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Matthew D. Parker

Abstract

Organized convection has long been recognized to have a nocturnal maximum over the central United States. The present study uses idealized numerical simulations to investigate the mechanisms for the maintenance, propagation, and evolution of nocturnal-like convective systems. As a litmus test for the basic governing dynamics, the experiments use horizontally homogeneous initial conditions (i.e., they include neither fronts nor low-level jet streams).

The simulated storms are allowed to mature as surface-based convective systems before the boundary layer is cooled. In this case it is then surprisingly difficult to cut the mature convective systems off from their source of near-surface inflow parcels. Even when 10 K of the low-level cooling has been applied, the preexisting system cold pool is sufficient to lift boundary layer parcels to their levels of free convection. The present results suggest that many of the nocturnal convective systems that were previously thought to be elevated may actually be surface based. With additional cooling, the simulated systems do, indeed, become elevated. First, the CAPE of the near-surface air goes to zero: second, as the cold pool’s temperature deficit vanishes, the lifting mechanism evolves toward a bore atop the nocturnal inversion. Provided that air above the inversion has CAPE, the system then survives and begins to move at the characteristic speed of the bore. Interestingly, as the preconvective environment is cooled and approaches the temperature of the convective outflow, but before the system becomes elevated, yet another distinct behavior emerges. The comparatively weaker cold pool entails slower system motion but also more intense lifting, apparently because it is more nearly balanced by the lower-tropospheric shear. This could explain the frequent observation of intensifying convective systems in the evening hours without the need for a nocturnal low-level jet. The governing dynamics of the simulated systems, as well as the behavior of low-level tracers and parcel trajectories, are addressed for a variety of environments and degrees of stabilization.

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Matthew D. Parker

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This article is the second of two describing convective lines with parallel stratiform (PS) precipitation. The PS mode appears to be the preferred organizational structure in environments with line-parallel vertical wind shear. This paper presents a detailed analysis of the processes that lead to the development of the PS structure within line-parallel shear, and the positive and negative feedbacks associated with the mature PS structure. As well, the particular importance of line-perpendicular and line-parallel wind shear, line-end effects, inertial stability, and patterns of convective initiation are investigated through a battery of sensitivity tests.

Convective lines with PS precipitation develop in environments with both significant line-perpendicular and line-parallel vertical wind shear. Although the studied environments are initially supportive of supercells, the merging of outflows soon renders a predominant linear forcing and the characteristic PS structure. The systems’ linearity in the presence of along-line wind shear makes the local wind field more dependent upon the mesoscale structure of the convective system. For example, the along-line transport of hydrometeors is required for the development of a line-parallel precipitation region, and yet this transport does not occur immediately down the convective line’s axis because it is interrupted by the pressure maxima associated with other convective cells that are farther down the line. However, the along-line flow within the line’s leading and trailing anvils is able to contribute substantially because there are along-line pressure gradient accelerations associated with the tilted mesoscale structure of the system’s buoyancy field.

This paper concludes the study by synthesizing its dynamical and sensitivity analyses with the overarching structures described in the companion article, yielding perhaps the first consolidated view of these little-studied systems.

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Matthew D. Parker

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Adiabatic lapse rates appear to be a common feature in the lower troposphere on tornado days. This article reviews physical reasons why lapse rates may influence surface vortex intensification and reports on numerical simulations designed to study the key processes. In the idealized numerical model, an initial mesocyclone-like vortex and nonvarying convection-like heat source are used in different environmental stability profiles. The scales of interest in these simulations typify those of a parent supercell, and the developing circulations constitute direct responses to the imposed heating.

Downward parcel displacements are needed for surface vortex development in environments with no preexisting surface vorticity. In the simulations, under neutral stratification there is strong heating-induced subsidence anchored near the storm edge, whereas under stable stratification there are instead gravity waves that propagate away to the far field. In addition, under weak or neutral low-level stratification there is very little resistance to downward parcel displacements. In the simulations, these two effects combine to bring high angular momentum air from aloft downward to the surface under neutral lapse rates; this in turn leads to surface vortex genesis, even without precipitation processes. When the lower troposphere is stable, surface vortex intensification is only simulated when there is already preexisting vertical vorticity at the ground. When the initial vortex is elevated (vertical vorticity falls off to zero above the ground), surface vortex intensification is only simulated under neutral low-level stability. The results are interpreted within the controlled experimental framework, after which the possible ramifications to processes in real storms are discussed.

