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 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.

scholarly journals Genesis of Diamond Dust and Thick Cloud Episodes observed above Dome C, Antarctica

2016 ◽  
Author(s):  
Philippe Ricaud ◽  
Eric Bazile ◽  
Massimo del Guasta ◽  
Christian Lanconelli ◽  
Paolo Grigioni ◽  
...  
Keyword(s):  
Water Vapour ◽  
Dust Cloud ◽  
Ice Crystals ◽  
Surface Radiation ◽  
Orthogonal Polarization ◽  
Tropospheric Temperature ◽  
Depolarization Ratio ◽  
Air Masses ◽  
Tropospheric Aerosol ◽  
Depolarization Ratios

Abstract. From 15 March to 8 April 2011 and from 4 to 5 March 2013, the atmosphere above Dome C (Concordia station, Antarctica, 75°06' S, 123°21' E, 3233 m amsl) has been probed by several instruments and model to study episodes of thick cloud and diamond dust (cloud constituted of suspended ice crystals). 1) A ground-based microwave radiometer (HAMSTRAD, H2O Antarctica Microwave Stratospheric and Tropospheric Radiometers) installed at Dome C that provided vertical profiles of tropospheric temperature and absolute humidity to calculate Integrated Water Vapour (IWV). 2) Daily radiosoundings launched at 12:00 UTC at Dome C. 3) A tropospheric aerosol Lidar that provides aerosol depolarization ratio along the vertical at Dome C. 4) Down- and upward short- and longwave radiations as provided by the Baseline Surface Radiation Network (BSRN) facilities. 5) Space-borne aerosol depolarization ratio from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) Lidar aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) platform along orbits close to the Dome C station. The time evolution of the atmosphere has also been evaluated by considering the outputs from the meso-scale AROME and the global-scale ARPEGE meteorological models. Two distinct periods are highlighted by all the datasets: the warm and wet periods (24–26 March 2011 and 4 March 2013) and the cold and dry periods (5 April 2011 and 5 March 2013). Combining radiation and active measurements of aerosols with nebulosity calculations, a thick cloud is detected during the warm and wet periods with high depolarization ratios (greater than 30 %) from the surface to 5–7 km altitude associated with precipitation of ice particles and the presence of a supercooled liquid water (depolarization of about 10 %) cloud. During the cold and dry periods, high depolarization ratios (greater than 30 %) to a maximum altitude of 100–500 m are measured suggesting that the cloud is constituted of ice crystals with no trace of precipitation. These ice crystals in suspension in the air are named diamond dust. Considering 5-day back trajectories from Dome C and global distributions of IWV over the Antarctic show that the thick-cloud episode is attributed to air masses with an oceanic origin whilst the diamond dust episode is attributed to air masses with continental origins. This is consistent with ARPEGE temperature and water vapour tendency favouring predominantly advection processes including microphysical processes for water vapour.

scholarly journals Probing ice clouds by broadband mid-infrared extinction spectroscopy: case studies from ice nucleation experiments in the AIDA aerosol and cloud chamber

2006 ◽  
Vol 6 (12) ◽  
pp. 4775-4800 ◽  
Author(s):  
R. Wagner ◽  
S. Benz ◽  
O. Möhler ◽  
H. Saathoff ◽  
U. Schurath
Keyword(s):  
Size Distribution ◽  
Particle Number ◽  
Ice Nucleation ◽  
Number Concentration ◽  
Cloud Chamber ◽  
Ice Crystals ◽  
Ice Clouds ◽  
Number Size Distribution ◽  
Ice Particles ◽  
Infrared Extinction

