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Robert P. Davies-Jones

Abstract

The area effectively covered by dual-Doppler radars is presented as a function of spatioal resolution and accuracy of horizontal velocity measurement. Implications of this relationship for the spacing of a dual-Doppler network tornadic storm research are discussed.

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Robert P. Davies-Jones

Abstract

Four soundings taken in high-speed updrafts of severe thunderstorms indicate moist adiabatic ascent to mid-levels. This implies that the cores of strong updrafts are undiluted by environmental air. The Squires-Turner entraining jet model substantiates this conclusion. Comparison of model and observed vertical velocities indicates in one case that actual velocities may be reduced by adverse pressure gradient forces which are not included in the model. However, the data from the other cases are not sufficient to verify this hypothesis.

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Robert P. Davies-Jones

Abstract

The nondimensional radius of the turbulent core in Ward's laboratory tornado model is shown to be primarily a function of swirl ratio alone. Thus, for given circulation and updraft radius, high-volume flow rate (but not necessarily high momentum flux) is required for the production of concentrated vortices.

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Robert P. Davies-Jones

Abstract

A theoretical approximation for condensation temperature is obtained. The approximate solution lies within 0.01 K of the corresponding iterative solution for dewpoint depressions up to 40 K. This approximation error is less than the uncertainty resulting from use of an empirical formula for saturation vapor pressure and the assumption that cp is constant for dry air.

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Robert P. Davies-Jones
and
Vincent T. Wood

Abstract

Exact solutions of the Navier–Stokes or Euler equations of motion and the continuity equation in cylindrical coordinates for 3D, axisymmetric, inviscid, or laminar flows are utilized to represent evolving vortices that roughly model tornado cyclones or misocyclones contracting to tornadoes. These solutions are unsteady versions of the diffusive Burgers–Rott vortex and the inviscid Rankine-combined vortex. They satisfy the free-slip condition at the ground. Different vortices are obtained by choosing different values of the constant eddy viscosity and uniform horizontal convergence while holding the circulation at infinity constant. A simulated Weather Surveillance Radar-1988 Doppler (WSR-88D) is employed to generate time-varying Doppler velocity signatures in uniform reflectivity of these analytical vortices at ranges of 25 and 50 km from the radar. Mean Doppler velocities are determined by computing 3D integrals over effective resolution volumes. Magnitudes of Doppler vortex signatures at different times in the evolution of the stationary vortices are computed for effective beamwidths of 1.02° and 1.39°, which correspond to azimuthal sampling intervals of 0.5° and 1.0°, respectively. Four tornado predictors—rotational velocity, shear, excess rotational kinetic energy, and circulation—are examined.

Results of the simulations show that for smaller effective beamwidths, Doppler vortex signatures are stronger and exceed fixed threshold values of rotational velocity and shear earlier. With finer azimuthal resolution, tornado cyclone, misocyclone, or tornado signatures switch to tornadic vortex signatures later. Circulations of the vortex signatures give good estimates of the circulations of the simulated tornadoes and tornado cyclones with relative insensitivity to range, effective beamwidth, and stage of evolution. High circulation and convergence values of a rotation signature reveal the potential for a tornado earlier than all the other predictors, which increase significantly during tornadogenesis.

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Yvette P. Richardson
,
Kelvin K. Droegemeier
, and
Robert P. Davies-Jones

Abstract

Severe convective storms are typically simulated using either an idealized, horizontally homogeneous environment (i.e., single sounding) or an inhomogeneous environment constructed using numerous types of observations. Representing opposite ends of the spectrum, the former allows for the study of storm dynamics without the complicating effects of either land surface or atmospheric variability, though arguably at the expense of physical realism, while the latter is especially useful for prediction and data sensitivity studies, though because of its physical completeness, determination of cause can be extremely difficult. In this study, the gap between these two extremes is bridged by specifying horizontal variations in environmental vertical shear in an idealized, controlled manner so that their influence on storm morphology can be readily diagnosed. Simulations are performed using the Advanced Regional Prediction System (ARPS), though with significant modification to accommodate the analytically specified environmental fields. Several steady-state environments are constructed herein that retain a good degree of physical realism while permitting clear interpretation of cause and effect. These experiments are compared to counterpart control simulations in homogeneous environments constructed using single wind profiles from selected locations within the inhomogeneous environment domain. Simulations in which steady-state vertical shear varies spatially are presented for different shear regimes (storm types). A gradient of weak shear across the storm system leads to preferred cell development on the flank with greater shear. In a stronger shear regime (i.e., in the borderline multicell/supercell regime), however, cell development is enhanced on the weaker shear flank while cell organization is enhanced on the strong shear side. When an entire storm system moves from weak to strong shear, changes in cell structure are influenced by local mesoscale forcing associated with the cold pool. In this particular experiment, cells near the leading edge of the cold pool, where gust front convergence occurs along a continuous line, evolve into a bow-echo structure as the shear increases. In contrast, simulated cells that remain relatively isolated on the flank of the cold pool tend to develop supercellular characteristics.

