. Until now, the processes involved in secondary ice production which generate high concentration... more . Until now, the processes involved in secondary ice production which generate high concentrations of ice crystals in clouds have been poorly understood. However, collisions that involve rimed ice particles or crystal aggregates have the potential to effectively produce secondary ice from their fragmentation. Unfortunately, there have only been a few laboratory studies on ice-ice collision, resulting in an inaccurate representation of this process in microphysical schemes. To address this issue, experiments were conducted at the Wind tunnel laboratory of the Johannes Gutenberg University, Mainz on graupel- graupel and graupel-snowflake collisions under still air conditions at -15 °C and over water saturation. All fragments resulting from graupel-graupel collisions were collected and investigated by means of a digital optical microscope, while fragments from graupel-snowflake collisions were observed and recorded instantly after collision using a holographic instrument. From these experiments, distributions were obtained for fragment sizes, cross sectional areas and aspect ratios. The results showed a higher number of fragments at lower kinetic energy compared to those presented in the literature. 150 to 600 fragments were observed for graupel-graupel collisions, and 70 to 500 fragments for graupel-snowflake collisions between 10−7 and 10−5 J. Parametrizations for fragment size distributions are provided with a mode at 75 µm for graupel-graupel collisions and at 400 µm for graupel-snowflake collisions. These results can be used to improve the representation of ice collision breakup in microphysical schemes.
For the purpose of testing our previously described theory of SO z scavenging a laboratory invest... more For the purpose of testing our previously described theory of SO z scavenging a laboratory investigation was carried out in the UCLA 33 m long rainshaft. Drops with radii between 250 and 2500 ~tm were allowed to come to terminal velocity, after which they passed through a chamber of variable length filled with various SO 2 concentrations in air. After falling through a gas separating chamber consisting of a fluorocarbon gas the drops were collected and analyzed for their total S content in order to determine the rate of SO 2 absorption.
An experimental investigation of the simultaneous absorption of NH3 and SO2 from the ambient atmo... more An experimental investigation of the simultaneous absorption of NH3 and SO2 from the ambient atmosphere by freely falling water drops has been carried out in the Mainz vertical wind tunnel. The experimental results were found to be in good agreement with the results derived from computations with the Kronig-Brink convective diffusion model and also with a model which assumes a drop to be well mixed at all times. Encouraged by this agreement, these computation schemes for the uptake of gas by single drops where incorporated in a pollution washout model with realistic SO2, NH3 and CO2 gas profiles. This model allows an entire raindrop size distribution to fall through a gas layer. The results of this 'plume-model' show that the SO2 uptake is strongly dependent on the NH3 concentration in the atmosphere and on the rainrate. We also lind that the small drops contribute more towards the washout of these gases. In the case of simultaneous presence of NH3 and SO2, desorption of these gases is negligible.
An experimental and theoretical study has been carried out to investigate the rate of desorption ... more An experimental and theoretical study has been carried out to investigate the rate of desorption of SO, from water drops falling at terminal velocity in air. The experiments were carried out in the Mainz vertical wind tunnel in which water drops of various sizes containing S(IV) in various concentrations were freely suspended in the vertical airstream of the tunnel. The results of these experiments were compared with the predictions of three theoretical models, and with the experiments of Walcek et al. This comparison shows that the predictions of the diffusion model of Kronig and Brink in the formulation given by Walcek and Pruppacher agree well with the experimental results for all relevant large and small raindrop sizes, and for all considered concentrations of S(IV) inside the drops. In contrast, the predictions of the diffusion model which assumes complete internal mixing inside a drop agrees with the experimental results only if the concentration of S(IV) inside the drop is less than that equivalent of an equilibrium SO 2 concentration of 15 ppbv. At larger concentrations, the theoretical predictions of the model for complete internal mixing progressively deviate from the experimental results. It is further shown that Barrie's double film model can he used to interpret the resistance to diffusion inside a drop in terms of a diffusion boundary layer inside the drop which increases in thickness with decreasing concentration of S(IV). Applying our results to the desorption of SO 2 from small and large rain drops falling below an assumed cloud base, shows that for typical contents of S(IV) inside the drops substantial amounts of SO2 will desorb from these drops unless H202 is present in the surrounding air.
