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- Author or Editor: J. Doyne Sartor x
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Problems in the design, preparation, and use of local objective forecasting studies have been evaluated while specific local objective forecasting studies were being made. As a result, a systematic procedure that may be used as a guide in designing and preparing local objective forecasting studies was developed and tested in a number of seminar-workshop groups. The systematic procedure is outlined and discussed with illustrations from several local objective forecasting studies. The procedure consists of defining the local forecasting problem in terms consistent with observing and forecasting capabilities, resolving the variability of the forecast element into its cyclic (seasonal and diurnal) and “synoptic” components, and finally, reassembling the components into a forecast scheme that allows the forecaster to exercise his training as a meteorologist.
Problems in the design, preparation, and use of local objective forecasting studies have been evaluated while specific local objective forecasting studies were being made. As a result, a systematic procedure that may be used as a guide in designing and preparing local objective forecasting studies was developed and tested in a number of seminar-workshop groups. The systematic procedure is outlined and discussed with illustrations from several local objective forecasting studies. The procedure consists of defining the local forecasting problem in terms consistent with observing and forecasting capabilities, resolving the variability of the forecast element into its cyclic (seasonal and diurnal) and “synoptic” components, and finally, reassembling the components into a forecast scheme that allows the forecaster to exercise his training as a meteorologist.
Abstract
The modification of thunderstorms to suppress hail requires a knowledge of where, when and how much to seed. We show that growth by accretion by precipitating particles (hail, rain and graupel) in a summer convective storm depends on the path that the particles take with respect to the cloud air circulation in which the cloud droplets are embedded, as well as on the ambient atmospheric parameters of temperature, moisture, stability and larger scale circulations. For this purpose we used a two-dimensional simulation of the circulation in which the most important features of one-dimensional time-dependent microphysics simulation can be incorporated into the calculations at each time step.
The effect of changes in the altitude of ice particle initiation is calculated using simulations of the clouds and their environment on two days during this period when the total amount of hail differed by more than an order of magnitude. The simulated hail size and amount varied in the same sense as the observed.
The simulation is applied to the case used by Browning and Foote (1976) to develop a conceptual model of a severe hailstorm. The results show that in order to obtain the tilt of the updraft shown by Browning and Foote, the presence of a squall line or gust front would be required—a situation that they say is possible from their observed meteorological data.
Abstract
The modification of thunderstorms to suppress hail requires a knowledge of where, when and how much to seed. We show that growth by accretion by precipitating particles (hail, rain and graupel) in a summer convective storm depends on the path that the particles take with respect to the cloud air circulation in which the cloud droplets are embedded, as well as on the ambient atmospheric parameters of temperature, moisture, stability and larger scale circulations. For this purpose we used a two-dimensional simulation of the circulation in which the most important features of one-dimensional time-dependent microphysics simulation can be incorporated into the calculations at each time step.
The effect of changes in the altitude of ice particle initiation is calculated using simulations of the clouds and their environment on two days during this period when the total amount of hail differed by more than an order of magnitude. The simulated hail size and amount varied in the same sense as the observed.
The simulation is applied to the case used by Browning and Foote (1976) to develop a conceptual model of a severe hailstorm. The results show that in order to obtain the tilt of the updraft shown by Browning and Foote, the presence of a squall line or gust front would be required—a situation that they say is possible from their observed meteorological data.
Abstract
The rate that vertical vorticity is created or retarded in electrified clouds is calculated from the cross product of the charge gradient and the electrical field and compared with the magnitude of the vertical vorticity produced dynamically. Calculations are made for clouds on the thunderstorm, mesoscale and synoptic scales for midlatitude and tropical conditions. The results show that in moderately electrified clouds with particle charges an order of magnitude less than the observed maxima, the production of vorticity due to electrostatic forces approaches or slightly exceeds the dynamic production in thunderstorm anvil clouds and on the mesoscale in the tropics. Highly electrified clouds with maximum particle charges can produce vorticity at comparable rates to its dynamic production on all scales except the synoptic scale in midlatitudes.
To the extent that cloud charging conditions due to the global electric field and mid-tropospheric conductivity conditions are perturbed by solar events or solar-included electromagnetic disturbances, some solar influence on the electrical conditions could be expected in the mid-troposphere where clouds of the suitable type and extent form as a consequence of the normal meteorological processes. Where the electrical conditions exceed the threshold required for the production of vorticity dynamically, organized circulations produced electrophysically are possible in a solar-disturbed large-scale electrical environment.
Abstract
The rate that vertical vorticity is created or retarded in electrified clouds is calculated from the cross product of the charge gradient and the electrical field and compared with the magnitude of the vertical vorticity produced dynamically. Calculations are made for clouds on the thunderstorm, mesoscale and synoptic scales for midlatitude and tropical conditions. The results show that in moderately electrified clouds with particle charges an order of magnitude less than the observed maxima, the production of vorticity due to electrostatic forces approaches or slightly exceeds the dynamic production in thunderstorm anvil clouds and on the mesoscale in the tropics. Highly electrified clouds with maximum particle charges can produce vorticity at comparable rates to its dynamic production on all scales except the synoptic scale in midlatitudes.
