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Abstract
A synoptic case study where a small-scale low, a so-called polar low, can be followed as a closed circulation on the surface maps around 2O00 km from Iceland to the North Sea is presented. Satellite images show that the polar low develops a cloud pattern of convective clouds very much like that of a tropical cyclone. The 1000–500 mb thickness field shows that the polar low has a warm core during the time when it is best developed.
It is concluded that the polar low discussed in this work probably is a phenomenon different from a comma cloud and also different from small-scale vortices associated with strong mid-tropospheric positive vorticity advection.
Abstract
A synoptic case study where a small-scale low, a so-called polar low, can be followed as a closed circulation on the surface maps around 2O00 km from Iceland to the North Sea is presented. Satellite images show that the polar low develops a cloud pattern of convective clouds very much like that of a tropical cyclone. The 1000–500 mb thickness field shows that the polar low has a warm core during the time when it is best developed.
It is concluded that the polar low discussed in this work probably is a phenomenon different from a comma cloud and also different from small-scale vortices associated with strong mid-tropospheric positive vorticity advection.
Abstract
This note updates a previous study that utilized a baseline climatology of soundings associated with large hail, significant tornadoes, and 10 or more cloud-to-ground lightning flashes from 1992. Expanding on the earlier analysis, it is shown that three modified forecast parameters have more value in distinguishing between environments that favor significant tornadoes and those that favor large hail but no significant tornadoes, in the climatological data. These parameters are storm-relative helicity in the 0–1-km layer adjacent to the ground, energy–helicity index computed from this measure of helicity, and the convective available potential energy that accrues from the surface to 3 km above ground level. In addition, this note provides caveats regarding the interpretation of the climatological findings.
Abstract
This note updates a previous study that utilized a baseline climatology of soundings associated with large hail, significant tornadoes, and 10 or more cloud-to-ground lightning flashes from 1992. Expanding on the earlier analysis, it is shown that three modified forecast parameters have more value in distinguishing between environments that favor significant tornadoes and those that favor large hail but no significant tornadoes, in the climatological data. These parameters are storm-relative helicity in the 0–1-km layer adjacent to the ground, energy–helicity index computed from this measure of helicity, and the convective available potential energy that accrues from the surface to 3 km above ground level. In addition, this note provides caveats regarding the interpretation of the climatological findings.
Abstract
It is hypothesized that the precipitation intensity beneath a supercell updraft is strongly influenced by the amount of hydrometeors that are reingested into the updraft after being transported away in the divergent upper-level flow of the anvil. This paper presents the results of a climatological analysis of soundings associated with three types of isolated supercells having distinctive precipitation distributions, the so-called classic, low-precipitation (LP), and high-precipitation (HP) storms. It is shown that storm-relative flow at 9–10 km above the ground is strongest in the environments of LP storms, and relatively weak in the environments of HP storms, with classic storms occurring in environments with intermediate magnitudes of upper storm-relative flow. It is plausible that comparatively strong flow in the anvil-bearing levels of LP storms transports hydrometeors far enough from the updraft that they are relatively unlikely to be reingested into the updraft, leading to greatly diminished precipitation formation in the updraft itself. Conversely, the weak upper flow near HP storms apparently allows a relatively large number of hydrometeors to return to the updraft, leading to the generation of relatively large amounts of precipitation in the updraft. It also is apparent that thermodynamic factors such as convective available potential energy, low-level mixing ratio, and mean relative humidity are of lesser importance in determining storm type from a climatological perspective, although important variations in humidity may not be well sampled in this study. This climatological analysis does not directly evaluate the stated hypothesis; however, the findings do indicate that further modeling and microphysical observations are warranted.
Abstract
It is hypothesized that the precipitation intensity beneath a supercell updraft is strongly influenced by the amount of hydrometeors that are reingested into the updraft after being transported away in the divergent upper-level flow of the anvil. This paper presents the results of a climatological analysis of soundings associated with three types of isolated supercells having distinctive precipitation distributions, the so-called classic, low-precipitation (LP), and high-precipitation (HP) storms. It is shown that storm-relative flow at 9–10 km above the ground is strongest in the environments of LP storms, and relatively weak in the environments of HP storms, with classic storms occurring in environments with intermediate magnitudes of upper storm-relative flow. It is plausible that comparatively strong flow in the anvil-bearing levels of LP storms transports hydrometeors far enough from the updraft that they are relatively unlikely to be reingested into the updraft, leading to greatly diminished precipitation formation in the updraft itself. Conversely, the weak upper flow near HP storms apparently allows a relatively large number of hydrometeors to return to the updraft, leading to the generation of relatively large amounts of precipitation in the updraft. It also is apparent that thermodynamic factors such as convective available potential energy, low-level mixing ratio, and mean relative humidity are of lesser importance in determining storm type from a climatological perspective, although important variations in humidity may not be well sampled in this study. This climatological analysis does not directly evaluate the stated hypothesis; however, the findings do indicate that further modeling and microphysical observations are warranted.
