Gust Front Research Articles

The 3 November Tornadic Event during Sydney 2000: Storm Evolution and the Role of Low-Level Boundaries

Several severe thunderstorms, including a tornadic supercell, developed on the afternoon of 3 November 2000, during the Sydney 2000 Forecast Demonstration Project. Severe weather included three tornadoes, damaging wind gusts, hail to 7-cm diameter, and heavy rain causing flash flooding. A unique dataset was collected including data from two Doppler radars, a surface mesonet, enhanced upper-air profiling, storm photography, and a storm damage survey. Synoptic-scale forcing was weak and mesoscale factors were central to the development of severe weather. In particular, low-level boundaries such as gust fronts and the sea-breeze front played critical roles in the initiation and enhancement of storms, the motion of storms, and the generation of rotation at low levels. The complex and often subtle boundary interactions that led to the development of the tornadic supercell in this case highlight the need for advanced detection and prediction tools to improve the warning capacity for such events.

Analysis and Forecasting of the Low-Level Wind during the Sydney 2000 Forecast Demonstration Project

Abstract During the Sydney 2000 Forecast Demonstration Project (FDP) a four-dimensional variational assimilation (4DVAR) scheme was run to analyze the low-level wind field with high spatial and temporal resolution. The 4DVAR scheme finds an optimal fit to the data and a background field under the constraints of a dry boundary layer model. During the FDP, the system assimilated data from two Doppler radars, a surface mesonet, and a boundary layer profiler, and provided low-level analyses every 10 min. After the FDP, a number of experiments have been performed to test the ability of the system to provide short-term forecasts (0–60 min) of the low-level wind and convergence. Herein, the performance of the system during the FDP and the forecast experiment's performed after the FDP are described. Two strong gust front cases and one sea-breeze case that occurred during the FDP are also examined. It is found that for the strong gust front cases, the numerical forecasts improve over persistence in the 1-h time fr...

An overview of first Doppler Weather Radar inducted in the cyclone detection network of India Meteorological Department

India Meteorological Department (IMD) maintains a wide network of radars for the detection and study of severe weather phenomena like cyclones, thunderstorms, gust front etc. and for deriving upper air wind profile. To keep pace with the fast developments in the field of weather radar technology, IMD is gradually replacing its conventional radars with digital radars, a few of them with Doppler capabilities. An S-band Doppler Weather Radar (DWR) has been inducted into India Meteorological Department’s (IMD) Cyclone Detection Radar (CDR) network recently at Chennai as a replacement to the outlived analogue S-band radar and is declared operational from 21 February 2002. Salient features, both hardware and software, of the radar are discussed in this article.

An Observational Study of Atmospheric Bore Formation from Colliding Density Currents

Observations from east-central Florida during the Convection and Precipitation/Electrification (CaPE) experiment are used to investigate the factors that influence atmospheric bore formation from colliding density currents. Ten cases involving the collision of a gust front with a sea-breeze front are analyzed with Doppler radar and sounding and surface mesonet data. The gust fronts in these collisions were generally deeper, denser, and faster propagating than their sea-breeze-front counterparts. Seven of the 10 cases produced dual boundaries that moved away from each other in a relative sense after the collision. Post-collision boundaries moving in the same direction and oriented similarly to the pre-collision gust front occurred in all 10 cases. They transported mass in the manner of a density current in six cases, while the others behaved more like bores or bore–density current hybrids, as they were characterized by little or no mass transport. Similarity of gust-front and sea-breeze-front propagation speed prior to the collision was the best indicator of bore character in post-collision boundaries induced by gust fronts. Sea-breeze-front-induced post-collision boundaries occurred in the seven cases in which the pre-collision sea-breeze front was nonstationary. These post-collision boundaries exhibited no purely mass transport behavior and were all categorized as bores or bore/density current hybrids. The potential ability to predict dual zones of convergence emanating from boundary collisions as demonstrated in this study may be of value to convective nowcasting in Florida and perhaps other locales, as 70% of the observed post-collision boundaries initiated new convection or enhanced existing convection.