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Edward A. Brandes
,
Robert P. Davies-Jones
, and
Brenda C. Johnson

Abstract

The structure and steadiness of radar-observed supercell thunderstorms are examined in terms of their particular distribution of vorticity. The data confirm that the vorticity vector in supercells points in the direction of the storm-relative velocity vector and that supercell updrafts contain large positive helicity (V·ω). The alignment of vorticity and velocity vectors dictates that low pressure associates not only with vorticity but also with helicity. Accelerating pressure gradients and helicity, both thought important for suppressing small-scale features within supercells, may combine with shear-induced vertical pressure gradient forces to organize and maintain the large-scale persistent background updrafts that characterize supercells.

Rear downdrafts possess weak positive or negative helicity. Thus, the decline of storm circulation may be hastened by turbulent dissipation when the downdraft air eventually mixes into supercell updrafts.

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Vincent T. Wood
,
Robert P. Davies-Jones
, and
Alan Shapiro

Abstract

Single-Doppler radar data are often missing in important regions of a severe storm due to low return power, low signal-to-noise ratio, ground clutter associated with normal and anomalous propagation, and missing radials associated with partial or total beam blockage. Missing data impact the ability of WSR-88D algorithms to detect severe weather. To aid the algorithms, we develop a variational technique that fills in Doppler velocity data voids smoothly by minimizing Doppler velocity gradients while not modifying good data. This method provides estimates of the analyzed variable in data voids without creating extrema. Actual single-Doppler radar data of four tornadoes are used to demonstrate the variational algorithm. In two cases, data are missing in the original data, and in the other two, data are voided artificially. The filled-in data match the voided data well in smoothly varying Doppler velocity fields. Near singularities such as tornadic vortex signatures, the match is poor as anticipated. The algorithm does not create any velocity peaks in the former data voids, thus preventing false triggering of tornado warnings. Doppler circulation is used herein as a far-field tornado detection and advance-warning parameter. In almost all cases, the measured circulation is quite insensitive to the data that have been voided and then filled. The tornado threat is still apparent.

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Howard B. Bluestein
,
Eugene W. McCaul Jr.
,
Gregory P. Byrd
,
Robert L. Walko
, and
Robert Davies-Jones

Abstract

This is a case study of deep, but narrow convective towers which split twice into right- and left-moving components in southwestern Oklahoma on 28 May 1985. Our analysis makes use of storm-intercept visual documentation, mobile soundings, surface mesonetwork data, and frequent soundings from special sites. The data show that the convective towers behaved in many respects like low-precipitation storms, having formed in an environment of large CAPE and moderately strong unidirectional shear. The observation of towers splitting even when there is no heavy precipitation at the surface implies that rain processes are not crucial to the splitting phenomenon. The tiny storms were confined to a region northeast of a surface cyclone and low-pressure area, near the intersection of the dryline and an old outflow boundary, where convective temperature was reached. Evidence is presented that the moist layer was deepened locally just prior to convective initiation, and that the deepening was related to low-level convergence associated with the westward motion of the dryline.

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Steven V. Vasiloff
,
Edward A. Brandes
,
Robert P. Davies-Jones
, and
Peter S. Ray

Abstract

Nearly 2½ hours of dual-Doppler radar data with high temporal and spatial resolution are used to examine the evolution and morphology of a thunderstorm that evolved from a complex of small cells into a supercell storm. Individual storm cells and updrafts moved east-northeastward, nearly with the mean wind, while the storm complex, which encompassed the individual cells, propagated toward the south–southeast. Cells were first detected at middle levels (5–10 km) on the storm's right flank and dissipated on the left flank. Generally, the storm contained three cells—a forming cell, a mature cell, and a dissipating cell; life stages were apparently dictated by the source of updraft air. During the growth stage, cell inflow had a southerly component. As the cell moved through the storm complex, it started ingesting stable air from the north and soon dissipated.

A storm-environment feedback mechanism of updraft–downdraft interactions, in conjunction with increasing environmental vertical wind shear and buoyancy, is deemed responsible for an increase in the size and intensity of successive cells and updrafts. With time, a large region of background updraft, containing the updrafts of individual cells, formed on the storm's right flank. Unlike the individual cells, which moved nearly parallel to the mean wind and low-level shear vector, the region of background updraft moved to the right of the mean wind and low-level shear vector. It is believed that the formation and rightward motion of the background updraft region led to strong rotation on the storm's right flank. The larger cell and updraft size, with the same center-to-center spacing as at earlier times, made individual cell identification difficult, resulting in a nearly steady-state reflectivity structure.

The data support a growing consensus that a continuum of storm types, rather than a dichotomy, exists.

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