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.

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.

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.

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.

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.

Abstract

A set of numerical simulations of supercell thunderstorms has been carried out with a range of low-level curvatures in the environmental hodograph and midlevel shears. They cover a range of hodograph “shape,” as measured by the integrated helicity of the lowest 3 km of the hodograph. The peak updraft occurs in the first hour of the storms and tends to be greater for larger values of environmental helicity. There is also a slight tendency for greater updraft intensity with lesser values of midlevel shear. Significantly, air in the core of the updrafts at midlevels (∼5 km) is not the most unstable air at the level. The most buoyant air rises in a region with a downward-directed pressure gradient force, which slows its ascent. Conversely, pressure gradient forces at lower levels (2–3 km) accelerate less buoyant air upward into the core of the midlevel updrafts. The pressure gradient force is larger in the cases with more curvature in the environmental wind than the low-curvature environments. This is consistent with predictions of the pressure gradient force derived from a simple Beltrami flow model of a rotating thunderstorm and a scale analysis.

Abstract

In this second paper in a series on outflow interactions, we use the three-dimensional model described in Part I to examine the effects of vertical wind shear variations on cloud development along intersecting thunderstorm outflow boundaries. Three wind shear profiles are used in this study: shear only above cloud base, shear only below cloud base, and shear both above and below cloud base. As in Part I, the shear is unidirectional and is oriented perpendicular to the line containing the two initial outflow-producing clouds (which are spaced 16 km apart). Using the environmental thermodynamic structure from the control simulation in Part I, we vary the shear magnitude in each profile and examine the properties of cloud development in the region where the two outflows collide (the outflow collision line or CL).

The model results show that the intensity and the time interval between successive cell updraft maxima of the first two clouds along the CL (both of which are triggered by the outflow collision) are controlled by the strength of the vertical wind shear. In strong shears, the upshear member of this pair of clouds has a head start in development and becomes the stronger cell of the two. The timing difference between these two clouds is a few minutes. In weaker shears, the two clouds grow at nearly the same rate, and therefore have similar intensities and a smaller timing difference. The presence of wind shear in the boundary layer is found to enhance the updrafts of these two cells in all cases.

The strength of the third and subsequent clouds which form along the CL is related to the speed at which the gust front moves away from the developing cells. The larger this separation speed, the more quickly the gust front-induced convergence is removed from the clouds, and thus the weaker they are. The third and subsequent cells along the CL are found to be more intense when shear is present in the cloud-bearing layer. The factors governing the timing difference of the third and successive cells to form along the outflow's leading edge are not clear at this time.

Abstract

One of the most recognizable features associated with a well-organized tropical system are spiral rainbands. These quasi-stationary rainbands often extend hundreds of kilometers from the storm center and have been well described in the literature. Observational studies have since identified additional banding structures, including outward-propagating small-scale spiral bands. These rainbands may have considerable implications for “core type” tornadoes, local wind maxima associated with downburst damage swaths, as well as a role in overall hurricane dynamics. As such, here a numerical simulation of Hurricane Opal (1995) is examined with unprecedented resolution necessary to capture these small-scale spiral bands. Opal was an intense landfalling hurricane that demonstrated small-scale spiral banding features analogous to those observational studies. The scale and characteristics of the simulated bands are consistent with observed small-scale spiral banding of intense hurricanes. A varietal of Kelvin–Helmholtz instability combined with boundary layer shear is offered as the most plausible dynamical mechanism for the generation and maintenance of these propagating bands outward of the eyewall region.

Abstract

A three-dimensional numerical simulation using a two-way interactive nested grid is to study tornado-genesis within a supercell. During a 40-minute period, two tornadoes grow and decay within the storm's mesocyclone. The tornadoes have life spans of approximately 10 minutes. Maximum ground-relative surface wind speeds exceed 60 m s⁻¹ during both tornadoes, and horizontal pressure gradients reach 18 hPa km⁻¹ during the second tornado. Comparison of the simulated storm evolution with Doppler and field observations of supercells and tornadoes shows many similar features.

Vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm. Tornadogenesis is initiated when mesocyclone rotation increase above cloud base. The increased rotation generates lower pressure in the mesocyclone, increasing the upward pressure gradient forces. The upward pressure gradient forces accelerate the vertical motions near cloud base, creating 20–30 m s⁻¹ updrafts at this level. As the updraft intensifies at cloud base, the convergence in the subcloud layer also increases rapidly. The vertical vorticity is the stretched in the convergent flow, creating the tornado vortex. Tornado decay begins when the vertical pressure gradient forces decrease or even reverse at cloud base, weakening the updraft above tornado. As the updraft weakens, the low-level flow advects the occlusion downdraft completely around the tornado, surrounding the vortex with downdraft and low-level divergence. Cut off from its source of positive vertical vorticity, the tornado then dissipates, leaving a broad low-level circulation behind.

Abstract

The Klemp–Wilhelmson three-dimensional numerical cloud model is used to investigate cloud development along intersecting thunderstorm outflow boundaries. The model initial environment is characterized by a temperature and moisture profile typically found in strong convective situations, and the initial wind field is prescribed by a constant unidirectional shear 2.9 m s⁻¹ km⁻¹ from 0.8 to 8.9 km, with a constant wind everywhere else. The wind shear vector is perpendicular to the line containing the two initial outflow-producing clouds (which are spaced 16 km apart and are triggered by thermal impulses centered at the top of the boundary layer).

The dynamics of the outflow collision are documented using time-dependent, kinematic air parcel trajectories and thermodynamic data. We find that ambient air in the outflow collision region is literally “squeezed” out of the way as the two outflows collide. Some of this air is lifted to saturation, triggering two convective clouds. The upshear member of the pair has a head start in development, and since the two clouds are growing close together and competing for the same air, the upshear cloud is the strongest. In addition to, the downshear cell is suppressed because it grows into the region occupied by the upshear cell's downdraft and rain region.

By looking at the various terms in the inviscid form of the vertical momentum equation, we find that low-level air approaching the gust front along the outflow collision line is forced to rise up and over the cold air pool due to a deflection by the pressure gradient force. A third cloud is triggered along the outflow collision line as a result of this frontal uplifting, which is in contrast to the first two cells which are triggered primarily by the forced uplifting from the outflow collision.

Air parcel trajectories indicate that even though the first two cells along the outflow collision line are triggered by a different mechanism than subsequent cells, the air comprising each updraft core is virtually undiluted, and comes from the same general region (z = 0 ∼ 0.3 km). On their way to the cloud updrafts, some low-level air parcels approaching the outflow cross the cold air interface. This is a manifestation of the well-known fact that the gust front is a region of turbulent mixing. Once above the outflow, these air parcels may pass through several updrafts and downdrafts as they traverse the cloud region.

The modeled clouds are found to be sensitive to the low-level (0–1 km) moisture. When the moisture in this layer is increased, the collision line clouds become stronger and the rapidity of new cell development increases markedly. Decreasing the low-level moisture has the opposite effect, to the point that only weak shallow clouds form along the outflow collision line. Furthermore, a decrease in the low-level moisture is accompanied by a decrease in the outflow temperature deficit. This in turn decreases the outflow speed, a result that is consistent with classical inviscid density current theory.