Vertical profiles of droplet size distributions derived from cloud-side observations by the research scanning polarimeter: Tests on simulated data

Alexandrov, Mikhail D.; Miller, D.J.; Rajapakshe, Chamara; Fridlind, A. M.; Diedenhoven, Bastiaan van; Cairns, Brian; Ackerman, A. S.; Zhang, Zhibo

doi:10.1016/j.atmosres.2020.104924

Cited by 14 publications

(18 citation statements)

References 41 publications

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“…While the RSP does retrieve r eff larger than 30 µm, no study has yet validated the RFT retrieval on droplets in this size range (cf. Alexandrov et al, 2018Alexandrov et al, , 2020, leaving the possibility that the RSP may be partially insensitive to droplets in this range.…”

Section: Discussionmentioning

confidence: 99%

Inference of Precipitation in Warm Stratiform Clouds Using Remotely Sensed Observations of the Cloud Top Droplet Size Distribution

Sinclair

Diedenhoven

Cairns

et al. 2021

Geophysical Research Letters

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Low stratiform clouds cover approximately one third of Earth's surface and substantially enhance the shortwave radiative effect (Klein & Hartmann, 1993). The shortwave radiative effects of these clouds are determined by macrophysical properties such as cloud coverage and liquid water paths (LWPs) as well as microphysical properties that relate to their droplet size distributions (DSDs). Precipitation influences these clouds' macro-and micro-physical properties and thereby their radiative effects. Precipitation amount and rates are in turn governed by a large number of microphysical, thermodynamic and dynamical processes (Austin et al., 1995;Wood, 2005a). For example, studies have found that drizzle rates correlate positively with cloud water content and droplet sizes (

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Section: Discussionmentioning

confidence: 99%

Inference of Precipitation in Warm Stratiform Clouds Using Remotely Sensed Observations of the Cloud Top Droplet Size Distribution

Sinclair

Diedenhoven

Cairns

et al. 2021

Geophysical Research Letters

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“…The boundary of the union of all such (usually overlapping) disks is then declared to be the shape of the cloud. In some cases (especially of elongated polygons) this technique does not work well, so the polygon itself provides a better representation of the cloud shape (Alexandrov et al, 2020). A similar "space carving" methodology has been developed in a 3D case for the Multi-angle Imaging SpectroRadiometer (MISR) measurements (Lee et al, 2018).…”

Section: Derivation Of Cloud Shapes From the Research Scanning Polarimeter Measurementsmentioning

confidence: 99%

“…This method is a further development of our operational algorithms (Alexandrov et al, 2012a;Alexandrov et al, 2012b) based on the polarized rainbow analyses. The only new feature introduced by Alexandrov et al (2020) was the aggregation of the RSP measurements to a range of points at the cloud's side instead of the stereo-derived cloud top. The "cloud's side" here is a part of the cloud-shape curve corresponding to some brightness threshold.…”

Section: Derivation Of Droplet Size Profiles Along the Cloud Sidementioning

confidence: 99%

“…The presented algorithm has been developed for the Research Scanning Polarimeter (RSP) measurements. The RSP (Cairns et al, 1999) is an airborne sensor which high-resolution along-track scanning provides sufficient number of view rays to constrain cloud shapes (Alexandrov et al, 2016a) and also to derive droplet size distribution (DSD) profiles along the cloud side (Alexandrov et al, 2020). The RSP served as a prototype for the satellite Aerosol Polarimetery Sensor (APS) built for the NASA Glory Project (Mishchenko et al, 2006;Mishchenko et al, 2007).…”

Section: Introductionmentioning

confidence: 99%

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Application of Radon Transform to Multi-Angle Measurements Made by the Research Scanning Polarimeter: A New Approach to Cloud Tomography. Part I: Theory and Tests on Simulated Data

Alexandrov

Emde

Diedenhoven

et al. 2021

Front. Remote Sens.

