scholarly journals Riming of Graupel: Wind Tunnel Investigations of Collection Kernels and Growth Regimes

2009 ◽  
Vol 66 (8) ◽  
pp. 2359-2366 ◽  
Author(s):  
Nadine von Blohn ◽  
Karoline Diehl ◽  
Subir K. Mitra ◽  
Stephan Borrmann
Keyword(s):  
Wind Tunnel ◽  
Liquid Water ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Fall Velocity ◽  
Liquid Drops ◽  
Growth Regime ◽  
Cloud Models ◽  
Ice Particles ◽  
Mixed Phase Clouds

Abstract Laboratory experiments were carried out in the vertical wind tunnel in Mainz, Germany, to study the collision coalescence growth of single spherical ice particles having initial radii between 290 and 380 μm while they were freely floated in a laminar flow containing a cloud of supercooled droplets with radii between 10 and 20 μm. The experiments were performed in a temperature range between −8 and −12°C, where riming proceeds in the atmosphere, and with cloud liquid water contents lying between 0.9 and 1.6 g m−3 (i.e., values typically found in mixed-phase clouds). The collection kernels were calculated from the mass increase of the rimed ice particles and the average liquid water content during the experiments. Surface temperature measurements of growing graupel indicated that a dry growth regime prevailed during the whole set of growth experiments. The collection kernels of rimed ice particles attained values between 0.9 and 2.3 cm3 s−1 depending on their collector momenta (mass × fall velocity of the riming ice particles), which had values between 0.04 and 0.1 g cm s−1. It was found that the collection kernels of ice particles determined from the present set of experiments were higher than the collection kernels of liquid drops. To correct for this discrepancy, an empirical factor depending on the cloud droplet radii was extracted from the newly measured data as well as from the old data. For the investigated size ranges of ice particles and droplets, these corrected collection kernels of ice particles can be incorporated in cloud models for the corresponding size ranges.

scholarly journals Wind tunnel experiments on the retention of trace gases during riming: nitric acid, hydrochloric acid, and hydrogen peroxide

2011 ◽  
Vol 11 (6) ◽  
pp. 17447-17472
Author(s):  
N. von Blohn ◽  
K. Diehl ◽  
S. K. Mitra ◽  
S. Borrmann
Keyword(s):  
Hydrogen Peroxide ◽  
Wind Tunnel ◽  
Hydrochloric Acid ◽  
Liquid Water ◽  
Trace Gases ◽  
Supercooled Liquid ◽  
Liquid Water Content ◽  
Ice Particles ◽  
Trace Species ◽  
Ice Phase

Abstract. Laboratory experiments were carried out in a vertical wind tunnel to study the retention of different atmospheric trace gases during riming. In the experiments, the rimed ice particles floated in a laminar air stream carrying a cloud of supercooled droplets with radii between 10 and 20 μm. Ice particles, dendritic ice crystals, and snow flakes with diameters between 6 mm and 1.5 cm were allowed to rime at temperatures between −5 and −12 °C where riming mainly proceeds in the atmosphere and with cloud liquid water contents between 1 and 1.5 g m−3 which are values typically found in atmospheric mixed phase clouds. Three trace species were investigated, nitric and hydrochloric acid, and hydrogen peroxide. They were present in the supercooled liquid droplets in concentrations from 1 to 120 ppmv, i.e. similar to the ones measured in cloud drops. The chemical analyses of the rimed ice particles allow to determine the trace species concentration in the ice phase. Together with the known liquid phase concentration the retention coefficients were calculated in terms of the amount of the species which remained in the ice phase after freezing. It was found that the highly soluble trace gases nitric and hydrochloric acid were retained nearly completely (98.6 ± 8 % and 99.7 ± 9 %, respectively) while for hydrogen peroxide a retention coefficient of 64.3 ± 11 % was determined. No influence of the riming temperature on the retention was found which can be explained by the fact that in the observed range of temperature and liquid water content riming proceeded in the dry growth regime.

scholarly journals Comparison of different droplet measurement techniques in the Braunschweig Icing Wind Tunnel

