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- Author or Editor: John D. Marwitz x
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Abstract
An investigation was made into the precipitation processes operating within an Alberta supercell hailstorm which occurred on 22 July 1979. The main research tools employed in the study were an instrumented aircraft and an S-band meteorological radar.
Five cloud penetrations were conducted in and around the bounded weak-echo region associated with the main updraft of the storm during a 40 min period while the storm was producing 2–5 cm diam hail at the surface. The main updraft consisted of a relatively smooth, steady flow with adiabatic temperature. The main updraft acted as an obstacle to the mid-level environmental flow and caused the winds to split and accelerate around the southern side. Several smaller turbulent updrafts associated with time-dependent, fine-scale convective cells (feeder clouds) existed adjacent to the main updraft and appeared to be superimposed onto the quasi-steady, broader-scale dynamically forced circulation of the main storm. Graupel particles which originated within the feeder clouds were transported by the mid-level winds into and across the weak-echo region.
This study provides evidence that feeder clouds are also found within storms classed as supercells and that they provide a viable source of hailstone embryos and present a vital link among hail formation processes within a broader-scale continuum of hailstorm structures.
Abstract
An investigation was made into the precipitation processes operating within an Alberta supercell hailstorm which occurred on 22 July 1979. The main research tools employed in the study were an instrumented aircraft and an S-band meteorological radar.
Five cloud penetrations were conducted in and around the bounded weak-echo region associated with the main updraft of the storm during a 40 min period while the storm was producing 2–5 cm diam hail at the surface. The main updraft consisted of a relatively smooth, steady flow with adiabatic temperature. The main updraft acted as an obstacle to the mid-level environmental flow and caused the winds to split and accelerate around the southern side. Several smaller turbulent updrafts associated with time-dependent, fine-scale convective cells (feeder clouds) existed adjacent to the main updraft and appeared to be superimposed onto the quasi-steady, broader-scale dynamically forced circulation of the main storm. Graupel particles which originated within the feeder clouds were transported by the mid-level winds into and across the weak-echo region.
This study provides evidence that feeder clouds are also found within storms classed as supercells and that they provide a viable source of hailstone embryos and present a vital link among hail formation processes within a broader-scale continuum of hailstorm structures.
Abstract
The 1977–78 Sierra Cooperative Pilot Project (SCPP) field season offered an opportunity for comparing several Particle Measuring Systems (PMS) probes. During that winter the University of Wyoming's King Air aircraft was instrumented with 1D cloud, 2D cloud and 2D precipitation probes. Data from the three probes were analyzed from several flights in stable oregraphic storms. Various temperature levels (and hence, hydrometeor habits) above and below 0°C were flown in order to compare the data for water drops and for different ice crystal habits. It was found that all three probes agree quite well when sampling water drops of any size; however, when sampling ice crystals the probes agreed only at sizes larger than about 1 mm.
Abstract
The 1977–78 Sierra Cooperative Pilot Project (SCPP) field season offered an opportunity for comparing several Particle Measuring Systems (PMS) probes. During that winter the University of Wyoming's King Air aircraft was instrumented with 1D cloud, 2D cloud and 2D precipitation probes. Data from the three probes were analyzed from several flights in stable oregraphic storms. Various temperature levels (and hence, hydrometeor habits) above and below 0°C were flown in order to compare the data for water drops and for different ice crystal habits. It was found that all three probes agree quite well when sampling water drops of any size; however, when sampling ice crystals the probes agreed only at sizes larger than about 1 mm.
Abstract
Synthetic dual-Doppler (SDD) is a single-Doppler analysis technique that combines measurements from two different times, provided the viewing angle changes significantly. In this study, the viability of the SDD technique is investigated through comparisons with dual-Doppler analyses. Three case studies are used for the comparisons: a mature gust front, a supercell thunderstorm, and a developing squall line. An attempt to internally establish the efficacy of SDD analyses is made by examining the temporal correlation of the single-Doppler reflectivity fields and by comparing wind fields derived from similar synthetic analyses.
Results indicate that the SDD technique may be practical for the estimation of mean velocity fields of certain quasi-steady phenomena. The similarity of the SDD and conventional dual-Doppler results were found to be sensitive to the time between the two volume scans used in the SDD analyses, the angle subtended by the event, and the radii of influence of the Cressman weighting function used for the interpolation of the data.
Abstract
Synthetic dual-Doppler (SDD) is a single-Doppler analysis technique that combines measurements from two different times, provided the viewing angle changes significantly. In this study, the viability of the SDD technique is investigated through comparisons with dual-Doppler analyses. Three case studies are used for the comparisons: a mature gust front, a supercell thunderstorm, and a developing squall line. An attempt to internally establish the efficacy of SDD analyses is made by examining the temporal correlation of the single-Doppler reflectivity fields and by comparing wind fields derived from similar synthetic analyses.
Results indicate that the SDD technique may be practical for the estimation of mean velocity fields of certain quasi-steady phenomena. The similarity of the SDD and conventional dual-Doppler results were found to be sensitive to the time between the two volume scans used in the SDD analyses, the angle subtended by the event, and the radii of influence of the Cressman weighting function used for the interpolation of the data.
Abstract
The “ripening process” occurs due to thermodynamic instability of droplet size spectra in clouds. This instability results from the existence of droplets with different salinity and size in the droplet spectra. The ripening process is independent of turbulent fluctuations of supersaturation for a closed cloud parcel. Because of the ripening process, droplet number concentration continuously decreases with time after the initial peak supersaturation, which occurs during the initial updraft. Droplet spectra broaden to large sizes by evaporation of small droplets. Both mean droplet size and the standard deviation of droplet size spectra increase with time. This mechanism is suggested to be a potential physical mechanism for the formation of droplet size spectra in stratiform clouds.