Abstract
Observations of the development of cumulus convection, which reached depths of several kilometers but failed to develop into sustained, precipitating, cumulonimbus clouds—an event the authors term “convection initiation failure”—are presented from the 12 June 2002 International H2O Project (IHOP) case. The investigation relies heavily on remote and in situ data obtained by mobile, truck-borne Doppler radars, mobile mesonets, mobile soundings, and stereo cloud photogrammetry.
Data collection was focused in northwestern Oklahoma near the intersection of an outflow boundary and dryline. Thunderstorms developed along the dryline during the late afternoon approximately 40 km east of the domain intensively observed by the ground-based observing systems. Farther west, within the region of dense observations analyzed herein, cumulus congestus clouds formed along an outflow boundary. Multiple-Doppler wind syntheses revealed that the boundary layer vertical velocity field was dominated by thermals rather than by circulations associated with the mesoscale boundaries. In spite of this observation, deep cumulus cloud development was confined to the mesoscale boundaries. Trajectories into the deep cumulus clouds that developed along the outflow boundary were much more vertical than those entering the shallow cumulus clouds observed away from the outflow boundary. It is hypothesized that the role of the outflow boundary in promoting deep cumulus cloud formation was to promote updrafts that were less susceptible to the dilution of equivalent potential temperature, which controls the potential buoyancy, vertical velocity, and depth that can be realized by the clouds. It is also hypothesized that the lack of a persistent, spatially continuous corridor of mesoscale ascent along the outflow boundary and associated moisture upwelling contributed to convection initiation failure along the outflow boundary.
Abstract
Observations of the development of cumulus convection, which reached depths of several kilometers but failed to develop into sustained, precipitating, cumulonimbus clouds—an event the authors term “convection initiation failure”—are presented from the 12 June 2002 International H2O Project (IHOP) case. The investigation relies heavily on remote and in situ data obtained by mobile, truck-borne Doppler radars, mobile mesonets, mobile soundings, and stereo cloud photogrammetry.
Data collection was focused in northwestern Oklahoma near the intersection of an outflow boundary and dryline. Thunderstorms developed along the dryline during the late afternoon approximately 40 km east of the domain intensively observed by the ground-based observing systems. Farther west, within the region of dense observations analyzed herein, cumulus congestus clouds formed along an outflow boundary. Multiple-Doppler wind syntheses revealed that the boundary layer vertical velocity field was dominated by thermals rather than by circulations associated with the mesoscale boundaries. In spite of this observation, deep cumulus cloud development was confined to the mesoscale boundaries. Trajectories into the deep cumulus clouds that developed along the outflow boundary were much more vertical than those entering the shallow cumulus clouds observed away from the outflow boundary. It is hypothesized that the role of the outflow boundary in promoting deep cumulus cloud formation was to promote updrafts that were less susceptible to the dilution of equivalent potential temperature, which controls the potential buoyancy, vertical velocity, and depth that can be realized by the clouds. It is also hypothesized that the lack of a persistent, spatially continuous corridor of mesoscale ascent along the outflow boundary and associated moisture upwelling contributed to convection initiation failure along the outflow boundary.
Abstract
The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Profiler Studies project, the Verification of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 field projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets define the fields of airflow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression.
The results of the present study confirm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three cases are approximately Gaussian, suggesting a peak expected cloud frequency 15 km east of the dryline. Deep mesoscale moisture convergence is inferred in cloudy regions, with either subsidence or a lack of deep convergence in cloud-free regions. The results document the modulating effect of vertical wind shear and elevated dry layers in combination with the depth and strength of mesoscale updrafts on convective initiation, supporting the notion that moist boundary layer air parcels must be lifted to their lifted condensation level and level of free convection prior to leaving the mesoscale updraft to form deep convection. By relaxing the overly restrictive assumptions of parcel theory, it is suggested that a modification of proximity soundings to account for mesoscale lift and westerly wind shear effects can improve the diagnosis of the mesoscale dryline environment and the prediction of convective initiation at the dryline. Conversely, proximity environmental soundings, taken by themselves with consideration of CAPE and convective inhibition values according to parcel theory but neglecting vertical boundary layer circulations, are found to have less prognostic value than is conventionally assumed.
