Prefrontal Squall Line Research Articles

Synoptic and Mesoscale Analysis of a Severe Weather Event in Southern Brazil at the End of June 2020

At the end of June 2020, an explosive extratropical cyclone was responsible for an environment in which a squall line developed and caused life and economic losses in Santa Catarina state, southern Brazil. The aims of this case study are the following: (a) to describe the drivers of the cyclogenesis; (b) to investigate through numerical simulations the contribution of sea–air interaction to the development of the cyclone as an explosive system; and (c) to present the physical properties of the clouds associated with the squall line. The cyclogenesis started at 1200 UTC on 30 June 2020 on the border of southern Brazil and Uruguay, having a trough at middle-upper levels as a forcing, which is a common driver of cyclogenesis in the studied region. In addition, the cyclone’s lifecycle followed Bjerknes and Solberg’s conceptual model of cyclone development. A special feature of this cyclone was its fast deepening, reaching the explosive status 12 h after its genesis. A comparison between numerical experiments with sensible and latent turbulent heat fluxes switched on and off showed that the sea–air interaction (turbulent heat fluxes) contributed to the cyclone’s deepening leading it to the explosive status. The cold front, which is a component of the cyclone, favored the development of a pre-frontal squall line, responsible for the rough weather conditions in Santa Catarina state. While satellite images do not clearly show the squall line located ahead of the cold front in the cyclone wave due to their coarse resolution, radar reflectivity data represent the propagation of the squall line over southern Brazil. On 30 June 2020, the clouds in the squall line had more than 10 km of vertical extension and a reflectivity higher than 40 dBZ in some parts of the storm; this is an indicator of hail and, consequently, is a required condition for storm electrification. In fact, electrical activity was registered on this day.

Thermodynamic Recovery of the Pressure and Temperature Fields over Complex Terrain Using Wind Fields Derived by Multiple-Doppler Radar Synthesis

Abstract A new thermodynamic retrieval scheme is developed by which one can use the wind fields synthesized from multiple-Doppler radars to derive the three-dimensional thermodynamic fields over complex terrain. A cost function consisting of momentum equations and a simplified thermodynamic equation is formulated. By categorizing the analysis domain into flow and terrain regions, the variational technique is applied to minimize this cost function only within the flow region, leading to the solutions for the three-dimensional pressure and temperature perturbations immediately over terrain. Using idealized datasets generated by a numerical model, an experiment is first conducted to assess the accuracy of the proposed algorithm. The retrieval scheme is then applied to a real case that occurred during the 2008 Southwestern Monsoon Experiment (SoWMEX) conducted in Taiwan. The retrieved thermodynamic fields, verified by radiosonde data, reveal the structure of a prefrontal squall line as it approaches a mountain. The retrieved three-dimensional high-resolution pressure and temperature along with the wind fields not only allow us to better understand the thermodynamic and kinematic structure of a heavy rainfall system, but can also be assimilated into a numerical model to improve the forecast.

The Kinematic and Microphysical Characteristics and Associated Precipitation Efficiency of Subtropical Convection during SoWMEX/TiMREX

Abstract Dual-Doppler, polarimetric radar observations and precipitation efficiency (PE) calculations are used to analyze subtropical heavy rainfall events that occurred in southern Taiwan from 14 to 17 June 2008 during the Southwest Monsoon Experiment/Terrain-Influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX) field campaign. Two different periods of distinct precipitation systems with diverse kinematic and microphysical characteristics were investigated: 1) prefrontal squall line (PFSL) and 2) southwesterly monsoon mesoscale convective system (SWMCS). The PFSL was accompanied by a low-level front-to-rear inflow and pronounced vertical wind shear. In contrast, the SWMCS had a low-level southwesterly rear-to-front flow with a uniform vertical wind field. The PFSL (SWMCS) contained high (low) lightning frequency associated with strong (moderate) updrafts and intense graupel–rain/graupel–small hail mixing (more snow and less graupel water content) above the freezing level. It is postulated that the reduced vertical wind shear and enhanced accretional growth of rain by high liquid water content at low levels in the SWMCS helped produce rainfall more efficiently (53.1%). On the contrary, the deeper convection of the PFSL had lower PE (45.0%) associated with the evaporative loss of rain and the upstream transport of liquid water to form larger stratiform regions. By studying these two events, the dependence of PE on the environmental and microphysical factors of subtropical heavy precipitation systems are investigated by observational data for the first time. Overall, the PE of the convective precipitation region (47.9%) from 14 to 17 June is similar to past studies of convective precipitation in tropical regions.

