated cloudy scenes, which showed that FRESCO-B and FRESCO retrievals indicate an altitude inside the cloud layer but well below the top, which was expected
Summary
The Global Ozone Monitoring Experiment-2 (GOME-2) is a spectrometer flying on the MetOp series of satellites: on MetOp-A since 2006, on MetOp-B since 2012 and on MetOp-C, which was launched in November 2018. The primary goal of GOME2 measurements is the study of ozone as well as atmospheric trace gases and pollutants (e.g. nitrogen dioxide, sulfur dioxide, water vapour, bromine oxide). The trace-gas retrieval algorithms rely on information on cloud properties for each ground pixel. Clouds strongly affect tracegas retrievals from satellite measurements because of their shielding effect, albedo effect and in-cloud absorption effect (Stammes et al, 2008). In order to ensure goodquality trace-gas retrievals, it is very important to retrieve various cloud properties; the cloud top height, cloud geometrical thickness, cloud fraction, cloud optical thickness and the number of cloud layers (Boersma et al, 2004)
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