Storm-relative Flow Research Articles

A Numerical Study of Hurricane Erin (2001). Part I: Model Verification and Storm Evolution

Abstract The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate Hurricane Erin (2001) at high resolution (4-km spacing) from its early development as a tropical depression on 7 September 2001, through a period of rapid intensification into a strong hurricane (8–9 September), and finally into a stage during which it maintains its intensity on 10 September. These three stages of formation, intensification, and maintenance in the simulation are in good agreement with the observed evolution of Erin. The simulation shows that during the formation and early portions of the intensification stages, intensification is favored because the environmental wind shear is weak and the system moves over a warm tongue of water. As Erin intensifies, the wind shear gradually increases with the approach of an upper-level trough and strengthening of a low-level high pressure system. By 10 September, the wind shear peaks and begins to decrease, the storm moves over slightly cooler waters, and the intensification ends. Important structural changes occur at this time as the outer precipitation shifts from the northeastern and eastern sides to the western side of the eye. A secondary wind maximum and an outer eyewall begin to develop as precipitation begins to surround the entire eye. The simulation is used to investigate the role of vertical wind shear in the changes of the precipitation structure that took place between 9 and 10 September by examining the effects of changes in storm-relative flow and changes in the shear-induced tilt. Qualitative agreement is found between the divergence pattern and advection of vorticity by the relative flow with convergence (divergence) generally associated with asymmetric inflow (outflow) in the eyewall region. The shift in the outer precipitation is consistent with a shift in the low-level relative inflow from the northeastern to the northwestern side of the storm. The changes in the relative flow are associated with changes in the environmental winds as the hurricane moves relative to the upper trough and the low-level high pressure system. Examination of the shear-induced tilt of the vortex shows that the change in the tilt direction is greater than that of the shear direction as the tilt shifts from a northerly orientation to northwesterly. Consistent with theory for adiabatic vortices, the maximum low-level convergence and upper-level divergence (and the maximum upward motion) occurs in the direction of tilt. Consequently, both mechanisms may play roles in the changes in the precipitation pattern.

Proximity sounding analysis for derechos and supercells: an assessment of similarities and differences

Proximity soundings (within 2 h and 167 km) of derechos (long-lived, widespread damaging convective windstorms) and supercells have been obtained. More than 65 derechos, accompanied by 115 proximity soundings, are identified during the years 1983 to 1993. The derechos have been divided into categories according to the synoptic situation: strong forcing (SF), weak forcing (WF), and “hybrid” cases (which are neither weakly nor strongly forced). Nearly 100 supercell proximity soundings have been found for the period 1998 to 2001, subdivided into nontornadic and tornadic supercells; tornadic supercells were further subdivided into those producing significant (>F1 rating) tornadoes and weak tornadoes (F0-F1 rating). WF derecho situations typically are characterized by warm, moist soundings with large convective available potential instability (CAPE) and relatively weak vertical wind shear. SF derechos usually have stronger wind shears, and cooler and less moist soundings with lower CAPE than the weakly forced cases. Most derechos exhibit strong storm-relative inflow at low levels. In WF derechos, this is usually the result of rapid convective system movement, whereas in SF derechos, storm-relative inflow at low levels is heavily influenced by relatively strong low-level windspeeds. “Hybrid” cases collectively are similar to an average of the SF and WF cases. Supercells occur in environments that are not all that dissimilar from those that produce SF derechos. It appears that some parameter combining instability and deep layer shear, such as the Energy–Helicity Index (EHI), can help discriminate between tornadic and nontornadic supercell situations. Soundings with significant tornadoes (F2 and greater) typically show high 0–1 km relative humidities, and strong 0–1 km shear. Results suggest it may not be easy to forecast the mode of severe thunderstorm activity (i.e., derecho versus supercell) on any particular day, given conditions that favor severe thunderstorm activity in general. It is possible that the convective initiation mechanism is an important factor, with linear initiation favoring derechos, whereas nonlinear forcing might favor supercells. Upper-level storm-relative flow in supercells tends to be rear-to-front, whereas for derechos, storm-relative flow tends to be front-to-rear through a deep surface-based layer. However, knowing the storm-relative hodograph requires knowledge of storm motion, which can be a challenge to predict. These results generally imply that probabilistic forecasts of convective mode could be a successful strategy.