The Formation of Multiple Squall Lines and the Impacts of WSR-88D Radial Winds in a WRF Simulation

Abstract A detailed observational and Weather Research and Forecasting (WRF) model analysis utilizing Weather Surveillance Radar-1988 Doppler (WSR-88D), surface, and upper-air observations, as well as Geostationary Operational Environmental Satellite (GOES) images, shows a chain of events that leads to the formation of two prefrontal squall lines along the western Gulf coast on 29–30 April 2005. An approaching surface cold front (CF) generated an atmospheric bore that propagated along an inversion layer and excited high-frequency, low-level tropospheric gravity waves, initiating a squall line 60 km east of the cold front. This sequence of events manifested itself as low-level convergence ahead of the CF, which was detected by nearby WSR-88D radars. Two WRF model experiments were conducted in which one assimilated conventional observations (CTRL), and another included radar radial winds from nine WSR-88D locations (denoted as RADAR). Better representation of the low-level kinematics in RADAR yielded a distinct convergence line associated with the primary squall line. The RADAR experiment, as well as observations (such as an 0600 UTC Slidell, Louisiana, sounding), show that the secondary squall line formed ahead of the primary squall line due to high water vapor and warm temperature advection from the Gulf of Mexico that, when combined with a deep dry layer above the atmospheric boundary layer (ABL), destabilized the atmosphere. Concurrently, a lower-tropospheric trough, propagating faster than the surface front, enhanced lifting in the region and instigated the formation of new convection. RADAR forecasted the secondary convection not only in the right place but also at about the right time, while the CTRL experiment completely missed this secondary convection.

Numerical modeling of severe convective storms occurring in the Carpathian Basin

Squall lines often cause serious damages due to the strong surface outflow, hail, or heavy precipitation in Hungary every summer. Squall lines in the Carpathian Basin can be classified into two main categories: pre-frontal squall-lines and frontal convective lines. In this paper, these two types of severe mesoscale phenomena are investigated using the high resolution numerical weather prediction model, the MM5. The case study for the first type of convective systems occurred on 18th May 2005 when two main convective lines with their embedded severe storms formed daytime and caused high-velocity wind events and extensive damages in the eastern part of Hungary. The second case study is a frontal squall line that hit Budapest on 20th August 2006 and the associated high precipitation (HP) supercells reached the capital of Hungary at same time when the traditional Constitution Day firework began. The consequences were catastrophic: five people were killed and more than one thousand were injured due to the extreme weather. The non-hydrostatic high resolution MM5 model was able to simulate and catch the severe weather events occurred on the days under discussion. Moreover, the model was able to compute the detailed structure of the supercells embedded in thunderstorm lines. By studying the equivalent potential temperature (EPT) fields at lower levels, we state that in the prefrontal case, there is a competition between the supercell thunderstorms for the wet and warm air. A thunderstorm that can collect the wet and warm air from larger area will have longer lifetime and more intense updraft. In the second case, the frontal squall lines, the movement and the behavior of the supercell storms embedded in the line was highly determined by the synoptic-scale motions and less affected by the EPT field of the prefrontal masses.

Comparison of GPS slant wet delay measurements with model simulations during the passage of a squall line

Slant wet delay (SWD) measurements from a ground‐based GPS observation network provide accurate, high‐resolution moisture information for mesoscale analysis. In this study, we compared SWD simulated from a 9‐km MM5 model in a prefrontal squall line event against actual GPS SWD observations by NOAA's GPS network. We found that the model simulation did not capture abrupt moisture changes and small‐scale variations observed in the nonisotropic component of slant wet delays associated with the squall line. This result suggests that SWD measurements can be very useful in improving model moisture analysis.

Severe convective storms and associated phenomena in Hungary

Convective activity dominates the weather of Hungary in the summer. Especially during the first part of the summer, the frequency of severe thunderstorms grows and associated phenomena such as wind storms, hail, sometimes even tornadoes cause serious damage. In this paper, an overview of the severe thunderstorm situation in the Carpathian Basin is presented with a focus on the most frequent phenomena: squall lines. To understand the physical background of these kinds of severe thunderstorms, the terminology of convective components is introduced. Making use of case studies, the roles of convective components are shown for different types of thunderstorm systems. Case studies also show that most of the tornado events are associated with organized thunderstorm systems, especially prefrontal squall lines.