The rate of change of mass of a hailstone by diffusion is affected by its motions. In a stationar... more The rate of change of mass of a hailstone by diffusion is affected by its motions. In a stationary pure diffusive case the water vapor distribution around a spherical hailstone is spherically symmetric having rather weak water vapor gradients. However, when a hailstone falls in the air, the flow field and hence the water vapor distribution around the hailstone is asymmetric showing much stronger water vapor gradients which are high at the upstream side and lower at the rear side of the hailstone. When averaged over the whole surface area of the hailstone the mass transfer to or from the falling hailstone surface is always higher compared to a pure diffusive case. This convective enhancement is given by the ventilation coefficient. Thus, to reliably quantify growth or sublimation rates of falling hailstones with models, it is necessary to know their ventilation coefficients. The rate of change of mass is proportional to the rate of change of heat. Therefore, the growth or sublimation...
Vertical wind tunnel experiments were carried out to investigate the melting of low-density lump ... more Vertical wind tunnel experiments were carried out to investigate the melting of low-density lump graupel while floating at their terminal velocities. The graupel characteristics such as maximum dimension, density, and axis ratio were 0.39 ± 0.06 cm, 0.41 ± 0.07 g cm−3, and 0.89 ± 0.06. The airstream of the wind tunnel was gradually heated simulating lapse rates between 4.5 and 3.21 K km−1. Each experimental run was performed at a constant relative humidity that was varied between 12% and 92% from one experiment to the other. From the image processing of video recordings, variations in minimum and maximum dimension, volume, aspect ratio, density, volume equivalent radius, and ice core radius were obtained. New parameterizations of the terminal velocity prior to melting and during melting were developed. It was found that mass and heat transfer in the dry stage is 2 times as high as that of liquid drops at the same Reynolds number. Based on the experimental results, a model was develo...
Abstract An experimental study was carried out to investigate the processes which occur during dr... more Abstract An experimental study was carried out to investigate the processes which occur during drop-to-particle conversion in atmospheric clouds whose drops are evaporating. The experiment was carried out in the Mainz vertical wind tunnel and in a 4-m high fall shaft. Drops consisting of aqueous solutions of NaCl, (NH 4 ) 2 SO 4 , and salts of artificial and natural ocean water, containing various kinds of solid, water-insoluble particles such as clays, were studied. The aerosol particles produced by the evaporation of drops had diameters in the range 0.1–1 μm, 5–20 μm, 40–80 μm and 100–300 μm. The shape and fall mode of these particles are discussed. It was found that the aerosol particles formed did not break up or splinter during the evaporation of drops. This led to the conclusion that, in evaporating atmospheric clouds, each evaporating drop produces by drop-to-particle conversion one aerosol particle of mass and chemistry given by the mass and chemistry of the foreign material present in the drop.
Atmospheric Environment. Part A. General Topics, 1990
A laboratory experiment is described where the uptake of SO 2 by dendritic snow crystals was stud... more A laboratory experiment is described where the uptake of SO 2 by dendritic snow crystals was studied. In a first experimental series the uptake of SO2 was investigated during the growth of the snow crystals from water vapor with and without the presence of H202 in the air. In a second series of experiments we studied the uptake of SO 2 by snow crystals which had completed their growth. The results of our experiments showed that under both conditions SO2 became scavenged by snow crystals. The uptake of SO2 was particularly pronounced during the growth of the snow crystals, and at temperatures close to 0°C where a quasi-liquid layer exists at the surface of ice. As expected, the SO 2 uptake becomes enhanced in the presence of H202. The present results are in qualitative agreement with previous studies involving bulk ice.
Page 1. ERAD 2008 - THE FIFTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY A Wind Tu... more Page 1. ERAD 2008 - THE FIFTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY A Wind Tunnel Study On The Oscillation of Freely Falling Raindrops Miklós Szakáll1 , Karoline Diehl1, Subir ...
