Employing a shadowgraph imaging technique for cloud microphysical measurements on a mountain observatory

2021 ◽  
Author(s):  
Moein Mohammadi ◽  
Jakub Nowak ◽  
Augustinus Bertens ◽  
Jan Molacek ◽  
Wojciech Kumala ◽  
...  
Keyword(s):  
Size Distribution ◽  
Field Experiments ◽  
Droplet Size Distribution ◽  
Number Concentration ◽  
Depth Of Field ◽  
Research Station ◽  
Mean Flow ◽  
Cloud Droplets ◽  
Probe Volume ◽  
Orographic Clouds

<p>Microphysical properties of cloud droplets, such as droplet size distribution and droplet<br>number concentration have been studied after performing a series of field experiments in<br>summer 2019 at Umweltforschungsstation Schneefernerhaus (UFS), an environmental<br>research station located just below the peak of Zugspitze in the German Alps.<br>“VisiSize D30” manufactured by Oxford Laser Ltd. is a shadowgraph imaging instrument<br>utilized for the first time to measure the size and velocity of cloud droplets during this<br>campaign. It applies a method called “Particle/Droplet Image Analysis” (PDIA) which<br>involves illuminating the region of interest from behind with an infrared pulse laser whilst<br>collecting shadow images of droplets passing through the measurement volume with a<br>high-resolution camera. Droplets detected inside the depth of field are then measured<br>based on their shadow images, and size distribution is built by analyzing a series of<br>images. Furthermore, while turbulent orographic clouds passing our measurement site<br>at UFS observatory during the campaign, a Phase Doppler Interferometer (PDI) device,<br>manufactured by Artium Tech. Inc., was also constantly measuring droplets passing<br>through its probe volume.<br>Analysis of simultaneously collected data from the two instruments, and applying<br>modifications to the original algorithms illustrate a reasonable agreement regarding the<br>droplet sizing and velocimetry between VisiSize D30 and PDI, at least for diameters<br>larger than 13 μm. Moreover, discrepancies have been observed concerning the<br>droplet number concentration results, especially in smaller sizes. Further investigation<br>by applying appropriate filters on data has allowed the attribution of discrepancies to<br>the different optical performance of the sensors regarding small droplets, and to high<br>turbulent velocity fluctuations relative to the mean flow resulting in an uncertain estimate<br>of the volume of air passing through the PDI probe volume.</p>

scholarly journals Cloud microphysical measurements at a mountain observatory: comparison between shadowgraph imaging and phase Doppler interferometry

2021 ◽  
Author(s):  
Moein Mohammadi ◽  
Jakub Lukasz Nowak ◽  
Guus Bertens ◽  
Jan Molacek ◽  
Wojciech Kumala ◽  
...  
Keyword(s):  
Field Experiments ◽  
Droplet Size Distribution ◽  
Number Concentration ◽  
Environmental Research ◽  
Research Station ◽  
Mean Flow ◽  
Cloud Droplets ◽  
Microphysical Properties ◽  
Phase Doppler Interferometry ◽  
Software Algorithms

Abstract. The microphysical properties of cloud droplets, such as droplet size distribution and droplet number concentration, were studied. A series of field experiments were performed in the summer of 2019 at the Umweltforschungsstation Schneefernerhaus (UFS), an environmental research station located just below the peak of Mount Zugspitze in the German Alps. A VisiSize D30 manufactured by Oxford Laser Ltd., which is a shadowgraph imaging instrument, was utilized for the first time to measure the size and velocity of cloud droplets during this campaign. Furthermore, a phase Doppler interferometer (PDI) device, manufactured by Artium Tech. Inc., was simultaneously measuring cloud droplets. After applying modifications to the built-in software algorithms, the results from the two instruments show reasonable agreement regarding droplet sizing and velocimetry for droplet diameters larger than 13 µm. Moreover, discrepancies were observed concerning the droplet number concentration results, especially with smaller droplet sizes. Further investigation by applying appropriate filters to the data allowed the attribution of the discrepancies to two phenomena: the different optical performance of the sensors with regard to small droplets and high turbulent velocity fluctuations relative to the mean flow that result in an uncertain estimate of the volume of air passing through the PDI probe volume.

