scholarly journals Characterising optical array particle imaging probes: implications for small ice crystal observations

2020 ◽  
Author(s):  
Sebastian O'Shea ◽  
Jonathan Crosier ◽  
James Dorsey ◽  
Louis Gallagher ◽  
Waldemar Schledewitz ◽  
...  
Keyword(s):  
Field Experiments ◽  
Field Measurements ◽  
Sample Volume ◽  
Ice Crystal ◽  
Cloud Particle ◽  
Cloud Properties ◽  
Imaging Probes ◽  
New Methodologies ◽  
Particle Imaging ◽  
Shape Data

Abstract. The cloud particle concentration, size and shape data from optical array probes (OAPs) are routinely used to parameterise cloud properties and constrain remote sensing retrievals. This paper characterises the optical response of OAPs using a combination of modelling, laboratory and field experiments. Significant uncertainties are found to exist with such probes for ice crystal measurements. We describe and test two independent methods to constrain a probe's sample volume that removes the most severely mis-sized particles: (1) greyscale image analysis and (2) co-location using stereoscopic imaging. These methods are tested using field measurements from three research flights in cirrus. For these cases, the new methodologies significantly improve agreement with a holographic imaging probe compared to conventional data processing protocols, either removing or significantly reducing the concentration of small ice crystals (

scholarly journals Characterising optical array particle imaging probes: implications for small-ice-crystal observations

2021 ◽  
Vol 14 (3) ◽  
pp. 1917-1939
Author(s):  
Sebastian O'Shea ◽  
Jonathan Crosier ◽  
James Dorsey ◽  
Louis Gallagher ◽  
Waldemar Schledewitz ◽  
...  
Keyword(s):  
Field Experiments ◽  
Field Measurements ◽  
Sample Volume ◽  
Ice Crystal ◽  
Cloud Particle ◽  
Ice Clouds ◽  
Cloud Properties ◽  
Imaging Probes ◽  
New Methodologies ◽  
Particle Imaging

Abstract. The cloud particle concentration, size, and shape data from optical array probes (OAPs) are routinely used to parameterise cloud properties and constrain remote sensing retrievals. This paper characterises the optical response of OAPs using a combination of modelling, laboratory, and field experiments. Significant uncertainties are found to exist with such probes for ice crystal measurements. We describe and test two independent methods to constrain a probe's sample volume that remove the most severely mis-sized particles: (1) greyscale image analysis and (2) co-location using stereoscopic imaging. These methods are tested using field measurements from three research flights in cirrus. For these cases, the new methodologies significantly improve agreement with a holographic imaging probe compared to conventional data-processing protocols, either removing or significantly reducing the concentration of small ice crystals (< 200 µm) in certain conditions. This work suggests that the observational evidence for a ubiquitous mode of small ice particles in ice clouds is likely due to a systematic instrument bias. Size distribution parameterisations based on OAP measurements need to be revisited using these improved methodologies.

scholarly journals Shattering during Sampling by OAPs and HVPS. Part I: Snow Particles

2005 ◽  
Vol 22 (5) ◽  
pp. 528-542 ◽  
Author(s):  
Alexei Korolev ◽  
George A. Isaac
Keyword(s):  
Remote Sensing ◽  
Wind Shear ◽  
High Volume ◽  
Sample Volume ◽  
Particle Sizes ◽  
Small Particles ◽  
Cloud Particle ◽  
Ice Particles ◽  
Imaging Probes ◽  
Cloud Parameterization

Abstract The data on cloud particle sizes and concentrations collected with the help of aircraft imaging probes [optical array probes OAP-2DC, OAP-2DP, and the High Volume Precipitation Spectrometer (HVPS)] are widely used for cloud parameterization and validation of remote sensing. The goal of the present work is to study the effect of shattering of ice particles during sampling. The shattering of ice particles may occur due to 1) mechanical impact with the probe arms prior to their entering the sample volume, and 2) fragmentation due to interaction with turbulence and wind shear generated by the probe housing. The effect of shattering is characterized by the shattering efficiency that is equal to the ratio of counts of disintegrated particles, to all counts. The shattering efficiency depends on the habit, size, and density of ice particles, probe inlet design, and airspeed. For the case of aggregates, the shattering efficiency may reach 10% or even more. The shattering of ice particles results in an overcounting of small particles and an undercounting of large ones. The number of fragments in the images of shattered particles may reach several hundreds. It was found that particles as small as 600 μm may shatter after impact with the probe arms. The effect of particle shattering should be taken into account during data analysis and carefully considered in future designs of airborne cloud particle size spectrometers.

