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 Derivation of physical and optical properties of midlatitude cirrus ice crystals for a size-resolved cloud microphysics model

2016 ◽  
Author(s):  
A. M. Fridlind ◽  
R. Atlas ◽  
B. van Diedenhoven ◽  
J. Um ◽  
G. M. McFarquhar ◽  
...  
Keyword(s):  
Optical Properties ◽  
Asymmetry Parameter ◽  
Near Infrared ◽  
Single Scattering ◽  
Cloud Microphysics ◽  
Single Scattering Albedo ◽  
Ice Crystals ◽  
Projected Area ◽  
Maximum Dimension ◽  
Plate Aspect Ratio

Abstract. Single-crystal images collected in mid-latitude cirrus are analyzed to provide internally consistent ice physical and optical properties for a size-resolved cloud microphysics model, including single-particle mass, projected area, fall speed, capacitance, single-scattering albedo, and asymmetry parameter. Using measurements gathered during two flights through a widespread synoptic cirrus shield, bullet rosettes are found to be the dominant identifiable habit among ice crystals with maximum dimension (Dmax) greater than 100 μm. Properties are therefore first derived for bullet rosettes based on measurements of arm lengths and widths, then for aggregates of bullet rosettes and for unclassified (irregular) crystals. Derived bullet rosette masses are substantially greater than reported in existing literature, whereas measured projected areas are similar or lesser, resulting in factors of 1.5–2 greater fall speeds, and, in the limit of large Dmax, near-infrared single-scattering albedo and asymmetry parameter (g) greater by ~ 0.2 and 0.05, respectively. A model that includes commonly imaged side plane growth on bullet rosettes exhibits relatively little difference in microphysical and optical properties aside from ~ 0.05 increase in mid-visible g primarily attributable to plate aspect ratio. In parcel simulations, ice size distribution and g are sensitive to assumed ice properties.

scholarly journals Derivation of physical and optical properties of mid-latitude cirrus ice crystals for a size-resolved cloud microphysics model

2016 ◽  
Vol 16 (11) ◽  
pp. 7251-7283 ◽  
Author(s):  
Ann M. Fridlind ◽  
Rachel Atlas ◽  
Bastiaan van Diedenhoven ◽  
Junshik Um ◽  
Greg M. McFarquhar ◽  
...  
Keyword(s):  
Optical Properties ◽  
Asymmetry Parameter ◽  
Near Infrared ◽  
Single Scattering ◽  
Cloud Microphysics ◽  
Single Scattering Albedo ◽  
Ice Crystals ◽  
Projected Area ◽  
Maximum Dimension ◽  
Plate Aspect Ratio

Abstract. Single-crystal images collected in mid-latitude cirrus are analyzed to provide internally consistent ice physical and optical properties for a size-resolved cloud microphysics model, including single-particle mass, projected area, fall speed, capacitance, single-scattering albedo, and asymmetry parameter. Using measurements gathered during two flights through a widespread synoptic cirrus shield, bullet rosettes are found to be the dominant identifiable habit among ice crystals with maximum dimension (Dmax) greater than 100 µm. Properties are therefore first derived for bullet rosettes based on measurements of arm lengths and widths, then for aggregates of bullet rosettes and for unclassified (irregular) crystals. Derived bullet rosette masses are substantially greater than reported in existing literature, whereas measured projected areas are similar or lesser, resulting in factors of 1.5–2 greater fall speeds, and, in the limit of large Dmax, near-infrared single-scattering albedo and asymmetry parameter (g) greater by  ∼  0.2 and 0.05, respectively. A model that includes commonly imaged side plane growth on bullet rosettes exhibits relatively little difference in microphysical and optical properties aside from  ∼ 0.05 increase in mid-visible g primarily attributable to plate aspect ratio. In parcel simulations, ice size distribution, and g are sensitive to assumed ice properties.