scholarly journals Applicability of the VisiSize D30 shadowgraph system for cloud microphysical measurements

2020 ◽  
Author(s):  
Jakub L. Nowak ◽  
Moein Mohammadi ◽  
Szymon P. Malinowski
Keyword(s):  
Size Distribution ◽  
Droplet Diameter ◽  
Strong Dependence ◽  
Field Tests ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Sample Volume ◽  
Number Concentration ◽  
Depth Of Field ◽  
Spatial Homogeneity

Abstract. The commercial shadowgraph system, Oxford Lasers VisiSize D30, originally designed to characterize industrial and agricultural sprays, was tested with respect to the application for measuring cloud microphysical properties, such as droplet size distribution and number concentration. Laboratory experiment with a dense stream of poly-disperse cloud-like droplets indicated strong dependence of the depth of field, thus also sample volume, on particle size. This relationship was determined and a suitable correction method was developed to improve estimations of droplet number concentration and size distribution. Spatial homogeneity of detection probability inside the sample volume and minimum droplet diameter providing uniform detection were examined. The second experiment with mono-disperse droplets produced by Flow-Focusing Monosized Aerosol Generator (FMAG) verified sizing accuracy and demonstrated reasonable agreement between the instruments. Effects of collisions and evaporation of droplets produced by FMAG were observed. Finally, the instrument was applied to sample atmospheric clouds at a ground-based mountain observatory and performed reliably during 3 week long field experiment. Based on the laboratory and field tests, recommendations concerning the use of the instrument for cloud droplet measurements were formulated.

scholarly journals Applicability of the VisiSize D30 shadowgraph system for cloud microphysical measurements

2021 ◽  
Vol 14 (4) ◽  
pp. 2615-2633
Author(s):  
Jakub L. Nowak ◽  
Moein Mohammadi ◽  
Szymon P. Malinowski
Keyword(s):  
Size Distribution ◽  
Droplet Diameter ◽  
Strong Dependence ◽  
Field Tests ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Sample Volume ◽  
Number Concentration ◽  
Depth Of Field ◽  
Spatial Homogeneity

Abstract. A commercial shadowgraph system, the Oxford Lasers VisiSize D30, originally designed to characterize industrial and agricultural sprays, was tested with respect to its application for measuring cloud microphysical properties such as droplet size distribution and number concentration. A laboratory experiment with a dense stream of polydisperse cloud-like droplets indicated a strong dependence of the depth of field, and thus also the sample volume, on particle size. This relationship was determined and a suitable correction method was developed to improve estimations of droplet number concentration and size distribution. The spatial homogeneity of the detection probability inside the sample volume and the minimum droplet diameter providing uniform detection were examined. A second experiment with monodisperse droplets produced by a Flow Focusing Monodisperse Aerosol Generator (FMAG) verified the sizing accuracy and demonstrated reasonable agreement between the instruments. Effects of collisions and the evaporation of droplets produced by the FMAG were observed. Finally, when the instrument was applied to sample atmospheric clouds at a mountain-based observatory, it performed reliably during a 3-week-long field experiment. Based on the laboratory and field tests, recommendations concerning the use of the instrument for cloud droplet measurements were formulated.

scholarly journals Quantifying the aerosol effect on droplet size distribution at cloud top

2019 ◽  
Vol 19 (11) ◽  
pp. 7839-7857
Author(s):  
Lianet Hernández Pardo ◽  
Luiz Augusto Toledo Machado ◽  
Micael Amore Cecchini ◽  
Madeleine Sánchez Gácita
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Initial Conditions ◽  
Cloud Condensation Nuclei ◽  
Droplet Size Distribution ◽  
Number Concentration ◽  
Population Characteristics ◽  
Aerosol Size Distribution ◽  
Effective Diameter ◽  
Distribution Sensitivity