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The Research Scanning Polarimeter (RSP) is an airborne along-track scanner measuring the polarized and total reflectances in 9 spectral channels. The RSP was a prototype for the Aerosol Polarimetery Sensor (APS) launched on-board the NASA Glory satellite. Currently the retrieval algorithms developed for the RSP are being adopted for the measurements of the space-borne polarimeters on the upcoming NASA’s Plankton, Aerosol, Cloud Ocean Ecosystem (PACE) satellite mission. The RSP’s uniquely high angular resolution coupled with the high frequency of measurements allows for characterization of liquid water cloud droplet sizes using the polarized rainbow structure. It also provides geometric constraints on the cumulus cloud’s 2D cross section yielding the cloud’s geometric shape estimates. In this study we further build on the latter technique to develop a new tomographic approach to retrieval of cloud internal structure from remote sensing measurements. While tomography in the strict definition is a technique based on active measurements yielding a tomogram (directional optical thickness as a function of angle and offset of the view ray), we developed a “semi-tomographic” approach in which tomogram of the cloud is estimated from passive observations instead of being measured directly. This tomogram is then converted into 2D spatial distribution of the extinction coefficient using inverse Radon transform (filtered backprojection) which is the standard tomographic procedure used e.g., in medical CT scans. This algorithm is computationally inexpensive compared to techniques relying on highly-multi-dimensional least-square fitting; it does not require iterative 3D RT simulations. The resulting extinction distribution is defined up to an unknown constant factor, so we discuss the ways to calibrate it using additional independent measurements. In the next step we use the profile of the droplet size distribution parameters from the cloud’s side (derived by fitting the polarized rainbows) to convert the 2D extinction distribution into that of the droplet number concentration. We illustrate and validate the proposed technique using 3D-RT-simulated RSP observations of a LES-generated Cu cloud. Quantitative comparisons between the retrieved and the original optical and microphysical parameters are presented.

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“…Then, a standard bi-spectral technique infers the vertical profile of cloud particle size along the side of the cloud. Alexandrov et al [39] used LES clouds and 3D vector RT to demonstrate cloud-side microphysical profiling with RSP's scanning polarimetric capability.…”

Section: Contextmentioning

confidence: 99%

Multi-View Polarimetric Scattering Cloud Tomography and Retrieval of Droplet Size

et al. 2020

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Tomography aims to recover a three-dimensional (3D) density map of a medium or an object. In medical imaging, it is extensively used for diagnostics via X-ray computed tomography (CT). We define and derive a tomography of cloud droplet distributions via passive remote sensing. We use multi-view polarimetric images to fit a 3D polarized radiative transfer (RT) forward model. Our motivation is 3D volumetric probing of vertically-developed convectively-driven clouds that are ill-served by current methods in operational passive remote sensing. Current techniques are based on strictly 1D RT modeling and applied to a single cloudy pixel, where cloud geometry defaults to that of a plane-parallel slab. Incident unpolarized sunlight, once scattered by cloud-droplets, changes its polarization state according to droplet size. Therefore, polarimetric measurements in the rainbow and glory angular regions can be used to infer the droplet size distribution. This work defines and derives a framework for a full 3D tomography of cloud droplets for both their mass concentration in space and their distribution across a range of sizes. This 3D retrieval of key microphysical properties is made tractable by our novel approach that involves a restructuring and differentiation of an open-source polarized 3D RT code to accommodate a special two-step optimization technique. Physically-realistic synthetic clouds are used to demonstrate the methodology with rigorous uncertainty quantification.

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Vertical profiles of droplet size distributions derived from cloud-side observations by the research scanning polarimeter: Tests on simulated data

Cited by 14 publications

References 41 publications

Inference of Precipitation in Warm Stratiform Clouds Using Remotely Sensed Observations of the Cloud Top Droplet Size Distribution

Inference of Precipitation in Warm Stratiform Clouds Using Remotely Sensed Observations of the Cloud Top Droplet Size Distribution

Application of Radon Transform to Multi-Angle Measurements Made by the Research Scanning Polarimeter: A New Approach to Cloud Tomography. Part I: Theory and Tests on Simulated Data

Multi-View Polarimetric Scattering Cloud Tomography and Retrieval of Droplet Size

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