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 STUDY OF AEROSOL OPTICAL PROPERTIES OVER TWO SITES IN THE FOOTHILLS OF THE CENTRAL HIMALAYAS

2018 ◽  
Vol XLII-3 ◽  
pp. 1493-1497 ◽  
Author(s):  
D. Rupakheti ◽  
S. Kang ◽  
Z. Cong ◽  
M. Rupakheti ◽  
L. Tripathee ◽  
...  
Keyword(s):  
Optical Properties ◽  
Biomass Burning ◽  
Asymmetry Parameter ◽  
Monsoon Season ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Aerosol Optical Properties ◽  
Central Himalayas ◽  
Aerosol Types ◽  
Volume Size

Atmospheric aerosol possesses impacts on climate system and ecological environments, human health and agricultural productivity. The environment over Himalayas and Tibetan Plateau region are continuously degraded due to the transport of pollution from the foothills of the Himalayas; mostly the Indo-Gangetic Plain (IGP). Thus, analysis of aerosol optical properties over two sites; Lumbini and Kathmandu (the southern slope of central Himalayas) using AERONET’s CIMEL sun photometer were conducted in this study. Aerosol optical depth (AOD at 500 nm), angstrom exponent (α or AE), volume size distribution (VSD), single scattering albedo (SSA) and asymmetry parameter (AP) were studied for 2013–2014 and the average AOD was found to be: 0.64 ± 0.41 (Lumbini) and 0.45 ± 0.30 (Kathmandu). The average AE was found to be: 1.25 ± 0.24 and 1.26 ± 0.18 respectively for two sites. The relation between AOD and AE was used to discriminate the aerosol types over these sites which indicated anthropogenic, mixed and biomass burning origin aerosol constituted the major aerosol types in Lumbini and Kathmandu. A clear bi-modal distribution of aerosol volume size was observed with highest volume concentration during the post-monsoon season in fine mode and pre-monsoon season in coarse mode (Lumbini) and highest value over both modes during pre-monsoon season in Kathmandu. The single scattering albedo (SSA) and asymmetry parameter (AP) analyses suggested aerosols over the Himalayan foothills sites are dominated by absorbing and anthropogenic aerosols from urban and industrial activities and biomass burning. Long-term studies are essential to understand and characterize the nature of aerosol over this research gap zone.

scholarly journals Simulations of Contrail Optical Properties and Radiative Forcing for Various Crystal Shapes

2011 ◽  
Vol 50 (8) ◽  
pp. 1740-1755 ◽  
Author(s):  
Krzysztof M. Markowicz ◽  
Marcin L. Witek
Keyword(s):  
Optical Properties ◽  
Asymmetry Parameter ◽  
Radiative Forcing ◽  
Surface Albedo ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Crystal Shape ◽  
Crystal Shapes ◽  
The Mean ◽  
The Asymmetry

AbstractThe aim of this study is to investigate the sensitivity of radiative-forcing computations to various contrail crystal shape models. Contrail optical properties in the shortwave and longwave ranges are derived using a ray-tracing geometric method and the discrete dipole approximation method, respectively. Both methods present good correspondence of the single-scattering albedo and the asymmetry parameter in a transition range (3–8 μm). There are substantial differences in single-scattering properties among 10 crystal models investigated here (e.g., hexagonal columns and plates with different aspect ratios, and spherical particles). The single-scattering albedo and the asymmetry parameter both vary by up to 0.1 among various crystal shapes. The computed single-scattering properties are incorporated in the moderate-resolution atmospheric radiance and transmittance model (MODTRAN) radiative transfer code to simulate solar and infrared fluxes at the top of the atmosphere. Particle shapes have a strong impact on the contrail radiative forcing in both the shortwave and longwave ranges. The differences in the net radiative forcing among optical models reach 50% with respect to the mean model value. The hexagonal-column and hexagonal-plate particles show the smallest net radiative forcing, and the largest forcing is obtained for the spheres. The balance between the shortwave forcing and longwave forcing is highly sensitive with respect to the assumed crystal shape and may even change the sign of the net forcing. The optical depth at which the mean diurnal radiative forcing changes sign from positive to negative varies from 4.5 to 10 for a surface albedo of 0.2 and from 2 to 6.5 for a surface albedo of 0.05. Contrails are probably never that optically thick (except for some aged contrail cirrus), however, and so will not have a cooling effect on climate.