Abstract. This work uses the number concentration-effective diameter phase-space to test cloud sensitivity to variations in the aerosol population characteristics, such as the aerosol size distribution, number concentration and hygroscopicity. It is based on the information from the top of a cloud simulated by a bin-microphysics single-column model, for initial conditions typical of the Amazon, using different assumptions regarding the entrainment and the aerosol size distribution. It is shown that the cloud-top evolution can be very sensitive to aerosol properties, but the relative importance of each parameter is variable. The sensitivity to each aerosol characteristic varies as a function of the parameter tested and is conditioned by the base values of the other parameters, showing a specific dependence for each configuration of the model. When both the entrainment and the bin treatment of the aerosol are allowed, the largest influence on the droplet size distribution sensitivity was obtained for the median radius of the aerosols and not for the total number concentration of aerosols. Our results reinforce that the cloud condensation nuclei activity can not be predicted solely on the basis of the w∕Na supersaturation-based regimes.

scholarly journals Influence of Microphysical Variability on Stochastic Condensation in a Turbulent Laboratory Cloud

2018 ◽  
Vol 75 (1) ◽  
pp. 189-201 ◽  
Author(s):  
N. Desai ◽  
K. K. Chandrakar ◽  
K. Chang ◽  
W. Cantrell ◽  
R. A. Shaw
Keyword(s):  
Relaxation Time ◽  
Size Distribution ◽  
Droplet Size ◽  
Droplet Size Distribution ◽  
Number Concentration ◽  
Droplet Growth ◽  
Diffusional Growth ◽  
Phase Relaxation ◽  
Distribution Width ◽  
Phase Relaxation Time

Diffusional growth of droplets by stochastic condensation and a resulting broadening of the size distribution has been considered as a mechanism for bridging the cloud droplet growth gap between condensation and collision–coalescence. Recent studies have shown that supersaturation fluctuations can lead to a broadening of the droplet size distribution at the condensational stage of droplet growth. However, most studies using stochastic models assume the phase relaxation time of a cloud parcel to be constant. In this paper, two questions are asked: how variability in droplet number concentration and radius influence the phase relaxation time and what effect it has on the droplet size distributions. To answer these questions, steady-state cloud conditions are created in the laboratory and digital inline holography is used to directly observe the variations in local number concentration and droplet size distribution and, thereby, the integral radius. Stochastic equations are also extended to account for fluctuations in integral radius and obtain new terms that are compared with the laboratory observations. It is found that the variability in integral radius is primarily driven by variations in the droplet number concentration and not the droplet radius. This variability does not contribute significantly to the mean droplet growth rate but does contribute significantly to the rate of increase of the size distribution width.

scholarly journals Microphysical processing of aerosol particles in orographic clouds

2015 ◽  
Vol 15 (16) ◽  
pp. 9217-9236 ◽  
Author(s):  
S. Pousse-Nottelmann ◽  
E. M. Zubler ◽  
U. Lohmann
Keyword(s):  
Cloud Droplet ◽  
Number Concentration ◽  
Mixed Phase ◽  
Cloud Formation ◽  
Small Scale ◽  
Cloud Droplets ◽  
Aerosol Mass ◽  
Aerosol Processing ◽  
Orographic Clouds ◽  
Aerosol Scavenging

