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Robert B. Wilhelmson

Abstract

A review is given of six equations that are used to approximate reversible saturated parcel ascent. These approximations are compared with the aid of several examples and their appropriateness for use in modeling deep clouds discussed.

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Robert B. Wilhelmson
and
Joseph B. Klemp

Abstract

A three-dimensional numerical storm model is used to investigate the observed splitting of several reflectivity echoes on 3 April 1964 in Oklahoma. Representative soundings from this day exhibit a nearly one-directional environmental wind shear vector and the presence of strong low-level wind shear. In the numerical simulation an initial cloud splits into two long-lived rotating storms, one that moves to the left of the mean winds and the other to the right. The left-moving storm develops more slowly than the right-moving one due to the deviation of the environmental wind hodograph from a straight line below 1 km. Further, the left mover eventually splits. Convergence induced by the cold, low-level storm outflow plays a major role in the development of both the first and second splits. However, the second split appears to be dynamically different than the first as the left-moving updraft remains essentially unchanged while a new updraft forms immediately adjacent to it. Because of the different propagational characteristics of the new storm it separates from the left mover. As the left-and right-moving storms move apart, new clouds develop in between them along an expanding cold outflow boundary. In this manner the evolving storm configuration becomes similar to that of a squall line, but has evolved from a single convective cell in the absence of imposed convergence. A comparison of the simulation with observed reflectivity and surface data reveals sufficient similarity to suggest that the explanations for the model storm development also may apply to some of the observed events.

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Robert B. Wilhelmson
and
Joseph B. Klemp

Abstract

We have used a three-dimensional cloud model to investigate the splitting of an initially isolated storm in a one-directional east-west shear. The simulated evolution of storm splitting in some cases follows all four stages suggested by Achtemeier (1969) after analysis of radar data, including the development of two self-sustaining storm. One of these storms moves to the right of the mean wind vector and the other to the left. In the right-moving storm the updraft rotates cyclonically and the downdraft anticyclonically, forming a vortex pair, as depicted in the schematic model of Fankhauser (1971). The vortex pair structure is also similar to that observed with Doppler radar and analyzed by Ray (1976). The downdraft-induced gust front interacts with the low-level environmental wind to produce the convergence necessary to sustain the storm. This convergence extends to the south and west of the storm, and if enough low-level moisture is available a flanking line develops. The distribution of rainwater within the updraft suggests the existence of an over-hang and book typically observed in severe storms.

To understand when splitting might occur the strength and distribution of the vertical wind shear were varied. The various simulations suggest that strong shear at and just above cloud base is important for the splitting process to be successful. For splitting to occur the low-level inflow from the cast in our simulations must be sufficiently strong to inhibit the propagation of the gust front toward the cast. If the gust front (or wind shift line) can propagate away from the storm toward the cast, the region of low-level convergence moves away from the storm and initial splitting in the lower updraft cannot he sustained. Further, without the precipitation-induced downdraft and associated low-level outflow splitting does not occur.

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Joseph B. Klemp
and
Robert B. Wilhelmson

Abstract

A new three-dimensional cloud model has been developed for investigating the dynamic character of convective storms. This model solves the compressible equations of motion using a splitting procedure which provides numerical efficiency by treating the sound wave modes separately. For the subgrid turbulence processes, a time-dependent turbulence energy equation is solved which depends on local buoyancy, shear and dissipation. First-order closure is applied to nearly conservative variables with eddy coefficients based on the computed turbulence energy. Open lateral boundaries are incorporated in the model that respond to internal forcing and permit gravity waves to propagate out of the integration domain with little apparent reflection. Microphysical processes are included in the model using a Kessler-type parameterization. Simulations conducted for an unsheared environment reveal that the updraft temperatures follow a moist adiabatic lapse rate and that the convection is dissipated by water loading of the updraft. The influence of a one-directional shear on the storm development is also investigated. A simulation with a veering and backing wind profile exhibits interesting features which include a double vortex circulation, cell splitting and, secondary cell formation.