scholarly journals Ice Crystal Sizes in High Ice Water Content Clouds. Part II: Statistics of Mass Diameter Percentiles in Tropical Convection Observed during the HAIC/HIWC Project

2017 ◽  
Vol 34 (1) ◽  
pp. 117-136 ◽  
Author(s):  
D. Leroy ◽  
E. Fontaine ◽  
A. Schwarzenboeck ◽  
J. W. Strapp ◽  
A. Korolev ◽  
...  
Keyword(s):  
Water Content ◽  
Ice Crystals ◽  
Ice Crystal ◽  
Potential Threat ◽  
Convective Systems ◽  
Imaging Probes ◽  
Ice Water Content ◽  
The Individual ◽  
Particle Imaging ◽  
Ice Water

AbstractHigh ice water content (IWC) regions in mesoscale convective systems (MCSs) are a potential threat to commercial aviation, as they are suspected to cause in-service engine power-loss events and air data probe malfunctions. To investigate this, the high-altitude ice crystals (HAIC)/high ice water content (HIWC) projects set up a first field campaign in Darwin (Australia) in 2014. The airborne instrumentation was selected to provide the most accurate measurements of both the bulk total water content (TWC), using a specially developed isokinetic evaporator, and the individual ice crystals properties, using particle imaging probes.This study focuses on determining the size ranges of ice crystals responsible for the mass in high IWC regions, defined here as cloud regions with IWC greater than 1.5 g m−3. It is shown that for high IWC areas in most of the encountered MCSs, median mass diameters (MMDs) of ice crystals range from 250 to 500 μm and decrease with increasing TWC and decreasing temperature. At the same time, the mass contribution of the smallest crystals (below 100 μm) remains generally low (below 15%).In contrast, data from two flight missions in a long-lasting quasi-stationary tropical storm reveal that high IWC values can also be associated with MMDs in the range 400–800 μm and peak values of up to 2 mm. Ice crystal images suggest a major growth contribution by vapor deposition (columns, capped columns) even for such larger MMD values.

scholarly journals Supplementary material to "Characterising optical array particle imaging probes: implications for small ice crystal observations"

2020 ◽  
Author(s):  
Sebastian O'Shea ◽  
Jonathan Crosier ◽  
James Dorsey ◽  
Louis Gallagher ◽  
Waldemar Schledewitz ◽  
...  
Keyword(s):  
Ice Crystal ◽  
Imaging Probes ◽  
Supplementary Material ◽  
Particle Imaging

scholarly journals Cloud Particle Phase Determination with the AVHRR

2000 ◽  
Vol 39 (10) ◽  
pp. 1797-1804 ◽  
Author(s):  
Jeffrey R. Key ◽  
Janet M. Intrieri
Keyword(s):  
Field Experiments ◽  
Climate Models ◽  
Accurate Determination ◽  
Sufficient Information ◽  
Particle Phase ◽  
Phase Determination ◽  
Cloud Particle ◽  
Ice Clouds ◽  
Cloud Properties

Abstract An accurate determination of cloud particle phase is required for the retrieval of other cloud properties from satellite and for radiative flux calculations in climate models. The physical principles underlying phase determination using the advanced very high resolution radiometer (AVHRR) satellite sensor are described for daytime and nighttime, cold cloud and warm cloud conditions. It is demonstrated that the spectral properties of cloud particles provide necessary, but not sufficient, information for phase determination, because the relationship between the cloud and surface temperatures is also important. Algorithms based on these principles are presented and tested. Validation with lidar and aircraft data from two Arctic field experiments shows the procedures to be accurate in identifying the phase of homogeneous water and ice clouds, though optically thin, mixed-phase, and multilayer clouds are problematic.