2021 ◽  
Vol 14 (2) ◽  
pp. 1761-1781
Author(s):  
Inken Knop ◽  
Stephan E. Bansmer ◽  
Valerian Hahn ◽  
Christiane Voigt
Keyword(s):  
Wind Tunnel ◽  
Mass Flow ◽  
Liquid Water ◽  
Measurement Techniques ◽  
Wind Tunnel Test ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Liquid Droplets ◽  
Spray System ◽  
Phase Doppler Interferometry

Abstract. The generation, transport and characterization of supercooled droplets in multiphase wind tunnel test facilities is of great importance for conducting icing experiments and to better understand cloud microphysical processes such as coalescence, ice nucleation, accretion and riming. To this end, a spray system has been developed, tested and calibrated in the Braunschweig Icing Wind Tunnel. Liquid droplets in the size range of 1 to 150 µm produced by pneumatic atomizers were accelerated to velocities between 10 and 40 m s−1 and supercooled to temperatures between 0 and −20 ∘C. Thereby, liquid water contents between 0.07 and 2.5 g m−3 were obtained in the test section. The wind tunnel conditions were stable and reproducible within 3 % standard variation for median volumetric diameter (MVD) and 7 % standard deviation for liquid water content (LWC). Different instruments were integrated in the icing wind tunnel measuring the particle size distribution (PSD), MVD and LWC. Phase Doppler interferometry (PDI), laser spectroscopy with a fast cloud droplet probe (FCDP) and shadowgraphy were systematically compared for present wind tunnel conditions. MVDs measured with the three instruments agreed within 15 % in the range between 8 and 35 µm and showed high coefficients of determination (R2) of 0.985 for FCDP and 0.799 for shadowgraphy with respect to PDI data. Between 35 and 56 µm MVD, the shadowgraphy data exhibit a low bias with respect to PDI. The instruments' trends and biases for selected droplet conditions are discussed. LWCs determined from mass flow calculations in the range of 0.07–1.5 g m−3 are compared to measurements of the bulk phase rotating cylinder technique (RCT) and the above-mentioned single-particle instruments. For RCT, agreement with the mass flow calculations of approximately 20 % in LWC was achieved. For PDI 84 % of measurement points with LWC<0.5 g m−3 agree with mass flow calculations within a range of ±0.1 g m−3. Using the different techniques, a comprehensive wind tunnel calibration for supercooled droplets was achieved, which is a prerequisite for providing well-characterized liquid cloud conditions for icing tests for aerospace, wind turbines and power networks.

scholarly journals Cloud Conditions Favoring Secondary Ice Particle Production in Tropical Maritime Convection

2014 ◽  
Vol 71 (12) ◽  
pp. 4500-4526 ◽  
Author(s):  
Andrew Heymsfield ◽  
Paul Willis
Keyword(s):  
Liquid Water ◽  
Particle Production ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Virgin Islands ◽  
Complex Interactions ◽  
Ice Particles ◽  
Ice Multiplication ◽  
The Many ◽  
Ice Production

Abstract Progress in understanding the formation of ice in lower-tropospheric clouds is slowed by the difficulties in characterizing the many complex interactions that lead to ice initiation and to the dynamic, non-steady-state nature of the clouds. The present study characterizes the conditions where secondary ice particles, specifically identified as needle or thin columnar types, are observed in tropical maritime convection with modest liquid water contents during the Ice in Clouds Experiment-Tropical (ICE-T), based out of St. Croix, U.S. Virgin Islands, and the NASA African Monsoon Multidisciplinary Analyses (NAMMA) in 2006 sampling from Cape Verde, Africa. The properties of the cloud droplet populations relevant to the secondary ice production process and the ice particle populations are characterized as a function of temperature and vertical velocity. These secondary ice particles are observed primarily in regions of low liquid water content and weak vertical velocities. Two situations are examined in detail. First, ice formation is examined by following the tops of a group of ICE-T chimney clouds as they ascend and cool from a temperature of +7° to −8°C, examining the production of the first ice. Then, using the data from a cloud system sampled during NAMMA, the authors elucidate a process that promotes ice multiplication. The intention is that this study will lead both to a better understanding of how secondary ice production proceeds in natural clouds and to more realistic laboratory studies of the processes involved.

scholarly journals Comparison of different droplet measurement techniques in the Braunschweig Icing Wind Tunnel

