Numerical Study of the 6 May 2012 Tsukuba Supercell Tornado: Vorticity Sources Responsible for Tornadogenesis

Remarkable progress has been achieved in understanding the vorticity source responsible for tornadogenesis. Nevertheless, the answer to this question remains elusive, particularly after introducing surface friction in realistic tornado simulations. In this study, a simulation using the Weather Research and Forecasting (WRF) Model is conducted based on the F3 supercell tornado that hit Tsukuba City, Japan, on 6 May 2012. The simulation uses triply nested domains, and the tornado is successfully reproduced in the innermost domain with 50-m horizontal grid spacing. The circulation analyses reveal that the frictional term is the dominant vorticity source responsible for the vortices at both the pretornadic and tornadogenesis times. The detailed vorticity source analyses of the air parcels show that the vorticity of the tornado at the genesis time mainly originates from the frictionally generated crosswise vorticity near the ground. The crosswise vorticity is directly tilted (or first exchanged into streamwise vorticity and then tilted) into vertical vorticity when the air parcels enter the tornado. A rear-flank downdraft (RFD) surge from the south and west sides of a primary low-level mesocyclone (LMC) may trigger tornadogenesis by increasing the convergence near the ground. The RFD surge is not necessarily associated with the baroclinically generated vorticity. In this study, the baroclinity is weak across the hook echo, which may cause a lack of baroclinically generated vorticity in the RFD surge.

A Numerical Study of the 6 May 2012 Tsukuba City Supercell Tornado. Part II: Mechanisms of Tornadogenesis

In Part I, the vorticity sources of midlevel and low-level mesocyclones in the 6 May 2012 Tsukuba City, Japan, tornadic supercell were investigated by using high-resolution simulation results with a 50-m horizontal grid spacing. In Part II, the analyses are extended to the mechanisms of tornadogenesis. The tornado was generated at the leading edge of a rear-flank downdraft (RFD) outflow surge. Backward-trajectory and vortex line analyses revealed that the RFD outflow surge was a triggering factor for tornadogenesis and that horizontal vorticity around the strong RFD outflow region was ingested into the tornado. To identify the vorticity source of the tornado, the evolution of circulation along a material circuit surrounding the tornado was investigated. Owing to baroclinity at the tip of a hook-shaped distribution of hydrometeors (hereafter hook echo), the circulation increased rapidly from a negative value when the core of the hydrometeors was descending, about 10 min prior to tornadogenesis. Analysis of the buoyancy field as well as a sensitivity experiment without diabatic cooling showed that baroclinity associated with cooling due to evaporation of rain and melting of ice-phase hydrometeors around the tip of the hook echo was the dominant vorticity source responsible for tornadogenesis.

On the Dynamics of Flows Induced by Topographic Ridges

This study describes a nonlocal mechanism for the generation of oceanic alternating jets by topographic ridges. The dynamics of these jets is examined using a baroclinic quasigeostrophic model configured with an isolated meridional ridge. The zonal topographic slopes of the ridge lead to the formation of a system of currents, consisting of mesoscale eddies, meridional currents over the ridge, and multiple zonal jets in the far field. Dynamical analysis shows that transient eddies are vital in sustaining the deep meridional currents over the ridge, which in turn play a key role in the upper-layer potential vorticity (PV) balance. The zonal jets in the rest of the domain owe their existence to the eddy forcing over the ridge but are maintained by the local Reynolds and form stress eddy forcing. The analysis further shows that a broad stable current that either becomes locally nonzonal or encounters a topographic ridge tends to become unstable. This instability provides a vorticity source and generates multiple zonal jets in the far field through a nonlocal mechanism.