Multiscale Overview of a Violent Tornado Outbreak with Attendant Flash Flooding

On 1 March 1997 violent tornadoes caused numerous fatalities and widespread damage across portions of central and eastern Arkansas and western Tennessee. In addition, the associated thunderstorms produced very heavy rainfall and flash flooding, with a few locations receiving up to 150 mm (6 in.) of rainfall in 3 h. The initial environment appeared favorable for strong tornadoes with unseasonably warm moist air at lower levels resulting in significant instability (convective available potential energy values between 1400 and 1800 J kg−1) where 0–2-km storm-relative helicities exceeded 300 m2 s−2 and the middle-tropospheric storm-relative flow was conducive for tornadic supercells. The most destructive tornadoes developed along a preexisting surface boundary where lower-tropospheric moisture convergence and frontogenesis were enhanced. Tornadoes and heaviest rainfall only ensue after upward motion associated with the direct circulation of an upper-tropospheric jet streak became collocated with lower-tropospheric upward forcing along the surface boundaries. From a flash flood perspective the event occurred in a hybrid mesohigh-synoptic heavy rain pattern as thunderstorms developed and moved along surface boundaries aligned nearly parallel to the mean wind. In addition, strong flow and associated moisture flux convergence in the lower troposphere favored the formation of cells to the southwest or upstream of the initial convection with thunderstorms, including a a tornadic supercell, traversing over the same area. The available moisture and ambient instability also supported both vigorous updrafts and high precipitation rates.

Interactions between a Developing Mesoscale Convective System and Its Environment. Part I: Observational Analysis

This paper is the first in a two part series in which the interactions between a growing mesoscale convective system (MCS) and its surrounding environment are investigated. The system studied here developed in northeastern Colorado on 19 July 1993 and propagated into Kansas as a long-lived nocturnal MCS. High-resolution dual-Doppler and surface mesonet data collected from this system are discussed in Part I, while the results of a numerical simulation are discussed in Part II. The observations show that organized mesoscale surface pressure and flow features appeared very early in the lifetime of this system, long before the development of any trailing stratiform precipitation. Most of the stratiform anvil advected ahead of the convective line in the strong upper-tropospheric westerlies. In accordance with this, most of the mid- and upper-tropospheric storm-relative flow behind the line remained westerly, or rear-to-front. Despite the westerlies, the strongest flow perturbations with respect to the ambient winds developed to the rear of the line. The structure of these perturbations was similar to the upper-tropospheric front-to-rear and midtropospheric rear-to-front flows typically found in more mature leading-line/trailing-stratiform systems. The presence of these perturbations on the upwind side of the convective line indicates that gravity wave propagation was primarily responsible for their development.

Variations in Supercell Morphology. Part I: Observations of the Role of Upper-Level Storm-Relative Flow

It is hypothesized that the precipitation intensity beneath a supercell updraft is strongly influenced by the amount of hydrometeors that are reingested into the updraft after being transported away in the divergent upper-level flow of the anvil. This paper presents the results of a climatological analysis of soundings associated with three types of isolated supercells having distinctive precipitation distributions, the so-called classic, low-precipitation (LP), and high-precipitation (HP) storms. It is shown that storm-relative flow at 9–10 km above the ground is strongest in the environments of LP storms, and relatively weak in the environments of HP storms, with classic storms occurring in environments with intermediate magnitudes of upper storm-relative flow. It is plausible that comparatively strong flow in the anvil-bearing levels of LP storms transports hydrometeors far enough from the updraft that they are relatively unlikely to be reingested into the updraft, leading to greatly diminished precipitation formation in the updraft itself. Conversely, the weak upper flow near HP storms apparently allows a relatively large number of hydrometeors to return to the updraft, leading to the generation of relatively large amounts of precipitation in the updraft. It also is apparent that thermodynamic factors such as convective available potential energy, low-level mixing ratio, and mean relative humidity are of lesser importance in determining storm type from a climatological perspective, although important variations in humidity may not be well sampled in this study. This climatological analysis does not directly evaluate the stated hypothesis; however, the findings do indicate that further modeling and microphysical observations are warranted.

