Bow Echo Research Articles

Advanced Method for Forecasting and Warning of Severe Convective Weather and Local-scale Hazards

Hurricane Ida ferociously affected many south-eastern and eastern parts of the United States, making it one of the strongest hurricanes in recent years. Advanced forecast and warning tool has been used to track the path of the ex-Hurricane, Ida, as it left New Orleans on its way towards the northeast, accurately predicting significant supercell development above New York City on September 01, 2021. This advanced method accurately detected the area with the highest possible level of convective instability with 24-h lead time and even Level 5, devised in the categorical outlooks legend of the system. Therefore, an extreme level implied a very high probability of the local-scale hazard occurring above the NYC. Cloud model output fields (updrafts and downdrafts, wind shear, near-surface convergence, the vertical component of relative vorticity) show the rapid development of a strong supercell storm with rotating updrafts and a mesocyclone. The characteristic hook-shaped echo signature visible in the reflectivity patterns indicates a signal for a highly precipitable (HP) supercell with the possibility of tornado initiation. Open boundary conditions represent a good basis for simulating a tornado that evolved from a supercell storm, initialized with initial data obtained from a real-time simulation in the period when the bow echo and tornado-like signature occurred. Тhe modeled results agree well with the observations.

Study of an Asymmetric and Anticyclonic Bow Echo Near Taiwan

On 2 April 2007, a strong bow echo struck southern Taiwan, with a peak surface wind speed of 26 m s−1. On observation, the rear inflow jet (RIJ) was located at the northern flank and only one anticyclonic vortex dominated behind the bow structure. This case was different from commonly occurring cyclonic–anticyclonic pairs of bookend vortices, and was investigated through data analysis, model simulation, and vorticity budget diagnostics. The present bow echo formed at the leading edge of a cold front, with favorable ingredients of instability, large west-southwesterly vertical wind shear, and dry air aloft. Farther behind the front, however, stable conditions could not support deep convection and the portion north of the RIJ was therefore missing. Within a frontal flow structure, the developing mechanism of the anticyclonic vortex also differed from typical cases. As the low-level (west-southwesterly) vertical shear pointed from right to left of the line, and the tilting effect of updrafts generated positive (negative) vorticity at the front (rear) side. South of and below the RIJ, the anticyclonic vorticity was enhanced by the stretching and tilting effect of system-generated horizontal vorticity via a sinking motion. These sources of vorticity were then advected downward and southwestward by the postfrontal flow near the surface.

Comparative study of the simulation and mechanism of two different severe thunderstorms embedded in a squall line in North China

Based on a cloud-permitting numerical simulation carried out using the Weather Research and Forecasting Model, version 4.1.5, a squall line (SL) system accompanied by severe thunderstorms in North China in July 2019 was simulated. Both the observation and simulation showed that the convective organizational modes of the thunderstorms experienced three main stages: a quasi-linear (QL), bow echo (BE) and SL stage. Across these three stages, the thunderstorms consisted of two different convective systems. One was a thunderstorm with severe wind gusts (SWGs), which was concentrated in the BE and SL stages, and the other was a thunderstorm without gales but that featured high frequency lightning (HFL), which initiated and enhanced in the QL and SL stages, respectively. In the BE stage, SWGs began to extend backwards from the apex of the bow structure. In the SL stage later, the extent of SWGs increased significantly and dominated over a wide range behind the convective line as the convection nearly died out. Further analysis showed that the features and formation mechanisms of the SWG system were significantly different between the two stages. During the BE period, SWGs were mainly caused by the downdrafts in the convective core, which could be attributed to the formation and development of the horizontal vortex pair in the lower level. In the SL stage, however, SWGs mainly attributed to the sinking of the rear inflow, which was associated with the wide extent of negative buoyancy in the uneven convective environment, enhanced by the evaporation of the rainwater near the surface. HFL enhanced strongly on an extreme cold surface in the SL stage as the thunderstorm grew vigorously. The outflow along the edge of the cold dome converged with the front inflow and continuously triggered convection. The development and the interaction between the multiple convective cells triggered lightning, which was enhanced by the strong condensation heating in the storms.

