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- Author or Editor: Roger Edwards x
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Abstract
This study tests hypothetical correspondences between size of severe hail, WSR-88D derived vertically integrated liquid water (VIL), and an array of thermodynamic variables derived from computationally modified sounding analyses. In addition, these associations are documented for normalized VIL using various sounding parameters, and statistical predictive value is assigned to the various VIL-based and sounding variables. The database was gathered from Weather Service Radar-1988 Doppler (WSR-88D) units nationwide from cases identified during real-time operations and consists of over 400 hail events, each associated with a radar-observed VIL value and a modified observational sounding.
Some parameters are found to increase in the mean with larger hail-size categories. Specific hail size, however, varies widely across the spectra of VIL, thermodynamic sounding variables, and combinations thereof, with only a few exceptions. No operationally useful parameters of value in hail-size prediction were discovered in the database of VIL and thermodynamic sounding data. These largely antihypothetical findings are compared with hail forecasting and warning techniques developed in the WSR-88D era—few in number and mostly regionalized and informal in nature—and with more widespread and empirical forecasting assumptions involving many of the same variables.
Abstract
This study tests hypothetical correspondences between size of severe hail, WSR-88D derived vertically integrated liquid water (VIL), and an array of thermodynamic variables derived from computationally modified sounding analyses. In addition, these associations are documented for normalized VIL using various sounding parameters, and statistical predictive value is assigned to the various VIL-based and sounding variables. The database was gathered from Weather Service Radar-1988 Doppler (WSR-88D) units nationwide from cases identified during real-time operations and consists of over 400 hail events, each associated with a radar-observed VIL value and a modified observational sounding.
Some parameters are found to increase in the mean with larger hail-size categories. Specific hail size, however, varies widely across the spectra of VIL, thermodynamic sounding variables, and combinations thereof, with only a few exceptions. No operationally useful parameters of value in hail-size prediction were discovered in the database of VIL and thermodynamic sounding data. These largely antihypothetical findings are compared with hail forecasting and warning techniques developed in the WSR-88D era—few in number and mostly regionalized and informal in nature—and with more widespread and empirical forecasting assumptions involving many of the same variables.
Abstract
An overview of conditions associated with the Oklahoma–Kansas tornado outbreak of 3 May 1999 is presented, with emphasis on the evolution of environmental and supercellular characteristics most relevant to the prediction of violent tornado episodes. This examination provides a unique perspective of the event by combining analyses of remote observational data and numerical guidance with direct observations of the event in the field by forecasters and other observers. The 3 May 1999 outbreak included two prolific supercells that produced several violent tornadoes, with ambient parameters comparable to those of past tornado outbreaks in the southern and central Great Plains. However, not all aspects leading to the evening of 3 May unambiguously favored a major tornado outbreak. The problems that faced operational forecasters at the Storm Prediction Center are discussed in the context of this outbreak, including environmental shear and instability, subtle processes contributing to convective initiation, the roles of preexisting boundaries, and storm-relative flow. This examination reveals several specific aspects where conceptual models are deficient and/or additional research is warranted.
Abstract
An overview of conditions associated with the Oklahoma–Kansas tornado outbreak of 3 May 1999 is presented, with emphasis on the evolution of environmental and supercellular characteristics most relevant to the prediction of violent tornado episodes. This examination provides a unique perspective of the event by combining analyses of remote observational data and numerical guidance with direct observations of the event in the field by forecasters and other observers. The 3 May 1999 outbreak included two prolific supercells that produced several violent tornadoes, with ambient parameters comparable to those of past tornado outbreaks in the southern and central Great Plains. However, not all aspects leading to the evening of 3 May unambiguously favored a major tornado outbreak. The problems that faced operational forecasters at the Storm Prediction Center are discussed in the context of this outbreak, including environmental shear and instability, subtle processes contributing to convective initiation, the roles of preexisting boundaries, and storm-relative flow. This examination reveals several specific aspects where conceptual models are deficient and/or additional research is warranted.
