Environmental Vertical Wind Shear Research Articles

Hodograph Variability within Analytically Modeled, Synoptic-Scale, Baroclinic Systems

Although the relationship between the behavior of convective storms and their environmental vertical wind shear has been examined using proximity soundings and idealized numerical modeling experiments, the manner in which the vertical shear profiles, as visualized by hodographs, is regulated by the larger-scale baroclinic wave structure has not been considered in detail. To examine this synoptic-scale dependence, a relatively simple, analytic model for baroclinic systems in midlatitudes having exact solutions for a frictionless, quasigeostrophic atmosphere is employed. The analytical model consists of a checkerboard of high and low pressure areas at 1000 mb, hydrostatically modulated above by a mean meridional temperature gradient and a checkerboard of warm and cold centers at 1000 mb. Aloft, the model atmosphere consists of a zonally oriented wave train. This approach allows a systematic examination of the dependence of hodographs on the following five synoptic-scale parameters included in the model: 1) mean meridional temperature gradient, 2) system wavelength, 3) phase lag between the height and temperature fields at 1000 mb, 4) magnitude of the temperature perturbation associated with the checkerboard of warm and cold centers at 1000 mb, and 5) magnitude of the 1000-mb height perturbation. It is seen that the phase lag between the height and temperature fields and the system wavelength have the greatest quantitative influence on the relative contribution of the ageostrophic wind component to the total wind. These two parameters are associated with significant clockwise curvature with height in the hodograph of the total wind, particularly if the deep-layer ageostrophic wind shear is oriented perpendicular and to the right of the geostrophic shear. Hodograph curvature, however, is not ubiquitous in the model, and despite the model’s simplicity, likely speaks to the importance of features departing from the model, mesoscale variability, and boundary layer friction in enhancing hodograph curvature.

Formation of tropical storms in an atmospheric general circulation model

The formation of tropical storms in a low-resolution Atmospheric General Circulation model is studied over the western North Pacific region during the June—October season. The model simulates the mean annual cycle of storm number in this basin quite well. Time-dependent composites of the storms are formed and analyzed, with a focus on the temporal evolution of quantities averaged in space around the storm centers. Day zero of each composite corresponds to the time at which the disturbance passes criteria for detection. The composites depict the model storms as convectively coupled, synoptic-scale vortices whose degree of coupling to convection increases at some point, leading to intensification. Variables related to disturbance intensity have significant anomalies at day —7, indicating a finite amplitude disturbance prior to “genesis”. Many of these variables show similar temporal evolution, with a local minimum two or three days before day zero, and a strong increase after that for several days, followed by an eventual decrease. The precipitation reaches its maximum on day 2, the net moist static energy forcing (surface fluxes minus net tropospheric radiative cooling, each of which has an anomaly of 20–30 W m—2 in the sense of warming the atmosphere) a day later, and dynamical variables such as vorticity and temperature still later, with broad plateaus centered around day 4 or 5. The vorticity increases at the surface at the same time as at midlevels, unlike in observed storms. The mean composite environmental vertical wind shear has a maximum amplitude on day —2 and then decreases. This could indicate a causal role of shear in limiting development, but would also be consistent with a coincidental storm motion to regions of lower shear, with development controlled by other factors. A signal in the skewness of the lower-level relative humidity distribution over the ensemble suggests that a dry lower troposphere can prevent development of a model tropical disturbance.

Low-Level Mesovortices within Squall Lines and Bow Echoes. Part II: Their Genesis and Implications

