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rain curtain in a supercell instigate tornadogenesis barotropically? J. Atmos. Sci. , 65 , 2469 – 2497 , doi: 10.1175/2007JAS2516.1 . 10.1175/2007JAS2516.1 Davies-Jones , R. , 2015 : A review of supercell and tornado dynamics . Atmos. Res. , 158–159 , 274 – 291 , doi: 10.1016/j.atmosres.2014.04.007 . 10.1016/j.atmosres.2014.04.007 Davies-Jones , R. , and H. E. Brooks , 1993 : Mesocyclogenesis from a theoretical perspective. The Tornado: Its Structure, Dynamics, Prediction, and
rain curtain in a supercell instigate tornadogenesis barotropically? J. Atmos. Sci. , 65 , 2469 – 2497 , doi: 10.1175/2007JAS2516.1 . 10.1175/2007JAS2516.1 Davies-Jones , R. , 2015 : A review of supercell and tornado dynamics . Atmos. Res. , 158–159 , 274 – 291 , doi: 10.1016/j.atmosres.2014.04.007 . 10.1016/j.atmosres.2014.04.007 Davies-Jones , R. , and H. E. Brooks , 1993 : Mesocyclogenesis from a theoretical perspective. The Tornado: Its Structure, Dynamics, Prediction, and
rotation near the ground ( Ludlam 1963 ; Fujita 1975 ; Burgess et al. 1977 ; Barnes 1978 ; Lemon and Doswell 1979 ), their precise role in the tornadogenesis process remains unclear. A lengthy review of observational, numerical modeling, and theoretical findings pertinent to hook echoes and RFDs recently has been completed by Markowski (2002) . In a companion paper by Markowski et al. (2002) , it was observed that the air parcels at the surface within the RFDs of tornadic supercells tend to be
rotation near the ground ( Ludlam 1963 ; Fujita 1975 ; Burgess et al. 1977 ; Barnes 1978 ; Lemon and Doswell 1979 ), their precise role in the tornadogenesis process remains unclear. A lengthy review of observational, numerical modeling, and theoretical findings pertinent to hook echoes and RFDs recently has been completed by Markowski (2002) . In a companion paper by Markowski et al. (2002) , it was observed that the air parcels at the surface within the RFDs of tornadic supercells tend to be
. Discussion An interesting question is how the above analysis relates to tornadogenesis. As discussed in section 3b , we have rather limited faith in the treatment of near-ground trajectories once they descend below the bottom scalar model level, so we cannot describe vortex genesis faithfully. However, when animated, the vertical vorticity field at the lowest scalar model level clearly shows how the rivers and lobes of positive ζ move downstream and feed into the developing vortex (see also Fig. 5
. Discussion An interesting question is how the above analysis relates to tornadogenesis. As discussed in section 3b , we have rather limited faith in the treatment of near-ground trajectories once they descend below the bottom scalar model level, so we cannot describe vortex genesis faithfully. However, when animated, the vertical vorticity field at the lowest scalar model level clearly shows how the rivers and lobes of positive ζ move downstream and feed into the developing vortex (see also Fig. 5
Abstract
This study is the first in a series that investigates the effects of turbulence in the boundary layer of a tornado vortex. In this part, axisymmetric simulations with constant viscosity are used to explore the relationships between vortex structure, intensity, and unsteadiness as functions of diffusion (measured by a Reynolds number Re
r
) and rotation (measured by a swirl ratio S
r
). A deep upper-level damping zone is used to prevent upper-level disturbances from affecting the low-level vortex. The damping zone is most effective when it overlaps with the specified convective forcing, causing a reduction to the effective convective velocity scale W
e
. With this damping in place, the tornado-vortex boundary layer shows no sign of unsteadiness for a wide range of parameters, suggesting that turbulence in the tornado boundary layer is inherently a three-dimensional phenomenon. For high Re
r
, the most intense vortices have maximum mean tangential winds well in excess of W
e
, and maximum mean vertical velocity exceeds 3 times W
e
. In parameter space, the most intense vortices fall along a line that follows 
Abstract
This study is the first in a series that investigates the effects of turbulence in the boundary layer of a tornado vortex. In this part, axisymmetric simulations with constant viscosity are used to explore the relationships between vortex structure, intensity, and unsteadiness as functions of diffusion (measured by a Reynolds number Re
r
) and rotation (measured by a swirl ratio S
r
). A deep upper-level damping zone is used to prevent upper-level disturbances from affecting the low-level vortex. The damping zone is most effective when it overlaps with the specified convective forcing, causing a reduction to the effective convective velocity scale W
e
. With this damping in place, the tornado-vortex boundary layer shows no sign of unsteadiness for a wide range of parameters, suggesting that turbulence in the tornado boundary layer is inherently a three-dimensional phenomenon. For high Re
r
, the most intense vortices have maximum mean tangential winds well in excess of W
e
, and maximum mean vertical velocity exceeds 3 times W
e
. In parameter space, the most intense vortices fall along a line that follows
1. Introduction The mechanisms by which tornadoes are formed within supercell thunderstorms (tornadogenesis) remain a significant scientific mystery. This is a problem of both great scientific interest and social impact, as the ability to understand tornado formation is critical in reducing the time between issuing of tornado warnings to potentially affected populations and tornado formation. However, the interplay of spatiotemporally varying processes within the highly dynamic supercell