Seasonality of instability phenomena (hailstorms and thunderstorms) in Padova, northern Italy, from archive and instrumental sources since AD 1300

In northern Italy, and especially in the Po Valley, instability phenomena such as hail and thunder storms are mainly associated with the penetration of Atlantic disturbances, especially cold fronts, or pre-frontal squall lines in the mid-seasons, and with the thermal convection of hot and moist air in the summer. Changes of their seasonal distribution may be considered as an index of anomalies in the synoptic pressure pattern, which determines the general circulation over Europe. Two different types of meteorological data have been analysed: (1) historical, non-specialist descriptions, taken from chronicles, annals or diaries after AD 1300, when people began to record, in addition to the most outstanding climatic events and natural hazards, minor observations including instability phenomena; (2) regular daily observations made by trained meteorologists after 1740. The historical descriptions show that in the Po Valley the highest thunderstorm frequency is in the summer, between June and August, but in the 1500s the distribution was anomalous. In that century, a broad maximum was found between March and August, and a secondary maximum was found in December. In the early period of observations made by trained meteorological personnel, i.e., 1740–1799, a summer maximum of thunderstorm frequency was found, almost equally distributed from June to August, followed by May and September. In the most recent period, i.e., 1940–1989, the seasonal distribution was very regular for three decades (1940 to 1969) with the mode in June, followed by July and August, and it then changed, with more late peaks. The historical written sources show that the seasonal distribution of hail was more homogeneous than thunderstorms. Hail was especially frequent in the summer, from June to August, except in the 1500s, when the seasonal distribution was practically one month earlier. In the early period of regular meteorological observations, hail was seasonally distributed less regularly than thunderstorms, with more skew and earlier in the year. The maximum frequency was in June, followed by April, May, July and August. In the most recent period, the distribution was still irregular, with marked fluctuations, the distribution being displaced later in the season.

The synoptic setting of a thundery low and associated prefrontal squall line in western Europe

A case study of a thundery low and associated prefrontal squall line in western Europe is presented. It is shown that the prefrontal squall line is linked to the vertical circulation associated with an intensifying cold front and a propagating midtropospheric jetstreak, with maximum wind speeds at levels between 700 and 500 hPa. The squall line is triggered in the updraught of the cross-frontal circulation, which can be observed at the earth's surface as a line of mass convergence or confluence stretching for more than 1000 km from southern Iberia to northern France. The intensification of the front and the destabilization of the atmosphere are interpreted by using the slope of isentropes as indicator of frontal intensity. An equation is derived for the rate of change of frontal intensity, which predicts a forward tilt of the cold front with increasing height due to the cross-frontal circulation in the area of warm air advection (the so-called Spanish plume) and, associated with this, a destabilization of the atmosphere at midtropospheric levels.

An Application of Potential Vorticity Inversion to Improving the Numerical Prediction of the March 1993 Superstorm

In this study, a methodology is proposed to improve the model initial conditions, based on available surface temperature observations from ships, buoys, and drifters. It is tested with the numerical prediction of the 12‐ 14 March 1993 superstorm that is initialized at its incipient stage over the Gulf of Mexico. In this methodology, the authors make use of the piecewise potential vorticity (PV) inversion technique and treat the temperature errors at the lowest level as a surrogate PV anomaly. After inverting the wind and mass perturbations from the surface thermal anomaly and its pertinent interior PV anomaly, a three-dimensional, dynamically consistent set of ‘‘errors’’ are obtained and added to the model initial conditions to improve the representation of the lower troposphere over the data-sparse ocean. It is found that the numerical model prediction, initialized with the modified initial conditions, exhibits significant improvements in the early rapid deepening and the track of the superstorm over ocean, the development of a prefrontal squall line, and the central sea level pressure traces during the life cycle of the cyclone, as verified against observations. These results show that the methodology proposed is promising in improving the representation of lower-tropospheric meteorological variables in the model initial conditions, based on available surface observations over data-sparse regions.

Numerical prediction of an intense convective system associated with the July 1987 montreal flood. part I: Gravity waves and the squall line

Abstract In this study, a 24‐h high‐resolution numerical prediction of a prefrontal squall line associated with the 14 July 1987 Montreal flood is employed to investigate the origin and role of mesoscale gravity waves in the development of the squall system. The 24‐h integration using an improved mesoscale version of the Canadian regional finite‐element model is first validated against available observations; then non‐observable features are diagnosed to reveal the relationship between deep convection and gravity wave events. It is shown that the model reproduces well many aspects of the squall line, such as the propagation and organization of the convective system, as well as its associated precipitation. It is found that gravity waves are first excited near Lake Erie, following the initiation of early convective activity. Then, these waves propagate eastward and northeastward at speeds of 20 and 35 m s‐1, respectively. As the waves propagate downstream, deep convection radiates rapidly behind the wave t...

