Bow Echo Research Articles

A Preliminary Examination of WRF Ensemble Prediction of Convective Mode Evolution

Abstract Accurately simulating convective mode evolution can assist forecasters in the severe weather warning process. A few prior studies have examined the skill of simulating convective modes using single, deterministic forecasts. The present study extends the earlier evaluations to a small, four-member ensemble, with each member incorporating varying initial and lateral boundary conditions, microphysics schemes, and planetary boundary layer schemes. Simulated convective modes from thirty-two 12-h simulations were categorized into nine classifications using a classification scheme developed from previous studies. Multiple methods were used to derive forecasts of these convective classifications, creating an hourly deterministic ensemble mode forecast and probabilistic forecasts for 1-, 6-, and 12-h periods. Forecasts were compared with observed radar reflectivity for verification. In general, hourly deterministic ensemble mode forecasts showed improvement over individual member forecasts. The small ensemble produced more skillful individual cellular convective mode forecasts than individual linear mode forecasts, with the least skill present for bow echoes and squall lines with trailing stratiform precipitation. In contrast, the ensemble was more skillful at forecasting the broader linear convective group than the broader cellular convective group. For a limited number of these cases, a test was performed using a larger 10-member ensemble run by the National Center for Atmospheric Research (NCAR) to examine what impacts the small ensemble size might have. The results did not differ substantially, suggesting the findings from the small ensemble can be generalized. Probabilistic forecasts for longer time periods were more skillful than shorter-term probabilistic forecasts.

The Mesoscale Convective Systems with bow echo radar signatures as an example of extremely severe and widespread geohazard in Poland

Abstract In the last two decades we can notice a significant increase of severe anemological events, which are mostly connected with mesoscale convective systems and a cold front of a deep low-pressure system. One of them are very strong winds with speeds more than 25 m/s. They caused material damage and threatening people's lives. The most dangerous are winds generated by mesoscale convective systems where radar reflectivity signatures of bow echo/derecho appeared. In this paper the area of occurrence of such phenomenon in Poland are described and the features of bow echo signatures on radar images are presented and explained. Additionally one of the most severe event and still very weakly known episode of 11th August 2017 derecho in Poland is analysed. The damage data from European Severe Weather Database (ESWD) were analysed to confirm if the August 11th storm met derecho criteria. To identify the radar reflectivity signatures inside MCC the data from the Polish Institute of Meteorology and Water Management shared on the radar-opadow.pl site were used. The CAPPI 1 km data were very useful to determine the convective forms. After that the data from synoptic station were examined for presenting the running of selected meteorological elements. Finally, some information about material damage in infrastructures and forests are mentioned.

Organizational Modes of Severe Wind-producing Convective Systems over North China

Severe weather reports and composite radar reflectivity data from 2010–14 over North China were used to analyze the distribution of severe convective wind (SCW) events and their organizational modes of radar reflectivity. The six organizational modes for SCW events (and their proportions) were cluster cells (35.4%), squall lines (18.4%), nonlinear-shaped systems (17.8%), broken lines (11.6%), individual cells (1.2%), and bow echoes (0.5%). The peak month for both squall lines and broken lines was June, whereas it was July for the other four modes. The highest numbers of SCW events were over the mountains, which were generally associated with disorganized systems of cluster cells. In contrast, SCW associated with linear systems occurred mainly over the plains, where stations recorded an average of less than one SCW event per year. Regions with a high frequency of SCW associated with nonlinear-shaped systems also experienced many SCW events associated with squall lines. Values of convective available potential energy, precipitable water, 0–3-km shear, and 0–6-km shear, were demonstrably larger over the plains than over the mountains, which had an evident effect on the organizational modes of SCW events. Therefore, topography may be an important factor in the organizational modes for SCW events over North China.

