Assessing the quality of active–passive satellite retrievals using broad‐band radiances

Abstract

Included in the Earth, Clouds, Aerosols, and Radiation Explorer (EarthCARE) satellite's array of instruments is a multi-view broad-band radiometer (BBR). BBR data will facilitate a radiative closure assessment of cloud and aerosol properties inferred from data gathered by EarthCARE's other passive and active sensors. The closure assessment will consist, in part, of comparisons between BBR radiances and radiances computed by three-dimensional (3D) radiative transfer models (RTM) that act on narrow 3D domains that derive from, and include, the retrieved cross-section of cloud and aerosol properties. Assessment domains D will be ∼100 km 2 . Following a brief outline of the closure experiment, a method is proposed for estimating the likelihood of BBR radiances providing a meaningful closure assessment of cloud and aerosol properties in D. The method capitalizes on the ability of Monte Carlo RTMs to compute contributions to radiances from any constituent in any given D. While this methodology introduces some circularity into the closure test, it might, nevertheless, be tolerable, given that the method's purpose is simply to identify, and thus avoid, assessments that are likely to be fruitless or misleading. A 3000 km long stretch of A-Train satellite data was used in this initial demonstration of the proposed methodology. Only results for solar radiation are shown. All radiative quantities used here were computed by a 3D Monte Carlo RTM. A control simulation provided proxy BBR measurements. Random 'errors' were introduced into the A-Train field to produce experimental fields that roughly mimic retrievals. Experimental and control radiances were compared in mock-closure assessments. Arbitrarily assuming that a fruitful assessment requires ∼75% of a BBR radiance to result from cloud and aerosol scattering events inside D, ∼70% of the (11 km) 2 domains were flagged as reliably testable for this example.

The Spectral Nature of Earth’s Reflected Radiation: Measurement and Science Applications

“…In a broad sense, this overall approach that uses atmospheric profile input is analogous to that developed for the CloudSat flux product (Hendersen et al, 2013), although on a different spatial scale, as well as closely following the EarthCARE approach. Both have been shown to perform similarly to CERES observed fluxes at the top of atmosphere while also delivering vertical flux profiles and related radiative heating rates (Barker et al, 2015;Tornow et al, 2018). Construction of broad band shortwave fluxes from spectral radiances has also been demonstrated in a few studies, such as in Doelling et al, 2013 who used geostationary spectral radiances to derive CERES-like time resolved fluxes and Oyola et al, 2019 who provide a case study field experiment example combining airborne HSRL lidar measurements with spectral radiance imagery to quantify radiative properties of aerosol.…”

On the use of simulated photon paths to co‐register top‐of‐atmosphere radiances in EarthCARE radiative closure experiments

“…EarthCARE will retrieve vertical profiles of cloud and aerosol properties by combining data from active and passive instruments. In order to check the verisimilitude of the retrieved properties, a radiative closure assessment is foreseen (see Barker et al , ; Illingworth et al , ) in which EarthCARE 's BBR radiances, measured at three viewing angles, will be compared with modelled radiances simulated by 1D and 3D radiative transfer models. Differences between measured and simulated radiances and fluxes will hopefully help retrieval algorithm developers and users to understand the performance characteristics of the algorithms and their retrieved products.…”

“…In general, however, clouds can be semi‐transparent with multiple reflecting layers present in D and clouds in D can be either blocked by clouds in front of D or their contribution to radiances overpowered by backlighting from reflecting layers or surfaces behind D (cf. Barker et al , ). Hence, when observing D from an off‐nadir perspective, the following question arises: which off‐nadir radiances are most useful for assessing retrievals in D ?…”

“…Using the same MC model as used here, Barker et al () used all radiance contributions leaving a domain D to estimate whether oblique TOA radiances receive sufficient radiation scattered from clouds and aerosols in order to carry out a meaningful radiative closure assessment of retrieval quantities in D . Here, on the other hand, radiance contributions stemming from scattering events are accumulated for each cell.…”

“…Arguably, these radiances afford the most stringent assessment of retrievals: not only do these views differ radically from that used by the retrieval algorithms, but regions well below cloud tops can, at times, determine off‐nadir radiances strongly, thus providing a stronger test of the retrieved profile than that afforded by nadir radiances alone (cf. Barker et al , ).…”