Abstract
The cyclone-relative location and variability in the number of tornadoes among tropical cyclones (TCs) are not completely understood. A key understudied factor that may improve our understanding is ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS), which impacts both the symmetry and strength of deep convection in TCs. This study conducts a climatological analysis of VWS impacts upon tornadoes in TCs from 1995 to 2018, using observed TC and tornado data together with radiosondes. TC tornadoes were classified by objectively defined VWS categories, derived from reanalyses, to quantify the sensitivity of tornado frequency, location, and their environments to VWS. The analysis shows that stronger VWS is associated with enhanced rates of tornado production—especially more damaging ones. Tornadoes also become localized to the downshear half of the TC as VWS strengthens, with tornado location in strongly sheared TCs transitioning from the downshear-left quadrant in the TC inner core to the downshear-right quadrant in the TC outer region. Analysis of radiosondes shows that the downshear-right quadrant in strongly sheared TCs is most frequently associated with sufficiently strong near-surface speed shear and veering aloft, and lower-tropospheric thermodynamic instability for tornadoes. These supportive kinematic environments may be due to the constructive superposition of the ambient and TC winds, and the VWS-induced downshear enhancement of the TC circulation among other factors. Together, this work provides a basis for improving forecasts of TC tornado frequency and location.
Abstract
The cyclone-relative location and variability in the number of tornadoes among tropical cyclones (TCs) are not completely understood. A key understudied factor that may improve our understanding is ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS), which impacts both the symmetry and strength of deep convection in TCs. This study conducts a climatological analysis of VWS impacts upon tornadoes in TCs from 1995 to 2018, using observed TC and tornado data together with radiosondes. TC tornadoes were classified by objectively defined VWS categories, derived from reanalyses, to quantify the sensitivity of tornado frequency, location, and their environments to VWS. The analysis shows that stronger VWS is associated with enhanced rates of tornado production—especially more damaging ones. Tornadoes also become localized to the downshear half of the TC as VWS strengthens, with tornado location in strongly sheared TCs transitioning from the downshear-left quadrant in the TC inner core to the downshear-right quadrant in the TC outer region. Analysis of radiosondes shows that the downshear-right quadrant in strongly sheared TCs is most frequently associated with sufficiently strong near-surface speed shear and veering aloft, and lower-tropospheric thermodynamic instability for tornadoes. These supportive kinematic environments may be due to the constructive superposition of the ambient and TC winds, and the VWS-induced downshear enhancement of the TC circulation among other factors. Together, this work provides a basis for improving forecasts of TC tornado frequency and location.
Abstract
This work investigates how the relationship between tropical cyclone (TC) tornadoes and ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS) varies between coastal and inland environments. Observed U.S. TC tornado track data are used to study tornado frequency and location, while dropsonde and radiosonde data are used to analyze convective-scale environments. To study the variability in the TC tornado–VWS relationship, these data are categorized by both 1) their distance from the coast and 2) reanalysis-derived VWS magnitude. The analysis shows that TCs produce coastal tornadoes regardless of VWS magnitude primarily in their downshear sector, with tornadoes most frequently occurring in strongly sheared cases. Inland tornadoes, including the most damaging cases, primarily occur in strongly sheared TCs within the outer radii of the downshear-right quadrant. Consistent with these patterns, dropsondes and coastal radiosondes show that the downshear-right quadrant of strongly sheared TCs has the most favorable combination of enhanced lower-tropospheric near-surface speed shear and veering, and reduced lower-tropospheric thermodynamic stability for tornadic supercells. Despite the weaker intensity farther inland, these kinematic conditions are even more favorable in inland environments within the downshear-right quadrant of strongly sheared TCs, due to the strengthened veering of the ambient winds and the lack of changes in the TC outer tangential wind field strength. The constructive superposition of the ambient and TC winds may be particularly important to inland tornado occurrence. Together, these results will allow forecasters to anticipate how the frequency and location of tornadoes and, more broadly, convection may change as TCs move inland.