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Matthew D. Parker

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In recent years there has been debate about whether squall lines have an “optimal state.” It has been repeatedly demonstrated that the slope of a squall line’s convective region is related to the comparative magnitudes of the squall line’s cold pool and the base-state vertical wind shear. The present work addresses a related assertion, that squall-line intensity ought to be maximized for an upright updraft zone. A simple demonstration shows that upright systems realize more of their buoyancy because their attendant downward-directed perturbation pressure gradient accelerations are weaker.

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Matthew D. Parker

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Three-dimensional composite analyses using 134 soundings from the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) reveal the nature of near-storm variability in the environments of supercell thunderstorms. Based upon the full analysis, it appears that vertical wind shear increases as one approaches a supercell within the inflow sector, providing favorable conditions for supercell maintenance (and possibly tornado formation) despite small amounts of low-level cooling near the storm. The seven analyzed tornadic supercells have a composite environment that is clearly more impressive (in terms of widely used metrics) than that of the five analyzed nontornadic supercells, including more convective available potential energy (CAPE), more vertical wind shear, higher boundary layer relative humidity, and lower tropospheric horizontal vorticity that is more streamwise in the near-storm inflow. The widely used supercell composite parameter (SCP) and significant tornado parameter (STP) summarize these differences well. Comparison of composite environments from early versus late in supercells' lifetimes reveals only subtle signs of storm-induced environmental modification, but potentially important changes associated with the evening transition toward a cooler and moister boundary layer with enhanced low-level vertical shear. Finally, although this study focused primarily on the composite inflow environment, it is intriguing that the outflows sampled by VORTEX2 soundings were surprisingly shallow (generally ≤500 m deep) and retained considerable CAPE (generally ≥1000 J kg−1). The numerous VORTEX2 near-storm soundings provide an unprecedented observational view of supercell–environment interactions, and the analyses are ripe for use in a variety of future studies.

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Matthew D. Parker

Abstract

This article, the first of two describing convective lines with parallel stratiform (PS) precipitation, addresses the basic kinematic and precipitation features of these systems. The PS mode appears to be the preferred organizational structure in environments with line-parallel vertical wind shear. This archetype for long-lived convective systems has received relatively little attention to date, and yet it is frequently implicated in flash flooding because it entails both the along-line movement of hydrometeors and back-building convective development. As a reality check, this paper presents conventional observations of the wind and reflectivity fields associated with an archetypal PS system from 2 May 1997. Thereafter, analyses of idealized numerical simulations serve as the basis for a more detailed investigation of PS systems’ internal structures and processes.

The observations and simulations suggest several unique aspects of the PS structure. The environment’s vertically sheared 3D wind profile helps to explain PS systems’ tendency to back-build, develop line-parallel precipitation, and evolve asymmetrically. Along-line flow within the system cold pool entails back-building on both the mesoscale and the convective scale. As well, along-line flow in the upper troposphere within the system entails along-line hydrometeor transports, especially in the leading and trailing anvils. These behaviors lead to the archetypal PS structure.

Along-line hydrometeor advection means that much of the system’s precipitation falls very near its outflow boundary, and that the convective cells can seed other updrafts farther down the line. As a result, PS systems in line-parallel shear can intensify their cold pools quite rapidly. As well, in time the PS structure is characterized by diminished upper-tropospheric along-line flow within its axis. These factors may hasten transition toward a predominantly rearward-sloped updraft and the production of trailing precipitation. Even in the absence of Coriolis accelerations, this evolutionary pathway leads to highly asymmetric structures, such as are commonly observed in midlatitudes.

The present introductory exposition of PS systems in deep tropospheric line-parallel wind shear sets the stage for a detailed investigation of their dynamics and sensitivities in a companion article.