Three Ovidian Tails

Three Ovidian Tails PAUL BAROLSKY Kneeling at the edge of a pond in push-up position, a beautiful nude boy crowned with flowers gazes down at the water in which he beholds his reflection. In love, he is enthralled. Thus, the image of Narcissus rendered by the Florentine painter Alessandro Allori in a work that has been largely overlooked until recently. Datable to the second half of the sixteenth century, it now hangs in the Turkish Embassy in Washington, DC. In the distant background of the picture we behold Echo, who extends an arm upward seemingly in a gesture of despair, since she is rejected by Narcissus. She stands before a mass of rock, her left leg beginning to turn to stone, the very stone into which she vanishes as only her voice remains when she echoes her beloved. The painting abounds in Alessandro Allori, Narcissus. Photograph © Patricia Rubin. arion 26.3 winter 2019 closely observed passages such as the little fish in the pond, one of which has captured a worm. There are other notable details such as the crab on a rock in the foreground, or the clusters of pendulous, ripe grapes, fruit of Bacchus, to whom androgynous Narcissus is likened in Ovid’s Metamorphoses. Allori also employs poetic simile in his picture, since the fingers of the youth’s left hand rhyme with the finger-like roots that emerge from the water. These roots are magnified in turn by their reflected image on the water’s surface. 136 three ovidian tails Abraham Janssens, Nymph and Pan Among the Reeds. AKG / Museen Böttcherstra ße. The apex of the picture, we might say, is baby Cupid who, hovering above, aims his dart at Narcissus and thus arouses the boy’s desire. The shape of Cupid’s bow echoes that of Narcissus ’s prominent buttocks which are framed by the attractive , bright golden drapery fluttering prominently around the youth’s posterior. One might well say that Allori features the buttocks of Narcissus’s body most provocatively in a kind of butt which recalls that of Cupid in the famous Venus, Cupid, Folly, and Time painted by the artist’s teacher, Bronzino. What Allori pictures is not exactly what Ovid writes. It is, however, how Allori imagines the Narcissus of Ovid’s tale. Allori is not alone among Ovidian artists in focusing on the backside of the human body. The posterior is especially prominent in another mythological picture, this one from the early seventeenth century, executed by a disciple of Rubens, Abraham Janssens. This work has found its way to Bremen, where it is now housed in the Ludwig Roselius Museum. It illustrates Pan’s pursuit of Syrinx, who is metamorphosed into reeds in a story told by Mercury in order to put Argus asleep. Ovid is Janssens’s point of departure. The artist depicts the moment when a muscular Pan with a ruddy, almost leathery face embraces Syrinx in her new form as a bunch of reeds. Janssens departs from Ovid with a most novel invention. He depicts a woman, seen from behind, who dominates the composition. With sharp contrasts of lights and darks, vestiges of Caravaggio, the painter emphasizes the female’s imposingly broad backside, which is framed by a billowing, bright, attention-grabbing red drapery that she seems to hold across the front of her body. Could it be that Janssens here alludes to the backside of Syrinx as Pan would have seen it when he chased her? It would seem that we are given not only a view of Pan embracing reeds, we are simultaneously given a Pan’s-eye view of Syrinx’s prominent and provocative backside before her metamorphosis. I wish to consider one final example of the derrière in Ovidian art, this one prominent in a picture painted well Paul Barolsky 137 over a hundred years after Janssens worked: Louis-JeanFran çois Lagrenée’s Abduction of Deianira by the Centaur Nessus, a work which is now in the Louvre. In Ovid’s story, Hercules arrives with his bride Deianira at the shore of the river Evenus where she is abducted by Nessus , who flees only to have Hercules shoot him with...

Structure and Evolution of a Convective System with Bow Echo Associated with Terrain on Jeju Island, Korea

Structure and Evolution of a Convective System with Bow Echo Associated with Terrain on Jeju Island, Korea

Skeleton matching with applications in severe weather detection

Severe weather conditions cause an enormous amount of damages around the globe. Bow echo patterns in radar images are associated with a number of these destructive conditions such as damaging winds, hail, thunderstorms, and tornadoes. They are detected manually by meteorologists. In this paper, we propose an automatic framework to detect these patterns with high accuracy by introducing novel skeletonization and shape matching approaches. In this framework, first we extract regions with high probability of occurring bow echo from radar images and apply our skeletonization method to extract the skeleton of those regions. Next, we prune these skeletons using our innovative pruning scheme with fuzzy logic. Then, using our proposed shape descriptor, Skeleton Context, we can extract bow echo features from these skeletons in order to use them in shape matching algorithm and classification step. The output of classification indicates whether these regions are bow echo with over 97% accuracy.

Synoptic Analysis and Hindcast of an Intense Bow Echo in Western Europe: The 9 June 2014 Storm

Abstract On Pentecost Monday, 9 June 2014, a severe linearly organized mesoscale convective system (MCS) hit Belgium and western Germany. This storm was one of the most severe thunderstorms in Germany in decades. The synoptic-scale and mesoscale characteristics of this storm are analyzed based on remote sensing data and in situ measurements. Moreover, the forecast potential of the storm is evaluated using sensitivity experiments with a regional climate model. The key ingredients for the development of the Pentecost storm were the concurrent presence of low-level moisture, atmospheric conditional instability, and wind shear. The synoptic and mesoscale analysis shows that the outflow of a decaying MCS above northern France triggered the storm, which exhibited the typical features of a bow echo like a bookend vortex and a rear-inflow jet. This resulted in hurricane-force wind gusts (reaching 40 m s−1) along a narrow swath in the Rhine–Ruhr region leading to substantial damage. Operational numerical weather prediction models mostly failed to forecast the storm, but high-resolution regional model hindcasts enable a realistic simulation of the storm. The model experiments reveal that the development of the bow echo is particularly sensitive to the initial wind field and the lower-tropospheric moisture content. Adequate initial and boundary conditions are therefore essential for realistic numerical forecasts of such a bow echo event. It is concluded that the Pentecost storm exhibited a comparable structure and a similar intensity to observed bow echo systems in the United States.