Abstract
This work investigates how the relationship between tropical cyclone (TC) tornadoes and ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS) varies between coastal and inland environments. Observed U.S. TC tornado track data are used to study tornado frequency and location, while dropsonde and radiosonde data are used to analyze convective-scale environments. To study the variability in the TC tornado–VWS relationship, these data are categorized by both 1) their distance from the coast and 2) reanalysis-derived VWS magnitude. The analysis shows that TCs produce coastal tornadoes regardless of VWS magnitude primarily in their downshear sector, with tornadoes most frequently occurring in strongly sheared cases. Inland tornadoes, including the most damaging cases, primarily occur in strongly sheared TCs within the outer radii of the downshear-right quadrant. Consistent with these patterns, dropsondes and coastal radiosondes show that the downshear-right quadrant of strongly sheared TCs has the most favorable combination of enhanced lower-tropospheric near-surface speed shear and veering, and reduced lower-tropospheric thermodynamic stability for tornadic supercells. Despite the weaker intensity farther inland, these kinematic conditions are even more favorable in inland environments within the downshear-right quadrant of strongly sheared TCs, due to the strengthened veering of the ambient winds and the lack of changes in the TC outer tangential wind field strength. The constructive superposition of the ambient and TC winds may be particularly important to inland tornado occurrence. Together, these results will allow forecasters to anticipate how the frequency and location of tornadoes and, more broadly, convection may change as TCs move inland.
Abstract
A sample of 1185 Rapid Update Cycle (RUC) model analysis (0 h) proximity soundings, within 40 km and 30 min of radar-identified discrete storms, was categorized by several storm types: significantly tornadic supercells (F2 or greater damage), weakly tornadic supercells (F0–F1 damage), nontornadic supercells, elevated right-moving supercells, storms with marginal supercell characteristics, and nonsupercells. These proximity soundings served as the basis for calculations of storm-relative helicity and bulk shear intended to apply across a broad spectrum of thunderstorm types. An effective storm inflow layer was defined in terms of minimum constraints on lifted parcel CAPE and convective inhibition (CIN). Sixteen CAPE and CIN constraint combinations were examined, and the smallest CAPE (25 and 100 J kg−1) and largest CIN (−250 J kg−1) constraints provided the greatest probability of detecting an effective inflow layer within an 835-supercell subset of the proximity soundings. Effective storm-relative helicity (ESRH) calculations were based on the upper and lower bounds of the effective inflow layer. By confining the SRH calculation to the effective inflow layer, ESRH values can be compared consistently across a wide range of storm environments, including storms rooted above the ground. Similarly, the effective bulk shear (EBS) was defined in terms of the vertical shear through a percentage of the “storm depth,” as defined by the vertical distance from the effective inflow base to the equilibrium level associated with the most unstable parcel (maximum θe value) in the lowest 300 hPa. ESRH and EBS discriminate strongly between various storm types, and between supercells and nonsupercells, respectively.
Abstract
A sample of 1185 Rapid Update Cycle (RUC) model analysis (0 h) proximity soundings, within 40 km and 30 min of radar-identified discrete storms, was categorized by several storm types: significantly tornadic supercells (F2 or greater damage), weakly tornadic supercells (F0–F1 damage), nontornadic supercells, elevated right-moving supercells, storms with marginal supercell characteristics, and nonsupercells. These proximity soundings served as the basis for calculations of storm-relative helicity and bulk shear intended to apply across a broad spectrum of thunderstorm types. An effective storm inflow layer was defined in terms of minimum constraints on lifted parcel CAPE and convective inhibition (CIN). Sixteen CAPE and CIN constraint combinations were examined, and the smallest CAPE (25 and 100 J kg−1) and largest CIN (−250 J kg−1) constraints provided the greatest probability of detecting an effective inflow layer within an 835-supercell subset of the proximity soundings. Effective storm-relative helicity (ESRH) calculations were based on the upper and lower bounds of the effective inflow layer. By confining the SRH calculation to the effective inflow layer, ESRH values can be compared consistently across a wide range of storm environments, including storms rooted above the ground. Similarly, the effective bulk shear (EBS) was defined in terms of the vertical shear through a percentage of the “storm depth,” as defined by the vertical distance from the effective inflow base to the equilibrium level associated with the most unstable parcel (maximum θe value) in the lowest 300 hPa. ESRH and EBS discriminate strongly between various storm types, and between supercells and nonsupercells, respectively.
