scholarly journals Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements

2012 ◽  
Vol 5 (10) ◽  
pp. 2361-2374 ◽  
Author(s):  
B. van Diedenhoven ◽  
B. Cairns ◽  
I. V. Geogdzhayev ◽  
A. M. Fridlind ◽  
A. S. Ackerman ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Optical Thickness ◽  
Asymmetry Parameter ◽  
Ice Crystal ◽  
Polarization Measurements ◽  
Aspect Ratios ◽  
Scattering Phase ◽  
Cloud Optical Thickness ◽  
The Asymmetry ◽  
Asymmetry Parameters

Abstract. We present a new remote sensing technique to infer the average asymmetry parameter of ice crystals near cloud top from multi-directional polarization measurements. The method is based on previous findings that (a) complex aggregates of hexagonal crystals generally have scattering phase matrices resembling those of their components; and (b) scattering phase matrices systematically vary with aspect ratios of crystals and their degree of microscale surface roughness. Ice cloud asymmetry parameters are inferred from multi-directional polarized reflectance measurements by searching for the closest fit in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with varying aspect ratios and roughness values. The asymmetry parameter of the hexagonal particle that leads to the best fit with the measurements is considered the retrieved value. For clouds with optical thickness less than 5, the cloud optical thickness must be retrieved simultaneously with the asymmetry parameter, while for optically thicker clouds the asymmetry parameter retrieval is independent of cloud optical thickness. Evaluation of the technique using simulated measurements based on the optical properties of a number of complex particles and their mixtures shows that the ice crystal asymmetry parameters are generally retrieved to within 5%, or about 0.04 in absolute terms. The retrieval scheme is largely independent of calibration errors, range and sampling density of scattering angles and random noise in the measurements. The approach can be applied to measurements of past, current and future airborne and satellite instruments that measure multi-directional polarized reflectances of ice-topped clouds.

scholarly journals Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements

2012 ◽  
Vol 5 (3) ◽  
pp. 4321-4359 ◽  
Author(s):  
B. van Diedenhoven ◽  
B. Cairns ◽  
I. V. Geogdzhayev ◽  
A. M. Fridlind ◽  
A. S. Ackerman ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Optical Thickness ◽  
Asymmetry Parameter ◽  
Ice Crystal ◽  
Polarization Measurements ◽  
Aspect Ratios ◽  
Scattering Phase ◽  
Cloud Optical Thickness ◽  
The Asymmetry ◽  
Asymmetry Parameters

Abstract. We present a new remote sensing technique to infer the average asymmetry parameter of ice crystals near cloud top from multi-directional polarization measurements. The method is based on previous findings that (a) complex aggregates of hexagonal crystals generally have scattering phase matrices resembling those of their components and (b) scattering phase matrices systematically vary with aspect ratios of crystals and their degree of microscale surface roughness. Ice cloud asymmetry parameters are inferred from multi-directional polarized reflectance measurements by searching for the closest fit in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual hexagonal columns and plates with varying aspect ratios and roughness values. The asymmetry parameter of the hexagonal particle that leads to the best fit with the measurements is considered the retrieved value. For clouds with optical thickness less than 5, the cloud optical thickness must be retrieved simultaneously with the asymmetry parameter, while for optically thicker clouds the asymmetry parameter retrieval is independent of cloud optical thickness. Evaluation of the technique using simulated measurements based on the optical properties of a number of complex particles and their mixtures shows that the ice crystal asymmetry parameters are generally retrieved to within 5%, or about 0.04 in absolute terms. The retrieval scheme is largely independent of calibration errors, range and sampling density of scattering angles and random noise in the measurements. The approach can be readily applied to measurements of past, current and future airborne and satellite instruments that measure multi-directional polarized reflectances of ice-topped clouds.

