New sampling strategy mitigates a solar-geometry-induced bias in sub-kilometre vapour scaling statistics derived from imaging spectroscopy
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Published:2022-01-05
Issue:1
Volume:15
Page:117-129
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ISSN:1867-8548
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Container-title:Atmospheric Measurement Techniques
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language:en
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Short-container-title:Atmos. Meas. Tech.
Author:
Richardson Mark T.ORCID, Thompson David R.ORCID, Kurowski Marcin J., Lebsock Matthew D.
Abstract
Abstract. Upcoming spaceborne imaging spectrometers will retrieve
clear-sky total column water vapour (TCWV) over land at a horizontal resolution of 30–80 m. Here we show how to obtain, from these retrievals,
exponents describing the power-law scaling of sub-kilometre horizontal variability in clear-sky bulk planetary boundary layer (PBL) water vapour (q) accounting
for realistic non-vertical sunlight paths. We trace direct solar beam paths
through large eddy simulations (LES) of shallow convective PBLs and show that retrieved 2-D water vapour fields are “smeared” in the direction of the solar azimuth. This changes the horizontal spatial scaling of the field
primarily in that direction, and we address this by calculating exponents
perpendicular to the solar azimuth, that is to say flying “across” the
sunlight path rather than “towards” or “away” from the Sun. Across 23
LES snapshots, at solar zenith angle SZA = 60∘ the mean bias in calculated exponent is 38 ± 12 % (95 % range) along the solar azimuth, while
following our strategy it is 3 ± 9 % and no longer significant. Both bias and root-mean-square error decrease with lower SZA. We include
retrieval errors from several sources, including (1) the Earth Surface Mineral Dust Source Investigation (EMIT) instrument noise model, (2) requisite assumptions about the atmospheric thermodynamic profile, and (3) spatially nonuniform aerosol distributions. By only considering the direct
beam, we neglect 3-D radiative effects such as light scattered into the field of view by nearby clouds. However, our proposed technique is necessary
to counteract the direct-path effect of solar geometries and obtain unique
information about sub-kilometre PBL q scaling from upcoming spaceborne spectrometer missions.
Funder
Jet Propulsion Laboratory
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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