scholarly journals Study of Horizontally Oriented Ice Crystals with CALIPSO Observations and Comparison with Monte Carlo Radiative Transfer Simulations

2012 ◽  
Vol 51 (7) ◽  
pp. 1426-1439 ◽  
Author(s):  
Chen Zhou ◽  
Ping Yang ◽  
Andrew E. Dessler ◽  
Yongxiang Hu ◽  
Bryan A. Baum
Keyword(s):  
Monte Carlo ◽  
Monte Carlo Model ◽  
Incident Beam ◽  
Mixed Phase ◽  
Ice Crystals ◽  
Orthogonal Polarization ◽  
Depolarization Ratio ◽  
Ice Clouds ◽  
Lidar Measurements ◽  
Mixed Phase Clouds

AbstractData from the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) indicate that horizontally oriented ice crystals (HOIC) occur frequently in both ice and mixed-phase clouds. When compared with the case for clouds consisting of randomly oriented ice crystals (ROIC), lidar measurements from clouds with HOIC, such as horizontally oriented hexagonal plates or columns, have stronger backscatter signals and smaller depolarization ratio values. In this study, a 3D Monte Carlo model is developed for simulating the CALIOP signals from clouds consisting of a mixture of quasi HOIC and ROIC. With CALIOP’s initial orientation with a pointing angle of 0.3° off nadir, the integrated attenuated backscatter is linearly related to the percentage of HOIC but is negatively related to the depolarization ratio. At a later time in the CALIOP mission, the pointing angle of the incident beam was changed to 3° off nadir to minimize the signal from HOIC. In this configuration, both the backscatter and the depolarization ratio are similar for clouds containing HOIC and ROIC. Horizontally oriented columns with two opposing prism facets perpendicular to the lidar beam and horizontally oriented plates show similar backscattering features, but the effect of columns is negligible in comparison with that of plates because the plates have relatively much larger surfaces facing the incident lidar beam. From the comparison between the CALIOP simulations and observations, it is estimated that the percentage of quasi-horizontally oriented plates ranges from 0% to 6% in optically thick mixed-phase clouds, from 0% to 3% in warm ice clouds (>−35°C), and from 0% to 0.5% in cold ice clouds.

CALIPSO/CALIOP Cloud Phase Discrimination Algorithm

2009 ◽  
Vol 26 (11) ◽  
pp. 2293-2309 ◽  
Author(s):  
Yongxiang Hu ◽  
David Winker ◽  
Mark Vaughan ◽  
Bing Lin ◽  
Ali Omar ◽  
...  
Keyword(s):  
Phase Retrieval ◽  
Spatial Coherence ◽  
Phase Relationship ◽  
Ice Crystals ◽  
Orthogonal Polarization ◽  
Depolarization Ratio ◽  
Threshold Method ◽  
Ice Clouds ◽  
Phase Discrimination ◽  
Ice Particles

Abstract The current cloud thermodynamic phase discrimination by Cloud-Aerosol Lidar Pathfinder Satellite Observations (CALIPSO) is based on the depolarization of backscattered light measured by its lidar [Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)]. It assumes that backscattered light from ice crystals is depolarizing, whereas water clouds, being spherical, result in minimal depolarization. However, because of the relationship between the CALIOP field of view (FOV) and the large distance between the satellite and clouds and because of the frequent presence of oriented ice crystals, there is often a weak correlation between measured depolarization and phase, which thereby creates significant uncertainties in the current CALIOP phase retrieval. For water clouds, the CALIOP-measured depolarization can be large because of multiple scattering, whereas horizontally oriented ice particles depolarize only weakly and behave similarly to water clouds. Because of the nonunique depolarization–cloud phase relationship, more constraints are necessary to uniquely determine cloud phase. Based on theoretical and modeling studies, an improved cloud phase determination algorithm has been developed. Instead of depending primarily on layer-integrated depolarization ratios, this algorithm differentiates cloud phases by using the spatial correlation of layer-integrated attenuated backscatter and layer-integrated particulate depolarization ratio. This approach includes a two-step process: 1) use of a simple two-dimensional threshold method to provide a preliminary identification of ice clouds containing randomly oriented particles, ice clouds with horizontally oriented particles, and possible water clouds and 2) application of a spatial coherence analysis technique to separate water clouds from ice clouds containing horizontally oriented ice particles. Other information, such as temperature, color ratio, and vertical variation of depolarization ratio, is also considered. The algorithm works well for both the 0.3° and 3° off-nadir lidar pointing geometry. When the lidar is pointed at 0.3° off nadir, half of the opaque ice clouds and about one-third of all ice clouds have a significant lidar backscatter contribution from specular reflections from horizontally oriented particles. At 3° off nadir, the lidar backscatter signals for roughly 30% of opaque ice clouds and 20% of all observed ice clouds are contaminated by horizontally oriented crystals.