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Matthew D. Parker

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Among forecasters and storm chasers, there is a common perception that hodographs with counterclockwise curvature or kinking in the midlevels (sometimes called backing aloft or veer–back–veer profiles) are unfavorable for long-lived supercells and tornadoes. This study reviews and then evaluates several possible explanations for the purported negative effect of backing aloft. As a controlled hypothesis test, simulated supercells are initiated within a range of idealized wind profiles, many of which include representative counterclockwise kinks or bends in their hodographs. In these experiments, the short-term, direct impacts of backing aloft upon supercell maintenance are generally small. Backing aloft does modify the component of vertical accelerations linked to updraft–shear interactions, but these changes generally occur well above the level of free convection (LFC), and they are generally offset by substantial upward accelerations attributable to other processes (e.g., within-storm rotation and positive buoyancy). In these simulations, the longevity of isolated supercells seems to be most directly hindered in environments with very low storm-relative helicity (SRH) or else (for a line of supercells) substantial along-line flow in the upper troposphere. Although these two disrupting properties can accompany backing aloft, they are neither universally nor exclusively associated with it. From the perspective of storm dynamics, it seems advisable to focus on SRH and along-line flow in the environment, rather than the presence (or absence) of backing aloft in the wind profile.

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Matthew D. Parker

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The Plains Elevated Convection at Night (PECAN) field project was designed to explain the evolution and structures of nocturnal mesoscale convective systems (MCSs) and relate them to specific mechanisms and environmental ingredients. The present work examines four of the strongest and best-organized PECAN cases, each numerically simulated at two different levels of complexity. The suite of simulations enables a longitudinal look at how nocturnal MCSs resemble (or differ from) more commonly studied diurnal MCSs. All of the simulations produce at least some surface outflow (“cold pools”), with stronger outflows occurring in environments with more CAPE and weaker near-ground stability. As these surface outflows emerge, the lifting of near-ground air occurs, causing each simulated nocturnal MCS to ultimately become “surface-based.” The end result in each simulation is a quasi-linear convective system (QLCS) that is most intense toward the downshear flank of its cold pool, with the classical appearance of many afternoon squall lines. This pathway of evolution occurs both in fully heterogeneous real-world-like simulations and horizontally homogeneous idealized simulations. One of the studied cases also exhibits a back-building “rearward off-boundary development” stage, and this more complex behavior is also well simulated in both model configurations. As a group, the simulations imply that a wide range of nocturnal MCS behaviors may be self-organized (i.e., not reliant on larger-scale features external to the convection).

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Keith D. Sherburn and Matthew D. Parker

Abstract

High-shear, low-CAPE (HSLC) environments, here characterized by surface-based CAPE ≤ 500 J kg−1, most unstable parcel CAPE ≤ 1000 J kg−1, and 0–6-km shear vector magnitude ≥ 18 m s−1, occur at all times of day, across all seasons, and throughout the entire United States. HSLC environments represent a unique challenge for forecasters, as they occur frequently but produce severe weather a relatively low percentage of the time. Recent studies have primarily focused on improving nowcasting and warnings for events through the identification of radar signatures commonly associated with HSLC tornadoes. Few studies have investigated the forecasting of HSLC severe weather, despite the acknowledged poor performance of traditional tools and techniques. A general climatology of HSLC significant severe weather is presented, focusing on regional, diurnal, and annual trends. Through this climatology, it becomes apparent that multiple types of HSLC environments are possible, including surface-based cases with low lifted condensation levels and high-based convection cases. A statistical analysis of HSLC events and nulls from the southeastern and mid-Atlantic states is utilized to assess the performance of conventional composite parameters in HSLC environments. Additionally, a new composite parameter is introduced that utilizes the product of the statistically most skillful parameters in HSLC environments: the 0–3-km lapse rate, the 700–500-hPa lapse rate, and multiple wind and shear metrics. The strengths and weaknesses of these ingredients-based techniques are then reviewed, with an eye toward improving future HSLC severe weather forecasts.

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