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Abstract
A technique is described whereby an investigator can obtain a more representative peak velocity for a tornadic vortex signature (TVS) than can be obtained by using the extreme measured Doppler velocities associated with the signature. This technique, which is based on theoretical curves derived by scanning a simulated radar through Rankine combined vortices, is applied to WSR-88D Doppler velocity data collected in the Garden City, Kansas, tornado of 16 May 1995. The technique produces peak TVS velocities that are more consistent in time and height than those computed directly from the extreme measured values.
Abstract
A technique is described whereby an investigator can obtain a more representative peak velocity for a tornadic vortex signature (TVS) than can be obtained by using the extreme measured Doppler velocities associated with the signature. This technique, which is based on theoretical curves derived by scanning a simulated radar through Rankine combined vortices, is applied to WSR-88D Doppler velocity data collected in the Garden City, Kansas, tornado of 16 May 1995. The technique produces peak TVS velocities that are more consistent in time and height than those computed directly from the extreme measured values.
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Abstract
The hydrodynamic instability characteristics of planetary zonal flows are investigated through use of a quasi-geostrophic numerical model of high spatial resolution. An initial-value technique is employed to obtain solutions of the linear problem.
Certain zonal flows containing both vertical and lateral shears, which are representative of those observed in the earth's atmosphere, are found to be unstable with respect to the large-scale quasi-geostrophic disturbances. Westerly currents, each characterized by a latitudinally symmetric jet containing absolute vorticity extrema at various latitudes, amplify perturbations of some scales through a dominating baroclinic mechanism, and amplify perturbations of other scales through a dominating barotropic mechanism. For these flows, the unstable perturbations of relatively short zonal wavelength convert zonal available potential energy into perturbation energy and simultaneously strengthen the zonal kinetic energy of the basic flow. On the other hand, the unstable perturbations of relatively long zonal wavelength reduce both the zonal kinetic and available potential energies of the basic flow, with the former reduction dominating. For certain flows, these combinations produce two distinct wavelengths of maximum instability. Flows which are similar but contain no vanishing meridional gradient of absolute vorticity are found to produce baroclinically unstable perturbations with a tendency toward barotropic damping.
Abstract
The hydrodynamic instability characteristics of planetary zonal flows are investigated through use of a quasi-geostrophic numerical model of high spatial resolution. An initial-value technique is employed to obtain solutions of the linear problem.
Certain zonal flows containing both vertical and lateral shears, which are representative of those observed in the earth's atmosphere, are found to be unstable with respect to the large-scale quasi-geostrophic disturbances. Westerly currents, each characterized by a latitudinally symmetric jet containing absolute vorticity extrema at various latitudes, amplify perturbations of some scales through a dominating baroclinic mechanism, and amplify perturbations of other scales through a dominating barotropic mechanism. For these flows, the unstable perturbations of relatively short zonal wavelength convert zonal available potential energy into perturbation energy and simultaneously strengthen the zonal kinetic energy of the basic flow. On the other hand, the unstable perturbations of relatively long zonal wavelength reduce both the zonal kinetic and available potential energies of the basic flow, with the former reduction dominating. For certain flows, these combinations produce two distinct wavelengths of maximum instability. Flows which are similar but contain no vanishing meridional gradient of absolute vorticity are found to produce baroclinically unstable perturbations with a tendency toward barotropic damping.
Abstract
A technique for the measurement of the ice water content (IWC) of cirrus clouds is described. The IWC is obtained by the measurement of the total water content (TWC) and the subtraction of the saturation specific humidity with respect to ice at the ambient pressure and temperature. The method is independent of the measurement of the crystal size spectrum and also of any assumptions about the bulk densities of various crystal habits. Examples of IWC measurements made during the International Cirrus Experiment are presented and compared with conventional measurements from a 2D optical array probe. The prime sources of error are the accuracy of the calibration of the TWC probe and the occurrence of subsaturated air, which invalidates one of the main principles of the technique.
