Analysis of Scattering Angle Sampling by Multi-Angle Imaging Polarimeters for Different Orbit Geometries

Thompson, Sabrina; Diedenhoven, Bastiaan van; Colarco, Peter R.; Castellanos, Patricia; Lian, Eric C.‐Y.; Martins, J. V.

doi:10.3389/frsen.2022.836262

Cited by 3 publications

(2 citation statements)

References 24 publications

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“…It should be noted that several EUREC 4 A HALO flights were partly conducted during nighttime (Konow et al, 2021), when the observation of the cloudbow is, of course, impossible. A recent study by Thompson et al (2022) evaluated the sampled scattering angle range of multiangle satellite instruments depending on different factors, such as the solar and view geometry or the location, season, and swath width, in more detail.…”

Section: Discussionmentioning

confidence: 99%

High-spatial-resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

Pörtge¹,

Kölling

Weber³

et al. 2023

Atmos. Meas. Tech.

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Abstract. The cloud droplet size distribution is often described by a gamma distribution defined by the effective radius and the effective variance. The effective radius is directly related to the cloud's optical thickness, which influences the radiative properties of a cloud. The effective variance affects, among other things, the evolution of precipitation. Both parameters can be retrieved from measurements of the cloudbow. The cloudbow (or rainbow) is an optical phenomenon that forms due to the single scattering of radiation by liquid cloud droplets at the cloud edge. The polarized radiance of the cloudbow crucially depends on the cloud droplet size distribution. The effective radius and the effective variance can be retrieved by fitting model simulations (stored in a lookup table) to polarized cloudbow observations. This study uses measurements from the wide-field polarization-sensitive camera of the spectrometer of the Munich Aerosol Cloud Scanner (specMACS) aboard the German “High Altitude and LOng range research aircraft” (HALO). Along with precise cloud geometry data derived by a stereographic method, a geolocalization of the observed clouds is possible. Observations of the same cloud from consecutive images are combined into one radiance measurement from multiple angles. Two case studies of trade-wind cumulus clouds measured during the EUREC4A (ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte) field campaign are presented, and the cloudbow technique is demonstrated. The results are combined into maps of the effective radius and the effective variance with a 100 m × 100 m spatial resolution and large coverage (across-track swath width of 8 km). The first case study shows a stratiform cloud deck with distinct patches of large effective radii up to 40 µm and a median effective variance of 0.11. specMACS measures at a very high angular resolution (binned to 0.3∘) which is necessary when large droplets are present. The second case study consists of small cumulus clouds (diameters of approximately 2 km). The retrieved effective radius is 7.0 µm, and the effective variance is 0.08 (both median values). This study demonstrates that specMACS is able to determine the droplet size distribution of liquid water clouds even for small cumulus clouds, which are a problem for traditional droplet size retrievals based on total reflectances.

show abstract

Section: Discussionmentioning

confidence: 99%

High-spatial-resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

Pörtge¹,

Kölling

Weber³

et al. 2023

Atmos. Meas. Tech.

View full text Add to dashboard Cite

show abstract

“…A recent study by Thompson et al (2022) evaluated the sampled scattering angle range of multi-angle satellite instruments depending on different factors such as solar and view geometry, or location, season, and swath width in more detail.…”

mentioning

confidence: 99%

High spatial resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

Pörtge¹,

Kölling²,

Weber³

et al. 2022

Preprint

View full text Add to dashboard Cite

Abstract. The cloud droplet size distribution is often described by a gamma distribution defined by the effective radius (reff) and the effective variance (veff). The effective radius is directly related to the cloud’s optical thickness which influences the radiative properties of a cloud. The effective variance affects, among other things, the evolution of precipitation. Both parameters can be retrieved from measurements of the cloudbow. The cloudbow or rainbow is an optical phenomenon, which forms by single scattering of radiation by liquid cloud droplets at the cloud edge. The polarized radiance of the cloudbow crucially depends on the cloud droplet size distribution. reff and veff can be retrieved by fitting model simulations (stored in a look-up table) to polarized cloudbow observations. This study uses measurements from the wide-field polarization-sensitive camera of the LMU spectral imager system specMACS onboard the German research airplane HALO. Together with precise cloud geometry data derived by a stereographic method, a geolocalization of the observed clouds is possible. Observations of the same cloud from consecutive images are combined into one radiance measurement from multiple angles. Two case studies of trade wind cumulus clouds measured during the EUREC4A field campaign are presented and the cloudbow technique is demonstrated. The results are combined into maps of reff and veff with a spatial resolution of 100 m by 100 m and large coverage (across-track swath width: 8 km). The first case study shows a stratiform cloud deck with distinct patches of large effective radii up to 40 µm and a median effective variance of 0.12. The second case study consists of small cumulus clouds (diameters of approximately 2 km). The retrieved reff is 6.92 µm and veff is 0.07 (both median values).

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Accounting for the effect of aerosols in GHGSat methane retrieval

Yu,

Jervis,

Huang

2024

Atmos. Meas. Tech.

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Abstract. GHGSat comprises a constellation of satellites with high spatial and spectral resolution that specialize in monitoring methane emissions at 1.65 µm. This study investigates the ability to accurately retrieve both the methane mixing-ratio enhancement (ΔXCH4) and the aerosol optical depth (AOD) simultaneously from simulated GHGSat observations that incorporate angle-dependent scattering information. Results indicate that the sign of the ΔXCH4 bias when neglecting aerosols changes from negative to positive as surface albedo increases, which is consistent with previous studies. The bias in ΔXCH4 is most pronounced when AOD is not simultaneously retrieved, ranging from −3.0 % to 6.3 % with an AOD of 0.1, a 60° solar zenith angle, and a surface albedo of 0.2 for the nadir-only retrieval. Using multiple satellite viewing angles during the GHGSat observation sequence with a scattering angle ranging from 100 to 140°, the study shows that the mean bias and standard deviation of ΔXCH4 are within 0.3 % and 2.8 % relative to the background. The correlation between simultaneously retrieved ΔXCH4 and AOD shifts from being positive to negative as surface albedo increases and the aerosol asymmetry factor decreases, signifying a transition of the dominant aerosol effect from aerosol-only scattering to aerosol–surface multiple scattering. The variety of scattering angle ranges has little impact on the performance of the multi-angle viewing method. This study improves the understanding of the impact of aerosols on the GHGSat ΔXCH4 retrieval and provides guidance for improving future GHGSat-like point-source imagers.

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Analysis of Scattering Angle Sampling by Multi-Angle Imaging Polarimeters for Different Orbit Geometries

Cited by 3 publications

References 24 publications

High-spatial-resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

High-spatial-resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

High spatial resolution retrieval of cloud droplet size distribution from polarized observations of the cloudbow

Accounting for the effect of aerosols in GHGSat methane retrieval

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