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 km2. 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.