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Paul Markowski and Christina Hannon

Abstract

Overdetermined, dual-Doppler wind syntheses are used to document the evolution, structure, and dynamics of vertical vorticity extrema observed in a convective boundary layer during the 12 June 2002 International H2O Project (IHOP) mission. Discrete vertical vorticity extrema having horizontal scales of 1–2 km can be observed continuously for periods exceeding an hour. The evolution of the vorticity field is characterized by complex interactions among vorticity extrema and between the vertical vorticity and vertical velocity fields. The most prominent vorticity maxima have amplitudes of approximately 0.01 s−1 and are associated with retrieved pressure deficits of order 0.1 mb. The vorticity extrema weaken with height and tilt in the presence of vertical wind shear. Advection and propagation both contribute substantially to the motion of the vorticity extrema.

Amplifications of vertical vorticity are closely linked to the intensification of updrafts. Both stretching and tilting can contribute significantly to the vorticity budgets of the air parcels comprising the vorticity extrema, and their relative importance varies with elevation, evolutionary stage, and from one vorticity extremum to another. It is therefore difficult to generalize about the dynamics of the vorticity extrema. It also is difficult to generalize about the helicity of the vorticity maxima and suppression of mixing for similar reasons. The weakening of vertical vorticity extrema is closely tied to the weakening of updrafts. In some cases, downward-directed vertical pressure gradient forces due to vertical gradients of rotation bring about updraft weakening and vorticity demise. An improved understanding of the nature of boundary layer vortices could have large relevance to convection initiation owing to feedbacks between vertical velocity and vorticity.

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Paul Markowski and Yvette Richardson

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Dual-Doppler wind syntheses from mobile radar observations obtained during the International H2O Project document some of the spatial variability of vertical wind profiles in convective boundary layers. Much of the variability of popular forecasting parameters such as vertical wind shear magnitude and storm-relative helicity is thought to result from pressure and temperature gradients associated with mesoscale boundaries (e.g., drylines, outflow boundaries, fronts). These analyses also reveal substantial heterogeneity even in the absence of obvious mesoscale wind shifts—in regions many might have classified as “horizontally homogeneous” with respect to these parameters in the past. This heterogeneity is closely linked to kinematic perturbations associated with boundary layer convection. When a mean wind is present, the large spatial variability implies significant temporal variability in the vertical wind profiles observed at fixed locations, with the temporal variability increasing with mean wind speed. Significant differences also can arise between true hodographs and “pseudohodographs” obtained from rawinsondes that are advected horizontally as they ascend. Some possible implications of the observed heterogeneity with respect to forecasting and simulating convective storms also are discussed.

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Paul M. Markowski

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Idealized simulations are used to investigate the contributions of frictionally generated horizontal vorticity to the development of near-surface vertical vorticity in supercell storms. Of interest is the relative importance of barotropic vorticity (vorticity present in the prestorm environment), baroclinic vorticity (vorticity that is principally generated by horizontal buoyancy gradients), and viscous vorticity (vorticity that originates from the subgrid-scale turbulence parameterization, wherein the effects of surface drag reside), all of which can be advected, tilted, and stretched. Equations for the three partial vorticities are integrated in parallel with the model. The partial vorticity calculations are complemented by analyses of circulation following material circuits, which are often able to be carried out further in time because they are less susceptible to explosive error growth.

Near-surface mesocyclones that develop prior to cold-pool formation (this only happens when the environmental vorticity is crosswise near the surface) are dominated by only barotropic vertical vorticity when the lower boundary is free slip, but both barotropic and viscous vertical vorticity when surface drag is included. Baroclinic vertical vorticity grows large once a cold pool is established, regardless of the lower boundary condition and, in fact, dominates at the time the vortices are most intense in all but one simulation (a simulation dominated early by a barotropic mode of vortex genesis that may not be relevant to real convective storms).

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Jeffrey Frame and Paul Markowski

Abstract

Numerical simulations of squall lines traversing sinusoidal mountain ridges are performed using the Advanced Regional Prediction System cloud-resolving model. Precipitation and updraft strength are enhanced through orographic ascent as a squall line approaches a ridge. The simulated squall line then weakens as it descends the ridge because some of the cold pool is blocked by the terrain, resulting in less lift along the gust front and weaker convective cells. The flow within the cold pool accelerates slightly and the depth of the cold air decreases owing to upstream blocking, transitioning the flow in the cold pool head from subcritical to supercritical, then back to subcritical at the bottom of the ridge. A hydraulic jump forms when the flow transitions the second time, enabling the development of a new convective line downwind of the mountain. These new updrafts grow and eventually replace the older updrafts that weakened during descent. This process results in the discrete propagation of a squall line just downstream of a ridge, resulting in the formation of rain shadows downstream from topographic features. Discrete propagation only occurs if a ridge is of sufficient height, however. This replacement process repeats itself if a squall line encounters multiple ridges. The risk of damaging winds from a squall line is greater on the lee side of ridges and on the top of high ridges. These terrain-forced intensity fluctuations increase with mountain height, because the higher terrain permits even less cold air to flow over it. A wider ridge results in a more gradual orographic enhancement and downslope-induced weakening.