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 Statistical evaluation of aerosol retrievals from AERONET using in-situ aircraft measurements

2011 ◽  
Vol 11 (10) ◽  
pp. 29003-29054 ◽  
Author(s):  
A. R. Esteve ◽  
J. A. Ogren ◽  
P. J. Sheridan ◽  
E. Andrews ◽  
B. N. Holben ◽  
...  
Keyword(s):  
Optical Properties ◽  
Asymmetry Parameter ◽  
Statistical Evaluation ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Aircraft Measurements ◽  
Aerosol Optical Properties ◽  
Total Light ◽  
The One

Abstract. Aerosol optical properties were measured by NOAA's Airborne Aerosol Observatory over Bondville, Illinois, during more than two years using a light aircraft. Measured properties included total light scattering, backscattering, and absorption, while calculated parameters included aerosol optical depth (AOD), Ångström exponent, single-scattering albedo, hemispheric backscatter fraction, asymmetry parameter, and submicrometer mode fraction of scattering. The in-situ aircraft measurements are compared here with AERONET measurements and retrievals of the aerosol optical properties at the same location. The comparison reveals discrepancies between the aerosol properties retrieved from AERONET and from in-situ aircraft measurements. These discrepancies are smaller for the AOD, while the biggest discrepancies are for the single-scattering albedo, hemispheric backscatter fraction, and asymmetry parameter. Possible sources of discrepancy between the AOD measured by AERONET and the one calculated from the in-situ aircraft measurements are investigated. The largest portion of the AOD discrepancy is likely due to an incorrect adjustment to ambient RH of the scattering coefficient. Another significant part (along with uncertain nephelometer truncation corrections) may come from the possibility that there might be less aerosol below the lowest flight altitude or that the aircraft inlet excludes aerosol particles larger than 5–7 μm diameter.

scholarly journals Aerosol single-scattering albedo and asymmetry parameter from MFRSR observations during the ARM Aerosol IOP 2003

2006 ◽  
Vol 6 (6) ◽  
pp. 13367-13386 ◽  
Author(s):  
E. I. Kassianov ◽  
T. P. Ackerman ◽  
J. C. Barnard ◽  
C. J. Flynn
Keyword(s):  
Optical Properties ◽  
Asymmetry Parameter ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Radiative Properties ◽  
Retrieval Technique ◽  
Temporal And Spatial Variability ◽  
Different Types ◽  
Atmospheric Radiation Measurement ◽  
Temporal And Spatial

Abstract. Multi-filter Rotating Shadowband Radiometers (MFRSRs) provide routine measurements of the aerosol optical depth (τ) at six wavelengths (0.415, 0.5, 0.615, 0.673, 0.870 and 0.94 μm). The single-scattering albedo (ϖ0) is typically estimated from the MFRSR measurements by assuming the asymmetry parameter (g). In most instances, however, it is not easy to set an appropriate value of g due to its strong temporal and spatial variability. Here, we introduce and validate an updated version of our retrieval technique that allows one to estimate simultaneously ϖ0 and g for different types of aerosol. We use the aerosol and radiative properties obtained during the Atmospheric Radiation Measurement (ARM) Aerosol Intensive Operational Period (IOP) to validate our retrieval in two ways. First, the MFRSR-retrieved optical properties are compared with those obtained from independent surface, Aerosol Robotic Network (AERONET) and aircraft measurements. The MFRSR-retrieved optical properties are in reasonable agreement with these independent measurements. Second, we perform radiative closure experiments using the MFRSR-retrieved optical properties. The calculated broadband values of the direct and diffuse fluxes are comparable (~5 W/m2) to those obtained from measurements.

scholarly journals Seasonality of aerosol optical properties in the Arctic

2018 ◽  
Vol 18 (16) ◽  
pp. 11599-11622 ◽  
Author(s):  
Lauren Schmeisser ◽  
John Backman ◽  
John A. Ogren ◽  
Elisabeth Andrews ◽  
Eija Asmi ◽  
...  
Keyword(s):  
Optical Properties ◽  
Absorption Coefficient ◽  
Asymmetry Parameter ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Scattering Coefficient ◽  
The Arctic ◽  
Aerosol Optical Properties ◽  
Ångström Exponent ◽  
The Mean