Cold Pools and MCS Propagation: Forecasting the Motion of Downwind-Developing MCSs

The primary factors that affect the direction of propagation and overall movement of surface-based mesoscale convective systems (MCSs) are discussed. It is shown that although propagation is indeed related to the strength and direction of the low-level jet as previous studies have shown, it is more specifically dependent upon the degree of cold-pool-relative flow and to the distribution of conditional instability present along a system’s gust front. An updated technique that may be used to forecast the short-term (3‐6 h) motion of MCS centroids based on these concepts is introduced. The procedure builds on the long-established observation that MCS motion is a function of 1) the advection of existing cells by the mean wind and 2) the propagation of new convection relative to existing storms. Observed wind and thermodynamic data, in conjunction with anticipated cold-pool motion and orientation, are used to assess the speed and direction of cell propagation, that is, whether propagation will be upwind, downwind, or some combination of the two. The technique ultimately yields an estimate of overall system movement and has application regardless of scale, season, or synoptic regime.

Mobile Doppler Radar Observations of a Tornado in a Supercell near Bassett, Nebraska, on 5 June 1999. Part I: Tornadogenesis

Abstract This two-part paper details an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar: The dataset captures the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The formation of the tornado vortex near the ground is described from a sequence of sector scans ranging from 30-s intervals prior to tornadogenesis to 10–15-s intervals during much of the lifetime of the tornado. Cyclonic vortices of 100–200 m width were found along a bow-shaped line of enhanced radar reflectivity, at what appears to have been the leading edge of a rear-flank gust front. At the time of tornadogenesis, one of these vortices was located just ahead of the nose of the bow-shaped radar echo and a jet, which were embedded within a larger-scale cyclone. At other times, small-scale cyclonic vortices coexisted with the tornado along an arc-shaped line extending to its north and northeast but did not appear to interact with the tornado. The ...

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.

A diagnostic study on the environmental influence of a mesoscale convective system over southern China in Meiyu season

¶During the Post-TAMEX forecast experiment of Taiwan in 1992, a mesoscale convective system (MCS) developed on June 5–6 over southern China. As this system matured, it produced readily apparent cirrus outflow on satellite imageries while the upper level flow also exhibited a diffluent pattern. The purpose of the current study is to examine the possible changes in its environment associated with the development of this MCS. By using 12-h data from 1200 UTC June 5 to 1200 UTC June 6, objective analyses were performed for a 1°×1° latitude/longitude grid using sounding data and a low-pass filter. To facilitate the diagnosis, a band-pass filter was further applied to separate mesoscale features from macroscale ones, while the apparent heat source and apparent moisture sink defined by Yanai et al (1973) were also calculated. Results suggest that the MCS exerted clearly discernable effects on its environment. The latent heat release led to the development of a warm core and mesoscale high-pressure disturbance at upper levels when the system matured. Ageostrophic winds and diffluent flow patterns together with strong anticyclonic vorticity at 200 hPa near the MCS were associated with the mesohigh. After the mature stage, weak cooling occurred above 350 hPa, likely due to radiative emission from the cloud top. However, a mid-level cyclonic vortex, often present in MCSs over the North America, was not apparent here due to weak environmental vorticity and small Coriolis parameter f. The level of maximum divergence was initially located at 500 hPa, but rose to 200 hPa as the MCS matured. In response, the upward motion not only intensified, but the level at which strongest rising occurred also ascended from 700 to 350 hPa. Results from the apparent heat source and moisture sink calculation suggest that this slow ascent of maximum heating was partially due to vertical transport of sensible heat by updrafts. During the MCS’s mature stage, under the stratiform clouds to the west of the strongest convection, a cold mesohigh formed at the surface due to evaporative cooling in downdrafts, and a gust front appeared along the leading edge of the outflow boundary. A trailing mesolow was also observed, likely due to near-adiabatic warming in drier downdrafts since no precipitation was associated with it.

Generation Mechanisms of Convectively Forced Internal Gravity Waves and Their Propagation to the Stratosphere

Abstract Characteristics of gravity waves induced by mesoscale convective storms and the gravity wave sources are investigated using a two-dimensional cloud-resolving numerical model. In a nonlinear moist (control) simulation, the convective system reaches a quasi-steady state after 4 h in which convective cells are periodically regenerated from a gust front updraft. In the convective storms, there are two types of wave forcing: nonlinear forcing in the form of the divergences of momentum and heat flux, and diabatic forcing. The magnitude of the nonlinear source is 2 to 3 times larger than the diabatic source, especially in the upper troposphere. Three quasi-linear dry simulations forced by the wave sources obtained from the control (CTL) simulation are performed to investigate characteristics of gravity waves induced by the various wave source mechanisms. In the three dry simulations, the magnitudes of the perturbations produced in the stratosphere are comparable, yet much larger than those in the CTL si...