. Until now, the processes involved in secondary ice production which generate high concentration... more . Until now, the processes involved in secondary ice production which generate high concentrations of ice crystals in clouds have been poorly understood. However, collisions that involve rimed ice particles or crystal aggregates have the potential to effectively produce secondary ice from their fragmentation. Unfortunately, there have only been a few laboratory studies on ice-ice collision, resulting in an inaccurate representation of this process in microphysical schemes. To address this issue, experiments were conducted at the Wind tunnel laboratory of the Johannes Gutenberg University, Mainz on graupel- graupel and graupel-snowflake collisions under still air conditions at -15 °C and over water saturation. All fragments resulting from graupel-graupel collisions were collected and investigated by means of a digital optical microscope, while fragments from graupel-snowflake collisions were observed and recorded instantly after collision using a holographic instrument. From these experiments, distributions were obtained for fragment sizes, cross sectional areas and aspect ratios. The results showed a higher number of fragments at lower kinetic energy compared to those presented in the literature. 150 to 600 fragments were observed for graupel-graupel collisions, and 70 to 500 fragments for graupel-snowflake collisions between 10−7 and 10−5 J. Parametrizations for fragment size distributions are provided with a mode at 75 µm for graupel-graupel collisions and at 400 µm for graupel-snowflake collisions. These results can be used to improve the representation of ice collision breakup in microphysical schemes.
For the purpose of testing our previously described theory of SO z scavenging a laboratory invest... more For the purpose of testing our previously described theory of SO z scavenging a laboratory investigation was carried out in the UCLA 33 m long rainshaft. Drops with radii between 250 and 2500 ~tm were allowed to come to terminal velocity, after which they passed through a chamber of variable length filled with various SO 2 concentrations in air. After falling through a gas separating chamber consisting of a fluorocarbon gas the drops were collected and analyzed for their total S content in order to determine the rate of SO 2 absorption.
An experimental investigation of the simultaneous absorption of NH3 and SO2 from the ambient atmo... more An experimental investigation of the simultaneous absorption of NH3 and SO2 from the ambient atmosphere by freely falling water drops has been carried out in the Mainz vertical wind tunnel. The experimental results were found to be in good agreement with the results derived from computations with the Kronig-Brink convective diffusion model and also with a model which assumes a drop to be well mixed at all times. Encouraged by this agreement, these computation schemes for the uptake of gas by single drops where incorporated in a pollution washout model with realistic SO2, NH3 and CO2 gas profiles. This model allows an entire raindrop size distribution to fall through a gas layer. The results of this 'plume-model' show that the SO2 uptake is strongly dependent on the NH3 concentration in the atmosphere and on the rainrate. We also lind that the small drops contribute more towards the washout of these gases. In the case of simultaneous presence of NH3 and SO2, desorption of these gases is negligible.
An experimental and theoretical study has been carried out to investigate the rate of desorption ... more An experimental and theoretical study has been carried out to investigate the rate of desorption of SO, from water drops falling at terminal velocity in air. The experiments were carried out in the Mainz vertical wind tunnel in which water drops of various sizes containing S(IV) in various concentrations were freely suspended in the vertical airstream of the tunnel. The results of these experiments were compared with the predictions of three theoretical models, and with the experiments of Walcek et al. This comparison shows that the predictions of the diffusion model of Kronig and Brink in the formulation given by Walcek and Pruppacher agree well with the experimental results for all relevant large and small raindrop sizes, and for all considered concentrations of S(IV) inside the drops. In contrast, the predictions of the diffusion model which assumes complete internal mixing inside a drop agrees with the experimental results only if the concentration of S(IV) inside the drop is less than that equivalent of an equilibrium SO 2 concentration of 15 ppbv. At larger concentrations, the theoretical predictions of the model for complete internal mixing progressively deviate from the experimental results. It is further shown that Barrie's double film model can he used to interpret the resistance to diffusion inside a drop in terms of a diffusion boundary layer inside the drop which increases in thickness with decreasing concentration of S(IV). Applying our results to the desorption of SO 2 from small and large rain drops falling below an assumed cloud base, shows that for typical contents of S(IV) inside the drops substantial amounts of SO2 will desorb from these drops unless H202 is present in the surrounding air.
