Broadband radiative quantities for the EarthCARE mission: the ACM-COM and ACM-RT products
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Abstract. The EarthCARE satellite mission's objective is to retrieve profiles of aerosol and water cloud physical properties from
measurements made by its cloud-profiling radar, backscattering lidar, and
passive multi-spectral imager. These retrievals, together
with other geophysical properties, are input into broadband (BB) radiative
transfer (RT) models that predict radiances and fluxes commensurate with
measurements made and inferred from EarthCARE's BB radiometer (BBR). The
scientific goal is that modelled and “observed” BB top-of-atmosphere (TOA) fluxes differ, on
average, by less than ±10 W m−2. When sound synergistic retrievals
from the ACM-CAP process (ACM: ATLID – backscattering lidar, CPR – cloud-profiling
radar, and MSI – multi-spectral imager; CAP: clouds, aerosols, and precipitation) are available, they are acted on by the RT models.
When they are not available, the RT models act on “composite” profiles of
properties retrieved from measurements made by individual sensors.
Compositing is performed in the ACM-COM (COM: composite) process. The majority of this report describes the RT models – and their products –
that make up EarthCARE's ACM-RT process. Profiles of BB shortwave (SW) and
longwave (LW) fluxes and heating rates (HRs) are computed by 1D RT models for each ∼ 1 km nadir column of inferred properties. Three-dimensional RT models
compute radiances for the BBR's three viewing directions, with the SW model
also computing flux and HR profiles; the 3D LW model produces upwelling flux
at just one level. All 3D RT products are averages over 5×21 km
“assessment domains” that are constructed using MSI data. Some of ACM-RT's products are passed forward to the “radiative closure assessment” process that quantifies, for each assessment domain, the likelihood that EarthCARE's
goal has been achieved. As EarthCARE represents the first mission to make
“operational” use of 3D RT models, emphasis is placed on differences
between 1D and 3D RT results. For upwelling SW flux at 20 km altitude, 1D
and 3D values can be expected to differ by more than EarthCARE's scientific
goal of ±10 W m−2 at least 50 % of the time.
