Thermodynamic correction of particle concentrations measured by underwing probes on fast flying aircraft

Abstract. Particle concentration measurements with underwing probes on aircraft are impacted by air compression upstream of the instrument body as a function of flight velocity. In particular for fast-flying aircraft the necessity arises to account for compression of the air sample volume. Hence, a correction procedure is needed to invert measured particle number concentrations to ambient conditions that is commonly applicable for different instruments to gain comparable results. In the compression region where the detection of particles occurs (i.e. under factual measurement conditions), pressure and temperature of the air sample are increased compared to ambient (undisturbed) conditions in certain distance away from the aircraft. Conventional procedures for scaling the measured number densities to ambient conditions presume that the particle penetration speed through the instruments' detection area equals the aircraft speed (True Air Speed, TAS). However, particle imaging instruments equipped with pitot-tubes measuring the Probe Air Speed (PAS) of each underwing probe reveal PAS values systematically below those of the TAS. We conclude that the deviation between PAS and TAS is mainly caused by the compression of the probed air sample. From measurements during two missions in 2014 with the German Gulfstream G-550 (HALO – High Altitude LOng range) research aircraft we develop a procedure to correct the measured particle concentration to ambient conditions using a thermodynamic approach. With the provided equation the corresponding concentration correction factor ξ is applicable to the high frequency measurements of each underwing probe which is equipped with its own air speed sensor (e.g. a pitot-tube). ξ-values of 1 to 0.85 are calculated for air speeds (i.e. TAS) between 60 and 260 m s−1. From HALO data it is found that ξ does not significantly vary between the different deployed instruments. Thus, for the current HALO underwing probe configuration a parameterisation of ξ as a function of TAS is provided for instances if PAS measurements are lacking. The ξ-correction yields higher ambient particle concentration by about 15–25 % compared to conventional procedures – an improvement which can be considered as significant for many research applications. The calculated ξ-values are specifically related to the considered HALO underwing probe arrangement and may differ for other aircraft or instrument geometries. Moreover, the ξ-correction may not cover all impacts originating from high flight velocities and from interferences between the instruments and, e.g., the aircraft wings and/or fuselage. Consequently, it is important that PAS (as a function of TAS) is individually measured by each probe deployed underneath the wings of a fast-flying aircraft.

Download Full-text

Thermodynamic correction of particle concentrations measured by underwing probes on fast-flying aircraft

Atmospheric Measurement Techniques ◽

10.5194/amt-9-5135-2016 ◽

2016 ◽

Vol 9 (10) ◽

pp. 5135-5162 ◽

Cited By ~ 20

Author(s):

Ralf Weigel ◽

Peter Spichtinger ◽

Christoph Mahnke ◽

Marcus Klingebiel ◽

Armin Afchine ◽

...

Keyword(s):