Multi-Scale Interaction between a Squall Line and a Supercell and Its Impact on the Genesis of the “0612” Gaoyou Tornado

The interaction between a squall line and a supercell and its impact on the genesis of a tornado that occurred in Gaoyou, Jiangsu Province, China on 12 June 2020 were analyzed using multi-source observations. The tornado formed as the result of an intensified meso-γ supercell in a favorable large-scale environment. The supercell developed in front of a squall line and slowly intensified after its formation. Due to its small size and weak intensity, the supercell did not produce any severe weather before the approaching of the squall line. As the squall line entered its mature stage with the formation of a well-organized bow echo, the supercell in front of the bow echo began to rapidly intensify and finally led to the tornado touchdown. The analysis of mesoscale and storm-scale wind fields indicated that the bow echo of the approaching squall line modified the kinematic fields near the supercell in such a way that was favorable for the intensification of the supercell. The interaction between the squall line and the supercell may have played a critical role in the occurrence of this tornado.

Detection of Bow Echoes in Kilometer-Scale Forecasts Using a Convolutional Neural Network

Abstract Bow echoes (BEs) are bow-shaped lines of convective cells that are often associated with swaths of damaging straight-line winds and small tornadoes. This paper describes a convolutional neural network (CNN) able to detect BEs directly from French kilometer-scale model outputs in order to facilitate and accelerate the operational forecasting of BEs. The detections are only based on the maximum pseudoreflectivity field predictor (“pseudo” because it is expressed in mm h−1 and not in dBZ). A preprocessing of the training database is carried out in order to reduce imbalance issues between the two classes (inside or outside bow echoes). A CNN sensitivity analysis against a set of hyperparameters is done. The selected CNN configuration has a hit rate of 86% and a false alarm rate of 39%. The strengths and weaknesses of this CNN are then emphasized with an object-oriented evaluation. The BE largest pseudoreflectivities are correctly detected by the CNN, which tends to underestimate the size of BEs. Detected BE objects have wind gusts similar to the hand-labeled BE. Most of the time, false alarm objects and missed objects are rather small (e.g., <1500 km2). Based on a cooperation with forecasters, synthesis plots are proposed that summarize the BE detections in French kilometer-scale models. A subjective evaluation of the CNN performances is also reported. The overall positive feedback from forecasters is in good agreement with the object-oriented evaluation. Forecasters perceive these products as relevant and potentially useful to handle the large amount of available data from numerical weather prediction models.

An Analysis of the 3 May 2020 Low-Predictability Derecho Using a Convection-Allowing MPAS Ensemble

Abstract This study analyzes the low short-range predictability of the 3 May 2020 derecho using a 40-member convection-allowing Model for Prediction Across Scales (MPAS) ensemble. Elevated storms formed in south-central Kansas late at night and evolved into a progressive mesoscale convective system (MCS) during the morning while moving across southern Missouri and northern Arkansas, and affected western and middle Tennessee and southern Kentucky in the afternoon. The convective initiation (CI) in south-central Kansas, the organization of a dominant bow echo MCS, and the MCS maintenance over Tennessee were identified as the three main predictability issues. These issues were explored using three MPAS ensemble members, observations, and the Rapid Refresh analyses. The MPAS members were classified as successful or unsuccessful with regard to each predictability issue. CI in south-central Kansas was sensitive to the temperature and dewpoint profiles in low levels, which were associated with greater elevated thermodynamic instability and lower level of free convection in the successful member. The subsequent organization of a dominant bowing MCS was well predicted by the member that had more widespread convection in the early stages and no detrimental interaction with other simulated convective systems. Last, the inability of MPAS ensemble members to predict the MCS maintenance over western and middle Tennessee was linked to a dry bias in low levels and much lower thermodynamic instability ahead of the MCS compared to observations. This case demonstrates the challenges in operational forecasting of warm-season derecho-producing progressive MCSs, particularly when ensemble numerical weather prediction guidance solutions differ considerably.