Control of confined vortex breakdown with partial rotating lids

Experiments were conducted to determine the effectiveness of controlling vortex breakdown in a confined cylindrical vessel using a small rotating disk, which was flush-mounted into the opposite endwall to the rotating endwall driving the primary recirculating flow. The results show that the control disk, with relatively little power input, can modify the azimuthal and axial flow significantly, changing the entire flow structure in the cylinder. Co-rotation was found to precipitate vortex breakdown onset whereas counter-rotation delays it. Furthermore, for the Reynolds-number range over which breakdown normally exists, co-rotation increases the bubble radial and axial dimensions, while shifting the bubble in the upstream direction. By contrast, counter-rotation tends to reduce the size of the bubble, or completely suppress it, while shifting the bubble in the downstream direction. These effects are amplified substantially by the use of larger control disks and higher rotation ratios. A series of numerical simulations close to the onset Reynolds number reveals that the control disk acts to generate a rotation-rate-invariant local positive or negative azimuthal vorticity source away from the immediate vicinity of the control disk but upstream of breakdown. Advection of this source along streamlines modifies the strength of the azimuthal vorticity ring, which effectively controls whether the flow reverses on the axis, and thus, in turn, whether vortex breakdown occurs. The vorticity source generated by the control disk scales approximately linearly with rotation ratio and cubically with disk diameter; this allows the observed variation of the critical Reynolds number to be approximately predicted.

The Bowdle, South Dakota, Cyclic Tornadic Supercell of 22 May 2010: Surface Analysis of Rear-Flank Downdraft Evolution and Multiple Internal Surges

Mobile mesonet sampling in the hook echo/rear-flank downdraft (RFD) region of a tornadic supercell near Bowdle, South Dakota, provided the opportunity to examine RFD thermodynamic and kinematic attributes and evolution. Focused analysis of the fifth low-level mesocyclone cycle that produced two significant tornadoes including a violent tornado, revealed four RFD internal surge (RFDIS) events. RFDISs appeared to influence tornado development, intensity, and demise by altering the thermodynamic and kinematic character of the RFD region bounding the pretornadic and tornadic circulations. Significant tornadoes developed and matured when the RFD, modulated by internal surges, was kinematically strong, only weakly negatively buoyant, and very potentially buoyant. In contrast, the demise of the Bowdle tornado was concurrent with a much cooler RFDIS that replaced more buoyant and far more potentially buoyant RFD air near the tornado. This surge also likely contributed to a displacement of the tornado from the storm updraft. Development of the first tornado and rapid intensification of the Bowdle tornado occurred when an RFDIS boundary convergence zone interacted with the pretornadic and tornadic circulations, respectively. In the latter case, a strong vertical vortex sheet along an RFDIS boundary appeared to be a near-surface cyclonic vorticity source for the tornado. A downdraft closely bounding the right flank of the developing first tornado and intensifying Bowdle tornado provided some of the inflow to these circulations. For the Bowdle tornado, parcels were also streaming toward the tornado from its immediate east and northeast. A cyclonic–anticyclonic vortex couplet was observed during a portion of each significant tornado cycle.

A deterministic model for the sublayer streaks in turbulent boundary layers for application to flow control

We present a relatively simple, deterministic, theoretical model for the sublayer streaks in a turbulent boundary layer based on an analogy with Klebanoff modes. Our approach is to generate the streamwise vortices found in the buffer layer by means of a vorticity source in the form of a fictitious body force. It is found that the strongest streaks correspond to a spanwise wavelength that lies within the range of the experimentally observed values for the statistical mean streak spacing. We also present results showing the effect of streamwise pressure gradient, Reynolds number and wall compliance on the sublayer streaks. The theoretical predictions for the effects of wall compliance on the streak characteristics agree well with experimental data. Our proposed theoretical model for the quasi-periodic bursting cycle is also described, which places the streak modelling in context. The proposed bursting process is as follows: (i) streamwise vortices generate sublayer streaks and other vortical elements generate propagating plane waves, (ii) when the streaks reach a sufficient amplitude, they interact nonlinearly with the plane waves to produce oblique waves that exhibit transient growth, and (iii) the oblique waves interact nonlinearly with the plane wave to generate streamwise vortices; these in turn generate the sublayer streaks and so the cycle is renewed.