Abstract. Series of infrared extinction spectra of ice crystals were recorded in the 6000–800 cm−1 wavenumber regime during expansion cooling experiments in the large aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe. Either supercooled sulphuric acid solution droplets or dry mineral dust particles were added as seed aerosols to initiate ice formation after having established ice supersaturated conditions inside the chamber. The various ice nucleation runs were conducted at temperatures between 237 and 195 K, leading to median sizes of the nucleated ice particles of 1–15 µm. The measured infrared spectra were fitted with reference spectra from T-matrix calculations to retrieve the number concentration as well as the number size distribution of the generated ice clouds. The precise evaluation of the time-dependent ice particle number concentrations, i.e., the rates of new ice particle formation, is of particular importance to quantitatively analyse the ice nucleation experiments in terms of nucleation rates and ice activation spectra. The ice particles were modelled as finite circular cylinders with aspect ratios ranging from 0.5 to 3.0. Benefiting from the comprehensive diagnostic tools for the characterisation of ice clouds which are available at the AIDA facility, the infrared retrieval results with regard to the ice particle number concentration could be compared to independent measurements with various optical particle counters. This provided a unique chance to quantitatively assess potential errors or solution ambiguities in the retrieval procedure which mainly originate from the difficulty to find an appropriate shape representation for the aspherical particle habits of the ice crystals. Based on these inter-comparisons, we demonstrate that there is no standard retrieval approach which can be routinely applied to all different experimental scenarios. In particular, the concept to account for the asphericity of the ice crystals, the a priori constraints which might be imposed on the unknown number size distribution of the ice crystals (like employing an analytical distribution function), and the wavenumber range which is included in the fitting algorithm should be carefully adjusted to each single retrieval problem.

scholarly journals Widespread polar stratospheric ice clouds in the 2015–2016 Arctic winter – implications for ice nucleation

2018 ◽  
Vol 18 (21) ◽  
pp. 15623-15641 ◽  
Author(s):  
Christiane Voigt ◽  
Andreas Dörnbrack ◽  
Martin Wirth ◽  
Silke M. Groß ◽  
Michael C. Pitts ◽  
...  
Keyword(s):  
Equilibrium Temperature ◽  
Ice Nucleation ◽  
The Arctic ◽  
Orthogonal Polarization ◽  
Depolarization Ratio ◽  
Data Set ◽  
Ice Clouds ◽  
Cold Temperatures ◽  
Back Trajectories ◽  
Depolarization Ratios

Abstract. Low planetary wave activity led to a stable vortex with exceptionally cold temperatures in the 2015–2016 Arctic winter. Extended areas with temperatures below the ice frost point temperature Tice persisted over weeks in the Arctic stratosphere as derived from the 36-year temperature climatology of the ERA-Interim reanalysis data set of the European Centre for Medium-Range Weather Forecasts (ECMWF). These extreme conditions promoted the formation of widespread polar stratospheric ice clouds (ice PSCs). The space-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on board the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite continuously measured ice PSCs for about a month with maximum extensions of up to 2×106 km2 in the stratosphere. On 22 January 2016, the WALES (Water Vapor Lidar Experiment in Space – airborne demonstrator) lidar on board the High Altitude and Long Range Research Aircraft HALO detected an ice PSC with a horizontal length of more than 1400 km. The ice PSC extended between 18 and 24 km altitude and was surrounded by nitric acid trihydrate (NAT) particles, supercooled ternary solution (STS) droplets and particle mixtures. The ice PSC occurrence histogram in the backscatter ratio to particle depolarization ratio optical space exhibits two ice modes with high or low particle depolarization ratios. Domain-filling 8-day back-trajectories starting in the high particle depolarization (high-depol) ice mode are continuously below the NAT equilibrium temperature TNAT and decrease below Tice∼10 h prior to the observation. Their matches with CALIPSO PSC curtain plots demonstrate the presence of NAT PSCs prior to high-depol ice, suggesting that the ice had nucleated on NAT. Vice versa, STS or no PSCs were detected by CALIPSO prior to the ice mode with low particle depolarization ratio. In addition to ice nucleation in STS potentially having meteoric inclusions, we find evidence for ice nucleation on NAT in the Arctic winter 2015–2016. The observation of widespread Arctic ice PSCs with high or low particle depolarization ratios advances our understanding of ice nucleation in polar latitudes. It further provides a new observational database for the parameterization of ice nucleation schemes in atmospheric models.