Analyses of gust fronts by means of limited area NWP model outputs

Two convective cases accompanied with propagating gust fronts and recorded over the Czech Republic (CR) in the year 2000 are numerically studied. The experimental integration of the Lokal Modell of the German Weather Service (LM DWD) is used in computations with the horizontal resolution 2.8 km. The prognostic fields of basic thermodynamic quantities as well as the fields of vertical velocities and precipitation rates are presented at various levels/cross-sections and at different times in order to clarify the effect of the cold evaporative outflow on the organization of severe convection. The position and movement of an outflow leading edge (a gust front) are determined with the aid of objective techniques based on a simple model of objective frontal analysis. The techniques include both mathematical and graphical tools. They allow for the automatic identification of gust fronts in atmospheric cross-sections by means of one locating equation and two masking criteria. The equation and criteria employ the moist potential temperature as an input thermal parameter. The appropriate regimes of outflow propagation, which may provide information about outflow dynamics, are assigned subjectively to examined gust fronts according to the properties of ambient flow. As compared with radar, the LM DWD runs prove to forecast precipitation in nearly the right subregions. The results of the numerical study confirm that the LM DWD, together with the objective techniques for gust front analyses, are capable of simulating thermodynamic processes associated with convective outflows. Thereby, they can represent the potential for improving the nowcasting and short-range forecasting of hazardous weather phenomena.

A DIAGNOSTIC STUDY OF STRENGTH OF DOWNDRAFTS IN A GUST FRONT AND THE ASSOCIATED DAMAGES TO AIRCRAFT AT PARKING BAY

A DIAGNOSTIC STUDY OF STRENGTH OF DOWNDRAFTS IN A GUST FRONT AND THE ASSOCIATED DAMAGES TO AIRCRAFT AT PARKING BAY

The Role of Gravity Waves in the Meso-.BETA.-Scale Cycle of Squall-Line Type Convective Systems

Observed or simulated mesoscale convective systems (MCSs) often show pulsations with time periods of a few hours. In this paper, results from a two-dimensional cloud model are used to determine the dynamical processes responsible for such self-modulating oscillations (meso-β-scale cycle, MBC) in squallline type convective systems. The simulated storms have weak cold pools and include notable pulsations with time periods of 3∼4 hours. These pulsations are manifested as periodic organizations of convective-scale cells into upsheartilted meso-β-scale cells (MCs). A rearward-propagating, dissipating MC and disturbances on the downshear side have the properties of vertically propagating gravity waves, with storm-relative time periods that are about 1.5 hour less than those of the MBC. Dry model simulations driven by prescribed thermal forcings show that the waves arise in response to the reduction of latent heating in the MC in its stationary stage. The evolution of storm structure following a convective burst is related to the phasing of the uppertropospheric wave disturbances relative to the lower-tropospheric disturbances. The storm exhibits multicellular structure so long as the downward and frontward acceleration phase of gravity waves exerts suppressing effects on convections behind the gust front. As the phase propagates to the rear, it in turn enhances the convergence at the terminus of the steady lower-tropospheric front-to-rear flow, aiding the cell reinvigoration there. The time period of MBC increases as MCs tilt more horizontally in their stationary growth stage, and as the storm-relative rear-to-front upper-tropospheric wind increases. The former effect increases the intrinsic period of excited waves, and the latter increases the storm-relative period of the waves by Doppler-shifting the phase speed. These results support the argument that gravity waves generated by tilted MCs are prominent in the dynamics of MBCs.

Nowcasting Storm Initiation and Growth UsingGOES-8and WSR-88D Data

The evolution of cumulus clouds over a variety of radar-detected, boundary layer convergence features in eastern Colorado has been examined using Geostationary Operational Environmental Satellite (GOES) imagery and Weather Surveillance Radar-1988 Doppler (WSR-88D) data. While convective storms formed above horizontal rolls in the absence of any additional surface forcing, the most intense storms initiated in regions above: gust fronts, gust front interaction with horizontal rolls, and terrain-induced stationary convergence zones. The onset of vigorous cloud growth leading to storm development was characterized by cloud tops that reached subfreezing temperatures and exhibited large cooling rates at cloud top 15 min prior to the first detection of 10dBZ radar echoes aloft and 30 min before 35 dBZ. The rate of cloud-top temperature change was found to be important for discriminating between weakly precipitating storms (,35 dBZ) and vigorous convective storms (.35 dBZ). Results from this study have been used to increase the lead time of thunderstorm initiation nowcasts with the NCAR automated, convective storm nowcasting system. This improvement is demonstrated at two operational forecast offices in Virginia and New Mexico.

