Abstract
Abstract. The FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A-band) algorithm has been used to retrieve cloud information from measurements of the O2 A-band around 760 nm by GOME, SCIAMACHY and GOME-2. The cloud parameters retrieved by FRESCO are the effective cloud fraction and cloud pressure, which are used for cloud correction in the retrieval of trace gases like O3 and NO2. To improve the cloud pressure retrieval for partly cloudy scenes, single Rayleigh scattering has been included in an improved version of the algorithm, called FRESCO+. We compared FRESCO+ and FRESCO effective cloud fractions and cloud pressures using simulated spectra and one month of GOME measured spectra. As expected, FRESCO+ gives more reliable cloud pressures over partly cloudy pixels. Simulations and comparisons with ground-based radar/lidar measurements of clouds show that the FRESCO+ cloud pressure is about the optical midlevel of the cloud. Globally averaged, the FRESCO+ cloud pressure is about 50 hPa higher than the FRESCO cloud pressure, while the FRESCO+ effective cloud fraction is about 0.01 larger. The effect of FRESCO+ cloud parameters on O3 and NO2 vertical column density (VCD) retrievals is studied using SCIAMACHY data and ground-based DOAS measurements. We find that the FRESCO+ algorithm has a significant effect on tropospheric NO2 retrievals but a minor effect on total O3 retrievals. The retrieved SCIAMACHY tropospheric NO2 VCDs using FRESCO+ cloud parameters (v1.1) are lower than the tropospheric NO2VCDs which used FRESCO cloud parameters (v1.04), in particular over heavily polluted areas with low clouds. The difference between SCIAMACHY tropospheric NO2 VCDs v1.1 and ground-based MAXDOAS measurements performed in Cabauw, The Netherlands, during the DANDELIONS campaign is about −2.12×1014molec cm−2.
Highlights
Clouds have significant effects on trace gas retrievals from satellite spectrometers operating in the UV/visible, such as GOME, SCIAMACHY, OMI and GOME-2
We found that the difference between the FRESCO+ and FRESCO cloud heights decreases with increasing of effective cloud fraction
To analyze the difference between the FRESCO+ and FRESCO cloud pressures, we show in Fig. 5 these cloud pressures as a function of effective cloud fraction
Summary
Clouds have significant effects on trace gas retrievals from satellite spectrometers operating in the UV/visible, such as GOME, SCIAMACHY, OMI and GOME-2. We have found that the FRESCO cloud pressures are often too low (cloud heights are too high) when the effective cloud fractions are less than 0.1 These are cases with a relatively large contribution from Rayleigh scattering, which is missing in the FRESCO algorithm. The FRESCO+ algorithm fits a simulated reflectance spectrum to the measured reflectance spectrum in the three windows, to retrieve the effective cloud fraction and cloud pressure. The FRESCO+ effective cloud fraction and cloud pressure are being used in O3 and NO2 total and tropospheric vertical column density (VCD) retrievals performed within the DUE TEMIS project (see http://www.temis.nl). The reason is probably that the OMI cloud heights are
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