Abstract
An analysis of 4 yr of Rapid Update Cycle-2 (RUC-2) derived soundings in proximity to radar-observed supercells and nonsupercells is conducted in an effort to answer two questions: 1) over what depth is the fixed-layer bulk wind differential (BWD; the vector difference between the wind velocity at a given level and the wind velocity at the surface) the best discriminator between supercell and nonsupercell environments and 2) does the upper-tropospheric storm-relative flow (UTSRF) discriminate between the environments of supercells and nonsupercells? Previous climatologies of sounding-based supercell forecast parameters have documented the ability of the 0–6-km BWD in delineating supercell from nonsupercell environments. However, a systematic examination of a wide range of layers has never been documented. The UTSRF has previously been tested as a parameter for discriminating between supercell and nonsupercell environments and there is some evidence that supercells may be sensitive to the UTSRF. However, this sensitivity may be a consequence of the correlation between UTSRF and the surface to midtropospheric BWD. Accurately assessing the ability of the UTSRF to distinguish between supercell and nonsupercell environments requires controlling for the surface to midtropospheric BWD.
It is shown that the bulk wind differential within the 0–5-km layer delineates best between supercell and nonsupercell environments. Analysis of the UTSRF demonstrates that even when not controlling for the BWD, the UTSRF has limited reliability in forecasting supercells. The lack of merit in using the UTSRF to forecast supercells is particularly evident when it is isolated from the BWD. Because the UTSRF and BWD are not independent, controlling for the BWD when examining the UTSRF reveals that the UTSRF is not a fundamental parameter that can be used to distinguish supercell from nonsupercell environments. Therefore, this work demonstrates that the UTSRF is an unreliable metric for forecasting supercell events.
Abstract
An analysis of 4 yr of Rapid Update Cycle-2 (RUC-2) derived soundings in proximity to radar-observed supercells and nonsupercells is conducted in an effort to answer two questions: 1) over what depth is the fixed-layer bulk wind differential (BWD; the vector difference between the wind velocity at a given level and the wind velocity at the surface) the best discriminator between supercell and nonsupercell environments and 2) does the upper-tropospheric storm-relative flow (UTSRF) discriminate between the environments of supercells and nonsupercells? Previous climatologies of sounding-based supercell forecast parameters have documented the ability of the 0–6-km BWD in delineating supercell from nonsupercell environments. However, a systematic examination of a wide range of layers has never been documented. The UTSRF has previously been tested as a parameter for discriminating between supercell and nonsupercell environments and there is some evidence that supercells may be sensitive to the UTSRF. However, this sensitivity may be a consequence of the correlation between UTSRF and the surface to midtropospheric BWD. Accurately assessing the ability of the UTSRF to distinguish between supercell and nonsupercell environments requires controlling for the surface to midtropospheric BWD.
It is shown that the bulk wind differential within the 0–5-km layer delineates best between supercell and nonsupercell environments. Analysis of the UTSRF demonstrates that even when not controlling for the BWD, the UTSRF has limited reliability in forecasting supercells. The lack of merit in using the UTSRF to forecast supercells is particularly evident when it is isolated from the BWD. Because the UTSRF and BWD are not independent, controlling for the BWD when examining the UTSRF reveals that the UTSRF is not a fundamental parameter that can be used to distinguish supercell from nonsupercell environments. Therefore, this work demonstrates that the UTSRF is an unreliable metric for forecasting supercell events.
Abstract
A sample of 413 soundings in close proximity to tornadic and nontornadic supercells is examined. The soundings were obtained from hourly analyses generated by the 40-km Rapid Update Cycle-2 (RUC-2) analysis and forecast system. A comparison of 149 observed soundings and collocated RUC-2 soundings in regional supercell environments reveals that the RUC-2 model analyses were reasonably accurate through much of the troposphere. The largest error tendencies were in temperatures and mixing ratios near the surface, primarily in 1-h forecast soundings immediately prior to the standard rawinsonde launches around 1200 and 0000 UTC. Overall, the RUC-2 analysis soundings appear to be a reasonable proxy for observed soundings in supercell environments.
Thermodynamic and vertical wind shear parameters derived from RUC-2 proximity soundings are evaluated for the following supercell and storm subsets: significantly tornadic supercells (54 soundings), weakly tornadic supercells (144 soundings), nontornadic supercells (215 soundings), and discrete nonsupercell storms (75 soundings). Findings presented herein are then compared to results from previous and ongoing proximity soundings studies. Most significantly, proximity soundings presented here reinforce the findings of previous studies in that vertical shear and moisture within 1 km of the ground can discriminate between nontornadic supercells and supercells producing tornadoes with F2 or greater damage. Parameters that combine measures of buoyancy, vertical shear, and low-level moisture show the strongest ability to discriminate between supercell classes.