1. Introduction The mechanisms by which tornadoes are formed within supercell thunderstorms (tornadogenesis) remain a significant scientific mystery. This is a problem of both great scientific interest and social impact, as the ability to understand tornado formation is critical in reducing the time between issuing of tornado warnings to potentially affected populations and tornado formation. However, the interplay of spatiotemporally varying processes within the highly dynamic supercell
evolution leading to a dynamic corner flow collapse (CFC) can naturally produce a very intense near-surface vortex from a much weaker larger-scale swirling flow, and it was argued that this might sometimes play a critical role in tornadogenesis and/or tornado variability. We begin by addressing important simulation concerns in section 2 , particularly the grid resolution requirements and interaction with the top boundary condition. We then sample the range of behavior encountered and address what
evolution leading to a dynamic corner flow collapse (CFC) can naturally produce a very intense near-surface vortex from a much weaker larger-scale swirling flow, and it was argued that this might sometimes play a critical role in tornadogenesis and/or tornado variability. We begin by addressing important simulation concerns in section 2 , particularly the grid resolution requirements and interaction with the top boundary condition. We then sample the range of behavior encountered and address what
determined to be temporally coincident withthe tornadic phase. Interpretation of tornadogenesis in supercells is made in the perspective of these observations and thefindings associated with fine-scale numerical simulations. A concluding hypothesis is that the major featuresof tornadogenesis are not particularly sensitive to many aspects of storm-scale circulations but rather theyrequire creation of specific localized conditions along the storm outflow boundary. One such condition is theexistence of a
determined to be temporally coincident withthe tornadic phase. Interpretation of tornadogenesis in supercells is made in the perspective of these observations and thefindings associated with fine-scale numerical simulations. A concluding hypothesis is that the major featuresof tornadogenesis are not particularly sensitive to many aspects of storm-scale circulations but rather theyrequire creation of specific localized conditions along the storm outflow boundary. One such condition is theexistence of a
, in final form 3 March 1995)ABSTRACT A three-dimensional numerical simulation using a two-way interactive nested grid is used to study tornadogenesis within a supercell. During a 40-minute period, two tornadoes grow and decay within the storm's mesocyclone. The tornadoes have life spans of approximately 10 minutes. Maximum ground-relative surface windspeeds exceed 60 m s-~ during both tornadoes, and horizontal pressure gradients reach 18 hPa km-~ during thesecond tornado. Comparison of the
, in final form 3 March 1995)ABSTRACT A three-dimensional numerical simulation using a two-way interactive nested grid is used to study tornadogenesis within a supercell. During a 40-minute period, two tornadoes grow and decay within the storm's mesocyclone. The tornadoes have life spans of approximately 10 minutes. Maximum ground-relative surface windspeeds exceed 60 m s-~ during both tornadoes, and horizontal pressure gradients reach 18 hPa km-~ during thesecond tornado. Comparison of the
sequence of events that leads to tornadogenesis. The "pseudostorm" is an idealized thunderstorm representation and emulates the storm-relative flow, into an updraft, of the horizontal streamwise vorticity that is baroclinically generated in cold air outflow. By not explicitly simulating the morphology of a tornadic thunderstorm, but instead concentrating on the development of low-level rotationand tornado-scale vortices, the authors are able to transcend many of the experimental limitations encountered
sequence of events that leads to tornadogenesis. The "pseudostorm" is an idealized thunderstorm representation and emulates the storm-relative flow, into an updraft, of the horizontal streamwise vorticity that is baroclinically generated in cold air outflow. By not explicitly simulating the morphology of a tornadic thunderstorm, but instead concentrating on the development of low-level rotationand tornado-scale vortices, the authors are able to transcend many of the experimental limitations encountered
to the explanation for the associative relationship between low-level vertical shear and the occurrence of significant tornadoes. Tornadogenesis, the formation of a concentrated vortex in contact with the cloud and the ground, requires amplification of surface vertical vorticity to tornado strength through convergence–stretching often associated with the storm’s airmass boundaries or self-generated as in the dynamic pipe effect ( Trapp and Davies-Jones 1997 ). Therefore, it can be deduced that
to the explanation for the associative relationship between low-level vertical shear and the occurrence of significant tornadoes. Tornadogenesis, the formation of a concentrated vortex in contact with the cloud and the ground, requires amplification of surface vertical vorticity to tornado strength through convergence–stretching often associated with the storm’s airmass boundaries or self-generated as in the dynamic pipe effect ( Trapp and Davies-Jones 1997 ). Therefore, it can be deduced that