scholarly journals Laboratory and Flight Tests of 2D Imaging Probes: Toward a Better Understanding of Instrument Performance and the Impact on Archived Data

2018 ◽  
Vol 35 (7) ◽  
pp. 1533-1553 ◽  
Author(s):  
Colin Gurganus ◽  
Paul Lawson
Keyword(s):  
Laboratory Tests ◽  
Sample Volume ◽  
Cloud Particle ◽  
Flight Tests ◽  
Data Archives ◽  
Imaging Probes ◽  
Research Aircraft ◽  
Water Drops ◽  
2D Imaging ◽  
Archived Data

AbstractTwo-dimensional (2D) imaging probes, such as the 2D stereo (2D-S) probe and the cloud imaging probe (CIP), are routinely used to provide in situ measurements of cloud particle properties. The basic measurement is shadowgraphs of water drops and ice particles from which particle size distributions, projected particle area, and mass concentrations are determined. These data permeate data archives of domestic and foreign government agencies, universities, and the private sector. This paper provides results from laboratory tests and flight tests on a Learjet research aircraft that give new insights into the performance of the 2D imaging probes, and how their performance may have impacted measurements collected in data archives. The laboratory tests are conducted with the aid of two devices: 1) a droplet generator that provides known concentrations of water drops from 15 to 65 µm ± 1 µm that can be positioned in the probe’s sample volume with 10-µm precision; and 2) a motorized spinning platform that supports transparent disks with small opaque features (i.e., a “spinning disk”), which replicates the effect of particles transecting the probe’s sample volume at translational speeds up to 190 m s−1. The flight tests were conducted with a Learjet research aircraft that collected cloud particle data at true airspeeds from 99 to 170 m s−1. The results provide new insights into how probe optics, time response, and data throughput of the 2D-S and CIP electro-optics impact the measurements of cloud particles. The results, summarized in the conclusions, suggest how archived data are impacted.

scholarly journals Ice supersaturations and cirrus cloud crystal numbers

2009 ◽  
Vol 9 (11) ◽  
pp. 3505-3522 ◽  
Author(s):  
M. Krämer ◽  
C. Schiller ◽  
A. Afchine ◽  
R. Bauer ◽  
I. Gensch ◽  
...  
Keyword(s):  
Low Temperatures ◽  
Field Experiments ◽  
Water Saturation ◽  
Field Measurements ◽  
Cloud Microphysics ◽  
Radiative Properties ◽  
Freezing Process ◽  
Ice Crystal ◽  
Data Set ◽  
Mixing Ratios

Abstract. Upper tropospheric observations outside and inside of cirrus clouds indicate water vapour mixing ratios sometimes exceeding water saturation. Relative humidities over ice (RHice) of up to and more than 200% have been reported from aircraft and balloon measurements in recent years. From these observations a lively discussion continues on whether there is a lack of understanding of ice cloud microphysics or whether the water measurements are tainted with large uncertainties or flaws. Here, RHice in clear air and in ice clouds is investigated. Strict quality-checked aircraft in situ observations of RHice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183–240 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super- or subsaturations inside of cirrus are frequently observed at low temperatures (<205 K) in our field data set. To explain persistent RHice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis – using conventional microphysics – of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus can possibly be explained by unexpected, frequent very low ice crystal numbers that could scarcely be caused by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations, with implications for the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.

scholarly journals Developing and bounding ice particle mass- and area-dimension expressions for use in atmospheric models and remote sensing

2015 ◽  
Vol 15 (20) ◽  
pp. 28517-28573 ◽  
Author(s):  
E. Erfani ◽  
D. L. Mitchell
Keyword(s):  
Remote Sensing ◽  
Field Measurements ◽  
Power Laws ◽  
Field Studies ◽  
Particle Mass ◽  
Atmospheric Models ◽  
Cloud Particle ◽  
Ice Clouds ◽  
Ice Cloud ◽  
Cloud Properties

