scholarly journals Laboratory and In-flight Evaluation of a Cloud Droplet Probe (CDP)

2018 ◽  
Author(s):  
Spencer Faber ◽  
Jeffrey R. French ◽  
Robert Jackson
Keyword(s):  
Laboratory Tests ◽  
Collection Efficiency ◽  
Droplet Diameter ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Weighted Mean ◽  
Droplet Concentration ◽  
Mean Diameter ◽  
Cloud Points ◽  
Generating System

Abstract. Laboratory and in-flight evaluations of measurements from a Cloud Droplet Probe (CDP) are presented. A description of a water droplet-generating device, similar to those used in previous studies, is provided along with validation of droplet sizing and positioning. Laboratory evaluations of a CDP using the droplet generating system indicate errors in sizing that depend on both droplet diameter and position within the sample area through which a droplet transited. For the smallest diameters tested, the CDP undersized droplets by 1–4 μm for the majority of those sampled. The remaining droplets were sized to within 1 μm of the actual diameter. Droplets with diameters of 17 and 24 μm were sized correctly, within 2 μm, which is the nominal CDP bin width for droplets of that size. For all larger diameters, the majority of droplets were oversized by 2–4 μm, while a small percentage were severely undersized, by as much as 30 μm. This combination leads to an artificial broadening of the spectra, although errors in higher order moments were generally less than 10 %. Comparisons of liquid water content (LWC) calculated from the CDP and that measured from a Nevzorov hotwire probe were conducted for 17,917 1 Hz in-cloud points. Although some differences were noted based on volume-weighted mean diameter and total droplet concentration, the CDP-estimated LWC exceeded that measured by the Nevzorov by approximately 20 %, more than twice the expected difference based on results of the laboratory tests and considerations of Nevzorov collection efficiency.

scholarly journals Laboratory and in-flight evaluation of measurement uncertainties from a commercial Cloud Droplet Probe (CDP)

2018 ◽  
Vol 11 (6) ◽  
pp. 3645-3659 ◽  
Author(s):  
Spencer Faber ◽  
Jeffrey R. French ◽  
Robert Jackson
Keyword(s):  
Collection Efficiency ◽  
Droplet Diameter ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Hot Wire ◽  
Weighted Mean ◽  
Wire Probe ◽  
Sample Area ◽  
Droplet Concentration ◽  
Cloud Points

Abstract. Laboratory and in-flight evaluations of uncertainties of measurements from a Cloud Droplet Probe (CDP) are presented. A description of a water-droplet-generating device, similar to those used in previous studies, is provided along with validation of droplet sizing and positioning. Seven experiments with droplet diameters of 9, 17, 24, 29, 34, 38, and 46 µm tested sizing and counting performance across a 10 µm resolution grid throughout the sample area of a CDP. Results indicate errors in sizing that depend on both droplet diameter and position within the sample area through which a droplet transited. The CDP undersized 9µm droplets by 1–4 µm. Droplets with diameters of 17 and 24 µm were sized to within 2 µm, which is the nominal CDP bin width for droplets of that size. The majority of droplets larger than 17 µm were oversized by 2–4 µm, while a small percentage were severely undersized, by as much as 30 µm. This combination led to an artificial broadening and skewing of the spectra such that mean diameters from a near-monodisperse distribution compared well (within a few percent), while the median diameters were oversized by 5–15 %. This has implications on how users should calibrate their probes. Errors in higher-order moments were generally less than 10 %. Comparisons of liquid water content (LWC) calculated from the CDP and that measured from a Nevzorov hot-wire probe were conducted for 17 917 1 Hz in-cloud points. Although some differences were noted based on volume-weighted mean diameter and total droplet concentration, the CDP-estimated LWC exceeded that measured by the Nevzorov by approximately 20 %, more than twice the expected difference based on results of the laboratory tests and considerations of Nevzorov collection efficiency.

scholarly journals Numerical Study of Super-cooled droplet impingement on Aeroengine

Mechanika ◽  
2020 ◽  
Vol 26 (1) ◽  
pp. 25-30
Author(s):  
Yan TAN ◽  
Wuguo WEI
Keyword(s):  
Flow Field ◽  
Data Exchange ◽  
Collection Efficiency ◽  
Numerical Study ◽  
Mixed Boundary ◽  
Droplet Diameter ◽  
Three Dimensional ◽  
Liquid Water Content ◽  
Euler Method ◽  
Nose Cone