A new wind zone map of Germany

The revised version of the wind zone map of Germany is based on a refined extreme value analysis for three physical phenomena which are leading to high gust wind speeds: the typical storms induced by strong frontal depressions, thunderstorms and additional gust fronts especially in weaker frontal depressions which are induced by down-drafts of cold air and/or by rain and which are not fully covered by the usual flow field models of frontal depressions. Individual ensembles for each of these storm phenomena have been sampled for a large number of stations. Furthermore, it has been tried to increase the statistical stability of the estimated representative values by summarizing stations with a similar wind climate. The fitting of the theoretical extreme value distribution is performed for only the right tail of the observed traces of non-exceedence probabilities. The proposed new map presents five wind zones with characteristic values from 22.5 to 32.5 m/s. The zoning follows mainly the boundaries of the administrative districts. Compared to the preceding wind zone map, the new map specifies for many sites smaller wind loads. Additionally, directional effects have been analysed, however, the results are too inhomogeneous to be included in the code.

Surface diurnal cycle and boundary layer structure over Rondônia during the rainy season

An integrated data set with simultaneous observations at the surface, from tethered balloons within the boundary layer and from rawinsonde ascents, was collected during the wet season experiment of the Large‐Scale Biosphere‐Atmosphere (LBA) Experiment in Amazonia during January and February of 1999 in support of the ground validation for the Tropical Rainfall Measuring Mission (TRMM). We analyze the surface diurnal cycles of temperature, humidity, lifting condensation level, equivalent potential temperature, and the surface fluxes of sensible and latent heat, ground heat flux and net radiation, for easterly and westerly wind regimes in the lower troposphere. During the easterly wind regimes, the diurnal evolution of mixing ratio shows that the flux of water vapor through cloud base exceeds the large surface evaporation. There is a trend toward a wetter and cooler subcloud layer as the rainy season progresses. Daytime surface Bowen ratio for this pasture site is about 0.4, and falls slightly as the rainy season progresses. Typically in the afternoon, evaporatively driven downdrafts from convective rainbands transform the boundary layer. The fall of equivalent potential temperature in the boundary layer is similar for both regimes, but the boundary layer cooling by convective events during the westerly regimes is reduced, because the subcloud layer is shallower on average. Tethersonde ascents through the edges of gust fronts show that subcloud air is first cooled and moistened by rainfall evaporation before the arrival of downdraft air at the surface. These measurements provide a detailed observational basis for the validation and improvement of parameterizations for shallow and deep convection in numerical forecast models.

Vertical variations in O3 concentrations before and after a gust front passage

Two consecutive increases in surface ozone, occurring after the cessation of photochemical ozone production, are investigated. A unique suite of instruments, including an ozone profiling lidar, a Doppler lidar, surface chemistry sensors, surface meteorological sensors, and a radar wind profiler, was deployed during the Southern Oxidants Study of 1999 in Nashville, Tennesee. Time series of ozone at 10, 405, and 1035 m above ground level (AGL) on 22 June 1999 showed variations in the vertical gradient of ozone over the course of the afternoon and evening. Analysis of time series of vertical velocity at 8 m AGL and its standard deviation, the variance in Doppler lidar calculated horizontal wind speed from near the surface to 500 m AGL, wind profiles, and meteorological surface station data, revealed mechanisms responsible for these changes in ozone. An early evening rise in surface ozone occurred with the passage of a thunderstorm gust front. Analysis showed that downward mixing of ozone‐rich air from a residual layer (RL) of ozone above 400 m caused a sharp rise in ozone at the surface and a decrease of ozone in the RL. Doppler lidar measurements showed details of the postgust front wind flow, such as the depth of the air mass behind the front, the turbulent wake region, and inferred vertical velocities throughout a layer several hundred meters deep. The second rise in surface ozone was caused by turbulent mixing due to elevated directional wind shear, which mixed ozone‐rich air down to the surface.