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Joseph B. Klemp
and
Robert B. Wilhelmson

Abstract

Using a three-dimensional numerical cloud model, self-sustaining right- and left-moving storms are simulated which arise through splitting of the original storm. The right-moving storm develops a structure which bears strong resemblance to Browning's (1964) conceptual model, while the left-moving storm has mirror image characteristics. By altering the direction of the environmental shear at low and middle levels, either the right- or the left-moving storm can be selectively enhanced. Specifically, if the wind hodograph turns clockwise with height, a single right-moving storm envolves from the splitting process. Conversely, counterclockwise turning of the hodograph favors development of the left-moving storm.

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Adam L. Houston
and
Robert B. Wilhelmson

Abstract

The sensitivity of storm longevity to the pattern of deep convection initiation (e.g., multiple, quasi-linearly arranged initial deep convective cells versus an isolated deep convective cell) is examined using idealized cloud-resolving simulations conducted with a low-shear initial environment. When multiple deep convective cells are initialized in close proximity to one another using either a line of thermals or a shallow airmass boundary, long-lived storms are produced. However, when isolated deep convection is initiated, the resultant storm steadily decays following initiation. These results illustrate that a quasi-linear mechanism, such as a preexisting airmass boundary, that initiates multiple deep convective cells in close proximity can lead to longer-lived storms than a mechanism that initiates isolated deep convection.

The essential difference between the experiments conducted is that an isolated initial storm produces a shallower cold pool than when a quasi-linear initiation is used. It is argued that the deep cold pools promote deep forced ascent, systematic convective cell redevelopment, and thus long-lived storms, even in environments with small values of vertical shear. The difference in cold pool depth between the simulations is attributed to differences in the horizontal flux of cold air to the gust front. With a single initial storm, the few convective cells that subsequently form provide only a limited source of cold air, leading to a cold pool that is shallow and incapable of fostering continued updraft redevelopment.

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Brian F. Jewett
and
Robert B. Wilhelmson

Abstract

This study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines. The forcing chosen was a cold front, simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front. Use of a realistic, balanced, and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing. Three-dimensional squall lines were modeled with and without the front present, in low and high bulk Richardson number environments. The forced convection evolved in significantly different ways than their isolated, unforced counterparts. In low-shear conditions, the line of isolated convective cells split, with the adjacent split cells interfering destructively with neighboring cells in the line. With forcing present, differences in anticyclonic cell intensity and propagation prevented this interaction from occurring, leading to longer-lived cyclonic convection despite a near-normal orientation between cloud-bearing shear and the convective line. The split-cell interaction also failed to occur under higher-shear conditions due to anticyclonic cell decay given the greater cyclonic hodograph curvature. In both low- and higher-shear environments, a strong bias toward cyclonic storms was noted with forcing present, due to shallower anticyclonic cells with the front present and to preexisting vorticity in the environment; updraft–vorticity correlations were skewed accordingly. Forcing also reduced the sensitivity of the evolving convection to detailed aspects of the initialization.

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Adam L. Houston
and
Robert B. Wilhelmson

Abstract

A suite of experiments conducted using a cloud-resolving model is examined to assess the role that preexisting airmass boundaries can play in regulating storm propagation. The 27 May 1997 central Texas tornadic event is used to guide these experiments. The environment of this event was characterized by multiple preexisting airmass boundaries, large CAPE, and weak vertical shear.

Only the experiments with preexisting airmass boundaries produce back-building storm propagation (storm motion in opposition to the mean wind). When both the cold front and dryline are present, storm maintenance occurs through the quasi-continuous maintenance of a set of long-lived updrafts and not through discrete updraft redevelopment. Since the cold front is not required for back building, it is clear that back building in this environment does not require quasi-continuous updraft maintenance. The back-building storm simulated with both the cold front and dryline is found to be anchored to the boundary zipper (the intersection of the cold front and dryline). However, multiple preexisting airmass boundaries are not required for back building since experiments with only a dryline also support back building. A conceptual model of back building and boundary zippering is developed that highlights the important role that preexisting boundaries can play in back-building propagation.