2020 ◽  
Author(s):  
Inken Knop ◽  
Stephan Bansmer ◽  
Valerian Hahn ◽  
Christiane Voigt
Keyword(s):  
Wind Tunnel ◽  
Liquid Water ◽  
Measurement Techniques ◽  
Wind Tunnel Test ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Liquid Droplets ◽  
Spray System ◽  
Phase Doppler Interferometry ◽  
Microphysical Processes

Abstract. The generation, transport and characterisation of supercooled droplets in multiphase wind tunnel-test facilities is of great importance for conducting icing experiments and to better understand cloud microphysical processes such as coalescence, ice nucleation, accretion and riming. To this end, a spray system has been developed, tested and calibrated in the Braunschweig Icing Wind Tunnel. Liquid droplets in the size range of 1 to 150 µm produced by pneumatic atomizers were accelerated to velocities between 10 and 40 m s−1 and supercooled to temperatures between 0 and −20 °C. Thereby, liquid water contents between 0.1 and 2.5 g m−3 were obtained in the test section. The wind tunnel conditions were stable and reproducible within 3 % standard variation for median volumetric diameter (MVD) and 7 % standard deviation for liquid water content (LWC). Different instruments were integrated in the icing wind tunnel measuring the particle size distribution (PSD), MVD and LWC. Phase Doppler Interferometry (PDI), laser spectroscopy with a Fast Cloud Droplet Probe (FCDP) and shadowgraphy were systematically compared for present wind tunnel conditions. MVDs measured with the three instruments agreed within 15 %, and showed high coefficients of determination (R2) of 0.985 for FCDP and 0.799 for shadowgraphy with respect to PDI data. The instruments' trends and biases for selected droplet conditions are discussed. LWCs determined from mass flow calculations are compared to measurements of the bulk phase rotating cylinder technique (RCT) and the above single particle instruments. For RCT and PDI, agreement of approximately 20 % in LWC was achieved, although in individual cases larger deviations depending on the flow conditions were detected. Using the different techniques, a comprehensive wind tunnel calibration for supercooled droplets was achieved, which is a prerequisite to provide well characterized liquid cloud conditions for icing tests for aerospace, wind turbines and power networks.

scholarly journals Wind tunnel experiments on the retention of trace gases during riming: nitric acid, hydrochloric acid, and hydrogen peroxide

2011 ◽  
Vol 11 (22) ◽  
pp. 11569-11579 ◽  
Author(s):  
N. von Blohn ◽  
K. Diehl ◽  
S. K. Mitra ◽  
S. Borrmann
Keyword(s):  
Hydrogen Peroxide ◽  
Wind Tunnel ◽  
Hydrochloric Acid ◽  
Liquid Water ◽  
Trace Gases ◽  
Supercooled Liquid ◽  
Liquid Water Content ◽  
Ice Particles ◽  
Trace Species ◽  
Ice Phase

Abstract. Laboratory experiments were carried out in a vertical wind tunnel to study the retention of different atmospheric trace gases during riming. In the experiments, the rimed ice particles floated in a laminar air stream carrying a cloud of supercooled droplets with radii between 10 and 20 μm. Ice particles, dendritic ice crystals, and snow flakes with diameters between 6 mm and 1.5 cm were allowed to rime at temperatures between −5 and −12 °C where riming mainly proceeds in the atmosphere and with cloud liquid water contents between 1 and 1.5 g m−3 which are values typically found in atmospheric mixed-phase clouds. Three trace species were investigated, nitric and hydrochloric acid, and hydrogen peroxide. They were present in the supercooled liquid droplets in concentrations from 1 to 120 ppmv, i.e. similar to the concentrations measured in cloud drops. The chemical analyses of the rimed ice particles allow one to determine the trace species concentration in the ice phase. Together with the known liquid phase concentration the retention coefficients were calculated in terms of the amount of the species which remained in the ice phase after freezing. It was found that the highly soluble trace gases, nitric and hydrochloric acid, were retained nearly completely (98.6±8% and 99.7±9%, respectively) while for hydrogen peroxide a retention of 64.3±11% was determined. No influence of the riming temperature on the retention was found which can be explained by the fact that in the observed range of temperature and liquid water content, riming proceeded in the dry growth regime.