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Jeffrey Frame and Paul Markowski

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Numerical simulations of supercell thunderstorms that include parameterized radiative transfer and surface fluxes are performed using the Advanced Regional Prediction System (ARPS) to investigate the effects of anvil shadows on the near-storm environment. If the simulated storm is nearly stationary, the maximum low-level air temperature deficits within the shadows are about 2 K, which is roughly half the cooling found in some previous observations. It is shown that the extinction of downwelling shortwave radiation by the anvil cloud creates a differential in the flux of downwelling shortwave radiation between the sun and the shade that is at least an order of magnitude greater than the differential of any other term in either the surface radiation or the surface energy budgets. The loss of strong solar heating of the model surface within the shaded regions leads to a reduction of surface temperatures and stabilization of the model surface layer beneath the anvil. The reduction in vertical mixing results in a shallow, strongly vertically sheared layer near the surface and calmer near-surface winds, which are limited to regions in the anvil shadow. This difference in radiative heating is shown not to affect the vertical thermodynamic or wind profiles above the near-surface layer (approximately the lowest 500 m). It is also found that these results are highly sensitive to the magnitude of the near-surface winds. If the initial hodograph is shifted such that the simulated storm acquires a substantial eastward propagation speed, the temperature deficit within the shadow is greatly diminished. This is due to both a weaker surface sensible heat flux and less time during which surface cooling and boundary layer stabilization can occur beneath the anvil.

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Paul Markowski and Yvette Richardson

Abstract

Vertical wind shear is commonly classified as “directional” or “speed” shear. In this note, these classifications are reviewed and their relevance discussed with respect to the dynamics of convective storms. In the absence of surface drag, storm morphology and evolution only depend on the shape and length of a hodograph, on which the storm-relative winds depend; that is, storm characteristics are independent of the translation and rotation of a hodograph. Therefore, traditional definitions of directional and speed shear are most relevant when applied to the storm-relative wind profile.

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Alan Shapiro and Paul Markowski

Abstract

Theoretical hydrodynamic models for the behavior of vortices with axially varying rotation rates are presented. The flows are inviscid, axisymmetric, and incompressible. Two flow classes are considered: (i) radially unbounded solid body–type vortices and (ii) vortex cores of finite radius embedded within radially decaying vortex profiles.

For radially unbounded solid body–type vortices with axially varying rotation rates, the von Kármán–Bödewadt similarity principle is applicable and leads to exact nonlinear solutions of the Euler equations. A vortex overlying nonrotating fluid, a vortex overlying a vortex of different strength, and more generally, a vortex with N horizontal layers of different rotation rate are considered. These vortices cannot exist in a steady state because continuity of pressure across the horizontal interface between the vortex layers demands that a secondary (meridional) circulation be generated. These similarity solutions are characterized by radial and azimuthal velocity fields that increase with radius and a vertical velocity field that is independent of radius. These solutions describe nonlinear interactions between the vortex circulations and the vortex-induced secondary circulations, and may play a role in the dynamics of the interior regions of broad mesoscale vortices. Decaying, amplifying, and oscillatory solutions are found for different vertical boundary conditions and axial distributions of vorticity. The oscillatory solutions are characterized by pulsations of vortex strength in lower and upper levels associated with periodic reversals in the sense of the secondary circulation. These solutions provide simple illustrations of the “vortex valve effect,” sometimes used to explain cyclic changes in updraft and rotation strength in tornadic storms.

A linear analysis of the Euler equations is used to describe the short-time behavior of an elevated vortex of finite radius embedded within a radially decaying vortex profile (i.e., elevated Rankine-type vortices). The linear solution describes the formation of a central updraft (as in the similarity solution) and an annular downdraft ringing the periphery of the vortex core (not accounted for in the similarity solution). Downdraft strength is sensitive to both the vortex core aspect ratio and outer vortex decay rate, being stronger and narrower for broader vortices and larger decay rates. It is hypothesized that this dynamically induced downdraft may facilitate the transport of mesocyclone vorticity down to low levels in supercell thunderstorms.

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Branden Katona and Paul Markowski

Abstract

Storms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective storm environments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

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