Evaluation and Modification of Microphysics Schemes on the Cold Pool Evolution for a Simulated Bow Echo in Southeast China

AbstractA merger‐formation bow echo (MFBE) in southeast China during the pre‐summer rainy season (PSRS) was simulated using three microphysics schemes including Thompson (THOM), Morrison (MORR), and Weather Research and Forecasting Double‐Moment 6‐Class (WDM6) schemes to compare against polarimetric observations and Variational Doppler Radar Analysis System (VDRAS) analyses. The three schemes captured the basic kinematic structures for this MFBE event after assimilating radar radial velocities, but all underpredicted the cold pool strength by ∼25%. Particularly, THOM produced the best raindrop size distributions (DSDs) and precipitation pattern compared with observed retrievals; further analyses indicated the larger raindrop size bias and the weak cold pool strength in THOM were owing to the relatively low rain breakup efficiency and inefficient rain evaporation, respectively. By decreasing the cutoff diameter of rain breakup parameterization from the default 1.6–1.2 mm (i.e., increasing breakup efficiency) and increasing evaporation efficiency by threefold in THOM, the simulated DSDs and precipitation were greatly improved, and the cold pool strength was significantly increased from 77% to 99% compared to that in VDRAS analyses. This study illustrated a plausible approach of combining polarimetric radar retrievals and VDRAS analyses as bases to adjust THOM default settings in simulating a MFBE event in southeast China with physical characteristics more consistent with observations. Since microphysical processes vary from convective organizations and climate regions, it is recognized more cases studies are needed in the future to examine the validity and approach in this study to improve simulations and predictions of MFBEs in southeast China.

A Merger-Formation Bow Echo Induced by low-level mesovortex in South China

A Merger-Formation Bow Echo Induced by low-level mesovortex in South China

Observational Analysis of a Wind Gust Event during the Merging of a Bow Echo and Mini-Supercell in Southeastern China

The merging of a fast-moving bow echo with a convective cell of a hook-echo signature was studied by using polarimetric radar detections. Gusts with wind speeds near 35 m s−1 were recorded by the surface station, which caused significant damage. A convective cell with a mesovortex signature, which is hereafter referred to as a mini-supercell, was observed over the northeast of the bow echo before the convective merging. It was found that the mesovortex possessed cyclonic circulation and resembled a supercell-like feature. The merging of the bow echo and the mini-supercell strengthened the updraft near the apex of the bow echo. The enhanced updraft was also demonstrated by the appearance of a differential reflectivity (ZDR) column with a topmost height of 4 km above the melting layer (~4 km). The bow was separated into northern and southern sectors after merging with the mini-supercell, leading to the gusty wind over the surface of the south sector.

Merger Formation of Bow Echo in South China

Merger Formation of Bow Echo in South China

Dancing Sprites Above a Lightning Mapping Array—An Analysis of the Storm and Flash/Sprite Developments

AbstractOne of the most enigmatic types of sprites is the dancing sprite which appears to dance above the storm as sequential luminous emissions a few 100 ms or less apart. Dancing sprites occur in relatively small proportion and many aspects of their generation remain unknown. We present a multi‐instrumental analysis of a 20‐hr duration Mesoscale Convective System (MCS) over the northwestern Mediterranean Sea on September 21, 2019, that produced 21 sprites recorded with a video camera, of which 19 (90) were dancing sprites. The asymmetric trailing stratiform MCS developed in strong convective conditions having a CAPE of 3,500 J . It formed several convective cores (up to 2,900 with cloud top temperature −C) and exhibited a bow echo structure during the sprite production period. Using Lightning Mapping Array data, we show that the sprite producing positive cloud‐to‐ground (SP + CG) flashes mainly initiated at the edge of the convective line on the side of the stratiform region. The flashes propagated 100–200 km across it, producing both positive and negative CG strokes. The 19 dancing sprite events included 49 sequences, of which 46 were associated with distinct SP + CG strokes and 3 with surges during the continuing current. An especially bright and wide sprite sequence was produced by three distinct SP + CG strokes that occurred within 3 ms and spread over 54 km. This sprite sequence could be classified as a new sprite category resembling a "wall" but structured in three groups, each associated with one of the +CG strokes.