In this work, a large bypass ratio engine was taken as the research object. The impingement property of droplets with various diameters including SLD (Super-cooled Large Droplet) was obtained by Euler method based on Mundo model. The grid models of nose cone, fan, guild vanes in bypass duct and core duct were established in segments firstly. The mixed boundary was used to realize the data exchange between different flow fields. Then the Spalart-Allmaras turbulence model was applied to calculate the three dimensional engine flow field. Based on the flow field result, the droplet trajectory was calculated by Euler multiphase flow method. The LWC (Liquid Water Content), droplet collection efficiency and the effect of droplet diameter on impingement law were obtained. The method used in this paper and results could provide some references for subsequent engine icing calculation and anti-icing system design.

scholarly journals Parameterization of the Autoconversion Process. Part II: Generalization of Sundqvist-Type Parameterizations

2006 ◽  
Vol 63 (3) ◽  
pp. 1103-1109 ◽  
Author(s):  
Yangang Liu ◽  
Peter H. Daum ◽  
R. McGraw ◽  
R. Wood
Keyword(s):  
Critical Radius ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Liquid Water Content ◽  
Smooth Transition ◽  
Relative Dispersion ◽  
Droplet Concentration ◽  
Single Function ◽  
Cloud Droplet Size Distribution ◽  
Part Structure

Abstract Existing Sundqvist-type parameterizations, which only consider dependence of the autoconversion rate on cloud liquid water content, are generalized to explicitly account for the droplet concentration and relative dispersion of the cloud droplet size distribution as well. The generalized Sundqvist-type parameterization includes the more commonly used Kessler-type parameterization as a special case, unifying the two different types of parameterizations for the autoconversion rate. The generalized Sundqvist-type parameterization is identical with the Kessler-type parameterization presented in Part I beyond the autoconversion threshold, but exhibits a more realistic, smooth transition in the vicinity of the autoconversion threshold (threshold behavior) in contrast to the discontinuously abrupt transition embodied in the Kessler-type parameterization. A new Sundqvist-type parameterization is further derived by applying the expression for the critical radius derived from the kinetic potential theory to the generalized Sundqvist-type parameterization. The new parameterization eliminates the need for defining the driving radius and for prescribing the critical radius associated with Kessler-type parameterizations. The two-part structure of the autoconversion process raises questions regarding model-based empirical parameterizations obtained by fitting simulation results from detailed collection models with a single function.

scholarly journals The relative dispersion of cloud droplets: its robustness with respect to key cloud properties

2015 ◽  
Vol 15 (4) ◽  
pp. 2009-2017 ◽  
Author(s):  
E. Tas ◽  
A. Teller ◽  
O. Altaratz ◽  
D. Axisa ◽  
R. Bruintjes ◽  
...  
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Measurement Data ◽  
Thermodynamic Characteristics ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Liquid Water Content ◽  
Relative Dispersion ◽  
Droplet Concentration ◽  
Cloud Droplet Size Distribution

Abstract. Flight data measured in warm convective clouds near Istanbul in June 2008 were used to investigate the relative dispersion of cloud droplet size distribution. The relative dispersion (ϵ), defined as the ratio between the standard deviation (σ) of the cloud droplet size distribution and cloud droplet average radius (⟨r⟩), is a key factor in regional and global models. The relationship between ε and the clouds' microphysical and thermodynamic characteristics is examined. The results show that ε is constrained with average values in the range of ~0.25–0.35. ε is shown not to be correlated with cloud droplet concentration or liquid water content (LWC). However, ε variance is shown to be sensitive to droplet concentration and LWC, suggesting smaller variability of ϵ in the clouds' most adiabatic regions. A criterion for use of in situ airborne measurement data for calculations of statistical moments (used in bulk microphysical schemes), based on the evaluation of ϵ, is suggested.

scholarly journals Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part II: Depletion of Cloud Droplets and Cloud Condensation Nuclei through Collision–Coalescence

2018 ◽  
Vol 75 (5) ◽  
pp. 1653-1673 ◽  
Author(s):  
Kuan-Ting O ◽  
Robert Wood ◽  
Christopher S. Bretherton
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Cloud Base ◽  
Condensation Nuclei ◽  
Cloud Droplets ◽  
Droplet Concentration ◽  
Cloud Thickness

In Part I, aircraft observations are used to show that ultraclean layers (UCLs) in the marine boundary layer (MBL) are a common feature of the stratocumulus-to-cumulus transition (SCT) region over the northeast Pacific. The ultraclean layers are defined as layers of either cloud or clear air in which the concentration of particles with diameter larger than 0.1 μm is below 10 cm−3. Here, idealized microphysical parcel modeling shows that in the cumulus regime, collision–coalescence can strongly deplete cloud droplet concentration in cumulus (Cu) updrafts, thereby removing cloud condensation nuclei (CCN) from the atmosphere, suggesting that collision scavenging is likely the key process causing the low particle concentration in UCLs. Furthermore, the model results suggest that the stratocumulus regime is typically not favorable for UCL formation, because condensate amounts are generally not large enough to deplete drops in the time it takes to loft air to the upper planetary boundary layer (PBL). A bulk parameterization of the coalescence-scavenging rate is derived based on in situ measurements. The fractional coalescence-scavenging rate is found to be strongly dependent upon liquid water content (LWC) and, hence, the height above cloud base, indicating that a higher cloud top and thus a greater cloud thickness in a Cu updraft is an important factor accounting for the observed sharp rise of UCL coverage in the SCT region. An important implication is that PBL height, which controls maximum cloud thickness, and therefore LWC in updrafts, could be a crucial factor constraining coalescence scavenging and thus the formation of UCLs in the MBL.

