A cloud algorithm based on the O 2 -O 2 477 nm absorption band featuring an advanced spectral fitting method and the use of surface geometry-dependent Lambertian-equivalent reflectivity
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.