Abstract. Ice particle mass- and projected area-dimension (m-D and A-D) power laws are commonly used in the treatment of ice cloud microphysical and optical properties and the remote sensing of ice cloud properties. Although there has long been evidence that a single m-D or A-D power law is often not valid over all ice particle sizes, few studies have addressed this fact. This study develops self-consistent m-D and A-D expressions that are not power laws, but can easily be reduced to power laws for the ice particle size (maximum dimension or D) range of interest, and they are valid over a much larger D range than power laws. This was done by combining field measurements of individual ice particle m and D formed at temperature T < −20 °C with 2-dimensional stereo (2D-S) and Cloud Particle Imager (CPI) probe measurements (or estimates) of D, A and m in synoptic and anvil ice clouds at similar temperatures. The resulting m-D and A-D expressions are functions of temperature and cloud type (synoptic vs. anvil), and are in good agreement with m-D power laws developed from recent field studies considering the same temperature range (−60 °C < T < −20 °C).

scholarly journals Ice supersaturations and cirrus cloud crystal numbers

2008 ◽  
Vol 8 (6) ◽  
pp. 21089-21128 ◽  
Author(s):  
M. Krämer ◽  
C. Schiller ◽  
A. Afchine ◽  
R. Bauer ◽  
I. Gensch ◽  
...  
Keyword(s):  
Low Temperatures ◽  
Field Experiments ◽  
Water Saturation ◽  
Field Measurements ◽  
Cloud Microphysics ◽  
Radiative Properties ◽  
Freezing Process ◽  
Ice Crystal ◽  
Data Set ◽  
Mixing Ratios

Abstract. Upper tropospheric observations outside and inside of cirrus clouds of water vapour mixing ratios sometimes exceeding water saturation, yielding up to more than 200% relative humidities over ice (RHice) have been reported from aircraft and balloon measurements in recent years. From these observations a lively continuous discussion arose on whether there is a lack of understanding of ice cloud microphysics or if the water measurements are tainted with large uncertainties or flaws. Here, RHice in clear air and in ice clouds is investigated: strictly quality checked aircraft in-situ observations of RHice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183–250 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super- or subsaturations inside of cirrus are frequently observed at low temperatures (<205 K) in our field data set. To explain persistent RHice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis – using conventional microphysics – of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus are caused by unexpected, frequent very low ice crystal numbers that could hardly be explained by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations with implications to the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.

scholarly journals Optical properties of pristine ice crystals in mid-latitude cirrus clouds: a case study during CIRCLE-2 experiment

2011 ◽  
Vol 11 (6) ◽  
pp. 2537-2544 ◽  
Author(s):  
J.-F. Gayet ◽  
G. Mioche ◽  
V. Shcherbakov ◽  
C. Gourbeyre ◽  
R. Busen ◽  
...  
Keyword(s):  
Optical Properties ◽  
Leading Edge ◽  
Ice Crystals ◽  
Ice Crystal ◽  
Cloud Particle ◽  
Cloud Band ◽  
Cloud Properties ◽  
Complex Shapes ◽  
Scattering Phase ◽  
Phase Functions

Abstract. In this paper, we describe in situ observations of mid-latitude cirrus cloud band carried out on 16 May 2007 during the CIRCLE-2 experiment. The Polar Nephelometer and the Cloud Particle Imager (CPI) instruments with PMS FSSP-300 and 2D-C probes were used for the description of the optical and microphysical cloud properties. Two selected cloud regions are compared and discussed in detail. Significant differences in optical properties are evidenced in terms of 22° halo occurrences even though prevalent planar-plate ice crystals are observed in both cloud regions. Featureless scattering phase functions are measured in the first cloud region located near the trailing edge of the cirrus-band at about 11 800 m/−57 °C. In contrast, well pronounced 22° halo peaks are observed with predominant similar-shaped ice crystals near the cirrus-band leading edge at 7100 m/−27 °C. CPI ice crystal images with Polar Nephelometer observations are carefully analysed and interpreted from a theoretical light scattering model in order to explain occurrence and non-occurrence of the 22° halo feature. The results highlight that the halo peaks are inherent only in perfect plate ice crystals (or pristine crystals). On the basis of previous datasets in mid-latitude cirrus, it is found that simple pristine crystals are uncommon whereas particles with imperfect or complex shapes are prevalent. As a result, phase functions that are smooth and featureless best represent cirrus scattering properties.

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