This two-part study proposes a fundamental explanation of the genesis, structure, and implications of lowlevel, meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such ‘‘mesovortices’’ are observed frequently, at times in association with tornadoes. Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs, especially when the environmental vertical wind shear is above a minimum threshold and when the Coriolis forcing is nonzero. As illustrated by a QLCS simulated in an environment of moderate vertical wind shear, mesovortexgenesis is initiated at low levels by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a 30-min period, the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative and, more notably, planetary vorticity is stretched vertically; hence, the Coriolis force plays a direct role in the low-level mesovortexgenesis. A downward-directed vertical pressure-gradient force is subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line. In moderate-to-strong environmental shear, the simulated QLCSs evolve into bow echoes with ‘‘straight line’’ surface winds found at the bow-echo apex and additionally in association with, and in fact induced by, the lowlevel mesovortices. Indeed, the mesovortex winds tend to be stronger, more damaging, and expand in area with time owing to a mesovortex amalgamation or ‘‘upscale’’ vortex growth. In weaker environmental shear—in which significant low-level mesovortices tend not to form—damaging surface winds are driven by a rear-inflow jet that descends and spreads laterally at the ground, well behind the gust front.

Low-Level Mesovortices within Squall Lines and Bow Echoes. Part I: Overview and Dependence on Environmental Shear

This two-part study proposes fundamental explanations of the genesis, structure, and implications of low-level meso-γ-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such “mesovortices” are observed frequently, at times in association with tornadoes. Idealized simulations are used herein to study the structure and evolution of meso-γ-scale surface vortices within QLCSs and their dependence on the environmental vertical wind shear. Within such simulations, significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s−1 or greater over a 0–2.5- or 0–5-km-AGL layer. As similarly found in observations of QLCSs, these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger-scale bow-shaped system. They generally develop first near the surface but can build upward to 6–8 km AGL. Vortex longevity can be several hours, far longer than individual convective cells within the QLCS; during this time, vortex merger and upscale growth is common. It is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme “straight line” wind damage, as has also been observed with some QLCSs. Surface vortices are also produced for weaker shears but remain shallow, weak, and short-lived. Although similar in size and strength to mesocyclones associated with supercell storms, and also sometimes producing similar hooklike structures in the rain field, it is also shown that the present vortices are quite distinct, structurally and dynamically. Most critically, such vortices are not associated with long-lived, rotating updrafts at midlevels and the associated strong, dynamically forced vertical accelerations, as occur within supercell mesocyclones.

Environmental Vertical Wind Shear with Hurricane Bertha (1996)

Abstract Hurricane Bertha (1996) was influenced by vertical wind shear with highly variable direction and magnitude. The paper describes a unique method for determining the vertical tilt of a tropical cyclone vortex using satellite and aircraft data. Hurricane Bertha's vortex tracks at three levels are shown during a period of intensification just prior to landfall. During this period, the hurricane vortex becomes more closely aligned in the vertical. Changes in asymmetries of satellite infrared (IR) cold cloud areas are shown to be related to the vortex alignment. Environmental vertical shear measurements throughout Hurricane Bertha's life cycle are presented using IR cloud asymmetries and numerical model analyses. Intensification periods are associated with more symmetric IR cloud measurements. The directions of the IR cloud asymmetric orientations are compared with numerical-model-derived vertical shear directions. The changes in the vertical shear analyses are discussed with respect to observed intens...

Nowcasting Thunderstorms: A Status Report

This paper reviews the status of forecasting convective precipitation for time periods less than a few hours (nowcasting). Techniques for nowcasting thunderstorm location were developed in the 1960s and 1970s by extrapolating radar echoes. The accuracy of these forecasts generally decreases very rapidly during the first 30 min because of the very short lifetime of individual convective cells. Fortunately more organized features like squall lines and supercells can be successfully extrapolated for longer time periods. Physical processes that dictate the initiation and dissipation of convective storms are not necessarily observable in the past history of a particular echo development; rather, they are often controlled by boundary layer convergence features, environmental vertical wind shear, and buoyancy. Thus, successful forecasts of storm initiation depend on accurate specification of the initial thermodynamic and kinematic fields with particular attention to convergence lines. For these reasons the ability to improve on simple extrapolation techniques had stagnated until the present national observational network modernization program. The ability to observe small-scale boundary layer convergence lines is now possible with operational Doppler radars and satellite imagery. In addition, it has been demonstrated that high-resolution wind retrievals can be obtained from single Doppler radar. Two methods are presently under development for using these modern datasets to forecast thunderstorm evolution: knowledge-based expert systems and numerical forecasting models that are initialized with radar data. Both these methods are very promising and progressing rapidly. Operational tests of expert systems are presently taking place in the United Kingdom and in the United States.