A Diagnostic Analysis of the Superstorm of March 1993

Abstract In this study, the synoptic evolution of the March 1993 superstorm is documented using conventional observations, and the predictability and deepening mechanisms of the storm are investigated using a newly implemented mesoscale version of the Canadian Regional Finite-Element (RFE) Mode. It is found that although the RFE model can predict well the large-scale background flow associated with the storm, it fails to predict various important mesoscale elements when it is initialized with fields that contain weak signals of the low-level circulations. However, when the storm is located close to the data-rich region, the model has considerable skill in predicting those mesoscale elements, such as a prefrontal squall line, upper- and low-level jets, as well as the quantitative aspects of their associated precipitation. Observational analysis reveals that the storm developed in a convectively unstable prestorm environment with ample moisture content. It first experienced an antecedent surface vorticity g...

Simulations of the 14 July 1987 squall line using a fully compressible model

Abstract Numerical simulations of the severe squall line of 14 July 1987 are discussed within the context of semi‐Lagrangian and semi‐implicit integrations. The fully compressible non‐hydrostatic Euler set of equations constitutes the basic dynamical framework of the numerical model. With elementary precipitation physics and with a generalized treatment of lateral boundary conditions, nested integrations simulate the observed structure of the squall line. The numerical solution and observations show a well organized precipitation system including a mesoscale fast‐propagating prefrontal squall line and a slow‐propagating system moving with the synoptic wave. The prefrontal squall line is seen to be a manifestation of the organization of an inertia‐gravity wave and has characteristics of a wave‐CISK mechanism. Owing to the interaction between the upper jet stream dynamics and moisture, the prefrontal perturbation is initiated locally and subsequently irradiates away from its point source. The high computational efficiency and the accuracy of the model emphasize its potential and demonstrate its value as an interesting tool for mesoscale modelling.

General Characteristics of Squall Lines Observed in TAMEX

Abstract Six cases of prefrontal squall lines were observed over the Taiwan Strait and western Taiwan during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The mean propagation speed was 10 m s−1, and the mean life span was 11.4 h for the six squall lines. All the lines occurred ahead of the Mei-Yu front and moved away from the front with time. The mean environmental conditions associated with the squall lines were analyzed by compositing the six cases. The environmental conditions observed during the TAMEX squall lines were found with characteristics between tropical and midlatitude squall lines. The steering level was near 7 km during the mature stage. A low-level jet at 3–4 km was present, with strong vertical shear in the presquall environment below 700 hPa. The squall lines were oriented 45° to the shear in the 1–3-km layer, like midlatitude cases. The CAPE, however, is similar to the tropical squall lines. The inflow ahead of the squall lines was deeper and stronger below 400 hPa, and the CAPE w...

Numerical Simulation and Analysis of a Prefrontal Squall Line. Part II: Propagation of the Squall Line as an Internal Gravity Wave

Abstract A numerical study of the squall line that occurred on 17–18 June 1978 was described in Part I of this paper. The squall line was collocated with a surface front during its initial development (at 0000 UTC 18 June 1978), but then propagated faster than the front, resulting in a separation of approximately 200 km by 0300 UTC and 300–400 km by 0600 UTC. In this paper (Part II), the movement of the squall line in the model is shown to be due to the propagation of a deep tropospheric internal gravity wave in a wave–CISK-like (Conditional Instability of the Second Kind) process. The thermal and dynamic perturbations associated with the hypothesized wave are shown to be consistent with internal gravity wave theory, and the characteristics of the wave are compared to similar results from other wave-CISK studies. The current literature favors the mechanism of gust front convergence to explain squall-line propagation, although there are other modeling studies that show specific instances of squall-line pro...

Numerical Simulation and Analysis of a Prefrontal Squall Line. Part I: Observations and Basic Simulation Results

Abstract An observational and numerical study of the squall line that occurred on 17–18 June 1978 is presented. This squall line was initially triggered by the strong surface convergence along a cold front and stretched from Illinois to the Texas Panhandle. The squall line was aligned with the surface front during its initial development (at 0000 UTC 18 June 1978), but then propagated faster than the front, resulting in a separation of approximately 200 km by 0300 UTC and 300–400 km by 0600 UTC. The Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS) is used to model the squall-line development and propagation. Results are described from several experiments that tested the sensitivity to the use of the Kuo-type cumulus parameterization scheme and grid-scale microphysical processes. The simulations that included the cumulus parameterization scheme accurately modeled the initial development of the squall line and its subsequent movement away from the front.