To the extent that cloud charging conditions due to the global electric field and mid-tropospheric conductivity conditions are perturbed by solar events or solar-included electromagnetic disturbances, some solar influence on the electrical conditions could be expected in the mid-troposphere where clouds of the suitable type and extent form as a consequence of the normal meteorological processes. Where the electrical conditions exceed the threshold required for the production of vorticity dynamically, organized circulations produced electrophysically are possible in a solar-disturbed large-scale electrical environment.
Abstract
The observational results from sailplane flights into the updrafts of developing cumulus clouds in north-eastern Colorado show some important variations in the microstructure of the cloud droplet and ice particle distributions. Some of these variations are apparently caused by the combined interactions of cloud droplets and precipitation particles with the horizontal and vertical components of the updraft and its horizontal and vertical structure.
Data from these observations are introduced into a circulation framework in an attempt to understand how the microphysics and the circulation can interact to give the features observed. The results cast doubt on the validity of the often made assumption that the microphysical properties of a cloud are distributed randomly with respect to each other on the smaller scales, and that this condition exists uniformly throughout the cloud.
The observed precipitation shafts with bimodal size distributions in the middle and lower parts of a cloud can be recreated in a two-dimensional simulation of the observed cloud air circulation with embedded microphysics. The observed and calculated frozen water content can increase by one to two orders of magnitude over the liquid water content when moving from cloudy air into a precipitation shaft. The observed change in concentration with height of the ice particles exceeds (by over two orders of magnitude) the expected ice nuclei concentration usually found in the atmosphere at comparable temperatures. The average concentrations of ice particles observed occasionally exceed 400 l −1.
Abstract
The observational results from sailplane flights into the updrafts of developing cumulus clouds in north-eastern Colorado show some important variations in the microstructure of the cloud droplet and ice particle distributions. Some of these variations are apparently caused by the combined interactions of cloud droplets and precipitation particles with the horizontal and vertical components of the updraft and its horizontal and vertical structure.
Data from these observations are introduced into a circulation framework in an attempt to understand how the microphysics and the circulation can interact to give the features observed. The results cast doubt on the validity of the often made assumption that the microphysical properties of a cloud are distributed randomly with respect to each other on the smaller scales, and that this condition exists uniformly throughout the cloud.
The observed precipitation shafts with bimodal size distributions in the middle and lower parts of a cloud can be recreated in a two-dimensional simulation of the observed cloud air circulation with embedded microphysics. The observed and calculated frozen water content can increase by one to two orders of magnitude over the liquid water content when moving from cloudy air into a precipitation shaft. The observed change in concentration with height of the ice particles exceeds (by over two orders of magnitude) the expected ice nuclei concentration usually found in the atmosphere at comparable temperatures. The average concentrations of ice particles observed occasionally exceed 400 l −1.
Abstract
Expressions are derived for ice particle charges acquired through successive collisions in a changing electric field. Our computations give lower particle charges, slower maximum field growth rates, and higher maxium fields than reported in recent literature. The minimum ice particle sizes and concentrations required for efficient electrification appear to lie well within the range observed in thunderstorms. If the mean radius of the large ice particles (graupel, hail, etc.) exceeds 0.2 cm, fields higher than 5000 V cm−1 can be generated.
Abstract
Expressions are derived for ice particle charges acquired through successive collisions in a changing electric field. Our computations give lower particle charges, slower maximum field growth rates, and higher maxium fields than reported in recent literature. The minimum ice particle sizes and concentrations required for efficient electrification appear to lie well within the range observed in thunderstorms. If the mean radius of the large ice particles (graupel, hail, etc.) exceeds 0.2 cm, fields higher than 5000 V cm−1 can be generated.
Abstract
There is evidence that the inductive charging process is likely to be efficient in regions of the cloud that contain relatively high concentrations of large ice particles (graupel, hail, etc.) and small ice particles or supercooled droplets, but not in regions where only liquid drops are present. Because of these spatial limitations the transport of charge centers by updrafts can be expected to affect the direction and effciency of the inductive charge transfer process. On the basis of some qualitative arguments we come to the following conclusions. 1) updrafts are necessary for the inductive charge separation process to be efficient, and 2) this charge separation mechanism need not always produce a bipolar charge distribution with the positive charge center above the negative charge center, but that in the presence of strong updrafts the negative charge center may extend all the way to the cloud top.
Abstract
There is evidence that the inductive charging process is likely to be efficient in regions of the cloud that contain relatively high concentrations of large ice particles (graupel, hail, etc.) and small ice particles or supercooled droplets, but not in regions where only liquid drops are present. Because of these spatial limitations the transport of charge centers by updrafts can be expected to affect the direction and effciency of the inductive charge transfer process. On the basis of some qualitative arguments we come to the following conclusions. 1) updrafts are necessary for the inductive charge separation process to be efficient, and 2) this charge separation mechanism need not always produce a bipolar charge distribution with the positive charge center above the negative charge center, but that in the presence of strong updrafts the negative charge center may extend all the way to the cloud top.