Abstract. An explicit and detailed treatment of cloud-borne particles allowing for the consideration of aerosol cycling in clouds has been implemented into COSMO-Model, the regional weather forecast and climate model of the Consortium for Small-scale Modeling (COSMO). The effects of aerosol scavenging, cloud microphysical processing and regeneration upon cloud evaporation on the aerosol population and on subsequent cloud formation are investigated. For this, two-dimensional idealized simulations of moist flow over two bell-shaped mountains were carried out varying the treatment of aerosol scavenging and regeneration processes for a warm-phase and a mixed-phase orographic cloud. The results allowed us to identify different aerosol cycling mechanisms. In the simulated non-precipitating warm-phase cloud, aerosol mass is incorporated into cloud droplets by activation scavenging and released back to the atmosphere upon cloud droplet evaporation. In the mixed-phase cloud, a first cycle comprises cloud droplet activation and evaporation via the Wegener–Bergeron–Findeisen (WBF) process. A second cycle includes below-cloud scavenging by precipitating snow particles and snow sublimation and is connected to the first cycle via the riming process which transfers aerosol mass from cloud droplets to snowflakes. In the simulated mixed-phase cloud, only a negligible part of the total aerosol mass is incorporated into ice crystals. Sedimenting snowflakes reaching the surface remove aerosol mass from the atmosphere. The results show that aerosol processing and regeneration lead to a vertical redistribution of aerosol mass and number. Thereby, the processes impact the total aerosol number and mass and additionally alter the shape of the aerosol size distributions by enhancing the internally mixed/soluble Aitken and accumulation mode and generating coarse-mode particles. Concerning subsequent cloud formation at the second mountain, accounting for aerosol processing and regeneration increases the cloud droplet number concentration with possible implications for the ice crystal number concentration.

scholarly journals Quantifying the aerosol effect on droplet size distribution at cloud-top

2018 ◽  
Author(s):  
Lianet Hernández Pardo ◽  
Luiz Augusto Toledo Machado ◽  
Micael Amore Cecchini ◽  
Madeleine Sánchez Gácita
Keyword(s):  
Size Distribution ◽  
Initial Conditions ◽  
Droplet Size Distribution ◽  
Cloud Microphysics ◽  
Number Concentration ◽  
Population Characteristics ◽  
Aerosol Size Distribution ◽  
Effective Diameter ◽  
Bin Microphysics ◽  
Aerosol Properties

Abstract. This work uses the number concentration-effective diameter phase-space to test cloud sensitivity to variations in the aerosol population characteristics, such as the aerosol size distribution, number concentration and hygroscopicity. It is based on the information from the top of a cloud simulated by a bin-microphysics single-column model, for initial conditions typical of the Amazon. It is shown that the cloud-top evolution can be very sensitive to aerosol properties, but the relative importance of each parameter is variable. The sensitivity to each aerosol characteristic varies as a function of the tested parameter and is conditioned by the base values of the other parameters. The median radius of the aerosols showed the largest influence on this sensitivity. We show that all aerosol properties can have significant impacts on cloud microphysics, especially if the median radius of the aerosol size distribution is smaller than 0.05 μm.

Turbulence Effects of Collision Efficiency and Broadening of Droplet Size Distribution in Cumulus Clouds

2018 ◽  
Vol 75 (1) ◽  
pp. 203-217 ◽  
Author(s):  
Sisi Chen ◽  
M. K. Yau ◽  
Peter Bartello
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Droplet Size Distribution ◽  
Droplet Growth ◽  
Small Scale ◽  
Cumulus Clouds ◽  
Collision Efficiency ◽  
Cloud Droplets ◽  
R Ratio ◽  
Condensational Growth

This paper aims to investigate and quantify the turbulence effect on droplet collision efficiency and explore the broadening mechanism of the droplet size distribution (DSD) in cumulus clouds. The sophisticated model employed in this study individually traces droplet motions affected by gravity, droplet disturbance flows, and turbulence in a Lagrangian frame. Direct numerical simulation (DNS) techniques are implemented to resolve the small-scale turbulence. Collision statistics for cloud droplets of radii between 5 and 25 μm at five different turbulence dissipation rates (20–500 cm2 s−3) are computed and compared with pure-gravity cases. The results show that the turbulence enhancement of collision efficiency highly depends on the r ratio (defined as the radius ratio of collected and collector droplets r/ R) but is less sensitive to the size of the collector droplet investigated in this study. Particularly, the enhancement is strongest among comparable-sized collisions, indicating that turbulence can significantly broaden the narrow DSD resulting from condensational growth. Finally, DNS experiments of droplet growth by collision–coalescence in turbulence are performed for the first time in the literature to further illustrate this hypothesis and to monitor the appearance of drizzle in the early rain-formation stage. By comparing the resulting DSDs at different turbulence intensities, it is found that broadening is most pronounced when turbulence is strongest and similar-sized collisions account for 21%–24% of total collisions in turbulent cases compared with only 9% in the gravitational case.