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.

scholarly journals An intercomparison of radar-based liquid cloud microphysics retrievals and implication for model evaluation studies

2011 ◽  
Vol 4 (6) ◽  
pp. 7109-7158 ◽  
Author(s):  
D. Huang ◽  
C. Zhao ◽  
M. Dunn ◽  
X. Dong ◽  
G. G. Mace ◽  
...  
Keyword(s):  
Great Plains ◽  
Liquid Water ◽  
Model Evaluation ◽  
Evaluation Studies ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Cloud Microphysics ◽  
Effective Radius ◽  
High Altitudes ◽  
Liquid Water Path

Abstract. To assess if current radar-based liquid cloud microphysical retrievals of the Atmospheric Radiation Measurement (ARM) program can provide useful constraints for modeling studies, this paper presents intercomparison results of three cloud products at the Southern Great Plains (SGP) site: the ARM MICROBASE, University of Utah (UU), and University of North Dakota (UND) products over the nine-year period from 1998 to 2006. The probability density and spatial autocorrelation functions of the three cloud Liquid Water Content (LWC) retrievals appear to be consistent with each other, while large differences are found in the droplet effective radius retrievals. The differences in the vertical distribution of both cloud LWC and droplet effective radius retrievals are found to be alarmingly large, with the relative difference between nine-year mean cloud LWC retrievals ranging from 20% at low altitudes to 100% at high altitudes. Nevertheless, the spread in LWC retrievals is much smaller than that in cloud simulations by climate and cloud resolving models. The MICROBASE effective radius ranges from 2.0 at high altitudes to 6.0 μm at low altitudes and the UU and UND droplet effective radius is 6 μm larger. Further analysis through a suite of retrieval experiments shows that the difference between MICROBASE and UU LWC retrievals stems primarily from the partition total Liquid Water path (LWP) into supercooled and warm liquid, and from the input cloud boundaries and LWP. The large differences between MICROBASE and UU droplet effective radius retrievals are mainly due to rain/drizzle contamination and the assumptions of cloud droplet concentration used in the retrieval algorithms. The large discrepancy between different products suggests caution in model evaluation with these observational products, and calls for improved retrievals in general.

scholarly journals Aerosol-landscape-cloud interaction: Signatures of topography effect on cloud droplet formation

2016 ◽  
Author(s):  
Sami Romakkaniemi ◽  
Zubair Maalick ◽  
Antti Hellsten ◽  
Antti Ruuskanen ◽  
Olli Väisänen ◽  
...  
Keyword(s):  
Liquid Water ◽  
Measurement Data ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Liquid Water Content ◽  
Droplet Formation ◽  
Cloud Base ◽  
Topography Effect ◽  
Boreal Environment ◽  
The Hill

Abstract. Long-term in situ measurements of aerosol-cloud interactions are usually performed in measurement stations residing on hills, mountains, or high towers. In such conditions, the surface topography of the surrounding area can affect the measured cloud droplet distributions by increasing turbulence or causing orographic flows and thus the observations might not be representative for a larger scale. The objective of this work is to analyse, how the local topography affects the observations at Puijo measurement station, which is located in the 75 m high Puijo tower, which itself stands on a 150 m high hill. The analysis of the measurement data shows that the observed cloud droplet number concentration mainly depends on the CCN concentration. However, when the wind direction aligns with the direction of the steepest slope of the hill, a clear topography effect is observed. This finding was further analysed by simulating 3D flow fields around the station and by performing trajectory ensemble modelling of aerosol- and wind-dependent cloud droplet formation. The results showed that in typical conditions, with geostrophic winds of about 10 m s−1, the hill can cause updrafts of up to 1 m s−1 in the air parcels arriving at the station. This is enough to produce in-cloud supersaturations higher than typically found at the cloud base (SS of ~ 0.2 %), and thus additional cloud droplets may form inside the cloud. In the observations, this is seen in the form of a bi-modal cloud droplet size distribution. The effect is strongest with high winds across the steepest slope of the hill and with low liquid water contents, and its relative importance quickly decreases as these conditions are relaxed. We therefore conclude that, after careful screening for wind speed and liquid water content, the observations at Puijo measurement station can be considered representative for clouds in a boreal environment.

scholarly journals Modelling the relationship between liquid water content and cloud droplet number concentration observed in low clouds in the summer Arctic and its radiative effects