Abstract
An electrostatic cloud droplet sizing device (electrostatic disdrometer) originally developed by Keily and Millen has been tested, modified extensively, and calibrated in our laboratory. The investigations have shown that soon after entry into the probe orifice, the incoming droplet is broken into many fragments. These impact and splash on an electrode raised to a 510 V potential. Measured pulses for a given droplet size give a reproducible calibration curve.
Airborne tests of the probe have shown it to operate reliably with minimal maintenance. Comparisons were made between values of the liquid water content measured by the electrostatic disdrometer and by the Johnson-Williams hot-wire, liquid-water-content meter and between the droplet size distributions measured by the disdrometer and by impaction slide replicas. The comparisons were satisfactory within the limits of instrument measuring and sampling errors and actual variations in the droplets spectra resulting from the separation of the instruments on the aircraft during the tests.
Abstract
An electrostatic cloud droplet sizing device (electrostatic disdrometer) originally developed by Keily and Millen has been tested, modified extensively, and calibrated in our laboratory. The investigations have shown that soon after entry into the probe orifice, the incoming droplet is broken into many fragments. These impact and splash on an electrode raised to a 510 V potential. Measured pulses for a given droplet size give a reproducible calibration curve.
Airborne tests of the probe have shown it to operate reliably with minimal maintenance. Comparisons were made between values of the liquid water content measured by the electrostatic disdrometer and by the Johnson-Williams hot-wire, liquid-water-content meter and between the droplet size distributions measured by the disdrometer and by impaction slide replicas. The comparisons were satisfactory within the limits of instrument measuring and sampling errors and actual variations in the droplets spectra resulting from the separation of the instruments on the aircraft during the tests.
Abstract
Most of the precipitation related theories on charge generation in thunderstorms fall into one of two categories: the inductive or polarization mechanism initiated by the ambient fair-weather field, and the non-inductive mechanism connected with certain electrochemical or thermoelectric particle characteristics. Our numerical study addresses the question of which mechanism gives more realistic results with regard to charge distribution and hold strength and what effect a combination of the two processes produces. The investigation is a first attempt using a simplified model.
In this model the microphysical processes of particle growth and simultaneous electrification are embedded in a steady state two-dimensional vortex circulation with and without vertical wind shear. The net space charge and potential are obtained everywhere in the cloud and the resulting electric fields are calculated. Computations are made for the collisions of growing solid precipitation (graupel) particles with either supercooled droplets (ice-water case) or with ice crystals (ice-ice case).
The results indicate that the non-inductive mechanism produces a rapid growth of the electric field in the early stages but tends to level out at a stable value considerably below the breakdown field strength. The inductive mechanism in turn shows a slow initial field growth with quickly varying charge distributions of often inverted polarity; however, it will reach breakdown field strength eventually due to its quasi-exponential growth character. Only the combination of the two processes achieves realistic thunderstorm conditions. It appears that the non-inductive mechanism controls the charge distribution and its polarity, and the inductive mechanism the field strength. both ice-water and ice-ice collisions give similar results, the only difference being a higher elevation of the charge dipole. in the ice-ice case. The opposite precipitation and cloud charges are always strongly masked.
The results permit some interesting conclusions on the origin of the fair-weather field.
Abstract
Most of the precipitation related theories on charge generation in thunderstorms fall into one of two categories: the inductive or polarization mechanism initiated by the ambient fair-weather field, and the non-inductive mechanism connected with certain electrochemical or thermoelectric particle characteristics. Our numerical study addresses the question of which mechanism gives more realistic results with regard to charge distribution and hold strength and what effect a combination of the two processes produces. The investigation is a first attempt using a simplified model.
In this model the microphysical processes of particle growth and simultaneous electrification are embedded in a steady state two-dimensional vortex circulation with and without vertical wind shear. The net space charge and potential are obtained everywhere in the cloud and the resulting electric fields are calculated. Computations are made for the collisions of growing solid precipitation (graupel) particles with either supercooled droplets (ice-water case) or with ice crystals (ice-ice case).
The results indicate that the non-inductive mechanism produces a rapid growth of the electric field in the early stages but tends to level out at a stable value considerably below the breakdown field strength. The inductive mechanism in turn shows a slow initial field growth with quickly varying charge distributions of often inverted polarity; however, it will reach breakdown field strength eventually due to its quasi-exponential growth character. Only the combination of the two processes achieves realistic thunderstorm conditions. It appears that the non-inductive mechanism controls the charge distribution and its polarity, and the inductive mechanism the field strength. both ice-water and ice-ice collisions give similar results, the only difference being a higher elevation of the charge dipole. in the ice-ice case. The opposite precipitation and cloud charges are always strongly masked.
The results permit some interesting conclusions on the origin of the fair-weather field.
