Abstract
The structure of natural tornadoes and simulated analogs are sensitive to the lower boundary condition for friction. Three-dimensional numerical simulations of storms require a choice for turbulence parameterizations and resolution of wind near the lower boundary. This article explores some of the consequences of choices of a surface drag coefficient on the structure of a mature simulated tornado, using a conventional axisymmetric model. The surface drag parameterization is explored over the range of the semi-slip condition, including the extremes of no-slip and free-slip. A moderate semi-slip condition allows for an extreme pressure deficit, but without the unrealistic vortex breakdown of the no-slip condition.
Highlights
In [1], we find this statement about including surface drag in a storm model: “Our philosophy is that the inclusion of modest surface drag represents a more physically consistent bottom boundary condition for tornadogenesis than does the habitually employed free-slip assumption.”
This article attempts something simpler: documenting the effect of surface roughness on the mature tornadoes that can occur in storm models
The purpose of this article is to help diagnose the effect of semi-slip boundary conditions on larger, average features of a mature tornado, rather than extreme events
Summary
In [1], we find this statement about including surface drag in a storm model: “Our philosophy is that the inclusion of modest surface drag represents a more physically consistent bottom boundary condition for tornadogenesis than does the habitually employed free-slip assumption.” Several other recent storm modeling efforts have investigated surface drag in storm evolution and tornadogenesis [2,3,4]. This article attempts something simpler: documenting the effect of surface roughness on the mature tornadoes that can occur in storm models. In [5] we read: “surface friction paradoxically instigates the extreme pressure deficits and upward and rotary winds in end-wall tornadoes.”. This paradoxical effect has long been recognized and studied in models. Axisymmetric transient events did occur in the model simulations here, but are not documented. The purpose of this article is to help diagnose the effect of semi-slip boundary conditions on larger, average features of a mature tornado, rather than extreme events. We present the model results in a dimensionless form, to allow for wider generalization
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