A Statistical Tropical Cyclone Intensity Forecast Technique Incorporating Environmental Wind and Vertical Wind Shear Information

Abstract An objective technique for forecasting the 24, 48 and 72 h intensity of western North Pacific tropical cyclones over the open ocean is proposed. In addition to the climatologicad and persistence information contained in present statistical intensity forecast technques environmental information is represented by empirical orthogonal functions of operationally-analyzed wind fields at 700, 400 and 250 mb and the vertical wind shears between these levels. The improvement over the conventional data only equations is small when a nonstratified dataset is used. When the sample is stratified by initial storm intensity, the separate sets of regression equations utilize more of the environmental predictors. Based on dependent sample verifications the combined results with the intensity-stratified sets appear to offer improvements relative to the Joint Typhoon Warning Center official forecasts at 48 and 72 h. Since the intensity-stratified results seem to be physically plausible, this technique appears to b...

An Investigation of the Transition from Multicell to Supercell Storms

Abstract Nearly 2½ hours of dual-Doppler radar data with high temporal and spatial resolution are used to examine the evolution and morphology of a thunderstorm that evolved from a complex of small cells into a supercell storm. Individual storm cells and updrafts moved east-northeastward, nearly with the mean wind, while the storm complex, which encompassed the individual cells, propagated toward the south–southeast. Cells were first detected at middle levels (5–10 km) on the storm's right flank and dissipated on the left flank. Generally, the storm contained three cells—a forming cell, a mature cell, and a dissipating cell; life stages were apparently dictated by the source of updraft air. During the growth stage, cell inflow had a southerly component. As the cell moved through the storm complex, it started ingesting stable air from the north and soon dissipated. A storm-environment feedback mechanism of updraft–downdraft interactions, in conjunction with increasing environmental vertical wind shear and ...

A Severe Frontal Rainband. Part I. Stormwide Hydrodynamic Structure

Abstract A narrow cold frontal band of intense precipitation is examined by means of triple Doppler radar and supporting observations. As the band passed through the Central Valley of California, it was accompanied by strong gusty winds, electrical activity, tornadoes and pressure jumps. Part I delineates the stormwide kinematic and thermodynamic structure. A highly two-dimensional pre-frontal updraft of 15–20 m s−1 results primarily from intense planetary boundary layer forcing of a low-level jet by the gravity-current propagation mechanism. Maximum updraft speed occurs at 2.1 km and the maximum radar echo depth is 6.6 km. Diabatic cooling, due to melting hydrometers, is proposed as a likely mechanism for control of gravity current depth and maintenance of density contrast together with synoptic-scale cold air advection. Available convective potential energy is shown to be small and kinetic energy of the environmental vertical wind shear is proposed as a likely source of energy on the updraft scale. Torn...

Precipitation Characteristics of a Sheared, Moderate Intensity, Supercell-Type Colorado Thunderstorm

Abstract A dense 450-station network of hail and rain gages in 2025 km2 of northeastern Colorado has provided surface precipitation data from a moderate intensity thunderstorm that crossed the network on 22 July 1972. The storm, which existed in an environmental vertical wind shear of 6 × 10−2 sec−1 for over 4 hr, exhibited characteristics of the supercell-type storm including a well-defined and continuing weak echo region, a deviate motion to the right of the local winds, a precipitation wall advancing toward the region of strongest inflow, and a forward overhang of high reflectivity as far as 10 km and more ahead of the precipitation wall. The surface precipitation network results from the storm are analyzed and correlated with other measurements obtained from precision radar, instrumented aircraft, multiple high-altitude released dropsondes, rawinsondes, and surface observations. Hail recorded at 12 network stations was associated with two inflow regions that existed simultaneously and moved in diverge...