Can Large Precipitating Cloud Hydrometeors Generate Secondary Cloud Droplets in its Wake?

2020 ◽  
Author(s):  
Taraprasad Bhowmick ◽  
Yong Wang ◽  
Gholamhossein Bagheri ◽  
Eberhard Bodenschatz
Keyword(s):  
Water Vapor ◽  
Thermal Diffusivity ◽  
Kinematic Viscosity ◽  
Droplet Diameter ◽  
Fluid Velocity ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Cloud Droplets ◽  
Lagrangian Tracking ◽  
Recirculation Length

<p>Atmospheric clouds play a very important role in the evolution of global atmosphere and climate through various interactive physical processes dynamically active over a huge range of scales [Devenish et al. QJRMS 2012, Grabowski and Wang. ARFM 2013]. However, many of such processed are yet to be understood; and in such context, we attempt to understand such a scientific question: whether large precipitating cloud drops can generate secondary droplets in it’s wake. Motivated by experimental investigation of large sedimenting cloud droplets [∼ mm radius] which showed presence of secondary cloud droplets in it’s wake [Prabhakaran et al. PRL 2017, ArXiv 2019]; we conduct direct numerical simulations of such precipitating hydrometeors using Lattice-Boltzmann method (LBM) to simulate cloud like ambient solving the evolution of the supersaturation field in the wake of the hydrometeor, and to investigate it’s impact on the nucleation of cloud aerosols. In our simulation results, we found various flow regimes based on the Reynolds number (Re = Droplet Diameter * Droplet Velocity / Kinematic Viscosity) in compliance with past researches. Steady axisymmetric wake for Re up to ∼ 220, after that steady oblique wake up to Re ∼ 280, then a transient oscillating nature of the wake up to Re ∼ 350, and beyond that Re, the wake is observed to become chaotic and turbulent. Comparison of drag coefficient, recirculation length and separation angles for fluid velocity at various Re shows good agreement with existing numerical and experimental simulations. The temperature profiles also fit well with other researches for similar Prandtl number (ratio of kinematic viscosity to thermal diffusivity). Evolution of the density of water vapor is similar to the temperature field, since both the equations show similar structure and the mass diffusivity of water vapor is almost same to the thermal diffusivity for atmospheric clouds. Distribution of the supersaturation field is computed using Clausius-Clapeyron Equation which gives saturation vapor pressure depending on temperature. In such simulations with background flow at -15<sup>o</sup> C temperature with 60% relative humidity (RH) and with the hydrometeor as a warm cloud droplet at 4<sup>o</sup> C temperature and 100% RH at it’s surface, the wake shows symmetric regions of supersaturation in the near vicinity of the hydrometeor at Re = 200. Whereas, at Re = 273, the wake is observed to become oblique, so the supersaturated region. Small pockets of supersaturated warm air parcels are observed to travel in the downstream direction when the hydrometeor started shedding vortices at higher Re. However, while traveling downstream, such supersaturated pockets also lost its’ excess of water vapor depending on the ambient cloud conditions. Due to higher supersaturation at the near vicinity of the warm hydrometeor, the cloud aerosols trapped inside the wake can be activated. However, whether such activated aerosols can become a drizzle drop, or may evaporate its liquid water content in subsaturated region, is to be understood by Lagrangian tracking of such aerosol tracers.</p>

Consistent Theoretical Model of Mean Diameter and Size Distribution by Liquid Sheet Atomization

2012 ◽  
Author(s):  
Chihiro Inoue ◽  
Toshinori Watanabe ◽  
Takehiro Himeno ◽  
Seiji Uzawa ◽  
Mitsuo Koshi
Keyword(s):  
Theoretical Model ◽  
Kinetic Energy ◽  
Droplet Diameter ◽  
Velocity Profiles ◽  
Liquid Sheet ◽  
Estimation Methods ◽  
Liquid Jets ◽  
Injection Velocity ◽  
Size Distributions ◽  
Mean Diameter