scholarly journals Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter

2012 ◽  
Vol 12 (12) ◽  
pp. 32063-32107
Author(s):  
B. van Diedenhoven ◽  
B. Cairns ◽  
A. M. Fridlind ◽  
A. S. Ackerman ◽  
T. J. Garrett
Keyword(s):  
Asymmetry Parameter ◽  
Climate Models ◽  
Continuous Selection ◽  
Ice Crystals ◽  
Shortwave Radiation ◽  
Convective Clouds ◽  
Ice Cloud ◽  
Aspect Ratios ◽  
The Asymmetry ◽  
Asymmetry Parameters

Abstract. A new method to retrieve ice cloud asymmetry parameters from multi-directional polarized reflectance measurements is applied to measurements of the airborne Research Scanning Polarimeter (RSP) obtained during the CRYSTAL-FACE campaign in 2002. The method assumes individual hexagonal ice columns and plates serve as proxies for more complex shapes and aggregates. The closest fit is searched in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with a virtually continuous selection of aspect ratios and distortion. The asymmetry parameter, aspect ratio and distortion of the hexagonal particle that leads to the best fit with the measurements are considered the retrieved values. Two cases of thick convective clouds and two cases of thinner anvil cloud layers are analyzed. Median asymmetry parameters retrieved by the RSP range from 0.76 to 0.78, and are generally smaller that those currently assumed in most climate models and satellite retrievals. In all cases the measurements indicate roughened ice crystals, which is consistent with previous findings. Retrieved aspect ratios in three of the cases range from 0.9 to 1.6, indicating compact particles dominate the cloud-top shortwave radiation. Retrievals for the remaining case indicate plate-like ice crystals with aspect ratios around 0.3. The RSP retrievals are qualitatively consistent with the CPI images obtained in the same cloud layers. Retrieved asymmetry parameters are compared to those determined in situ by the Cloud Integrating Nephelometer (CIN). For two cases, the median values of asymmetry parameter retrieved by CIN and RSP agree within 0.01, while for the two other cases RSP asymmetry parameters are about 0.03–0.05 greater than those obtained by the CIN. Part of this bias might be explained by vertical variation of the asymmetry parameter.

scholarly journals Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter

2013 ◽  
Vol 13 (6) ◽  
pp. 3185-3203 ◽  
Author(s):  
B. van Diedenhoven ◽  
B. Cairns ◽  
A. M. Fridlind ◽  
A. S. Ackerman ◽  
T. J. Garrett
Keyword(s):  
Asymmetry Parameter ◽  
Climate Models ◽  
Continuous Selection ◽  
Ice Crystals ◽  
Shortwave Radiation ◽  
Convective Clouds ◽  
Ice Cloud ◽  
Aspect Ratios ◽  
The Asymmetry ◽  
Asymmetry Parameters

Abstract. A new method to retrieve ice cloud asymmetry parameters from multi-directional polarized reflectance measurements is applied to measurements of the airborne Research Scanning Polarimeter (RSP) obtained during the CRYSTAL-FACE campaign in 2002. The method assumes individual hexagonal ice columns and plates serve as proxies for more complex shapes and aggregates. The closest fit is searched in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with a virtually continuous selection of aspect ratios and distortion. The asymmetry parameter, aspect ratio and distortion of the hexagonal particle that leads to the best fit with the measurements are considered the retrieved values. Two cases of thick convective clouds and two cases of thinner anvil cloud layers are analyzed. Median asymmetry parameters retrieved by the RSP range from 0.76 to 0.78, and are generally smaller than those currently assumed in most climate models and satellite retrievals. In all cases the measurements indicate roughened or distorted ice crystals, which is consistent with previous findings. Retrieved aspect ratios in three of the cases range from 0.9 to 1.6, indicating compact particles dominate the cloud-top shortwave radiation. Retrievals for the remaining case indicate plate-like ice crystals with aspect ratios around 0.3. The RSP retrievals are qualitatively consistent with the CPI images obtained in the same cloud layers. Retrieved asymmetry parameters are compared to those determined in situ by the Cloud Integrating Nephelometer (CIN). For two cases, the median values of asymmetry parameter retrieved by CIN and RSP agree within 0.01, while for the two other cases RSP asymmetry parameters are about 0.03–0.05 greater than those obtained by the CIN. Part of this bias might be explained by vertical variation of the asymmetry parameter or ice shattering on the CIN probe, or both.