Oriented particles in microwave and submillimeter radiative transfer simulations of ice clouds

2020 ◽  
Author(s):  
Manfred Brath ◽  
Robin Ekelund ◽  
Patrick Eriksson ◽  
Oliver Lemke ◽  
Stefan A. Buehler
Keyword(s):  
Radiative Transfer ◽  
Incidence Angle ◽  
Ice Crystals ◽  
Particle Orientation ◽  
Scattering Data ◽  
Reasonable Assumption ◽  
Random Orientation ◽  
Ice Clouds ◽  
Ice Cloud ◽  
Ice Particles

<div>Observations of Global Precipitation Measurement Microwave Imager (GMI) at 166 GHz consistently show polarized scattering signals of ice clouds. Conceptual models indicate that these signals emerge from oriented ice particles. Existing databases of scattering data of realistically shaped ice crystals for microwave and submillimeter typically assume total random orientation of ice particles. This is often a very reasonable assumption, but cannot explain the polarized ice cloud signals. Only few works considering oriented ice crystals exist, but they only consider microwave. With the upcoming Ice Cloud Imager (ICI) on board of Metop-SG B satellite, there will be additional dual-polarization measurements at 243 GHz and 664 GHz. These measurements will deliver new insights about clouds and their structure, if we know the scattering properties of oriented and realistically shaped ice crystals.</div><div>We provide publicly available scattering data for 51 different sized hexagonal plates and 18 different sized plate aggregates for 35 frequencies between 1 GHz and 864 GHz. The ice particles are assumed to be azimuthally randomly oriented with a fixed but arbitrary tilt angle. The scattering data for azimuthal random orientation is much more complex than for total random orientation. The scattering data of azimuthally randomly oriented particles depends in general on the incidence angle and two scattering angles compared to only one scattering angle for total random orientation. The scattering data is based on discrete dipole approximation simulations in combination with a self-developed orientation averaging approach.</div><div>We present detailed radiative transfer simulations of polarized GMI observations at 166 GHz and ICI observations at 243 GHz and at 664 GHz using our scattering data. The simulations of GMI recreate the observed polarization patterns. Analysis shows that not only orientation affects the polarization signal but also the hydrometeor composition. Furthermore, particle orientation also affects single polarized observations. Ignoring orientation can cause a negative bias for vertically polarized observations and a positive bias for horizontally polarized observations.</div>

scholarly journals Characteristics of CALIPSO and CloudSat Backscatter at the Top Center Layers of Mesoscale Convective Systems and Relation to Cloud Microphysics

2011 ◽  
Vol 50 (2) ◽  
pp. 368-378 ◽  
Author(s):  
C. M. R. Platt ◽  
M. A. Vaughan ◽  
R. T. Austin
Keyword(s):  
Satellite Observation ◽  
Cloud Microphysics ◽  
Effective Radius ◽  
Ice Crystals ◽  
Mesoscale Convective Systems ◽  
Orthogonal Polarization ◽  
Depolarization Ratio ◽  
Space Technology ◽  
Convective Systems ◽  
Mesoscale Convective

Abstract Following the discovery of anomalously high values of lidar integrated attenuated backscatter near the top center layers of mesoscale convective systems (MCSs) observed by the NASA Lidar In-Space Technology Experiment (LITE), a search of Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) data on board the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) platform revealed the same phenomena in a sample of eight MCSs investigated. The backscatter depolarization ratio also showed changes concurrent with the high integrated backscatter and either increased or decreased concurrently with the anomalous backscatter. Simultaneous CloudSat data in the A-Train formation showed a cloud-top altitude similar to that measured by CALIOP, indicating fairly large ice crystals were reaching cloud top. Based on previous work, the CALIOP and CloudSat returns were likely due to a mix of small ice droxtals or frozen drops extending in a continuous spectrum to large crystals composed of well-formed hexagonal columns, thick hexagonal plates, spheroids, and irregular particles. The CALIOP lidar would detect the whole spectrum whereas CloudSat would detect ice crystals greater than ∼30 μm in effective radius; there were apparently enough of such crystals to allow CloudSat to detect a cloud-top height similar to that found by CALIOP. Using such a model, it was estimated that the measured backscatter phase function in the most active part of the cloud could be reconciled approximately with theoretical values of the various crystal habits. However, it was harder to reconcile the changes in depolarization ratio given the absence of values of this parameter for small droxtal crystals.

scholarly journals Probing ice clouds by broadband mid-infrared extinction spectroscopy: case studies from ice nucleation experiments in the AIDA aerosol and cloud chamber