The rate of change of mass of a hailstone by diffusion is affected by its motions. In a stationar... more The rate of change of mass of a hailstone by diffusion is affected by its motions. In a stationary pure diffusive case the water vapor distribution around a spherical hailstone is spherically symmetric having rather weak water vapor gradients. However, when a hailstone falls in the air, the flow field and hence the water vapor distribution around the hailstone is asymmetric showing much stronger water vapor gradients which are high at the upstream side and lower at the rear side of the hailstone. When averaged over the whole surface area of the hailstone the mass transfer to or from the falling hailstone surface is always higher compared to a pure diffusive case. This convective enhancement is given by the ventilation coefficient. Thus, to reliably quantify growth or sublimation rates of falling hailstones with models, it is necessary to know their ventilation coefficients. The rate of change of mass is proportional to the rate of change of heat. Therefore, the growth or sublimation...
Vertical wind tunnel experiments were carried out to investigate the melting of low-density lump ... more Vertical wind tunnel experiments were carried out to investigate the melting of low-density lump graupel while floating at their terminal velocities. The graupel characteristics such as maximum dimension, density, and axis ratio were 0.39 ± 0.06 cm, 0.41 ± 0.07 g cm−3, and 0.89 ± 0.06. The airstream of the wind tunnel was gradually heated simulating lapse rates between 4.5 and 3.21 K km−1. Each experimental run was performed at a constant relative humidity that was varied between 12% and 92% from one experiment to the other. From the image processing of video recordings, variations in minimum and maximum dimension, volume, aspect ratio, density, volume equivalent radius, and ice core radius were obtained. New parameterizations of the terminal velocity prior to melting and during melting were developed. It was found that mass and heat transfer in the dry stage is 2 times as high as that of liquid drops at the same Reynolds number. Based on the experimental results, a model was develo...
Abstract An experimental study was carried out to investigate the processes which occur during dr... more Abstract An experimental study was carried out to investigate the processes which occur during drop-to-particle conversion in atmospheric clouds whose drops are evaporating. The experiment was carried out in the Mainz vertical wind tunnel and in a 4-m high fall shaft. Drops consisting of aqueous solutions of NaCl, (NH 4 ) 2 SO 4 , and salts of artificial and natural ocean water, containing various kinds of solid, water-insoluble particles such as clays, were studied. The aerosol particles produced by the evaporation of drops had diameters in the range 0.1–1 μm, 5–20 μm, 40–80 μm and 100–300 μm. The shape and fall mode of these particles are discussed. It was found that the aerosol particles formed did not break up or splinter during the evaporation of drops. This led to the conclusion that, in evaporating atmospheric clouds, each evaporating drop produces by drop-to-particle conversion one aerosol particle of mass and chemistry given by the mass and chemistry of the foreign material present in the drop.
Atmospheric Environment. Part A. General Topics, 1990
A laboratory experiment is described where the uptake of SO 2 by dendritic snow crystals was stud... more A laboratory experiment is described where the uptake of SO 2 by dendritic snow crystals was studied. In a first experimental series the uptake of SO2 was investigated during the growth of the snow crystals from water vapor with and without the presence of H202 in the air. In a second series of experiments we studied the uptake of SO 2 by snow crystals which had completed their growth. The results of our experiments showed that under both conditions SO2 became scavenged by snow crystals. The uptake of SO2 was particularly pronounced during the growth of the snow crystals, and at temperatures close to 0°C where a quasi-liquid layer exists at the surface of ice. As expected, the SO 2 uptake becomes enhanced in the presence of H202. The present results are in qualitative agreement with previous studies involving bulk ice.
Page 1. ERAD 2008 - THE FIFTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY A Wind Tu... more Page 1. ERAD 2008 - THE FIFTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY A Wind Tunnel Study On The Oscillation of Freely Falling Raindrops Miklós Szakáll1 , Karoline Diehl1, Subir ...
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