scholarly journals Combined retrieval of Arctic liquid water cloud and surface snow properties using airborne spectral solar remote sensing

2017 ◽  
Vol 10 (9) ◽  
pp. 3215-3230 ◽  
Author(s):  
André Ehrlich ◽  
Eike Bierwirth ◽  
Larysa Istomina ◽  
Manfred Wendisch
Keyword(s):  
Remote Sensing ◽  
Grain Size ◽  
Sea Ice ◽  
Optical Thickness ◽  
Liquid Water ◽  
Retrieval Algorithm ◽  
Continuous Transition ◽  
Cloud Properties ◽  
Cloud Optical Thickness ◽  
The Impact

Abstract. The passive solar remote sensing of cloud properties over highly reflecting ground is challenging, mostly due to the low contrast between the cloud reflectivity and that of the underlying surfaces (sea ice and snow). Uncertainties in the retrieved cloud optical thickness τ and cloud droplet effective radius reff, C may arise from uncertainties in the assumed spectral surface albedo, which is mainly determined by the generally unknown effective snow grain size reff, S. Therefore, in a first step the effects of the assumed snow grain size are systematically quantified for the conventional bispectral retrieval technique of τ and reff, C for liquid water clouds. In general, the impact of uncertainties of reff, S is largest for small snow grain sizes. While the uncertainties of retrieved τ are independent of the cloud optical thickness and solar zenith angle, the bias of retrieved reff, C increases for optically thin clouds and high Sun. The largest deviations between the retrieved and true original values are found with 83 % for τ and 62 % for reff, C. In the second part of the paper a retrieval method is presented that simultaneously derives all three parameters (τ, reff, C, reff, S) and therefore accounts for changes in the snow grain size. Ratios of spectral cloud reflectivity measurements at the three wavelengths λ1 = 1040 nm (sensitive to reff, S), λ2 = 1650 nm (sensitive to τ), and λ3 = 2100 nm (sensitive to reff, C) are combined in a trispectral retrieval algorithm. In a feasibility study, spectral cloud reflectivity measurements collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART) during the research campaign Vertical Distribution of Ice in Arctic Mixed-Phase Clouds (VERDI, April/May 2012) were used to test the retrieval procedure. Two cases of observations above the Canadian Beaufort Sea, one with dense snow-covered sea ice and another with a distinct snow-covered sea ice edge are analysed. The retrieved values of τ, reff, C, and reff, S show a continuous transition of cloud properties across snow-covered sea ice and open water and are consistent with estimates based on satellite data. It is shown that the uncertainties of the trispectral retrieval increase for high values of τ, and low reff, S but nevertheless allow the effective snow grain size in cloud-covered areas to be estimated.

scholarly journals Potential of remote sensing of cirrus optical thickness by airborne spectral radiance measurements in different viewing angles and nadir geometry

2016 ◽  
Author(s):  
Kevin Wolf ◽  
André Ehrlich ◽  
Tilman Hüneke ◽  
Klaus Pfeilsticker ◽  
Frank Werner ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Radiative Transfer ◽  
Optical Thickness ◽  
Convective Cloud ◽  
Sensitivity Study ◽  
Mean Value ◽  
Spectral Radiance ◽  
Crystal Shape ◽  
Ice Crystal ◽  
The North