Correction Factor ◽

Particle Concentration ◽

Sample Volume ◽

Time Interval ◽

Ambient Conditions ◽

Measured Concentration ◽

Air Sample ◽

Cloud Particles ◽

Air Speed ◽

Measured Particle

Abstract. Particle concentration measurements with underwing probes on aircraft are impacted by air compression upstream of the instrument body as a function of flight velocity. In particular, for fast-flying aircraft the necessity arises to account for compression of the air sample volume. Hence, a correction procedure is needed to invert measured particle number concentrations to ambient conditions that is commonly applicable to different instruments to gain comparable results. In the compression region where the detection of particles occurs (i.e. under factual measurement conditions), pressure and temperature of the air sample are increased compared to ambient (undisturbed) conditions in certain distance away from the aircraft. Conventional procedures for scaling the measured number densities to ambient conditions presume that the air volume probed per time interval is determined by the aircraft speed (true air speed, TAS). However, particle imaging instruments equipped with pitot tubes measuring the probe air speed (PAS) of each underwing probe reveal PAS values systematically below those of the TAS. We conclude that the deviation between PAS and TAS is mainly caused by the compression of the probed air sample. From measurements during two missions in 2014 with the German Gulfstream G-550 (HALO – High Altitude LOng range) research aircraft we develop a procedure to correct the measured particle concentration to ambient conditions using a thermodynamic approach. With the provided equation, the corresponding concentration correction factor ξ is applicable to the high-frequency measurements of the underwing probes, each of which is equipped with its own air speed sensor (e.g. a pitot tube). ξ values of 1 to 0.85 are calculated for air speeds (i.e. TAS) between 60 and 250 m s−1. For different instruments at individual wing position the calculated ξ values exhibit strong consistency, which allows for a parameterisation of ξ as a function of TAS for the current HALO underwing probe configuration. The ability of cloud particles to adopt changes of air speed between ambient and measurement conditions depends on the cloud particles' inertia as a function of particle size (diameter Dp). The suggested inertia correction factor μ (Dp) for liquid cloud drops ranges between 1 (for Dp < 70 µm) and 0.8 (for 100 µm < Dp < 225 µm) but it needs to be applied carefully with respect to the particles' phase and nature. The correction of measured concentration by both factors, ξ and μ (Dp), yields higher ambient particle concentration by about 10–25 % compared to conventional procedures – an improvement which can be considered as significant for many research applications. The calculated ξ values are specifically related to the considered HALO underwing probe arrangement and may differ for other aircraft. Moreover, suggested corrections may not cover all impacts originating from high flight velocities and from interferences between the instruments and e.g. the aircraft wings and/or fuselage. Consequently, it is important that PAS (as a function of TAS) is individually measured by each probe deployed underneath the wings of a fast-flying aircraft.

Download Full-text

Response Time Characteristics of the Fast-2D Optical Array Probe Detector Board

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-16-0062.1 ◽

2016 ◽

Vol 33 (12) ◽

pp. 2569-2583 ◽

Cited By ~ 5

Author(s):

Matthew Hayman ◽

Katie J. McMenamin ◽

Jørgen B. Jensen

Keyword(s):

Response Time ◽

Particle Concentration ◽

Detection System ◽

Sample Volume ◽

Angle Of Incidence ◽

Response Characteristics ◽

Array Probe ◽

Research Aircraft ◽

Volume Estimates ◽

Probe Response

AbstractTwo-dimensional optical array probes are commonly used for imaging raindrops and ice particles on research aircraft. The ability of these probes to accurately measure particle concentration and size partly depends on the response characteristics of the detection system. If the response characteristics are too slow, then small particles are less likely to be detected and the associated effective sample volume decreases. In an effort to better understand the sample volumes of optical array probes at the National Center for Atmospheric Research, the temporal response of the Fast-2D optical array probe detector board from optical input on the detector to digitization was characterized. The analysis suggests that the board electronics have a response time constant consistently near 50 ns. However, there is also a slow decay term that conforms to a decay rate. The amplitude of this slow function can impact the probe response, varying the minimum detectable pulse width between 60 and 150 ns. Also, the amplitude of the slow function is largely dictated by the illumination angle of incidence. The effects of the response time characteristics are analyzed using a simulator for a 2D cloud (2D-C) probe with 25-μm photodiode spacing. The results show the greatest sensitivity to response time characteristics when particles are smaller than 150 μm, where 10% uncertainty in the slow fraction is likely to produce sample volume uncertainties near 10%. Ignoring response time effects may bias sample volume estimates in the small size regime by as much as 25%.

Download Full-text

Optical Atmospheric Air-Speed Sensor for EDL Operations

Optical Sensors and Sensing Congress ◽

10.1364/sensors.2020.sm1b.2 ◽

2020 ◽

Author(s):

Elie R. Salameh ◽

Jaime Da Silva ◽

M. Volkan Otugen ◽

Dominique Fourguette

Keyword(s):

Atmospheric Air ◽

Speed Sensor ◽

Air Speed

Download Full-text

Stable Floating Drops of Liquid

Journal of Fluids Engineering ◽

10.1115/1.3242525 ◽

1985 ◽

Vol 107 (4) ◽

pp. 530-533

Author(s):

Peter Dransfield ◽

D. C. Davis

Keyword(s):

Rotational Speed ◽

Ambient Conditions ◽

Speed Sensor ◽

Floating Drops

There are few reported situations in which a drop of liquid will remain intact in ambient conditions for an indefinite period of time. The paper describes a situation where this happens. The phenomenon was noticed during experiments concerned with the development of a novel rotational speed sensor. It proved to be a substantial rather than a passing occurrence.

