Difficulties in early ice detection with the Small Ice Detector 2 HIAPER (SID-2H) in maritime cumuli

© Copyright 2014 American Meteorological Society (AMS).The Small Ice Detector 2 HIAPER (SID-2H) was used to attempt to detect small ice particles in the early stages of ice formation in the high liquid water environment of tropical maritime cumulus clouds sampled during the Ice in Clouds Experiment - Tropical (ICE-T) field campaign. Its performance in comparison to other probes, and the development of new corrections applied to the data, are presented. The SID-2H detected small ice crystals among larger particles. It correctly identified water drops, and discriminated between round and irregular particle shapes in water-dominated clouds with errors less than 5%. Remaining uncertainties in the sensing volume, and the volume over which coincidence of particles occurred, result in the data being used here in a qualitative manner to identify the presence of ice, its habits and sizes.Peer reviewe

Modelling light scattering by absorbing smooth and slightly rough facetted particles

A method for approximating light scattering properties of strongly absorbing facetted particles which are large compared to the wavelength is presented. It consists in adding the approximated external diffraction and reflection far fields and is demonstrated for a smooth hexagonal prism. This computationally fast method is extended towards prisms with slightly rough surfaces by introducing a surface scaling factor in order to account for edge effects on subfacets forming the rough surface. These effects become more pronounced with decreasing subfacet dimension to wavelength ratio. Azimuthally resolved light scattering patterns, phase functions and degree of linear polarisation obtained by this method and by the Discrete Dipole Approximation are compared for hexagonal prisms with smooth and slightly rough surfaces, respectively.Peer reviewedSubmitted Versio

The Universal Cloud and Aerosol Sounding System (UCASS): a low-cost miniature optical particle counter for use in dropsonde or balloon-borne sounding systems

© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. An earlier version of this work was published in Atmospheric Measurement Techniques Discussions: https://dx.doi.org/10.5194/amt-2019-70.A low-cost miniaturized particle counter has been developed by The University of Hertfordshire (UH) for the measurement of aerosol and droplet concentrations and size distributions. The Universal Cloud and Aerosol Sounding System (UCASS) is an optical particle counter (OPC), which uses wide-angle elastic light scattering for the high-precision sizing of fluid-borne particulates. The UCASS has up to 16 configurable size bins, capable of sizing particles in the range 0.4–40 µm in diameter. Unlike traditional particle counters, the UCASS is an open-geometry system that relies on an external air flow. Therefore, the instrument is suited for use as part of a dropsonde, balloon-borne sounding system, as part of an unmanned aerial vehicle (UAV), or on any measurement platform with a known air flow. Data can be logged autonomously using an on-board SD card, or the device can be interfaced with commercially available meteorological sondes to transmit data in real time. The device has been deployed on various research platforms to take measurements of both droplets and dry aerosol particles. Comparative results with co-located instrumentation in both laboratory and field settings show good agreement for the sizing and counting ability of the UCASS.Peer reviewe

Design and field campaign validation of a multi-rotor unmanned aerial vehicle and optical particle counter

© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0/).Small unmanned aircraft (SUA) have the potential to be used as platforms for the measurement of atmospheric particulates. The use of an SUA platform for these measurements provides benefits such as high manoeuvrability, re-usability, and low-cost when compared with traditional techniques. However, the complex aerodynamics of an SUA (particularly for multirotor airframes), combined with the miniaturisation of particle instruments poses difficulties for accurate and representative sampling of particulates. The work presented here relies on computational fluid dynamics with Lagrangian particle tracking (CFD-LPT) simulations to influence the design of a bespoke meteorological sampling system: the UH-AeroSAM. This consists of a custom built airframe, designed to reduce sampling artefacts due to the propellers, and a purpose built open-path optical particle counter–the Ruggedised Cloud and Aerosol Sounding System (RCASS). OPC size distribution measurements from the UH-AeroSAM are compared with the Cloud and Aerosol Precipitation Spectrometer (CAPS) for measurements of Stratus cloud during the Pallas Cloud Experiment (PaCE) in 2019. Good agreement is demonstrated between the two instruments. The integrated dN/dlog(Dp) is shown to have a coefficient of determination of 0.8, and a regression slope of 0.9 when plotted 1:1.Peer reviewe

AIITS: Preliminary light scattering data from Tropical Tropopause Layer cirrus

The new optical particle spectrometer AIITS (Aerosol Ice Interface Transition Spectrometer) is the next instrument in the Small Ice Detector (SID) family. Like SID3, it acquires two-dimensional forward scattering patterns from particles in the size range from about one to a few hundred micrometers (depending on variable settings). The patterns allow quantifying the phase, habit and fine surface features of large aerosol and ice crystals, which are frequently too small to be adequately characterised using traditional imaging techniques.Two 2D-forward scattering patterns are recorded per particle using two high-resolution cameras. The cameras fire simultaneously, recording the scattering pattern via a beamsplitter. AIITS can be configured such that the cameras measure either perpendicular polarisations (i.e. P-polarisation with one camera, S-polarisation with the other) or to have a different gain setting on each camera to encompass a larger dynamic range. The incident beam can be either circularly or linearly polarised. Backscatter depolarisation is also measured. The camera and beam configuration must be selected pre-flight.The probe was deployed on board the NASA Global Hawk aircraft during a recent ATTREX/CAST campaign over the tropical eastern Pacific. We present preliminary results from a case study from the 5th of March 2015 which showed the existence of a variety of particles, including rough surfaced ice crystals, some regular, hexagonal ones, as well as particles with smooth, curved surfaces (but not spherical). We compare AIITS data with co-located particle imaging from the SPEC Hawkeye probe.The Hawkeye probe combines a 2D-Stereo optical array probe, a Cloud Particle Imager (CPI), and a Fast Cloud Droplet Probe (FCDP) to provide high resolution images (2.3 micron pixel resolution) and particle size distributions of concentration, area, and mass for particles with diameter between one micron and a few centimeters.The TTL is known to be of importance due to the presence of subvisual cirrus, which contributes to net climate radiative feedback. Knowledge of the processes involved in the creation and persistence of such clouds is limited due to sparse observational data.Non peer reviewe