Synoptic-Scale Precursors, Characteristics and Typing of Nocturnal Mesoscale Convective Complexes in the Great Plains

Mesoscale convective complexes (MCCs) occur frequently during the warm season in the central U.S. and can produce flooding rains, hail and tornadoes. Previous work has found that the synoptic-scale environment can greatly affect, and be affected by, the development and maintenance of MCCs. Ninety-two MCC cases from 2006-2011 are manually identified using infrared satellite imagery and partitioned into three types (upstream trough, zonal and ridge) using a unique manual synoptic typing based on 500-hPa height patterns. Upstream trough cases feature an amplified longwave 500-hPa trough upstream of the MCC genesis region (GR), while the 500-hPa flow is relatively flat in zonal cases, and a strong 500-hPa ridge is present over the Rockies in ridge cases. Individual case and storm-relative composite analyses of a subset of 28 cases show that of the three types, upstream trough cases feature both the strongest quasigeostrophic forcing for ascent and lower-tropospheric frontogenesis, the latter of which enhances ascent and is associated with a strong southerly low-level jet (LLJ). Zonal and ridge cases feature smaller magnitudes (in descending order) of all ascent-forcing parameters. Ridge cases, in particular, are characterized by weak Q-vector convergence, but easterly upslope flow likely acts as a compensating ascent mechanism. A thermodynamic analysis shows that high-θe air is advected into the GR in all three MCC types, and serves as fuel for development and maintenance. However, while the southerly LLJ advects high-θe air from the Gulf of Mexico in the upstream trough and zonal cases, such air is already pooled in the High Plains in the ridge cases and advected into the GR by easterly flow. In accordance with the synoptic-dynamic analysis, upstream trough cases have the longest duration and largest impact on the synoptic-scale environment, while ridge cases are the shortest-lived. The various underlying precipitation structures of each group are also explored; zonal cases, for example, appear to preferentially be associated with bow echoes.

Operational Uses of Spectrum Width

Spectrum width is a WSR-88D product that has been available to operational forecasters since the radar was deployed. In 2008, super-high-resolution reflectivity, velocity and spectrum width data became available. Six cases exemplifying operational use of spectrum width are presented; five are from after the upgrade. The cases were selected to depict the wide array of uses of spectrum width (SW). In one case, use of SW improved forecaster capability to evaluate the strength of horizontal shear within a bow echo’s mesovortex. One case shows that SW can be extremely helpful in determining location of boundaries, which aids in overall situational awareness. In another case, SW aided forecaster confidence to issue a tornado warning with lead time. If a storm is close to the radar (55 km in this example), SW can be used to clarify the location of the rear flank downdraft, assess where its wind damage may be a threat, and discern subsequent cutoff of the tornado from the warm, moist inflow. Finally, when used in a derecho case, SW helped a forecaster to identify more quickly where wind damage threats were likely.

A Multi-scale Analysis of the Atmospheric Conditions Associated with the Daytona Beach Tornado of Christmas Day, 2006

This case study describes a severe-storm event over Florida, Georgia, and South Carolina on 25 December 2006, with a particular focus on an F2 tornado that struck Daytona Beach, FL and caused over $50 million in damages. The severe weather occurred over a 12-h period and was associated with a deep upper-level trough and surface front moving through the southeastern U.S. Morning soundings over Florida showed low to moderate CAPE and strong vertical wind shear, consistent with seasonal composite tornadic soundings for the region. A quasi-linear convective system moved onshore near Tampa during the midmorning hours, the northern half of which accelerated and produced bow echoes that resulted in two tornadoes and nontornadic wind damage over Pasco, Sumter, and Lake Counties between 1620 and 1725 UTC. This portion of the line then moved into Volusia County and spawned F2 tornadoes in Deland and Daytona Beach after 1800 UTC. Data from the Melbourne National Weather Service Forecast Office’s Weather Surveillance Radar-1988D (WSR-88D), Daytona Beach International Airport (DAB) Automated Surface Observing System (ASOS), and DAB Low Level Wind Shear Alert System (LLWAS) were integrated to analyze conditions at the east end of the DAB runway 7L/25R complex, where the tornado first appeared. The LLWAS is normally used by air traffic control personnel for monitoring airport wind-shear conditions. The 10-s LLWAS wind data filled critical temporal and spatial gaps in the WSR-88D and ASOS data, and captured evidence of strong winds and cyclonic curvature nearly coincident with the locations of the radar-identified velocity couplet and tornado itself.

