Wind‐speed limits in numerically simulated tornadoes with suction vortices

Abstract

Three-dimensional numerical simulations of tornado-like vortices are presented. Of particular interest is a regime with multiple suction vortices withn a larger cyclone. The simulations are designed to assess the magnitude and mechanism by which low pressure is sustained in a suction vortex. The pressure in the transient suction vortices can be nine times that supported by the buoyancy of the 'storm'. The low pressure is sustained by transient axial flow; the pressure-gradient force along the axis is balanced by the inertia of the axial flow, rather than by buoyancy.

Low-Level Mesocyclonic Concentration by Nonaxisymmetric Transport. Part II: Vorticity Dynamics

“…Since neither can be maintained to infinity, we must interpret them as conditions that at some level are maintained by the storm system aloft (e.g., Fiedler 1995). We note that the intense vortices of Wicker and Wilhelmson (1995) and Grasso and Cotton (1995) both were accompanied by strong vertical motion, possibly dynamically induced, near cloud base, and we speculate that this vertical motion may explain why their vortices, as well as many actual tornadoes, are stronger than ours.…”

“…We note that the intense vortices of Wicker and Wilhelmson (1995) and Grasso and Cotton (1995) both were accompanied by strong vertical motion, possibly dynamically induced, near cloud base, and we speculate that this vertical motion may explain why their vortices, as well as many actual tornadoes, are stronger than ours. Though we have emphasized that the vortex concentration described here occurs independently of conditions aloft, if the resultant pressure deficit exceeds that which would be consistent with the "thermodynamic speed limit" of the storm (Kessler 1970;Snow and Pauley 1984;Fiedler and Rotunno 1986;Walko 1988), then a downdraft will be induced that will eventually dissipate the vortex. That being stated, for a period of time the thermodynamic speed limit may not be a direct influence on the low-level dynamics, because the descent of the downdraft is not instantaneous (Fiedler 1998), and the downdraft's own translational motion may delay its interaction with the vortex center (Part I).…”

“…Vorticity is contoured every 0.02 s Ϫ1 , beginning with 0.005 s Ϫ1 . axisymmetric vortex models that generate the intense near-surface vertical motion characteristic of tornadoes (e.g., Burggraf et al 1971;Rotunno 1980;Howells et al 1988;Fiedler and Rotunno 1986;Nolan and Farrell 1999) generally include a concentrated region of vorticity surrounded by a large region of nearly irrotational flow. The process described in this study can help generate these conditions, and so may help generate the "parent" tornado vortex as described in Rotunno (1986).…”

Suction vortices and spiral breakdown in numerical simulations of tornado‐like vortices

“…Three‐dimensional simulations of laminar vortices under a perpetual buoyancy updraft were presented in Fiedler (1998). The vortices were offered as idealized tornadoes, from which a relationship between the environment and the maximum wind speed could be learned.…”

“…These simulations are being updated with modern computing power, for the purpose of further refining the knowledge of the factors that control maximum wind speed, and towards that purpose the recent developments in the concept of corner flow swirl ratio have been fruitfully applied. Simulations are quadruple in both resolution and Reynolds number, as compared with those used in Fiedler (1998). The current simulations commonly contain intense suction vortices capped by a spiral vortex breakdown.…”