2020 ◽  
Vol 20 (1) ◽  
pp. 29-43
Author(s):  
Joelle Dionne ◽  
Knut von Salzen ◽  
Jason Cole ◽  
Rashed Mahmood ◽  
W. Richard Leaitch ◽  
...  
Keyword(s):  
Linear Relationship ◽  
Water Content ◽  
Liquid Water ◽  
Cloud Droplet ◽  
Liquid Water Content ◽  
Number Concentration ◽  
Radiative Effect ◽  
Low Clouds ◽  
Cloud Radiative Effect ◽  
Cloud Droplet Number Concentration

Abstract. Low clouds persist in the summer Arctic with important consequences for the radiation budget. In this study, we simulate the linear relationship between liquid water content (LWC) and cloud droplet number concentration (CDNC) observed during an aircraft campaign based out of Resolute Bay, Canada, conducted as part of the Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments study in July 2014. Using a single-column model, we find that autoconversion can explain the observed linear relationship between LWC and CDNC. Of the three autoconversion schemes we examined, the scheme using continuous drizzle (Khairoutdinov and Kogan, 2000) appears to best reproduce the observed linearity in the tenuous cloud regime (Mauritsen et al., 2011), while a scheme with a threshold for rain (Liu and Daum, 2004) best reproduces the linearity at higher CDNC. An offline version of the radiative transfer model used in the Canadian Atmospheric Model version 4.3 is used to compare the radiative effects of the modelled and observed clouds. We find that there is no significant difference in the upward longwave cloud radiative effect at the top of the atmosphere from the three autoconversion schemes (p=0.05) but that all three schemes differ at p=0.05 from the calculations based on observations. In contrast, the downward longwave and shortwave cloud radiative effect at the surface for the Wood (2005b) and Khairoutdinov and Kogan (2000) schemes do not differ significantly (p=0.05) from the observation-based radiative calculations, while the Liu and Daum (2004) scheme differs significantly from the observation-based calculation for the downward shortwave but not the downward longwave fluxes.

scholarly journals Chemistry of riming: the retention of organic and inorganic atmospheric trace constituents

2017 ◽  
Vol 17 (16) ◽  
pp. 9717-9732 ◽  
Author(s):  
Alexander Jost ◽  
Miklós Szakáll ◽  
Karoline Diehl ◽  
Subir K. Mitra ◽  
Stephan Borrmann
Keyword(s):  
Wind Tunnel ◽  
Empirical Model ◽  
Free Fall ◽  
Liquid Water Content ◽  
Growth Surface ◽  
Phase Zone ◽  
Convective Clouds ◽  
Cloud Models ◽  
Trace Constituents ◽  
Semi Empirical

Abstract. During free fall in clouds, ice hydrometeors such as snowflakes and ice particles grow effectively by riming, i.e., the accretion of supercooled droplets. Volatile atmospheric trace constituents dissolved in the supercooled droplets may remain in ice during freezing or may be released back to the gas phase. This process is quantified by retention coefficients. Once in the ice phase the trace constituents may be vertically redistributed by scavenging and subsequent precipitation or by evaporation of these ice hydrometeors at high altitudes. Retention coefficients of the most dominant carboxylic acids and aldehydes found in cloud water were investigated in the Mainz vertical wind tunnel under dry-growth (surface temperature less than 0 °C) riming conditions which are typically prevailing in the mixed-phase zone of convective clouds (i.e., temperatures from −16 to −7 °C and a liquid water content (LWC) of 0. 9 ± 0. 2 g m−3). The mean retention coefficients of formic and acetic acids are found to be 0. 68 ± 0. 09 and 0. 63 ± 0. 19. Oxalic and malonic acids as well as formaldehyde show mean retention coefficients of 0. 97 ± 0. 06, 0. 98 ± 0. 08, and 0. 97 ± 0. 11, respectively. Application of a semi-empirical model on the present and earlier wind tunnel measurements reveals that retention coefficients can be well interpreted by the effective Henry's law constant accounting for solubility and dissociation. A parameterization for the retention coefficients has been derived for substances whose aqueous-phase kinetics are fast compared to mass transport timescales. For other cases, the semi-empirical model in combination with a kinetic approach is suited to determine the retention coefficients. These may be implemented in high-resolution cloud models.

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