Abstract
The “ripening process” occurs due to thermodynamic instability of droplet size spectra in clouds. This instability results from the existence of droplets with different salinity and size in the droplet spectra. The ripening process is independent of turbulent fluctuations of supersaturation for a closed cloud parcel. Because of the ripening process, droplet number concentration continuously decreases with time after the initial peak supersaturation, which occurs during the initial updraft. Droplet spectra broaden to large sizes by evaporation of small droplets. Both mean droplet size and the standard deviation of droplet size spectra increase with time. This mechanism is suggested to be a potential physical mechanism for the formation of droplet size spectra in stratiform clouds.
Abstract
A number of low-level flights were conducted with an instrumented aircraft to investigate wind characteristics in the planetary boundary layer over the low regions of the continental divide in southern Wyoming. The airflow upwind of the continental divide is stably-stratified and as it converses through Wyoming's “wind corridor&rdquo, a strong horizontal thermal gradient or baroclinic zone develops. The coldest air moves through the northern part of the wind corridor and is ovelain by a very stable layer. Trapped lee waves are prevalent in the planetary boundary layer of the wind corridor. Through the corridor the forces normal to the flow an in approximate balance and the flow is an anomalous anticyclonic flow around a low. Downwind of the wind corridor the airflow forms a convergence zone on its southern boundary with air from north-central Colorado. The convergence zone appears to remain distinct for ∼ 100 km downwind of the Medicine Bow Mountains.
The flow through smaller gaps was also investigated. Flows with hydraulic jump characteristics were observed.
Abstract
A number of low-level flights were conducted with an instrumented aircraft to investigate wind characteristics in the planetary boundary layer over the low regions of the continental divide in southern Wyoming. The airflow upwind of the continental divide is stably-stratified and as it converses through Wyoming's “wind corridor&rdquo, a strong horizontal thermal gradient or baroclinic zone develops. The coldest air moves through the northern part of the wind corridor and is ovelain by a very stable layer. Trapped lee waves are prevalent in the planetary boundary layer of the wind corridor. Through the corridor the forces normal to the flow an in approximate balance and the flow is an anomalous anticyclonic flow around a low. Downwind of the wind corridor the airflow forms a convergence zone on its southern boundary with air from north-central Colorado. The convergence zone appears to remain distinct for ∼ 100 km downwind of the Medicine Bow Mountains.
The flow through smaller gaps was also investigated. Flows with hydraulic jump characteristics were observed.
Abstract
Data were obtained for three High Plains thunderstorms in which penetrations were made of the weak echo region by an instrumented aircraft. The data from one of the cases are presented in detail. Two of the storms were steady state, as revealed by chaff analysis and subsequent subcloud passes. The third storm dissipated during the penetrations. The three storms were each characterized by negatively buoyant air at cloud base. Chaff released into the updrafts of the storms did not decelerate below the level of free convection (LFC). A vertical pressure perturbation gradient, therefore, existed below the LFC and within the weak echo region which acted to accelerate the air parcels in the presence of negative buoyancy. The analysis of the equivalent potential temperature fields for the two steady storm cases revealed considerable entrainment of environment air into the weak echo region. The mixing of the entrained parcels probably caused the observed increase of turbulence with height.
Abstract
Data were obtained for three High Plains thunderstorms in which penetrations were made of the weak echo region by an instrumented aircraft. The data from one of the cases are presented in detail. Two of the storms were steady state, as revealed by chaff analysis and subsequent subcloud passes. The third storm dissipated during the penetrations. The three storms were each characterized by negatively buoyant air at cloud base. Chaff released into the updrafts of the storms did not decelerate below the level of free convection (LFC). A vertical pressure perturbation gradient, therefore, existed below the LFC and within the weak echo region which acted to accelerate the air parcels in the presence of negative buoyancy. The analysis of the equivalent potential temperature fields for the two steady storm cases revealed considerable entrainment of environment air into the weak echo region. The mixing of the entrained parcels probably caused the observed increase of turbulence with height.
Abstract
Several encounters with hail and graupel (often inadvertent) have occurred while flying an aircraft through the organized updrafts at the base of thunderstorms in the High Plains area. These encounters normally occurred while entering or exiting the organized updrafts and while flying in the vicinity of the strong horizontal reflectivity gradients which commonly border organized updrafts. On several occasions mobile ground crews were beneath the organized updrafts and confirmed the observations from the aircraft. These crews also noted the sequence of precipitation events in this region. The hail which falls in this particular region has the following characteristics: it has a narrow size range, it is large hail (often the largest which falls from the storm), it is commonly not accompanied by rain, and it has a small range of concentrations for a given diameter. The precipitation sequence at the ground over which an organized updraft passes is large hail followed by smaller hail and rain.
Abstract
Several encounters with hail and graupel (often inadvertent) have occurred while flying an aircraft through the organized updrafts at the base of thunderstorms in the High Plains area. These encounters normally occurred while entering or exiting the organized updrafts and while flying in the vicinity of the strong horizontal reflectivity gradients which commonly border organized updrafts. On several occasions mobile ground crews were beneath the organized updrafts and confirmed the observations from the aircraft. These crews also noted the sequence of precipitation events in this region. The hail which falls in this particular region has the following characteristics: it has a narrow size range, it is large hail (often the largest which falls from the storm), it is commonly not accompanied by rain, and it has a small range of concentrations for a given diameter. The precipitation sequence at the ground over which an organized updraft passes is large hail followed by smaller hail and rain.