Abstract. Spectral radiance measurements from two airborne passive solar remote sensing instruments, the Spectral Modular Airborne Radiation measurement sysTem (SMART) and the Differential Optical Absorption Spectrometer (mini-DOAS), are used to compare the remote sensing of cirrus optical thickness τ in nadir and off-nadir geometry. The comparison is based on a sensitivity study using radiative transfer simulations and on measurements during the North Atlantic Rainfall VALidation (NARVAL) mission, the Mid-Latitude Cirrus Experiment (ML-CIRRUS) and the Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems (ACRIDICON) campaign. Radiative transfer simulations are used to quantify the sensitivity of measured upward radiance I with respect to cirrus optical thickness τ, effective radius τeff, viewing angle of the sensor σL, surface albedo α and ice crystal shape. From the calculations it is concluded that off-nadir measurements at wavelengths larger than λ = 900 nm significantly improve the ability to measure clouds of low optical thickness. The comparison of nadir and off-nadir retrievals of τ from mini-DOAS, SMART and independent estimates by the Water Vapour Lidar Experiment in Space (WALES) show general agreement within the range of measurement uncertainties. For the selected example case a mean optical thickness of 0.54±0.2 is derived by SMART and 0.49±0.2 by mini-DOAS nadir channels, while WALES obtained a mean value of 0.32 at 532 nm wavelength respectively. The mean of τ derived from the scanning mini-DOAS channels is 0.26. For the few simultaneous measurements, the scanning mini-DOAS measurements systematically underestimate (−17.6 %) the nadir observations from SMART and mini-DOAS, most likely due to the different probed scenes. The different values of τ derived by SMART, mini-DOAS and WALES can be potentially linked to spatial averages, ice crystal shape and the measurement strategies. The agreement of the simulations and retrievals indicate that off-nadir measurements are generally suited better to retrieve τ of thin clouds.

scholarly journals On the interpretation of an unusual in-situ measured ice crystal scattering phase function

2012 ◽  
Vol 12 (19) ◽  
pp. 9355-9364 ◽  
Author(s):  
A. J. Baran ◽  
J.-F. Gayet ◽  
V. Shcherbakov
Keyword(s):  
Phase Function ◽  
Asymmetry Parameter ◽  
Ice Crystals ◽  
Climate Modelling ◽  
Ice Crystal ◽  
Ice Particles ◽  
Scattering Phase ◽  
Scattering Phase Function ◽  
Phase Functions

Abstract. In-situ Polar Nephelometer (PN) measurements of unusual ice crystal scattering phase functions, obtained near the cloud-top of a mid-latitude anvil cloud, at a temperature of about −58 °C, were recently reported by Gayet et al. (2012). The ice crystal habits that produced the phase functions consisted of aggregates of ice crystals and aggregates of quasi-spherical ice particles. The diameters of the individual quasi-spherical ice particles were estimated to be between about 15 μm and 20 μm. The measured-averaged scattering phase functions were featureless, at scattering angles less than about 100°, but an ice bow-like feature was noted between the scattering angles of about 120° to 160°. The estimated asymmetry parameter was 0.78 ± 0.04. In this paper, the averaged scattering phase function is interpreted in terms of a weighted habit mixture model. The model that provides the best overall fit to the measured scattering phase function comprises of highly distorted ten-element hexagonal ice aggregates and quasi-spherical ice particles. The smaller quasi-spherical ice crystals are represented by Chebyshev ice particles of order 3, and were assumed to have equivalent spherical diameters of 24 μm. The asymmetry parameter of the best overall model was found to be 0.79. It is argued that the Chebyshev-like ice particles are responsible for the ice bow-like feature and mostly dominate the scattered intensity measured by the PN. The results from this paper have important implications for climate modelling (energy balance of anvils), cloud physics and the remote sensing of cirrus properties.

scholarly journals Photopolarimetric retrievals of snow properties

2015 ◽  
Vol 9 (3) ◽  
pp. 3055-3074
Author(s):  
M. Ottaviani ◽  
B. van Diedenhoven ◽  
B. Cairns
Keyword(s):  
Remote Sensing ◽  
Asymmetry Parameter ◽  
Total Reflection ◽  
Spectral Signatures ◽  
Snow Properties ◽  
Aspect Ratios ◽  
Aerosol Properties ◽  
Nm Range ◽  
Shape And Size