scholarly journals Chemical composition and droplet size distribution of cloud at the summit of Mount Tai, China

2017 ◽  
Author(s):  
Jiarong Li ◽  
Xinfeng Wang ◽  
Jianmin Chen ◽  
Chao Zhu ◽  
Weijun Li ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Size Distribution ◽  
Droplet Size ◽  
Atmospheric Aerosols ◽  
Ph Value ◽  
Inorganic Ions ◽  
Droplet Size Distribution ◽  
Dilution Effect ◽  
Water Soluble ◽  
Cloud Droplets

Abstract. Chemical composition of 39 cloud samples and droplet size distribution in 24 cloud events were investigated at the summit of Mt. Tai from July to October 2014. Inorganic ions, organic acids, metals, HCHO, H2O2, sulfur(IV), organic carbon, element carbon as well as pH and electrical conductivity were analyzed. The acidity of the cloud water significantly decreased from a reported value of pH 3.86 in 2007–2008 (Guo et al., 2012) to pH 5.87 in the present study. The concentrations of nitrate and ammonium were both increased since 2007–2008, but the overcompensation of ammonium led to the increase of the mean pH value. The microphysical properties showed that cloud droplets were smaller than 26.0 μm and the most were in the range of 6.0–9.0 μm. The maximum droplet number concentration (Nd) was associated with droplet sizes of 7.0 μm. Cloud droplets exhibited a strong interaction with atmospheric aerosols. High PM2.5 level resulted in higher concentrations of water soluble ions and smaller sizes with more numbers of cloud droplets, and further gave rise to relatively high acidity. High degrees of relative humidity facilitated the formation of large cloud droplets and led to high liquid water contents under low PM2.5 level. The cloud droplets to wet deposition acted as an important sink of soluble material in the atmosphere and the dilution effect of the water content should be considered when estimating concentrations of soluble components in the cloud phase.

scholarly journals Characterization and first results of an ice nucleating particle measurement system based on counterflow virtual impactor technique

2014 ◽  
Vol 7 (10) ◽  
pp. 10585-10617 ◽  
Author(s):  
L. P. Schenk ◽  
S. Mertes ◽  
U. Kästner ◽  
F. Frank ◽  
B. Nillius ◽  
...  
Keyword(s):  
Field Experiments ◽  
Water Saturation ◽  
Chemical Characterization ◽  
Number Concentration ◽  
Research Station ◽  
Virtual Impactor ◽  
First Results ◽  
Aerosol Instrumentation ◽  
Set Up

Abstract. A specific instrument combination was developed to achieve a better microphysical and chemical characterization of atmospheric aerosol particles that have the potential to act as ice nucleating particles (INP). For this purpose a pumped counterflow virtual impactor system called IN-PCVI was set up and characterized to separate ice particles that had been activated on INP in the Fast Ice Nucleus Chamber (FINCH) from interstitial, non-activated particles. This coupled setup consisting of FINCH (ice particle activation and counting), IN-PCVI (INP separation and preparation), and further aerosol instrumentation (INP characterization) had been developed for the application in field experiments. The separated INP were characterized on-line with regard to their total number concentration, number size distribution and chemical composition, especially with the Aircraft-based Laser Ablation Aerosol Mass Spectrometer ALABAMA. Moreover, impactor samples for electron microscopy were taken. Due to the coupling the IN-PCVI had to be operated with different flow settings than known from literature, which required a further characterization of its cut-off-behavior. Taking the changed cut-off-behavior into account, the INP number concentration measured by the IN-PCVI system was in good agreement with the one detected by the FINCH optics for water saturation ratios up to 1.01 (ice saturation ratios between 1.21–1.34 and temperatures between −18 and −26 °C). First field results of INP properties are presented which were gained during the INUIT-JFJ/CLACE 2013 campaign at the high altitude research station Jungfraujoch in the Bernese Alps, Switzerland (3580 m a.s.l.).

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