scholarly journals Cloud phase identification of low-level Arctic clouds from airborne spectral radiation measurements: test of three approaches

2008 ◽  
Vol 8 (4) ◽  
pp. 15901-15939 ◽  
Author(s):  
A. Ehrlich ◽  
E. Bierwirth ◽  
M. Wendisch ◽  
J.-F. Gayet ◽  
G. Mioche ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Mixed Phase ◽  
Ice Crystals ◽  
The Arctic ◽  
Pure Liquid ◽  
Phase Identification ◽  
Ice Clouds ◽  
Mixed Phase Clouds

Abstract. Boundary layer clouds were investigated with a complementary set of remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds that formed in a cold air outbreak over the open Greenland sea showed a variety in their thermodynamic state. Beside the predominant mixed-phase clouds pure liquid and ice clouds were observed. Utilizing the measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud were defined and compared. Two ice indices IS and IP were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500–1800 nm wavelength) characterized by ice and water absorption. A third ice index IA is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of cloud albedo and reflectance. Radiative transfer simulations showed that IS, IP and IA range between 5 to 80, 0 to 20 and 1 to 1.25, respectively, with lowest values indicating pure liquid water clouds and highest values pure ice clouds. IS and IP were found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. IA has the disadvantage that this index is mainly dominated by the uppermost cloud layer (τ<1.5). Typical boundary layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. The comparison with independent in situ microphysical measurements showed a good agreement in identifying the dominant mixed-phase clouds and a pure ice cloud at the edge of the cloud field.

scholarly journals Physical properties of High Arctic tropospheric particles during winter

2009 ◽  
Vol 9 (2) ◽  
pp. 7781-7823 ◽  
Author(s):  
L. Bourdages ◽  
T. J. Duck ◽  
G. Lesins ◽  
J. R. Drummond ◽  
E. W. Eloranta
Keyword(s):  
Boundary Layer ◽  
Inversion Layer ◽  
Lower Troposphere ◽  
High Arctic ◽  
Mixed Phase ◽  
Ice Crystals ◽  
Blowing Snow ◽  
Ice Clouds ◽  
Arctic Conditions ◽  
Mixed Phase Clouds

Abstract. A climatology of particle properties in the wintertime High Arctic troposphere is constructed using measurements from a lidar and cloud radar located at Eureka, Nunavut Territory (80° N, 86° W). Four different particle groupings are considered: aerosols, mixed-phase clouds, ice clouds and boundary-layer ice crystals. Two-dimensional histograms of occurrence probabilities against depolarization and radar/lidar colour ratio, as well as their vertical distributions, are presented. The largest ice crystals originate from mixed-phase clouds, whereas the smallest are topographic blowing snow residuals in the boundary layer. Ice cloud crystals have depolarization and size decreasing with height. The depolarization trend is associated with the large ice crystal sub-population. Small crystals depolarize more than large ones in ice clouds at a given altitude, and show constant modal depolarization with height. Ice clouds in the mid-troposphere are sometimes observed to precipitate to the ground. Water clouds are constrained to the lower troposphere and are associated with the surface inversion layer depth. Aerosols are most abundant near the ground and are frequently mixed with the other particle types. The data are used to construct a classification chart for particle scattering in wintertime Arctic conditions.