The 8 June 1995 McLean, Texas, Storm. Part I: Observations of Cyclic Tornadogenesis

Abstract On 8 June 1995 scientists participating in the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) collected airborne Doppler radar data in a storm that produced a family of tornadoes near McLean, Texas. The Electra Doppler Radar (ELDORA) scanned three significant tornadoes during their formative and mature stages; one of the tornadoes was of F4/F5 intensity. Evidence from pseudo-dual-Doppler analyses of the ELDORA data reveals a process of cyclic tornado formation qualitatively similar to that depicted in previous conceptual models. In particular, the rear-flank gust front appears to play a major role in determining the location of the next vortex in the series. When a tornado forms, a small region (3–5 km wide) of outflow surges ahead of the tornado, producing a local bulge in the gust front. A new vorticity maximum may form near the leading edge of the outflow. In contrast to what is suggested by earlier conceptual models, intersection of the rear-flank gust front with a w...

Structure of thunderstorm gust fronts with topographic effects

Surface meteorological observations, associated with gust fronts produced by thunderstorm outflows over Tehran, an area surrounded by mountains, have been analyzed. Distinctive features are sudden drop in air temperature, up to 10°C, sharp increase in wind speed, up to 30 m s−1, with wind shift, to northwesterly, pressure jump, up to 4 hPa, humidity increase, up to 40%, and rain after some 20 min. Gust fronts which often occur in spring time, have a typical thickness of about 1.5 km and produce vertical wind shear of the order of 10−2s−1. Although these features seem to be common for most of the events, their intensities differ from one event to another, indicating that the gust fronts may occur in different sizes and shapes. Apart from a dominant effect on the formation of the original thunderstorms, topography appears to break up the frontal structure of the gust fronts. The internal Rossby radius of deformation for these flows is small enough (∼ 100 km) for rotational effects to be minor.

A Further Study of the Mechanisms of Cell Regeneration, Propagation, and Development within Two-Dimensional Multicell Storms

The mechanisms of cell regeneration, development, and propagation within a two-dimensional multicell storm proposed by Lin, Deal, and Kulie (hereafter LDK) were further investigated by conducting a series of sensitivity tests. LDK’s advection mechanism was reexamined by performing simulations utilizing a plateau with five additional wind profiles having a wider range of shear. All five cases gave results that show that the cell regeneration period decreases with the storm-relative midlevel inflow, similar to that proposed by LDK. It was also found that a rigid lid is not an appropriate upper boundary condition for multicell storm simulations. In order to test whether the advection mechanism is responsible for cell regeneration with a different sounding, an idealized sounding was used. A multicell storm was produced along with a strong density current and gust front updraft. Investigation of this storm supports the advection mechanism within the growing mode and a gravity wave mechanism in the propagation mode, as proposed by LDK. From further investigation, the relaxation mechanism proposed by Fovell and Tan was shown to exist within these simulations yet found to be dependent on the advection mechanism to cause cell regeneration. To avoid some problems that occurred when using a plateau, a prescribed heat sink was used to produce a more realistic density current. This experiment demonstrates that the advection mechanism is responsible for cell regeneration and the gravity wave mechanism is responsible for cell propagation within the storm. It was found that without precipitation loading, an individual cell is still able to split. In this case, the compensating downdraft produced by vertical differential advection is responsible for cell splitting and merging.

Simple Adjoint Retrievals of Wet-Microburst Winds and Gust-Front Winds from Single-Doppler Radar Data

Abstract The simple adjoint method for retrieving the low-altitude winds from single-Doppler scans is modified and tested for two events: the wet-microburst event and the gust-front event observed simultaneously by two radars during the Federal Aviation Administration–1990 summer field experiment at Orlando Airport. For the wet microburst, 42 consecutive wind fields are retrieved from each radar and the rms errors (measured by the rms difference between the retrieved and dual-Doppler analyzed winds) range from 1.1 to 4.9 m s−1, and the averaged error is 2.4 m s−1. For the gust front, 22 consecutive wind fields are retrieved from each radar, the rms errors range from 1.2 to 2.9 m s−1, and the averaged error is 1.9 m s−1. The rms error and spatial correlation between the retrieved and analyzed cross-beam tangential winds exhibit strong temporal variations for the wet microburst and moderate temporal variations for the gust front. The rms error is found to be temporally correlated to the rms amplitude of the...