Miniature, low-cost optical particle counters

A range of miniature, low-cost optical particle counters for characterizing atmospheric particles has been developed. They are intended for use with disposable balloon-borne radiosondes, dropsondes or in dense ground-based sensor networks. Versions exist that are suitable for determining the size distributions and number concentrations of cloud particles or atmospheric aerosols such as mineral dust or volcanic ash.Peer reviewe

Measurements of atmospheric volcanic ash using an aerosol radiosonde

A low-cost, miniature aerosol particle counter has been developed, intended for use with balloon-borne radiosondes. The counter provides a size distribution in five size bins and is interfaced to a Vaisala RS92 radiosonde, which transmits data from the counter together with meteorological parameters and GPS position to a ground receiver at 1 Hz rate. The counter was used together with an electric charge sensor to profile ash layers from the Eyjafjalla eruption. The layer over Stranraer, Scotland, profiled on 19 April 2010 was centered on 4000m altitude and was about 600m thick, with fairly uniform, sharply defined aerosol concentration. The mean diameter by volume was 3 micrometres and the effective diameter 2.2 micrometres. Estimated optical thickness of the layer was 0.12 at 780nm. The mass concentration peaked around 0.3 mg per cubic metre, assuming the density of silica. The total mass load was 140 mg per square metre. The electric charge density within the layer was positive, and peaked around 0.5pC per cubic metre.Peer reviewe

Radiosonde aerosol counter for vertical profiling of atmospheric dust

A low-cost, miniature aerosol particle counter has been developed, intended for use with balloon-borne meteorological radiosondes. It is particularly suitable for airborne mineral dust measurements. Ambient air is drawn into the counter using a diaphragm pump at a rate of 0.5 litre per minute. The counter detects particles in the airstream using a diode laser and a photodiode. Output from the photodiode is digitised into 5 size bins, with minimum particle diameters equivalent to 0.6, 1.4, 2.6, 5.4 and 10.6 micrometers. The counter is interfaced to a Vaisala RS92 radiosonde, which transmits data from the counter together with meteorological parameters and GPS-derived position to a ground based receiver at 1 Hz rate. Statistically significant particle size distributions can be obtained once a second for number concentrations down to about 100,000 particle per litre (within the measured size range), or correspondingly less at lower temporal resolutions. At the same time, the counter is capable of measuring dust number concentrations exceeding a million per litre without incurring significant errors. Soundings during the DREAME campaign in Kuwait (Ulanowski et al. EGU 2010, AS4.7) and on Cape Verde Islands (Nicoll et al. EGU 2010, AS4.7) provided dust concentration profiles with a typical vertical resolution of 4 m. Comparisons with integrated dust column size distribution measurements from AERONET sun photometers showed good agreement in two out of three cases where near-simultaneous retrievals were available. Optical thickness calculations based on the size distributions measured in Kuwait, with the assumption that the dust particles were prolate spheroids, agreed with the AERONET optical thickness at 675 nm to within 15%.Non peer reviewe

Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements

© Author(s) 2014. This work is distributed under the Creative Commons Attribution 3.0 LicenseThe knowledge of properties of ice crystals such as size, shape, concavity and roughness is critical in the context of radiative properties of ice and mixed-phase clouds. Limitations of current cloud probes to measure these properties can be circumvented by acquiring two-dimensional lightscattering patterns instead of particle images. Such patterns were obtained in situ for the first time using the Small Ice Detector 3 (SID-3) probe during several flights in a variety of mid-latitude mixed-phase and cirrus clouds. The patterns are analysed using several measures of pattern texture, selected to reveal the magnitude of particle roughness or complexity. The retrieved roughness is compared to values obtained from a range of well-characterized test particles in the laboratory. It is found that typical in situ roughness corresponds to that found in the rougher subset of the test particles, and sometimes even extends beyond the most extreme values found in the laboratory. In this study we do not differentiate between small-scale, fine surface roughness and large-scale crystal complexity. Instead, we argue that both can have similar manifestations in terms of light-scattering properties and also similar causes. Overall, the in situ data are consistent, with ice particles with highly irregular or rough surfaces being dominant. Similar magnitudes of roughness were found in growth and sublimation zones of cirrus. The roughness was found to be negatively correlated with the halo ratio, but not with other thermodynamic or microphysical properties found in situ. Slightly higher roughness was observed in cirrus forming in clean oceanic air masses than in a continental, polluted one. Overall, the roughness and complexity are expected to lead to increased shortwave cloud reflectivity, in comparison with cirrus composed of more regular, smooth ice crystal shapes. These findings put into question suggestions that climate could be modified through aerosol seeding to reduce cirrus cover and optical depth, as the seeding may result in decreased shortwave reflectivityPeer reviewe