Bow Echo and Mesovortex Evolution during the 2 May 2007 North Texas Derecho

On 2 May 2007, a derecho-producing mesoscale convective system tracked across west-central and north Texas, producing damaging wind speeds up to 40 m s-1 (78 kt). Damage included loss of roofs, overturning of trucks and manufactured homes, and several downed electrical towers. Some of the most intense damage occurred in association with deep mesovortices (≥3 km AGL) along the leading edge of the line of storms. The event began as a cluster of storms in eastern New Mexico and west Texas that evolved into a line-echo wave pattern, and finally transitioned into a larger bow echo as it moved into north Texas. The derecho occurred in a thermodynamic environment characterized as moderately unstable, with weak vertical wind shear and weak synoptic-scale forcing for vertical motions. Some studies suggest that the thermodynamic environment on this day supported a derecho, but other research utilizing numerical simulations indicate that the vertical wind shear was too weak to sustain the elevated rear-inflow jet and deep mesovortices that occurred in this case. Several mesovortices, with average lifetimes of 12 min and depths ≥3 km, were identified within 97 km (60 mi) of the nearest network radar. This event both supports and contradicts various aspects of prior research on derecho evolution and behavior.

A Preliminary Study on the Organizational Modes of Mesoscale Convective Systems Associated With Warm‐Sector Heavy Rainfall in South China

AbstractThis work examines the organizational modes of mesoscale convective systems (MCSs) associated with warm‐sector heavy rainfall events in South China based on composite radar reflectivities during April–June from 2007 to 2020. The results showed that these MCSs can be classified into the nonlinear mode (NL) and eight linear modes: trailing stratiform precipitation (TS), no stratiform precipitation (NS), leading stratiform precipitation (LS), parallel stratiform precipitation (PS), embedded lines (EL), bow echoes (BE), training line/adjoining stratiform (TL/AS), and multiple rain bands (MRB). The NL mode accounts for 40% of the total, while linear modes account for 60%, with the TS mode being the most frequent linear mode. The detailed features of the top three most frequent organizational modes (NL, TS, and NS) were examined in this work. First, the MCSs of the top three organizational modes have different formation modes. Eighty‐four percent of NL systems and 77% of TS systems form via the broken areal mode, while 83% of NS systems form via the broken line mode. Second, relative to TS and NS systems, NL systems develop faster and have shorter lifespans but slower movements with higher 3‐h and 6‐h maximum precipitation values. Third, environmental analysis indicates that the Total Totals index (TTI) is significantly different among all the top three organizational modes, with the highest value in TS mode and the lowest value in NS mode. These features are apparently different from their counterparts in central East China due to different synoptic environments.

Numerical Simulation and the Underlying Mechanism of a Severe Hail‐Producing Convective System in East China

AbstractA severe convective system that produced egg‐sized hail in Jiangsu province in eastern China on April 28, 2015 was simulated using a cloud‐resolving Weather Research and Forecasting model with the spectral bin microphysics scheme. The simulation reproduced the two stages of the convective system: the linear convective system (LCS) and the bow echo system (BES) stages. During the LCS stage, the intensity of updrafts was slightly stronger than that in the BES stage, with abundant supercooled water above the −20°C layer, and graupel and hail mainly generated in this period. The microphysical budget analyses and size distribution characteristics of hail were studied within one cell that produced hail during the LCS. It was suggested that hail mainly formed through water‐graupel collision and increased in size by collecting liquid water. Hail particles with smaller diameter were located around the perimeter of the main updrafts, while larger ones were distributed at the left edge of the updrafts. Trajectories and size growth processes of hailstones within the chosen cell were calculated by a three‐dimensional hail growth model, and it was found that hail originated from the middle levels on the east side of the updrafts, and were transformed westward and downward to the lower levels. They continued to ascend along the left edge of the updrafts and experienced one or more up‐down recycles and fell down to the ground on the west side of the updrafts.