The Taiwan Area Mesoscale Experiment (TAMEX): An Overview

The Taiwan Area Mesoscale Experiment (TAMEX) is a research program conducted jointly by scientists of Taiwan, the Republic of China, and the United States to improve, through better understanding, the forecasting of heavy precipitation events that lead to flash floods. In order to achieve this objective the field phase of TAMEX was launched to collect the data necessary to study The field phase extended from 1 May to 29 June 1987, covering 13 operational missions. In addition, soundings were taken every 6 h within the special network for a 1-month period from 15 May to 15 June. Meteorological phenomena on which special observations were collected include the Mei-Yu front, low-level jet, prefrontal squall lines, open-ocean mesoscale convective systems, mountain convection, terrain-induced mesolows, frontal deformation due to orography, nocturnal convergence lines, and land-sea breeze. In a few cases, heavy precipitation (6-h rainfall exceeding 150 mm) was recorded within the observing network. The field program of TAMEX was an operational success, and an excellent dataset was collected to study the diurnal cycle and other mesoscale circulations over and around Taiwan during undisturbed, as well as disturbed periods. In this paper we review the scientific objectives of TAMEX, describe the experiment area and special observing systems of the field operations, and present highlights of preliminary scientific results. We also discuss the management and archiving of TAMEX datasets, which are open to all interested scientists.

Summertime Cold Fronts in Southeast Australia—Behavior and Low-Level Structure of Main Frontal Types

Abstract The Cold Fronts Research Programme (CFRP) concentrated upon cold frontal systems (synoptic scale fronts) occurring in southeast Australia during late spring and early summer (November and December); many of these tend to the complex, with change lines associated with sea breezes, prefrontal squall lines and related cold outflows (Garratt et al.). The present paper extends the earlier work to include additional observations of frontal events from field phase 3 of the CFRP held in late 1984. In addition we distinguish between this frontal system (Type 1) and a frontal system (Type 2) occurring throughout summer, but predominantly late in the season, which is often dry with a single major change line identified as a mesoscale (coastal) front. 0bservations from the CFRP and other sources are described covering a wide range of frontal intensities, with frontal speeds varying between 5 and 25 m s−1. The nature of the low level flow behind, and in the vicinity of, the active surface cold front (SCF) is ...

The Pre-storm Environment of Midlatitude Prefrontal Squall Lines

Abstract Composite environmental profiles Of wind and temperature are presented for squall line cases that are spatially removed from frontal disturbances. The motions of the squall lines are inferred from radar data, and the mean environmental wind profile is presented in a coordinate system relative to the squall line. The dataset consists of 60 cases from all seasons and from various geographical regions of the continental United States. The results are compared to a similar study by Bluestein and Jain.

Wave-CISK in a Baroclinic Basic State

Prefrontal squall lines are mesoscale convective systems that often cannot be linked to any preexisting organizing mechanism. This suggests the possibility that they are manifestations of a mesoscale instability involving the interaction between convective and larger scales. To investigate this hypothesis, a wave-CISK model is developed for two-dimensional disturbances in a baroclinic basic state with constant vertical wind shear. The governing equations are the linearized Boussinesq equations for an inviscid and hydrostatic fluid on an f-plane. The model domain is infinite in the horizontal and consists of two layers in the vertical representing the troposphere and the stratosphere. The model stratosphere has a larger static stability and no wind shear. The convective heating is confined to the troposphere. Normal mode solutions are assumed, and the convective heating is parameterized in the standard simple fashion its vertical structure is specified, and it is set proportional to the low-level vertical velocity. The model allows for arbitrary orientations of the disturbance axis in the horizontal plane. Results show the existence of two modes: large scale Eady modes, which are amplified slightly by heating, and smaller scale wave-USK model The wave-OSK modes have their maximum growth rates near the symmetric axis, i.e., with disturbance axes approximately parallel to the shear vector. For heating amplitudes that are not unrealistically large, wavelengths of maximum growth are finite and on the mesoscale (on the order of 500 km). Sensitivity experiments for these wave-CISK modes show that the value of the maximum growth rates and the wavelength of maximum growth, are not very sensitive to the form of the vertical heating profile, while other characteristics of the fastest growing mode are. In particular, the orientation of the disturbance axis depends on the heating profile: for maximum heating in the middle troposphere, the disturbance axis is rotated 20°–30° clockwise from the symmetric axis, implying upshear propagation, while for higher levels of maximum heating the disturbance is more nearly aligned with the shear vector. If low level cooling is included in the heating profile, disturbance axes are rotated counterclockwise. Comparisons with observations of squall lines in the atmosphere show some aspects of the solution, such as its vertical structure, to be in qualitative agreement. The orientation angle of the fastest growing mode, however, is near to observed values only if beating profiles with heating maxima at upper levels and cooling at lower levels are used. Predicted phase speeds at observed orientation angles are then too high by a factor of two to five. Reasons for this failure of the wave-CISK theory are discussed.