scholarly journals Falling Mixed-Phase Ice Virga and their Liquid Parent Cloud Layers as Observed by Ground-Based Lidars

2020 ◽  
Vol 12 (13) ◽  
pp. 2094
Author(s):  
Chong Cheng ◽  
Fan Yi
Keyword(s):  
Local Maximum ◽  
Water Layer ◽  
Supercooled Liquid ◽  
Mixed Phase ◽  
Ice Crystals ◽  
Cloud Base ◽  
Depolarization Ratio ◽  
Ice Crystal ◽  
Liquid Drops ◽  
The Mean

Falling mixed-phase virga from a thin supercooled liquid layer cloud base were observed on 20 occasions at altitudes of 2.3–9.4 km with ground-based lidars at Wuhan (30.5 °N, 114.4 °E), China. Polarization lidar profile (3.75-m) analysis reveals some ubiquitous features of both falling mixed-phase virga and their liquid parent cloud layers. Each liquid parent cloud had a well-defined base height where the backscatter ratio R was ~7.0 and the R profile had a clear inflection point. At an altitude of ~34 m above the base height, the depolarization ratio reached its minimum value (~0.04), indicating a liquid-only level therein. The thin parent cloud layers tended to form on the top of a broad preexisting aerosol/liquid water layer. The falling virga below the base height showed firstly a significant depolarization ratio increase, suggesting that most supercooled liquid drops in the virga were rapidly frozen into ice crystals (via contact freezing). After reaching a local maximum value of the depolarization ratio, both the values of the backscatter ratio and depolarization ratio for the virga exhibited an overall decrease with decreasing height, indicating sublimated ice crystals. The diameters of the ice crystals in the virga were estimated based on an ice particle sublimation model along with the lidar and radiosonde observations. It was found that the ice crystal particles in these virga cases tended to have smaller mean diameters and narrower size distributions with increasing altitude. The mean diameter value is 350 ± 111 µm at altitudes of 4–8.5 km.

scholarly journals Ice Particle Production in Mid-level Stratiform Mixed-phase Clouds Observed with Collocated A-Train Measurements

2017 ◽  
Author(s):  
Damao Zhang ◽  
Zhien Wang ◽  
Pavlos Kollias ◽  
Andrew M. Vogelmann ◽  
Kang Yang ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
Particle Production ◽  
Climate Models ◽  
Global Scale ◽  
Number Concentration ◽  
Mixed Phase ◽  
Liquid Water Path ◽  
Lidar Measurements ◽  
Dust Events ◽  
Mixed Phase Clouds

Abstract. Collocated CloudSat radar and CALIPSO lidar measurements between 2006 and 2010 are analyzed to study primary ice particle production characteristics in mid-level stratiform mixed-phase clouds on a global scale. For similar clouds in terms of cloud top temperature and liquid water path, Northern Hemisphere latitude bands have layer-maximum radar reflectivity (ZL) that is ~1 to 8 dBZ larger than their counterparts in the Southern Hemisphere. The systematically larger ZL under similar cloud conditions suggests larger ice number concentrations in mid-level stratiform mixed-phase clouds over the Northern Hemisphere, which is possibly related to higher background aerosol loadings. Furthermore, we show that northern mid- and high-latitude springtime has ZL that is larger by up to 8 dBZ (a factor of 6 higher ice number concentration) than other seasons, which might be related to more dust events that provide effective ice nucleating particles. Our study suggests that aerosol-dependent ice number concentration parameterizations are required in climate models to improve mixed-phase cloud simulations, especially over the Northern Hemisphere.

scholarly journals Some effects of ice crystals on the FSSP measurements in mixed phase clouds

2012 ◽  
Vol 12 (19) ◽  
pp. 8963-8977 ◽  
Author(s):  
G. Febvre ◽  
J.-F. Gayet ◽  
V. Shcherbakov ◽  
C. Gourbeyre ◽  
O. Jourdan
Keyword(s):  
Particle Size ◽  
Mixed Phase ◽  
Ice Crystals ◽  
Crystal Surfaces ◽  
Hexagonal Ice ◽  
Ice Particles ◽  
Second Mode ◽  
Ice Production ◽  
Bulk Parameters ◽  
Mixed Phase Clouds