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Adam L. Houston
and
Robert B. Wilhelmson

Abstract

A detailed analysis of the 27 May 1997 central Texas tornadic storm complex is undertaken in an attempt to document the prestorm environment and identify the roles played by preexisting boundaries on storm maintenance/propagation and rotation. Analysis is carried out using a broad suite of synoptic and subsynoptic data but focuses on the level-II and -III Weather Surveillance Radar-1988 Doppler (WSR-88D) data from three Texas radars. The 27 May 1997 event was characterized by a back-building (propagation against the mean flow) storm complex that produced at least 12 tornadoes including the F5 Jarrell, Texas, tornado. Furthermore, five of the eight longest-lived cells during the analysis period are shown to contain midlevel mesocyclones. However, one-dimensional metrics calculated using representative vertical profiles of state variables reveal that, despite the extreme values of CAPE in place (>6500 J kg−1), the (1D) environment associated with this event is best classified as only marginally favorable for supercells and unfavorable for significant, supercellular tornadoes. Furthermore, the observed wind shear values are shown to be more in line with the vertical shear values typically associated with nonsevere back-building storms. Examples of propagation controlled by quasi-continuous maintenance of a single cell as well as successive discrete cell redevelopment are found. In all situations, two preexisting boundaries in place during this event (a cold front and a dryline) are found to have been necessary for the maintenance/propagation of the storm complex. Specifically, it is argued that the “zippering” of the cold front and dryline (the collision of the dryline and cold front that allowed the cold front to overtake the dryline and penetrate into the most unstable air to the east) was essential for the back-building of the storm complex in this event since it resulted in new cell development at points farther south. While midlevel mesocyclones were present in five of the eight longest-lived and well-sampled cells, analysis of the relationship between observed cell motion, expected cell motion, expected supercellular deviation, and boundary motion for the longest-lived and well-sampled cells reveals little evidence that deviate motions generated through supercellular dynamics governed cell motions. Instead, it is shown that boundary motions largely controlled the propagation of individual cells.

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Adam L. Houston
and
Robert B. Wilhelmson

Abstract

The 27 May 1997 central Texas tornadic event has been investigated in a two-part observational study. As demonstrated in Part I, the 1D environment associated with this event was unfavorable for significant (≥F2) tornadoes. Yet, the storm complex produced at least six significant tornadoes, including one rated F5 (the Jarrell, Texas, tornado). The purpose of this article is to examine the spatiotemporal interrelationships between tornadoes, preexisting boundaries, antecedent low-level mesocyclones, convective cells, and midlevel mesocyclones. It is shown that each of the six observed tornadoes that produced greater than F0 damage formed along the storm-generated gust front, not along preexisting boundaries. Half of these tornadoes formed on the distorted gust front, the portion of the storm-generated gust front whose orientation was deformed largely by the horizontal shear across the cold front. The remaining three tornadoes developed at the gust front cusp (the persistent gust front inflection located at the northeast end of the gust front distortion). Unlike the tornadoes south of the gust front cusp, these tornadoes are found to be associated with antecedent mesocyclones located in the low levels above the boundary layer. Furthermore, these mesocyclonic tornadoes are found to be larger and more destructive than the three nonmesocyclonic tornadoes. The formation of the Jarrell tornado is found to occur as a nearly stationary convective cell became collocated with a south-southwestward-moving low-level mesocyclone near the gust front cusp—a behavior that resembles the formation of nonsupercell tornadoes. It is argued that the back-building propagation/maintenance of the storm complex enabled this juxtaposition of convective cells with vorticity along the distorted gust front and may have therefore enabled tornado formation. Each of the convective cells without midlevel mesocyclones was found to remain farther from the boundaries than the mesocyclonic cells. Since the cells nearest to the boundaries were longer lived than the remaining cells, it is argued that cells near the boundaries were mesocyclonic because the boundaries yielded cells that were more likely to support temporally coherent midlevel rotation.

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