A consistent theoretical model is proposed and validated for calculating droplet diameters and size distributions. The model is derived based on the energy conservation law including the surface free energy and the Laplace pressure. Under several hypotheses, the law derives an equation indicating that atomization results from kinetic energy loss. Thus, once the amount of loss is determined, the droplet diameter is able to be calculated without the use of experimental parameters. When the effects of ambient gas are negligible, injection velocity profiles of liquid jets are the essential cause of the reduction of kinetic energy. The minimum Sauter mean diameter produced by liquid sheet atomization is inversely proportional to the injection Weber number when the injection velocity profiles are laminar or turbulent. A non-dimensional distribution function is also derived from the mean diameter model and Nukiyama-Tanasawa’s function. The new estimation methods are favorably validated by comparing with corresponding mean diameters and the size distributions, which are experimentally measured under atmospheric pressure.

scholarly journals Icing Measurements on Fixed and Rotating Cylinders

1983 ◽  
Vol 4 ◽  
pp. 174-179
Author(s):  
P. McComber ◽  
J.-L. Laforte ◽  
D. Bouchard ◽  
D. D. Nguyen
Keyword(s):  
Transmission Lines ◽  
Liquid Water ◽  
Collection Efficiency ◽  
Droplet Diameter ◽  
Ice Accretion ◽  
Rotating Cylinders ◽  
Wind Velocities ◽  
Method Of Measurement ◽  
Temperature Constant ◽  
Water Contents

There is at present a need to develop a better technique for measuring the rate of icing on structures such as, for example, overhead transmission lines. For aircraft and helicopter icing, the most widely used method of measurement is the rotating cylinder. However, for measuring the icing of structures, this method is difficult to apply and also less accurate due to lower wind velocities. Different approaches are now being developed using fixed cylinders.Icing tests were conducted with fixed and rotating cylinders in a wind tunnel. The rate of icing was obtained through measurements of volume, accretion cross-section and time of deposition. Tests were made using five different liquid water contents and droplet diameter spectra, and four cylinder diameters, keeping the wind velocity and temperature constant. The rate of icing is presented as a function of the diameters of the fixed and rotating cylinders for each of the liquid water contents tested. Results indicate that at lower wind velocities the accretion rate is overestimated for the smaller rotating cylinders. This difference is probably due to the variation of the collection efficiency with diameter. From these results it is suggested that the rate of ice accretion on structures should be based on at least two fixed cylinders of different small sizes in order to take into account the effect of the collection efficiency.

scholarly journals A depolarisation lidar based method for the determination of liquid-cloud microphysical properties

2014 ◽  
Vol 7 (9) ◽  
pp. 9917-9992 ◽  
Author(s):  
D. P. Donovan ◽  
H. Klein Baltink ◽  
J. S. Henzing ◽  
S. R. de Roode ◽  
A. P. Siebesma
Keyword(s):  
Water Content ◽  
Multiple Scattering ◽  
Liquid Water ◽  
Cloud Droplet ◽  
Optimal Estimation ◽  
Liquid Water Content ◽  
Cloud Base ◽  
Base Region ◽  
Large Eddy Simulation Model ◽  
Inversion Procedure

Abstract. The fact that polarisation lidars measure a depolarisation signal in liquid clouds due to the occurrence of multiple-scattering is well-known. The degree of measured depolarisation depends on the lidar characteristics (e.g. wavelength and receiver field-of-view) as well as the cloud macrophysical (e.g. liquid water content) and microphysical (e.g. effective radius) properties. Efforts seeking to use depolarisation information in a quantitative manner to retrieve cloud properties have been undertaken with, arguably, limited practical success. In this work we present a retrieval procedure applicable to clouds with (quasi-)linear liquid water content (LWC) profiles and (quasi-)constant cloud droplet number density in the cloud base region. Thus limiting the applicability of the procedure allows us to reduce the cloud variables to two parameters (namely the derivative of the liquid water content with height and the extinction at a fixed distance above cloud-base). This simplification, in turn, allows us to employ a fast and robust optimal-estimation inversion using pre-computed look-up-tables produced using extensive lidar Monte-Carlo multiple-scattering simulations. In this paper, we describe the theory behind the inversion procedure and successfully apply it to simulated observations based on large-eddy simulation model output. The inversion procedure is then applied to actual depolarisation lidar data corresponding to a range of cases taken from the Cabauw measurement site in the central Netherlands. The lidar results were then used to predict the corresponding cloud-base region radar reflectivities. In non-drizzling condition, it was found that the lidar inversion results can be used to predict the observed radar reflectivities with an accuracy within the radar calibration uncertainty (2–3 dBZ). This result strongly supports the accuracy of the lidar inversion results. Results of a comparison between ground-based aerosol number concentration and lidar-derived cloud droplet number densities are also presented and discussed. The observed relationship between the two quantities is seen to be consistent with the results of previous studies based on aircraft-based in situ measurements.

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