Abstract. In this paper, we show that in mixed phase clouds, the presence of ice crystals may induce wrong FSSP 100 measurements interpretation especially in terms of particle size and subsequent bulk parameters. The presence of ice crystals is generally revealed by a bimodal feature of the particle size distribution (PSD). The combined measurements of the FSSP-100 and the Polar Nephelometer give a coherent description of the effect of the ice crystals on the FSSP-100 response. The FSSP-100 particle size distributions are characterized by a bimodal shape with a second mode peaked between 25 and 35 μm related to ice crystals. This feature is observed with the FSSP-100 at airspeed up to 200 m s−1 and with the FSSP-300 series. In order to assess the size calibration for clouds of ice crystals the response of the FSSP-100 probe has been numerically simulated using a light scattering model of randomly oriented hexagonal ice particles and assuming both smooth and rough crystal surfaces. The results suggest that the second mode, measured between 25 μm and 35 μm, does not necessarily represent true size responses but corresponds to bigger aspherical ice particles. According to simulation results, the sizing understatement would be neglected in the rough case but would be significant with the smooth case. Qualitatively, the Polar Nephelometer phase function suggests that the rough case is the more suitable to describe real crystals. Quantitatively, however, it is difficult to conclude. A review is made to explore different hypotheses explaining the occurrence of the second mode. However, previous cloud in situ measurements suggest that the FSSP-100 secondary mode, peaked in the range 25–35 μm, is likely to be due to the shattering of large ice crystals on the probe inlet. This finding is supported by the rather good relationship between the concentration of particles larger than 20 μm (hypothesized to be ice shattered-fragments measured by the FSSP) and the concentration of (natural) ice particles (CPI data). In mixed cloud, a simple estimation of the number of ice crystals impacting the FSSP inlet shows that the ice crystal shattering effect is the main factor in observed ice production.

scholarly journals Effects of Entrainment and Mixing on the Wegener–Bergeron–Findeisen Process

2020 ◽  
Vol 77 (6) ◽  
pp. 2279-2296
Author(s):  
Fabian Hoffmann
Keyword(s):  
Molecular Diffusion ◽  
Scale Effects ◽  
Mixed Phase ◽  
Ice Crystals ◽  
Small Scale ◽  
Liquid Droplets ◽  
Ice Crystal ◽  
Slowing Down ◽  
Linear Eddy Model ◽  
Mixed Phase Clouds

Abstract The growth of ice crystals at the expense of water droplets, the Wegener–Bergeron–Findeisen (WBF) process, is of major importance for the production of precipitation in mixed-phase clouds. The effects of entrainment and mixing on WBF, however, are not well understood, and small-scale inhomogeneities in the thermodynamic and hydrometeor fields are typically neglected in current models. By applying the linear eddy model, a millimeter-resolution representation of turbulent deformation and molecular diffusion, we investigate these small-scale effects on WBF. While we show that entrainment is accelerating WBF by contributing to the evaporation of liquid droplets, entrainment may also cause aforementioned inhomogeneities, particularly regions filled with exclusively ice or liquid hydrometeors, which tend to decelerate WBF if the ice crystal concentration exceeds 100 L−1. At lower ice crystal concentrations, even weak turbulence can homogenize hydrometeor and thermodynamic fields sufficiently fast so as to not affect WBF. Independent of the ice crystal concentration, it is shown that a fully resolved entrainment and mixing process may delay the nucleation of entrained aerosols to ice crystals, thereby delaying the uptake of water vapor by the ice phase, further slowing down WBF. All in all, this study indicates that, under specific conditions, small-scale inhomogeneities associated with entrainment and mixing counteract the accelerated WBF in entraining clouds. However, further research is required to assess the importance of the newly discovered processes more broadly in fully coupled, evolving mixed-phase cloud systems.

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