A seven-year climatology of the initiation, decay and morphology of severe convective storms during the warm season over North China

AbstractThe present work established a 7-year climatology of the initiation, decay, and morphology of severe convective storms (SCSs) during the warm seasons (May–September) of 2011–2018 (except 2014) over North China. This was achieved by using severe weather reports, precipitation observations, and composite Doppler radar reflectivity data. A total of 371 SCSs were identified. SCSs primarily initiated around noon with the highest frequency over the high terrain of Mount Taihang, and they mostly decayed over the plains at night. The storm morphologies were classified into three types of cellular storms (individual cells, clusters of cells, and broken lines), six types of linear systems (convective lines with no stratiform, with trailing stratiform, leading stratiform, parallel stratiform, embedded lines, and bow echoes), and nonlinear systems. Three types of severe convective weather, namely, short-duration heavy rainfall, hail, and thunderstorm high winds associated with these morphologies were investigated. Nonlinear systems were the most frequent morphology, followed by clusters of cells. Convective lines with trailing stratiform were the most frequent linear morphology. A total of 1,429 morphology samples from the 371 SCSs were found to be responsible for 15,966 severe convective weather reports. Linear (nonlinear) systems produced the most short-duration heavy rainfall (hail and thunderstorm high wind) reports. Bow echos were most efficient in producing both short-duration heavy rainfall and thunderstorm high wind reports whereas broken lines had the highest efficiency for hail production. The results in the present study are helpful for local forecasters to better anticipate the storm types and associated hazardous weather.

A five-year climatological lightning characteristics of linear mesoscale convective systems over North China

The lightning characteristics of 89 linear mesoscale convective systems (MCSs) occurred over the Beijing area from the year of 2007 to 2011 were analyzed by lightning data and S-band radar information. According to different morphology of radar echo, linear MCSs were classified into six categories: leading convective lines with a trailing stratiform region (TS for short), leading stratiform region with a trailing convective region (LS), leading convective lines with no stratiform region (NS), bow echo of leading lines (BE), leading convective lines with a parallel stratiform region (PS), and broken line stratiform (BL). The results showed that linear MCSs occurred frequently over the Beijing area at the summer time, in particular, TS, LS and PS modes totally accounted for 73% of linear MCSs. On the average, lightning mainly concentrated in the linear convective region, and small amount of lightning occurred in the stratiform region. At the mature stage, lightning mainly located in the linear convective region with strong radar echo of TS, LS, and PS MCSs. At the dissipating stage, the lightning gradually increased in the stratiform region of TS MCSs, whereas fewer lightning occurred in the mode of LS MCSs, and positive cloud-to-ground (+CG) lightning accounted for a large percentage in the mode of PS MCSs. From the distribution of +CG/CG in three archetypes of linear MCSs, it is found that PS MCSs with highest the proportion of +CG/CG, and then followed by LS MCSs while the lowest ratio appeared in TS MCSs. The relationship between lightning frequency and volume of 40 dBZ radar echo exhibited linear correlation in three distinct archetypes of linear MCS. Environmental factors of linear MCSs exhibited that high CAPE (Convective Available Potential Energy) value and the larger wind shear of 0–6 km played an important role to sustain the convective development of TS, LS and PS MCSs.

Differences between Severe and Nonsevere Warm-Season, Nocturnal Bow Echo Environments

AbstractNocturnal bow echoes can produce wind damage, even in situations where elevated convection occurs. Accurate forecasts of wind potential tend to be more challenging for operational forecasters than for daytime bows because of incomplete understanding of how elevated convection interacts with the stable boundary layer. The present study compares the differences in warm-season, nocturnal bow echo environments in which high intensity [>70 kt (1 kt ≈ 0.51 m s−1)] severe winds (HS), low intensity (50–55 kt) severe winds (LS), and nonsevere winds (NS) occurred. Using a sample of 132 events from 2010 to 2018, 43 forecast parameters from the SPC mesoanalysis system were examined over a 120 km × 120 km region centered on the strongest storm report or most pronounced bowing convective segment. Severe composite parameters are found to be among the best discriminators between all severity types, especially derecho composite parameter (DCP) and significant tornado parameter (STP). Shear parameters are significant discriminators only between severe and nonsevere cases, while convective available potential energy (CAPE) parameters are significant discriminators only between HS and LS/NS bow echoes. Convective inhibition (CIN) is among the worst discriminators for all severity types. The parameters providing the most predictive skill for HS bow echoes are STP and most unstable CAPE, and for LS bow echoes these are the V wind component at best CAPE (VMXP) level, STP, and the supercell composite parameter. Combinations of two parameters are shown to improve forecasting skill further, with the combination of surface-based CAPE and 0–6-km U shear component, and DCP and VMXP, providing the most skillful HS and LS forecasts, respectively.