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Abstract
A dynamic pipe effect (DPE) has been used previously to explain the descent from aloft of tornadic vortex signatures (TVSs), and presumably embryonic tornadoes, prior to the near-ground spinup of the tornado. But for many tornadoes the TVS appears to form simultaneously over a depth spanning the lowest few kilometers. A numerical model is used to determine the conditions under which a tornado is or is not preceded by a DPE. The model governs two-dimensional, axisymmetric, forced convection inside a closed, impermeable cylinder that rotates at a constant rate. Motion relative to the tank is initiated by a time-independent buoyancy field that is varied in a suite of experiments. The need for a DPE in vortex development in the model is shown to depend on trajectories of high-angular-momentum air, driven at least initially by this buoyancy field. Indeed, when buoyancy is confined primarily to midlevels, convergence at the foot of the vertical axis is weak initially, parcels with high angular momentum approach closest to the axis first at midlevels, and the vortex forms aloft (mode I). As the vortex intensifies and becomes cyclostrophically balanced, its central pressure drops and lateral motion into its core is inhibited. The resultant vertical pressure gradient and radial convergence below the vortex core increase, allowing parcels to approach the axis—and thus affording vortex spinup—at progressively lower levels. This process is the DPE. When significant buoyancy is present at low levels, air parcels that nearly conserve angular momentum are transported close to the axis over a relatively deep layer inclusive of the lower levels. The vortex in this case forms at low and midlevels at the same time, precluding a need for a DPE (mode II).
A simple analytical model is used to illustrate the two modes of vortex formation, and to generalize the conclusions drawn from the numerical model. A time-dependent version of the Burgers–Rott vortex due to Rott demonstrates vortex formation without a DPE. In this exact solution of the Navier–Stokes equations, horizontal convergence and angular momentum are independent of height, and the meridional flow is steady. A simple analytical solution for the DPE has not been found. However, it can be shown qualitatively that a vortex that develops aloft, because either the large-scale convergence or the ambient angular momentum increases with height, induces below itself an axial jet and increased radial inflow at low levels.
Abstract
A dynamic pipe effect (DPE) has been used previously to explain the descent from aloft of tornadic vortex signatures (TVSs), and presumably embryonic tornadoes, prior to the near-ground spinup of the tornado. But for many tornadoes the TVS appears to form simultaneously over a depth spanning the lowest few kilometers. A numerical model is used to determine the conditions under which a tornado is or is not preceded by a DPE. The model governs two-dimensional, axisymmetric, forced convection inside a closed, impermeable cylinder that rotates at a constant rate. Motion relative to the tank is initiated by a time-independent buoyancy field that is varied in a suite of experiments. The need for a DPE in vortex development in the model is shown to depend on trajectories of high-angular-momentum air, driven at least initially by this buoyancy field. Indeed, when buoyancy is confined primarily to midlevels, convergence at the foot of the vertical axis is weak initially, parcels with high angular momentum approach closest to the axis first at midlevels, and the vortex forms aloft (mode I). As the vortex intensifies and becomes cyclostrophically balanced, its central pressure drops and lateral motion into its core is inhibited. The resultant vertical pressure gradient and radial convergence below the vortex core increase, allowing parcels to approach the axis—and thus affording vortex spinup—at progressively lower levels. This process is the DPE. When significant buoyancy is present at low levels, air parcels that nearly conserve angular momentum are transported close to the axis over a relatively deep layer inclusive of the lower levels. The vortex in this case forms at low and midlevels at the same time, precluding a need for a DPE (mode II).
A simple analytical model is used to illustrate the two modes of vortex formation, and to generalize the conclusions drawn from the numerical model. A time-dependent version of the Burgers–Rott vortex due to Rott demonstrates vortex formation without a DPE. In this exact solution of the Navier–Stokes equations, horizontal convergence and angular momentum are independent of height, and the meridional flow is steady. A simple analytical solution for the DPE has not been found. However, it can be shown qualitatively that a vortex that develops aloft, because either the large-scale convergence or the ambient angular momentum increases with height, induces below itself an axial jet and increased radial inflow at low levels.
Abstract
Severe tornadoes caused much damage in eastern Oklahoma on 5 December 1975. Although synoptic conditions were favorable for tornadoes, the storms in eastern Oklahoma ahead of a strong dryline had no appearance of being tornadic when viewed on radar and on satellite photographs. This case and others like it indicate that dryline tornadic storms often have a unique structure which fails to fit the classic supercell model and results in a misleadingly weak appearance on radar (small, weak low-level echo without hook).
Abstract
Severe tornadoes caused much damage in eastern Oklahoma on 5 December 1975. Although synoptic conditions were favorable for tornadoes, the storms in eastern Oklahoma ahead of a strong dryline had no appearance of being tornadic when viewed on radar and on satellite photographs. This case and others like it indicate that dryline tornadic storms often have a unique structure which fails to fit the classic supercell model and results in a misleadingly weak appearance on radar (small, weak low-level echo without hook).
