Abstract
Abstract. We discuss a new cloud algorithm that retrieves an effective cloud pressure, also known as cloud optical centroid pressure (OCP), from oxygen dimer (O2-O2) absorption at 477 nm after determining an effective cloud fraction (ECF) at 466 nm, a wavelength not significantly affected by trace-gas absorption and rotational Raman scattering. The retrieved cloud products are intended for use as inputs to the operational nitrogen dioxide (NO2) retrieval algorithm for the Ozone Monitoring Instrument (OMI) flying on the Aura satellite. The cloud algorithm uses temperature-dependent O2-O2 cross sections and incorporates flexible spectral fitting techniques that account for specifics of the surface reflectivity. The fitting procedure derives O2-O2 slant column densities (SCDs) from radiances after O3, NO2, and H2O absorption features have been removed based on estimates of the amounts of these species from independent OMI algorithms. The cloud algorithm is based on the frequently used mixed Lambertian-equivalent reflectivity (MLER) concept. A geometry-dependent Lambertian-equivalent reflectivity (GLER), which is a proxy of surface bidirectional reflectance, is used for the ground reflectivity in our implementation of the MLER approach. The OCP is derived from a match of the measured O2-O2 SCD to that calculated with the MLER method. Temperature profiles needed for computation of vertical column densities are taken from the Global Modeling Initiative (GMI) model. We investigate the effect of using GLER instead of climatological LER on the retrieved ECF and OCP. For evaluation purposes, the retrieved ECFs and OCPs are compared with those from the operational OMI cloud product, which is also based on the same O2-O2 absorption band. Impacts of the application of the newly developed cloud algorithm to the OMI NO2 retrieval are discussed.
Highlights
Satellite ultraviolet and visible (UV–vis) nadir backscattered sunlight trace-gas algorithms need accurate estimates of cloud parameters in order to produce high-quality data sets
To account for the bidirectional reflectance distribution function (BRDF), we developed a new model of geometry-dependent Lambertian equivalent reflectivity (GLER) that was implemented within the existing Ozone Monitoring Instrument (OMI) cloud and NO2 retrieval algorithms (Vasilkov et al, 2017)
In a case-by-case study of the presumably cloud-free areas, we have found that various combinations of linear functions fitting the flanks of the O2-O2 profile lead to gross underestimates or overestimates of the retrieved scene pressures that are directly linked to the biases in the slant column densities (SCDs) evaluations
Summary
Satellite ultraviolet and visible (UV–vis) nadir backscattered sunlight trace-gas algorithms need accurate estimates of cloud parameters in order to produce high-quality data sets. Because of complexity of cloud effects on the radiation field in the atmosphere, clouds in trace-gas algorithms are treated using multiple simplifying assumptions. The fundamental assumptions are (1) the independent pixel approximation (IPA) that neglects the horizontal transport of radiative energy between the clear-sky and overcast subpixels, and (2) the assumption of horizontally and vertically homogeneous clouds that substantially simplifies radiative transfer in the clouds. For trace-gas retrievals it is important to estimate the photon path lengths in the atmosphere that determine trace-gas absorption and affect the measured top-of-atmosphere (TOA) radiance. The photon path lengths in a cloudy atmosphere are determined by the following most important cloud parameters, the geometrical cloud fraction and the cloud vertical extinction profile (Stammes et al, 2008).
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