Abstract. Given the sensitivity of the Arctic climate to short-lived climate forcers, long-term in situ surface measurements of aerosol parameters are useful in gaining insight into the magnitude and variability of these climate forcings. Seasonality of aerosol optical properties – including the aerosol light-scattering coefficient, absorption coefficient, single-scattering albedo, scattering Ångström exponent, and asymmetry parameter – are presented for six monitoring sites throughout the Arctic: Alert, Canada; Barrow, USA; Pallas, Finland; Summit, Greenland; Tiksi, Russia; and Zeppelin Mountain, Ny-Ålesund, Svalbard, Norway. Results show annual variability in all parameters, though the seasonality of each aerosol optical property varies from site to site. There is a large diversity in magnitude and variability of scattering coefficient at all sites, reflecting differences in aerosol source, transport, and removal at different locations throughout the Arctic. Of the Arctic sites, the highest annual mean scattering coefficient is measured at Tiksi (12.47 Mm−1), and the lowest annual mean scattering coefficient is measured at Summit (1.74 Mm−1). At most sites, aerosol absorption peaks in the winter and spring, and has a minimum throughout the Arctic in the summer, indicative of the Arctic haze phenomenon; however, nuanced variations in seasonalities suggest that this phenomenon is not identically observed in all regions of the Arctic. The highest annual mean absorption coefficient is measured at Pallas (0.48 Mm−1), and Summit has the lowest annual mean absorption coefficient (0.12 Mm−1). At the Arctic monitoring stations analyzed here, mean annual single-scattering albedo ranges from 0.909 (at Pallas) to 0.960 (at Barrow), the mean annual scattering Ångström exponent ranges from 1.04 (at Barrow) to 1.80 (at Summit), and the mean asymmetry parameter ranges from 0.57 (at Alert) to 0.75 (at Summit). Systematic variability of aerosol optical properties in the Arctic supports the notion that the sites presented here measure a variety of aerosol populations, which also experience different removal mechanisms. A robust conclusion from the seasonal cycles presented is that the Arctic cannot be treated as one common and uniform environment but rather is a region with ample spatiotemporal variability in aerosols. This notion is important in considering the design or aerosol monitoring networks in the region and is important for informing climate models to better represent short-lived aerosol climate forcers in order to yield more accurate climate predictions for the Arctic.

scholarly journals A Flexible Parameterization for Shortwave Optical Properties of Ice Crystals*

2014 ◽  
Vol 71 (5) ◽  
pp. 1763-1782 ◽  
Author(s):  
Bastiaan van Diedenhoven ◽  
Andrew S. Ackerman ◽  
Brian Cairns ◽  
Ann M. Fridlind
Keyword(s):  
Optical Properties ◽  
Aspect Ratio ◽  
Single Scattering ◽  
Ice Crystals ◽  
Geometric Optics ◽  
Atmospheric Models ◽  
Projected Area ◽  
Extinction Cross Section ◽  
Practical Applications ◽  
Hexagonal Crystals

Abstract A parameterization is presented that provides extinction cross section σe, single-scattering albedo ω, and asymmetry parameter g of ice crystals for any combination of volume, projected area, aspect ratio, and crystal distortion at any wavelength in the shortwave. Similar to previous parameterizations, the scheme makes use of geometric optics approximations and the observation that optical properties of complex, aggregated ice crystals can be well approximated by those of single hexagonal crystals with varying size, aspect ratio, and distortion levels. In the standard geometric optics implementation used here, σe is always twice the particle projected area. It is shown that ω is largely determined by the newly defined absorption size parameter and the particle aspect ratio. These dependences are parameterized using a combination of exponential, lognormal, and polynomial functions. The variation of g with aspect ratio and crystal distortion is parameterized for one reference wavelength using a combination of several polynomials. The dependences of g on refractive index and ω are investigated and factors are determined to scale the parameterized g to provide values appropriate for other wavelengths. The parameterization scheme consists of only 88 coefficients. The scheme is tested for a large variety of hexagonal crystals in several wavelength bands from 0.2 to 4 μm, revealing absolute differences with reference calculations of ω and g that are both generally below 0.015. Over a large variety of cloud conditions, the resulting root-mean-squared differences with reference calculations of cloud reflectance, transmittance, and absorptance are 1.4%, 1.1%, and 3.4%, respectively. Some practical applications of the parameterization in atmospheric models are highlighted.

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