Storm-Relative Winds and Helicity in the Tornadic Thunderstorm Environment

Environmental flow relative to tornado-producing thunderstorms is examined through the use of the large tornado proximity sounding dataset compiled at the University of Missouri. It is believed that the 184 soundings gleaned from this collection represent the largest, most restrictive database of its kind with observed storm velocities, as determined from microfilm of conventional National Weather Service radar. Using these storm velocities, mean storm-relative wind profiles were derived for the entire data sample and sample subsets based on tornadic intensity, strength of the mean environmental flow, magnitude of CAPE, and direction of storm motion with respect to the mean environmental wind vector. Although it is apparent that a number of tornadoes occur independent of the larger-scale flow, the mean storm-relative wind profiles suggest that there is a preferred storm-relative flow structure for tornadic thunderstorms. Tornadic intensity in association with this structure appears to strengthen as 1) the magnitude of storm-relative helicity grows through an increasingly deep layer of the lower through midtroposphere and 2) mid- and upper-level storm-relative winds strengthen while possessing decreasing directional variability at their respective heights above ground level (4–12 km AGL).

Tornado-like Vortexgenesis in a Simplified Numerical Model

A novel approach to the modeling of tornado-like vortexgenesis has been developed and is used to articulate the sequence of events that leads to tornadogenesis. The “pseudostorm” is an idealized thunderstorm representation and emulates the storm-relative flow, into an updraft, of the horizontal streamwise vorticity that is baroclinically generated in cold air outflow. By not explicitly simulating the morphology of a tornadic thunderstorm, but instead concentrating on the development of low-level rotation and tornado-scale vortices, the authors are able to transcend many of the experimental limitations encountered by cloud modelers. Intense, near-ground, cyclonic vortices, which are classified herein as “tornado like,” evolve solely from horizontal streamwise vorticity due to buoyancy gradients and friction, if present, at the lower boundary. Regardless of the lower boundary condition, none of the vortices exceed the thermodynamic speed limit based on the vertically integrated buoyancy (or convective available potential energy). Sensitivity experiments reveal that the tornado-like pseudostorm vortices develop only when certain updraft propagation rates (and thus storm-relative flow strengths), downdraft intensifies, and fluid viscosities are well matched. The simplicity of the pseudostorm allows one to look closely at the actual genesis of a vortex. In particular, it is found that the such genesis is not “triggered,” but is instead the outcome of a continuous (albeit rapid) process of amplification of vertical vorticity generated initially through tilting. Also, vertical vorticity intensification proceeds with a large degree of vertical uniformity, obviating the need for a mechanism like the “dynamic pipe effect” to advance the incipient tornado-like vortex toward the ground.

Interpretation of Doppler Weather Radar Displays of Midlatitude Mesoscale Convective Systems

Abstract The utility of color displays of Doppler-radar data in revealing real-time kinematic information has been demonstrated in past studies, especially for extratropical cyclones and severe thunderstorms. Such displays can also indicate aspects of the circulation within a certain type of mesoscale convective system—the squall line with trailing “stratiform” rain. Displays from a single Doppler radar collected in two squall-line storms observed during the Oklahoma-Kansas PRE-STORM project conducted in May and June 1985 reveal mesoscale-flow patterns in the stratiform rain region of the squall line, such as front-to-rear storm-relative flow at upper levels, a subsiding storm-relative rear inflow at middle and low levels, and low-level divergent flow associated with strong mesoscale subsidence. “Dual-Doppler” analysis further illustrates these mesoscale-flow features and, in addition, shows the structure of the convective region within the squall line and a mesoscale vortex in the “stratiform” region tra...