Download Full-text

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

Atmospheric Measurement Techniques ◽

10.5194/amt-14-1917-2021 ◽

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.

Download Full-text

The Influence of Particle Concentration on the Formation of Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds

Frontiers in Earth Science ◽

10.3389/feart.2021.640090 ◽

2021 ◽

Vol 9 ◽

Author(s):

Allan Fries ◽

Jonathan Lemus ◽

Paul A. Jarvis ◽

Amanda B. Clarke ◽

Jeremy C. Phillips ◽

...

Keyword(s):

Volcanic Ash ◽

Particle Concentration ◽

Initial Conditions ◽

Initial Density ◽

Fluid Phase ◽

Dimensionless Number ◽

Narrow Region ◽

Gravitational Instabilities ◽

Volcanic Clouds ◽

Particle Imaging

Settling-driven gravitational instabilities observed at the base of volcanic ash clouds have the potential to play a substantial role in volcanic ash sedimentation. They originate from a narrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms at the lower cloud-atmosphere interface and generates downward-moving ash fingers that enhance the ash sedimentation rate. We use scaled laboratory experiments in combination with particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques to investigate the effect of particle concentration on PBL and finger formation. Results show that, as particles settle across an initial density interface and are incorporated within the dense underlying fluid, the PBL grows below the interface as a narrow region of small excess density. This detaches upon reaching a critical thickness, that scales with (ν2/g′)1/3, where ν is the kinematic viscosity and g′ is the reduced gravity of the PBL, leading to the formation of fingers. During this process, the fluid above and below the interface remains poorly mixed, with only small quantities of the upper fluid phase being injected through fingers. In addition, our measurements confirm previous findings over a wider set of initial conditions that show that both the number of fingers and their velocity increase with particle concentration. We also quantify how the vertical particle mass flux below the particle suspension evolves with time and with the particle concentration. Finally, we identify a dimensionless number that depends on the measurable cloud mass-loading and thickness, which can be used to assess the potential for settling-driven gravitational instabilities to form. Our results suggest that fingers from volcanic clouds characterised by high ash concentrations not only are more likely to develop, but they are also expected to form more quickly and propagate at higher velocities than fingers associated with ash-poor clouds.

Download Full-text

Revisiting particle sizing using greyscale optical array probes: evaluation using laboratory experiments and synthetic data

Atmospheric Measurement Techniques ◽

10.5194/amt-12-3067-2019 ◽

2019 ◽

Vol 12 (6) ◽

pp. 3067-3079

Author(s):

Sebastian J. O'Shea ◽

Jonathan Crosier ◽

James Dorsey ◽

Waldemar Schledewitz ◽

Ian Crawford ◽

...

Keyword(s):

Climate Models ◽

Synthetic Data ◽

Mie Scattering ◽

Sample Volume ◽

Research Aircraft ◽

Order Of Magnitude ◽

Ambient Data ◽

In Situ Observations ◽

Synthetic Datasets

Abstract. In situ observations from research aircraft and instrumented ground sites are important contributions to developing our collective understanding of clouds and are used to inform and validate numerical weather and climate models. Unfortunately, biases in these datasets may be present, which can limit their value. In this paper, we discuss artefacts which may bias data from a widely used family of instrumentation in the field of cloud physics, optical array probes (OAPs). Using laboratory and synthetic datasets, we demonstrate how greyscale analysis can be used to filter data, constraining the sample volume of the OAP and improving data quality, particularly at small sizes where OAP data are considered unreliable. We apply the new methodology to ambient data from two contrasting case studies: one warm cloud and one cirrus cloud. In both cases the new methodology reduces the concentration of small particles (<60 µm) by approximately an order of magnitude. This significantly improves agreement with a Mie-scattering spectrometer for the liquid case and with a holographic imaging probe for the cirrus case. Based on these results, we make specific recommendations to instrument manufacturers, instrument operators and data processors about the optimal use of greyscale OAPs. The data from monoscale OAPs are unreliable and should not be used for particle diameters below approximately 100 µm.

Download Full-text