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 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 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 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 Small Irregular Ice Crystals in Tropical Cirrus

2011 ◽  
Vol 68 (11) ◽  
pp. 2614-2627 ◽  
Author(s):  
Timo Nousiainen ◽  
Hannakaisa Lindqvist ◽  
Greg M. McFarquhar ◽  
Junshik Um
Keyword(s):  
Asymmetry Parameter ◽  
Covariance Function ◽  
Three Dimensional ◽  
Warm Pool ◽  
Single Scattering ◽  
Department Of Energy ◽  
Ice Crystals ◽  
Three Dimensional Model ◽  
Cloud Particle ◽  
The Asymmetry

Abstract Images acquired by a Cloud Particle Imager (CPI) are analyzed to compile a statistical covariance function of radius for an ensemble of small, irregular, quasi-spherical ice crystals in tropical cirrus measured during the Department of Energy Atmospheric Radiation Measurement Program’s Tropical Warm Pool International Cloud Experiment. The infrequent occurrence of multiple particles in single CPI frames suggests that most crystals sampled were natural ice crystals rather than artifacts from large particles shattering on probe tips. The covariance function is used in conjunction with the Gaussian random sphere geometry to generate three-dimensional model ice particles for ray-optics simulations at 550-nm wavelength. The crystal shapes and single-scattering properties are compared with those obtained by the same methodology for midlatitude cirrus sampled over Oklahoma. The small tropical ice crystals are closer to spherical than their midlatitude counterparts and, consequently, their asymmetry parameters are larger, but the differences are not significant from the standpoint of climate studies. Because some quasi-spherical ice crystals seem partially faceted, a convex hull transformation is introduced that increases the asymmetry parameter from 0.785 to 0.808. Further modifying the covariance function to promote sixfold symmetry in the model crystals increases the asymmetry parameter to 0.818. The introduction of internal scatterers, such as air bubbles, has a larger impact, decreasing the asymmetry parameter by up to tens of percent, depending on their amount and characteristics. Unfortunately, no data are available to determine realistic values for the internal scatterers to assess their likely actual impact.

scholarly journals Evaluation of Hydrometeor Phase and Ice Properties in Cloud-Resolving Model Simulations of Tropical Deep Convection Using Radiance and Polarization Measurements

2012 ◽  
Vol 69 (11) ◽  
pp. 3290-3314 ◽  
Author(s):  
Bastiaan van Diedenhoven ◽  
Ann M. Fridlind ◽  
Andrew S. Ackerman ◽  
Brian Cairns
Keyword(s):  
Asymmetry Parameter ◽  
Near Infrared ◽  
Deep Convection ◽  
Effective Radius ◽  
Ice Crystals ◽  
Model Simulations ◽  
Brightness Temperatures ◽  
Aspect Ratios ◽  
Tropical Deep Convection ◽  
Cloud Resolving Model

Abstract Satellite measurements are used to evaluate the glaciation, particle shape, and effective radius in cloud-resolving model simulations of tropical deep convection. Multidirectional polarized reflectances constrain the ice crystal geometry and the thermodynamic phase of the cloud tops, which in turn are used to calculate near-infrared reflectances so as to constrain the simulated ice effective radius, thereby avoiding inconsistencies between retrieval algorithms and model simulations. Liquid index values derived from Polarization and Directionality of the Earth’s Reflectances (POLDER) measurements indicate only ice-topped clouds at brightness temperatures (BTs) lower than −40°C, only liquid clouds at BT > −20°C, and both phases occurring at temperatures in between. Liquid index values calculated from model simulations generally reveal too many ice-topped clouds at BT > −20°C. The model assumption of platelike ice crystals with an aspect ratio of 0.7 is found consistent with POLDER measurements for BT < −40°C when very rough ice crystals are assumed, leading to an asymmetry parameter of 0.74, whereas measurements indicate more extreme aspect ratios of ~0.15 at higher temperatures, yielding an asymmetry parameter of 0.84. MODIS-retrieved ice effective radii are found to be 18–28 μm at BT < −40°C, but biased low by about 5 μm owing primarily to the assumption of pristine crystals in the retrieval. Simulated 2.13-μm reflectances at BT < −40°C are found to be about 0.05–0.1 too large compared to measurements, suggesting that model-simulated effective radii are 7–15 μm too small. Two simulations with contrasting ice nucleation schemes showed little difference in simulated effective radii at BT < −40°C, indicating that homogeneous nucleation is dominating in the simulations. Changes around −40°C in satellite observations suggest a change in cloud-top ice shape and/or size in natural deep convection possibly related to a change in the freezing mechanism.

The Asymmetry Parameter and Aggregate Particles

1997 ◽  
Author(s):  
Gorden Videen ◽  
Ronald G. Pinnick ◽  
Dat Ngo ◽  
Qiang Fu ◽  
Petr Chylek
Keyword(s):  
Asymmetry Parameter ◽  
The Asymmetry ◽  
Aggregate Particles

On The Phase Structure of the Asymmetric Three-State Potts Model

1982 ◽  
Vol 21 ◽  
Author(s):  
G. v. Gehlen
Keyword(s):  
Phase Boundary ◽  
Potts Model ◽  
Phase Structure ◽  
Asymmetry Parameter ◽  
Finite Size ◽  
Critical Index ◽  
Incommensurate Phase ◽  
Finite Size Scaling ◽  
Lifshitz Point ◽  
The Asymmetry

ABSTRACTFinite-size scaling is applied to the Hamiltonian version of the asymmetric Z3-Potts model. Results for the phase boundary of the commensurate region and for the corresponding critical index ν are presented. It is argued that there is no Lifshitz point, the incommensurate phase extending down to small values of the asymmetry parameter.

scholarly journals Relationship between cloud radiative forcing, cloud fraction and cloud albedo, and new surface-based approach for determining cloud albedo

2011 ◽  
Vol 11 (14) ◽  
pp. 7155-7170 ◽  
Author(s):  
Y. Liu ◽  
W. Wu ◽  
M. P. Jensen ◽  
T. Toto
Keyword(s):  
Radiative Forcing ◽  
Climate Models ◽  
Cloud Fraction ◽  
Shortwave Radiation ◽  
Cloud Radiative Forcing ◽  
Analytical Expression ◽  
Cloud Albedo ◽  
Annual Variations ◽  
Hourly Data ◽  
Radiation Parameterizations

Abstract. This paper focuses on three interconnected topics: (1) quantitative relationship between surface shortwave cloud radiative forcing, cloud fraction, and cloud albedo; (2) surface-based approach for measuring cloud albedo; (3) multiscale (diurnal, annual and inter-annual) variations and covariations of surface shortwave cloud radiative forcing, cloud fraction, and cloud albedo. An analytical expression is first derived to quantify the relationship between cloud radiative forcing, cloud fraction, and cloud albedo. The analytical expression is then used to deduce a new approach for inferring cloud albedo from concurrent surface-based measurements of downwelling surface shortwave radiation and cloud fraction. High-resolution decade-long data on cloud albedos are obtained by use of this surface-based approach over the US Department of Energy's Atmospheric Radiaton Measurement (ARM) Program at the Great Southern Plains (SGP) site. The surface-based cloud albedos are further compared against those derived from the coincident GOES satellite measurements. The three long-term (1997–2009) sets of hourly data on shortwave cloud radiative forcing, cloud fraction and cloud albedo collected over the SGP site are analyzed to explore the multiscale (diurnal, annual and inter-annual) variations and covariations. The analytical formulation is useful for diagnosing deficiencies of cloud-radiation parameterizations in climate models.

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