Abstract
A technique for the measurement of the ice water content (IWC) of cirrus clouds is described. The IWC is obtained by the measurement of the total water content (TWC) and the subtraction of the saturation specific humidity with respect to ice at the ambient pressure and temperature. The method is independent of the measurement of the crystal size spectrum and also of any assumptions about the bulk densities of various crystal habits. Examples of IWC measurements made during the International Cirrus Experiment are presented and compared with conventional measurements from a 2D optical array probe. The prime sources of error are the accuracy of the calibration of the TWC probe and the occurrence of subsaturated air, which invalidates one of the main principles of the technique.
Abstract
Composite profiles of thermodynamic and kinematic variables are prepared to represent the characteristics of the environment within which a particular atmospheric phenomenon occurs. During the averaging process, it is desirable to retain the dominant features and associated gradients found in the individual profiles so that representative values of quantities such as flux parameters, energy budgets, convective available potential energy, and various stability indices can be computed from the composite profiles. The conventional compositing approach, where averages are computed at common heights, reduces or even smooths out a significant feature when the height and vertical extent of the feature differ from one individual profile to the next.
To retain a desirable feature in the composite profile, it is necessary to compute averages at the heights where the feature occurs and to compute the average height of the feature itself. As an example of the capabilities of this scaling or feature-preserving approach, the technique was applied to a set of 33 hodographs from supercell thunderstorm environments as documented in the literature. The feature-preserving technique retained realistic wind-shear values, including a midlatitude minimum-shear layer that disappeared when the conventional compositing technique was used.
Abstract
Composite profiles of thermodynamic and kinematic variables are prepared to represent the characteristics of the environment within which a particular atmospheric phenomenon occurs. During the averaging process, it is desirable to retain the dominant features and associated gradients found in the individual profiles so that representative values of quantities such as flux parameters, energy budgets, convective available potential energy, and various stability indices can be computed from the composite profiles. The conventional compositing approach, where averages are computed at common heights, reduces or even smooths out a significant feature when the height and vertical extent of the feature differ from one individual profile to the next.
To retain a desirable feature in the composite profile, it is necessary to compute averages at the heights where the feature occurs and to compute the average height of the feature itself. As an example of the capabilities of this scaling or feature-preserving approach, the technique was applied to a set of 33 hodographs from supercell thunderstorm environments as documented in the literature. The feature-preserving technique retained realistic wind-shear values, including a midlatitude minimum-shear layer that disappeared when the conventional compositing technique was used.
Abstract
Extratropical cyclones are responsible for significant weather in the form of heavy precipitation and strong winds. The capability of numerical weather prediction models to predict the synoptic-scale structure of such cyclones has improved greatly over recent years but much of the significant weather itself is associated with small and mesoscale processes not properly represented even in today's relatively high-resolution models. As a result, the detailed prediction of significant weather, even for the period 1–12 h ahead, still falls far short of requirements. In order to find out what improvements are needed by way of increased model resolution, better parameterizations, and/or improved observations/assimilation, it is first necessary to learn more about the structure, mechanism, and interaction of the small-scale and mesoscale processes. This is the subject of this review.
The review focuses on the structure and organization of slantwise and upright convection within extratropical cyclones, particularly cold-season maritime cyclones. These subsynoptic-scale features are set within a broader context using the conveyor-belt and frontal-fracture paradigms. It is shown that there is a common tendency for slantwise convection to occur in the form of vertically stacked multiple circulations, sometimes associated with lines of upright convection that are themselves broken into chains of line elements.
The review also examines the nature and significance of evaporation/sublimation and shearing instability within frontal zones. These processes play opposing roles in, respectively, sharpening and diffusing the individual slantwise convective circulations. Although shearing instability occurs mostly at very small scales, individual events can lead to breakdown of laminar flow over layers as much as 1 km deep. Such events may be attributed to potential shearing instability, which, like its counterpart in convective instability, can suddenly be released where a layer of air is lifted to saturation.
The studies of the above processes described in this review make extensive use of observations from many different types of radar. Although it is generally necessary to interpret radar observations within the context of other information, the pivotal role of radar in these studies is clear. The writer owes a debt of gratitude of Dave Atlas who from an early stage inspired him to attempt to use the full potential of radar.