Abstract
A sample of 413 soundings in close proximity to tornadic and nontornadic supercells is examined. The soundings were obtained from hourly analyses generated by the 40-km Rapid Update Cycle-2 (RUC-2) analysis and forecast system. A comparison of 149 observed soundings and collocated RUC-2 soundings in regional supercell environments reveals that the RUC-2 model analyses were reasonably accurate through much of the troposphere. The largest error tendencies were in temperatures and mixing ratios near the surface, primarily in 1-h forecast soundings immediately prior to the standard rawinsonde launches around 1200 and 0000 UTC. Overall, the RUC-2 analysis soundings appear to be a reasonable proxy for observed soundings in supercell environments.
Thermodynamic and vertical wind shear parameters derived from RUC-2 proximity soundings are evaluated for the following supercell and storm subsets: significantly tornadic supercells (54 soundings), weakly tornadic supercells (144 soundings), nontornadic supercells (215 soundings), and discrete nonsupercell storms (75 soundings). Findings presented herein are then compared to results from previous and ongoing proximity soundings studies. Most significantly, proximity soundings presented here reinforce the findings of previous studies in that vertical shear and moisture within 1 km of the ground can discriminate between nontornadic supercells and supercells producing tornadoes with F2 or greater damage. Parameters that combine measures of buoyancy, vertical shear, and low-level moisture show the strongest ability to discriminate between supercell classes.
Abstract
Over 400 vertical wind profiles in close proximity to nontornadic and tornadic supercell thunderstorms are examined. The profiles were obtained from the Rapid Update Cycle (RUC) model/analysis system. Ground-relative wind speeds throughout the lower and middle troposphere are larger, on average, in tornadic supercell environments than in nontornadic supercell environments. The average vertical profiles of storm-relative wind speed, vertical wind shear, hodograph curvature, crosswise and streamwise vorticity, and storm-relative helicity are generally similar above 1 km in the tornadic and nontornadic supercell environments, with differences that are either not statistically significant or not what most would regard as meteorologically significant. On the other hand, considerable differences are found in these average vertical profiles within 1 km of the ground, with environments associated with significantly tornadic supercells (those producing tornadoes of at least F2 intensity) having substantially larger low-level vertical wind shear, streamwise vorticity, and storm-relative helicity compared to environments associated with nontornadic supercells and weakly tornadic supercells (those producing F0 or F1 tornadoes). These findings may partly explain the extraordinary difficulty in discriminating between tornadic and nontornadic supercell environments in a forecasting setting, given the low temporal and spatial frequency of wind observations in the lowest 1 km. It is believed that it would be a worthwhile investment to augment low-level wind profiling capabilities, in addition to taking a closer look at the dynamical sensitivities of supercell storms to near-surface wind shear by way of high-resolution numerical simulations.
Abstract
Over 400 vertical wind profiles in close proximity to nontornadic and tornadic supercell thunderstorms are examined. The profiles were obtained from the Rapid Update Cycle (RUC) model/analysis system. Ground-relative wind speeds throughout the lower and middle troposphere are larger, on average, in tornadic supercell environments than in nontornadic supercell environments. The average vertical profiles of storm-relative wind speed, vertical wind shear, hodograph curvature, crosswise and streamwise vorticity, and storm-relative helicity are generally similar above 1 km in the tornadic and nontornadic supercell environments, with differences that are either not statistically significant or not what most would regard as meteorologically significant. On the other hand, considerable differences are found in these average vertical profiles within 1 km of the ground, with environments associated with significantly tornadic supercells (those producing tornadoes of at least F2 intensity) having substantially larger low-level vertical wind shear, streamwise vorticity, and storm-relative helicity compared to environments associated with nontornadic supercells and weakly tornadic supercells (those producing F0 or F1 tornadoes). These findings may partly explain the extraordinary difficulty in discriminating between tornadic and nontornadic supercell environments in a forecasting setting, given the low temporal and spatial frequency of wind observations in the lowest 1 km. It is believed that it would be a worthwhile investment to augment low-level wind profiling capabilities, in addition to taking a closer look at the dynamical sensitivities of supercell storms to near-surface wind shear by way of high-resolution numerical simulations.