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Changes in tornado structure were accompanied by corresponding changes in damage intensity and debris configuration. Initially, damage was light over a 200 m wide path but the vortical nature of the winch was clearly evident. During the mature stage, damage was severe and still showed signs of circulation Intriguing and mysterious microscale patterns were observed in wheat fields. In the shrinking and decaying stages, heavy damage occurred over a 100 m wide path and there was evidence of strong radial inflow in the lowest meter above the surface. Generally, debris was thrown ahead of the vortex, with heavy objects coming to rest on the right forward side. Signs of circulation were no longer apparent in the debris configuration. Flow relative to the moving vortex appeared asymmetrical with strongest winds on the right side of the funnel.
Abstract
Changes in tornado structure were accompanied by corresponding changes in damage intensity and debris configuration. Initially, damage was light over a 200 m wide path but the vortical nature of the winch was clearly evident. During the mature stage, damage was severe and still showed signs of circulation Intriguing and mysterious microscale patterns were observed in wheat fields. In the shrinking and decaying stages, heavy damage occurred over a 100 m wide path and there was evidence of strong radial inflow in the lowest meter above the surface. Generally, debris was thrown ahead of the vortex, with heavy objects coming to rest on the right forward side. Signs of circulation were no longer apparent in the debris configuration. Flow relative to the moving vortex appeared asymmetrical with strongest winds on the right side of the funnel.
This paper describes the Verification of the Origins of Rotation in Tornadoes Experiment planned for 1994 and 1995 to evaluate a set of hypotheses pertaining to tornadogenesis and tornado dynamics. Observations of state variables will be obtained from five mobile mesonet vehicles, four mobile ballooning laboratories, three movie photography teams, portable Doppler radar teams, two in situ tornado instruments deployment teams, and the T-28 and National Atmospheric and Oceanic Administration P-3 aircraft. In addition, extensive use will be made of the new generation of observing systems, including the WSR-88D Doppler radars, demonstration wind profiler network, and National Weather Service rawinsondes.
This paper describes the Verification of the Origins of Rotation in Tornadoes Experiment planned for 1994 and 1995 to evaluate a set of hypotheses pertaining to tornadogenesis and tornado dynamics. Observations of state variables will be obtained from five mobile mesonet vehicles, four mobile ballooning laboratories, three movie photography teams, portable Doppler radar teams, two in situ tornado instruments deployment teams, and the T-28 and National Atmospheric and Oceanic Administration P-3 aircraft. In addition, extensive use will be made of the new generation of observing systems, including the WSR-88D Doppler radars, demonstration wind profiler network, and National Weather Service rawinsondes.
Abstract
In November 2016, an unprecedented epidemic thunderstorm asthma event in Victoria, Australia, resulted in many thousands of people developing breathing difficulties in a very short period of time, including 10 deaths, and created extreme demand across the Victorian health services. To better prepare for future events, a pilot forecasting system for epidemic thunderstorm asthma (ETSA) risk has been developed for Victoria. The system uses a categorical risk-based approach, combining operational forecasting of gusty winds in severe thunderstorms with statistical forecasts of high ambient grass pollen concentrations, which together generate the risk of epidemic thunderstorm asthma. This pilot system provides the first routine daily epidemic thunderstorm asthma risk forecasting service in the world that covers a wide area, and integrates into the health, ambulance, and emergency management sector. Epidemic thunderstorm asthma events have historically occurred infrequently, and no event of similar magnitude has impacted the Victorian health system since. However, during the first three years of the pilot, 2017–19, two high asthma presentation events and four moderate asthma presentation events were identified from public hospital emergency department records. The ETSA risk forecasts showed skill in discriminating between days with and without health impacts. However, even with hindsight of the actual weather and airborne grass pollen conditions, some high asthma presentation events occurred in districts that were assessed as low risk for ETSA, reflecting the challenge of predicting this unusual phenomenon.
Abstract
In November 2016, an unprecedented epidemic thunderstorm asthma event in Victoria, Australia, resulted in many thousands of people developing breathing difficulties in a very short period of time, including 10 deaths, and created extreme demand across the Victorian health services. To better prepare for future events, a pilot forecasting system for epidemic thunderstorm asthma (ETSA) risk has been developed for Victoria. The system uses a categorical risk-based approach, combining operational forecasting of gusty winds in severe thunderstorms with statistical forecasts of high ambient grass pollen concentrations, which together generate the risk of epidemic thunderstorm asthma. This pilot system provides the first routine daily epidemic thunderstorm asthma risk forecasting service in the world that covers a wide area, and integrates into the health, ambulance, and emergency management sector. Epidemic thunderstorm asthma events have historically occurred infrequently, and no event of similar magnitude has impacted the Victorian health system since. However, during the first three years of the pilot, 2017–19, two high asthma presentation events and four moderate asthma presentation events were identified from public hospital emergency department records. The ETSA risk forecasts showed skill in discriminating between days with and without health impacts. However, even with hindsight of the actual weather and airborne grass pollen conditions, some high asthma presentation events occurred in districts that were assessed as low risk for ETSA, reflecting the challenge of predicting this unusual phenomenon.