2006 ◽  
Vol 6 (4) ◽  
pp. 5711-5771 ◽  
Author(s):  
R. Wagner ◽  
S. Benz ◽  
O. Möhler ◽  
H. Saathoff ◽  
U. Schurath
Keyword(s):  
Size Distribution ◽  
Ice Nucleation ◽  
Number Concentration ◽  
Cloud Chamber ◽  
Ice Crystals ◽  
Dust Particles ◽  
Ice Clouds ◽  
Number Size Distribution ◽  
Ice Particles ◽  
Infrared Extinction

Abstract. Series of infrared extinction spectra of ice crystals were recorded in the 6000–800 cm-1 wavenumber regime during expansion cooling experiments in the large aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe. Either supercooled sulphuric acid solution droplets or dry mineral dust particles were added as seed aerosols to initiate ice formation after having established ice supersaturated conditions inside the chamber. The various ice nucleation runs were conducted at temperatures between 237 and 195 K, leading to median sizes of the nucleated ice particles of 1–15 µm. The measured infrared spectra were fitted with reference spectra from T-matrix calculations to retrieve the number concentration as well as the number size distribution of the generated ice clouds. The ice particles were modelled as finite circular cylinders with aspect ratios ranging from 0.5 to 3.0. Benefiting from the comprehensive diagnostic tools for the characterisation of ice clouds which are available at the AIDA facility, the infrared retrieval results with regard to the ice particle number concentration could be compared to independent measurements with various optical particle counters. This provided a unique chance to quantitatively assess potential errors or solution ambiguities in the retrieval procedure which mainly originate from the difficulty to find an appropriate shape representation for the aspherical particle habits of the ice crystals. Based on these inter-comparisons, we demonstrate that there is no standard retrieval approach which can be routinely applied to all different experimental scenarios. In particular, the concept to account for the asphericity of the ice crystals, the a priori constraints which might be imposed on the unknown number size distribution of the ice crystals (like employing an analytical distribution function), and the wavenumber range which is included in the fitting algorithm should be carefully adjusted to each single retrieval problem.

scholarly journals Microphysical properties and fall speed measurements of snow ice crystals using the Dual Ice Crystal Imager (D-ICI)

2020 ◽  
Vol 13 (3) ◽  
pp. 1273-1285 ◽  
Author(s):  
Thomas Kuhn ◽  
Sandra Vázquez-Martín
Keyword(s):  
Remote Sensing ◽  
Vertical Direction ◽  
Ice Crystals ◽  
Ice Crystal ◽  
Double Exposure ◽  
Shape Information ◽  
Microphysical Properties ◽  
Ice Particles ◽  
Crystal Properties ◽  
Fall Speed

Abstract. Accurate predictions of snowfall require good knowledge of the microphysical properties of the snow ice crystals and particles. Shape is an important parameter as it strongly influences the scattering properties of the ice particles, and thus their response to remote sensing techniques such as radar measurements. The fall speed of ice particles is another important parameter for both numerical forecast models as well as representation of ice clouds and snow in climate models, as it is responsible for the rate of removal of ice from these models. We describe a new ground-based in situ instrument, the Dual Ice Crystal Imager (D-ICI), to determine snow ice crystal properties and fall speed simultaneously. The instrument takes two high-resolution pictures of the same falling ice particle from two different viewing directions. Both cameras use a microscope-like setup resulting in an image pixel resolution of approximately 4 µm pixel−1. One viewing direction is horizontal and is used to determine fall speed by means of a double exposure. For this purpose, two bright flashes of a light-emitting diode behind the camera illuminate the falling ice particle and create this double exposure, and the vertical displacement of the particle provides its fall speed. The other viewing direction is close-to-vertical and is used to provide size and shape information from single-exposure images. This viewing geometry is chosen instead of a horizontal one because shape and size of ice particles as viewed in the vertical direction are more relevant than these properties viewed horizontally, as the vertical fall speed is more strongly influenced by the vertically viewed properties. In addition, a comparison with remote sensing instruments that mostly have a vertical or close-to-vertical viewing geometry is favoured when the particle properties are measured in the same direction. The instrument has been tested in Kiruna, northern Sweden (67.8∘ N, 20.4∘ E). Measurements are demonstrated with images from different snow events, and the determined snow ice crystal properties are presented.

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.

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