Abstract
Extratropical cyclones are responsible for significant weather in the form of heavy precipitation and strong winds. The capability of numerical weather prediction models to predict the synoptic-scale structure of such cyclones has improved greatly over recent years but much of the significant weather itself is associated with small and mesoscale processes not properly represented even in today's relatively high-resolution models. As a result, the detailed prediction of significant weather, even for the period 1–12 h ahead, still falls far short of requirements. In order to find out what improvements are needed by way of increased model resolution, better parameterizations, and/or improved observations/assimilation, it is first necessary to learn more about the structure, mechanism, and interaction of the small-scale and mesoscale processes. This is the subject of this review.
The review focuses on the structure and organization of slantwise and upright convection within extratropical cyclones, particularly cold-season maritime cyclones. These subsynoptic-scale features are set within a broader context using the conveyor-belt and frontal-fracture paradigms. It is shown that there is a common tendency for slantwise convection to occur in the form of vertically stacked multiple circulations, sometimes associated with lines of upright convection that are themselves broken into chains of line elements.
The review also examines the nature and significance of evaporation/sublimation and shearing instability within frontal zones. These processes play opposing roles in, respectively, sharpening and diffusing the individual slantwise convective circulations. Although shearing instability occurs mostly at very small scales, individual events can lead to breakdown of laminar flow over layers as much as 1 km deep. Such events may be attributed to potential shearing instability, which, like its counterpart in convective instability, can suddenly be released where a layer of air is lifted to saturation.
The studies of the above processes described in this review make extensive use of observations from many different types of radar. Although it is generally necessary to interpret radar observations within the context of other information, the pivotal role of radar in these studies is clear. The writer owes a debt of gratitude of Dave Atlas who from an early stage inspired him to attempt to use the full potential of radar.
Abstract
The use of the holographic cloud particle imaging system developed by the Cloud Physics Branch of the Meteorological Office and carried on the C-130 Hercules aircraft of the Meteorological Research Flight (MRF) has hitherto been limited by the extremely labor intensive data extraction process. A new image reconstruction system has now been developed that enables numerous holograms from a single flight to be analyzed. A brief description of this system is given, and some of its uses and limitations are demonstrated by examples of both droplet and ice-crystal data. In each case, the holographic data are compared with those from what are now conventional cloud microphysical probes, principally the ASSP and 2-D Optical Array Probe. Results show that the holographic system can measure gross features of the droplet size spectrum in conditions when the ASSP data may be unreliable. Ice crystal measurements confirm the ability of the holographic technique to produce data down to sizes of about 60 μm, well below the practical limit for the 2-D Cloud probe. Holographic ice concentrations appear to be systematically larger than those from the 2-D, typically by a factor of about half an order of magnitude. Some possible sources of error in each system have been examined but the exact cause of the discrepancy remains unproven. The relative unambiguity of the holographic sample volume suggests that this system will give the most reliable results, particularly for columns.
Abstract
The use of the holographic cloud particle imaging system developed by the Cloud Physics Branch of the Meteorological Office and carried on the C-130 Hercules aircraft of the Meteorological Research Flight (MRF) has hitherto been limited by the extremely labor intensive data extraction process. A new image reconstruction system has now been developed that enables numerous holograms from a single flight to be analyzed. A brief description of this system is given, and some of its uses and limitations are demonstrated by examples of both droplet and ice-crystal data. In each case, the holographic data are compared with those from what are now conventional cloud microphysical probes, principally the ASSP and 2-D Optical Array Probe. Results show that the holographic system can measure gross features of the droplet size spectrum in conditions when the ASSP data may be unreliable. Ice crystal measurements confirm the ability of the holographic technique to produce data down to sizes of about 60 μm, well below the practical limit for the 2-D Cloud probe. Holographic ice concentrations appear to be systematically larger than those from the 2-D, typically by a factor of about half an order of magnitude. Some possible sources of error in each system have been examined but the exact cause of the discrepancy remains unproven. The relative unambiguity of the holographic sample volume suggests that this system will give the most reliable results, particularly for columns.
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