Improvements to the OMI O<sub>2</sub>–O<sub>2</sub> operational cloud algorithm and comparisons with ground-based radar–lidar observations

Abstract

Abstract. The OMI (Ozone Monitoring Instrument on board NASA's Earth Observing System (EOS) Aura satellite) OM-CLDO2 cloud product supports trace gas retrievals of for example ozone and nitrogen dioxide. The OMCLDO2 algorithm derives the effective cloud fraction and effective cloud pressure using a DOAS (differential optical absorption spectroscopy) fit of the O 2 -O 2 absorption feature around 477 nm. A new version of the OMI OMCLDO2 cloud product is presented that contains several improvements, of which the introduction of a temperature correction on the O 2 -O 2 slant columns and the updated look-up tables have the largest impact. Whereas the differences in the effective cloud fraction are on average limited to 0.01, the differences of the effective cloud pressure can be up to 200 hPa, especially at cloud fractions below 0.3. As expected, the temperature correction depends on latitude and season. The updated look-up tables have a systematic effect on the cloud pressure at low cloud fractions. The improvements at low cloud fractions are very important for the retrieval of trace gases in the lower troposphere, for example for nitrogen dioxide and formaldehyde. The cloud pressure retrievals of the improved algorithm are compared with ground-based radar-lidar observations for three sites at mid-latitudes. For low clouds that have a limited vertical extent the comparison yields good agreement. For higher clouds, which are vertically extensive and often contain several layers, the satellite retrievals give a lower cloud height. For high clouds, mixed results are obtained.

The operational cloud retrieval algorithms from TROPOMI on board Sentinel-5 Precursor

“…The cloud fraction of this product is based on the OMI O 2 -O 2 absorption feature around 477 nm (Veefkind et al, 2016). …”

“…This algorithm is based on the mixed Lambertian equivalent reflectivity (MLER) assumption. The second algorithm (OMI/Aura Cloud Pressure and Fraction O 2 -O 2 Absorption, or OMCLDO2) uses reflectances in and around the O 2 -O 2 absorption band near 477 nm (Acarreta et al, 2004;Veefkind et al, 2016); differential optical absorption spectroscopy (DOAS)-retrieved O 2 -O 2 slant columns are compared with simulated look-up table entries to obtain effective cloud fraction and cloud pres-D. G. Loyola et al: The operational cloud retrieval algorithms from TROPOMI sure. The third algorithm (OMI/Aura Near UV Aerosol Optical Depth and Single Scattering Albedo, or OMAERUV) derives aerosol optical depth and single-scattering albedo (Torres et al, 2007) from radiances at 354 and 388 nm; the cloud fraction is computed as an intermediate step.…”

An exploratory study on the aerosol height retrieval from OMI measurements of the 477 nm O<sub>2</sub> − O<sub>2</sub> spectral band using a neural network approach

“…the high frequency part) present in passive UV-vis spectral measurements of atmospheric radiation (Platt and Stutz, 2008). Here, the DOAS fit follows the same approach as in the OMI O 2 −O 2 cloud algorithm (Acarreta et al, 2004;Veefkind et al, 2016): i.e. the absorption cross-section spectrum of O 2 −O 2 is fitted together with a first-order polynomial:…”

“…The OMI O 2 −O 2 477 nm absorption band is currently operationally exploited by the OMI O 2 −O 2 cloud algorithm to derive effective cloud fraction and pressure (Acarreta et al, 2004;Veefkind et al, 2016). Park et al (2016) applied a lookup table (LUT) approach on this band to retrieve aerosol effective height over ocean, close to East Asia, within the error range of 1 km (compared to CALIOP).…”

Quality assessment of the Ozone_cci Climate Research Data Package (release 2017): 1. Ground-based validation of total ozone column data products

“…To account for contamination by clouds and/or aerosols, an effective scene approach is used (Coldewey-Egbers et al, 2005) in which the effective albedo of a scene located in between the cloud top height and the ground surface is fitted during the retrieval. The altitude of this effective scene depends on both the effective cloud fraction 30 and cloud top altitude provided by independent cloud algorithms (FRESCO v7, Wang et al, 2008 or the O2-O2 product, Veefkind et al, 2016). Radiances are simulated on-the-fly with the scalar radiative transfer model LIDORT for GOME, SCIAMACHY and GOME-2.…”