Abstract
The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Profiler Studies project, the Verification of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 field projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets define the fields of airflow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression.
The results of the present study confirm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three cases are approximately Gaussian, suggesting a peak expected cloud frequency 15 km east of the dryline. Deep mesoscale moisture convergence is inferred in cloudy regions, with either subsidence or a lack of deep convergence in cloud-free regions. The results document the modulating effect of vertical wind shear and elevated dry layers in combination with the depth and strength of mesoscale updrafts on convective initiation, supporting the notion that moist boundary layer air parcels must be lifted to their lifted condensation level and level of free convection prior to leaving the mesoscale updraft to form deep convection. By relaxing the overly restrictive assumptions of parcel theory, it is suggested that a modification of proximity soundings to account for mesoscale lift and westerly wind shear effects can improve the diagnosis of the mesoscale dryline environment and the prediction of convective initiation at the dryline. Conversely, proximity environmental soundings, taken by themselves with consideration of CAPE and convective inhibition values according to parcel theory but neglecting vertical boundary layer circulations, are found to have less prognostic value than is conventionally assumed.
Abstract
All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level also appears to be generally lower for supercells with significant tornadoes than those without. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even with the best discriminators, are discussed.
Abstract
All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level also appears to be generally lower for supercells with significant tornadoes than those without. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even with the best discriminators, are discussed.
Abstract
The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0–800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.
Abstract
The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0–800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.
Abstract
Prognostic equations are proposed for use in gridpoint models for the purpose of providing Lagrangian information without the need for computing Lagrangian trajectories. The information provided by the proposed methods might lead to improved representations of microphysical conversion processes. For example, the proposed methods could help improve the timing and location of the onset of precipitation in cloud models.
Abstract
Prognostic equations are proposed for use in gridpoint models for the purpose of providing Lagrangian information without the need for computing Lagrangian trajectories. The information provided by the proposed methods might lead to improved representations of microphysical conversion processes. For example, the proposed methods could help improve the timing and location of the onset of precipitation in cloud models.
Abstract
Doppler radar observations that establish common patterns in the evolution of the reflectivity and flow structures of squall lines are described. A number of squall lines have been analyzed with unprecedented time resolution in order to identity these patterns. All of the squall lines appeared to be approximately two-dimensional and featured a solid leading edge at some time during their life cycle, instead of being composed of discrete cells separated by echo-free regions. A large variety of intensities and evolution lime scales was observed.
It is shown that squall lines of this type evolve through identifiable stages of reflectivity structure. This evolution appears to be strongly related to changes that occur in the kinematic structure. As a typical system evolves, a rearward-sloping zone of horizontal vorticity, predominantly associated with vertical shear, develops on the scale of the system, presumably driven by the horizontal buoyancy gradients across the system. The vorticity that is generated allows further generation to take place by causing the superposition of a saturated, precipitating anvil cloud aloft over potentially cooler air below in the trailing region. The rearward-sloping vorticity zone gradually tilts toward the horizontal. The rate at which this zone tilts seems to be the primary difference between the systems studied. To a first approximation, the inflow streamlines parallel the sloping vorticity zone, so as it approaches a horizontal slope, vertical motion becomes smaller. Eventually, convective-scale ascent ceases, giving the impression that the gust front has surged out ahead of the precipitation.
Abstract
Doppler radar observations that establish common patterns in the evolution of the reflectivity and flow structures of squall lines are described. A number of squall lines have been analyzed with unprecedented time resolution in order to identity these patterns. All of the squall lines appeared to be approximately two-dimensional and featured a solid leading edge at some time during their life cycle, instead of being composed of discrete cells separated by echo-free regions. A large variety of intensities and evolution lime scales was observed.
It is shown that squall lines of this type evolve through identifiable stages of reflectivity structure. This evolution appears to be strongly related to changes that occur in the kinematic structure. As a typical system evolves, a rearward-sloping zone of horizontal vorticity, predominantly associated with vertical shear, develops on the scale of the system, presumably driven by the horizontal buoyancy gradients across the system. The vorticity that is generated allows further generation to take place by causing the superposition of a saturated, precipitating anvil cloud aloft over potentially cooler air below in the trailing region. The rearward-sloping vorticity zone gradually tilts toward the horizontal. The rate at which this zone tilts seems to be the primary difference between the systems studied. To a first approximation, the inflow streamlines parallel the sloping vorticity zone, so as it approaches a horizontal slope, vertical motion becomes smaller. Eventually, convective-scale ascent ceases, giving the impression that the gust front has surged out ahead of the precipitation.