Abstract. Polarimetric observations of snow surfaces, obtained in the 410–2264 nm range with the Research Scanning Polarimeter onboard the NASA ER-2 high-altitude aircraft, are analyzed and presented. These novel measurements are of interest to the remote sensing community because the overwhelming brightness of snow plagues aerosol and cloud retrievals based on air- and space-borne total reflection measurements. The spectral signatures of the polarized reflectance of snow are therefore worthwhile investigating in order to provide guidance for the adaptation of algorithms currently employed for the retrieval of aerosol properties over soil and vegetated surfaces. At the same time, the increased information content of polarimetric measurements allows for a meaningful characterization of the snow medium. In our case, the grains are modeled as hexagonal prisms of variable aspect ratios and microscale roughness, yielding retrievals of the grains' scattering asymmetry parameter, shape and size. The results agree with our previous findings based on a more limited dataset, with the majority of retrievals leading to moderately rough crystals of extreme aspect ratios, for each scene corresponding to a single value of the asymmetry parameter.

scholarly journals A Flexible Parameterization for Shortwave and Longwave Optical Properties of Ice Crystals and Derived Bulk Optical Properties for Climate Models

2020 ◽  
Vol 77 (4) ◽  
pp. 1245-1260 ◽  
Author(s):  
Bastiaan van Diedenhoven ◽  
Brian Cairns
Keyword(s):  
Optical Properties ◽  
Aspect Ratio ◽  
Asymmetry Parameter ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Ice Crystals ◽  
Reference Database ◽  
Extinction Efficiency ◽  
Aspect Ratios ◽  
Asymmetry Parameters

Abstract We provide a parameterization of the extinction efficiency, single-scattering albedo, and asymmetry parameter of single ice crystals with any combination of particle volume, projected area, component aspect ratio, and crystal distortion at any wavelength between 0.2 and 100 μm. The parameterization is an extension of the one previously published by van Diedenhoven et al. In addition, the parameterized optical properties are integrated over size distributions yielding bulk extinction efficiencies, single-scattering albedos, and asymmetry parameters for large ranges of effective radii, particle component aspect ratios, and crystal distortion values. The parameterization of single-particle optical properties is evaluated with a reference database. The bulk optical properties are evaluated against the ice model selected for the Moderate Resolution Imaging Spectroradiometer (MODIS) collection 6 products, for which accurate optical properties are available. Mean absolute errors in parameterized extinction efficiency, asymmetry parameter, and single-scattering albedo are shown to be 0.0272, 0.008 90, and 0.004 68, respectively, for shortwave wavelengths, while they are 0.0641, 0.0368, and 0.0200 in the longwave. Shortwave and longwave asymmetry parameters and single-scattering albedos are shown to vary strongly with particle component aspect ratio and distortion, resulting in substantial variation in shortwave fluxes, but relatively small variations in longwave cloud emissivity. The parameterization and bulk optical properties are made publicly available.

scholarly journals On the interpretation of an unusual in-situ measured ice crystal scattering phase function

2012 ◽  
Vol 12 (5) ◽  
pp. 12485-12502 ◽  
Author(s):  
A. J. Baran ◽  
J.-F. Gayet ◽  
V. Shcherbakov
Keyword(s):  
Phase Function ◽  
Asymmetry Parameter ◽  
Ice Crystals ◽  
Climate Modelling ◽  
Ice Crystal ◽  
Ice Particles ◽  
Scattering Phase ◽  
Scattering Phase Function ◽  
Phase Functions

Abstract. In-situ Polar Nephelometer (PN) measurements of unusual ice crystal scattering phase functions were recently reported by Gayet et al. (2012). The ice crystal habits that produced the phase functions were small chain-like aggregates, which had on their surfaces, smaller quasi-spherical ice crystals. The measured-averaged phase functions were featureless, at scattering angles less than about 100°, but an ice bow-like feature was noted between the scattering angles of about 120° to 160°. The estimated asymmetry parameter was 0.78 ± 0.04. In this paper, the phase function is interpreted in terms of a weighted habit mixture model. The best-fit model comprises of highly distorted ten element hexagonal ice aggregates, and the smaller quasi-spherical ice crystals are represented by Chebyshev ice particles. The weighted mean asymmetry parameter was found to be 0.81. It is argued that the Chebyshev-like ice particles are responsible for the ice bow-like feature and mostly dominate the scattered intensity measured by the PN. The results of this paper have important implications for climate modelling (energy balance of anvils) and the remote sensing of cirrus properties.

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