How’s the Weather? Severe Weather Classifications in Syndromic Surveillance

ObjectiveTo report the results of the application of New Jersey’s SevereWeather Classifier in New Jersey’s syndromic surveillance systemduring two extreme weather events.IntroductionHurricane ‘Superstorm’ Sandy struck New Jersey on October 29,2012, causing harm to the health of New Jersey residents and billionsof dollars of damage to businesses, transportation, and infrastructure.Monitoring health outcomes for increased illness and injury due toa severe weather event is important in measuring the severity ofconditions and the efficacy of state response, as well as in emergencyresponse preparations for future severe weather events. Followingthe experience with Hurricane Sandy, NJDOH initiated a projectto develop a suite of 19 indicators, known as the Severe WeatherClassifier (SWC) in EpiCenter, an online system which collectsemergency department chief complaint data in real-time, to performsyndromic surveillance of extreme weather–related conditions.NJDOH has since used these classifiers in more recent events tomonitor for weather-related visits to storm-affected area emergencydepartments (ED’s).In June, 2015, a squall line of damaging thunderstorms, known asa “bow echo,” caused downed wires and multi-day power outagesin Camden and Gloucester counties in southern New Jersey. Almostexactly seven months later, in January, 2016, Winter Storm Jonasdropped more than a foot of snow over New Jersey. These eventsprovided an opportunity to assess the indicators within SWC.MethodsThe impact of these storms on ED visits was assessed in EpiCenterby using the SWC sub-classifications for disrupted outpatientmedical care (dialysis and oxygen needs, and medication refills).Rates per 1,000 ED visits were calculated on two weeks of EDvisits by classification for each storm. For the June 2015 bow echostorm, this assessment focused on Gloucester and Camden counties,the two hardest hit by the storm. For Winter Storm Jonas, rates per1,000 ED visits were calculated statewide since all counties wereimpacted.ResultsAfter the June, 2015 bow echo storm, both Camden and Gloucestercounty ED’s experienced increases in disrupted medical care, themost notable being for oxygen needs (Figures 1 and 2). Duringand after Winter Storm Jonas, ED visits for oxygen assistance andmedicine refills were the most impacted (Figure 3). It is speculatedthat ED visits for dialysis were not noticeably higher since the stormoccurred over a weekend when, generally, treatments take placeduring weekdays.ConclusionsWhile not every classification in the suite that makes up theSWC would be relevant in every extreme weather event, having the19 various elements available provides tools for state and local usersto monitor storm impacts both locally and at the state level.

Polarimetric Doppler radar analysis of squall line systems crossing Salento Peninsula

ABSTRACTA detailed observational study of squall line thunderstorms, a particular form of mesoscale convective systems arranged in a line that repeatedly affected Salento Peninsula (southern Apulia, Italy) throughout 2015, is presented and discussed. Specifically, three relevant events that occurred in January, at the end of September and in October are described, combining thermodynamic and synoptic analysis with radar imagery delivered by the cutting‐edge polarimetric Doppler radar recently installed in Torchiarolo, in the central part of Salento Peninsula. Reported observational information reveals that, regardless of the season of the year, convergence along an approaching cold front represents the common initiating mechanism leading to the formation of linear convective systems. Furthermore, high resolution Doppler data provided by Torchiarolo radar allows better detection and prompt recognition of severe weather signatures, such as bow echoes and cyclonic mesovortices, commonly associated with squall line thunderstorms. Therefore, the present study clearly confirms the key role of the new advanced weather radar installed in Torchiarolo which thus represents a powerful tool giving researchers and forecasters valuable information about kinematic and microphysical properties of severe weather systems crossing Apulia region, especially Salento Peninsula.

The occurrence of convective systems with a bow echo in warm season in Poland

The characteristics of occurrence of convective systems with a bow echo in Poland in the warm season between 2007 and 2014 were presented. Using the identification criteria proposed by Fujita (1978), Burke and Schultz (2004), Klimowski et al. (2000, 2004), and supplemented by Gatzen (2013), 91 bow echo cases were identified in the analysed period. Depending on the year, the maximum number of cases usually occurred in July or August. From the multi-annual perspective, 28 and 30 cases occurred in those months. The diurnal variation of bow echo occurrences showed that it developed, or entered the Polish territory, usually between the hours of 13:00 UTC and 21:00 UTC, while it disappeared or receded beyond the country border in the hours between 15:00 UTC and 23:00 UTC. The areas most exposed to the occurrence of bow echo included the northern part of Lubuskie and Wielkopolska provinces, the southern part of West Pomerania province, Łódź province and Silesia province. In the period studied, the south-western direction of movement of convective systems with a bow echo was prevalent. This direction changed, however, depending on the region and the month of occurrence. The type and development mode of a bow echo, as well as synoptic conditions conducive to its occurrence were defined for selected cases. The results showed that BECs (bow-echo complex) and BEs (classic bow echo) were the predominant types (respectively 43 and 29 cases). Bow echoes developed most frequently from a squall line, or from a combination of a few, often weakly organized convective cells.

Structure and Motion of Severe-Wind-Producing Mesoscale Convective Systems and Derechos in Relation to the Mean Wind

Abstract The goal of this study is to document differences in the convective structure and motion of long-track, severe-wind-producing MCSs from short-track severe-wind-producing MCSs in relation to the mean wind. An ancillary goal is to determine if these differences are large enough that some criterion for MCS motion relative to the mean wind could be used in future definitions of “derechos.” Results confirm past investigations that well-organized MCSs, including those that produce derechos, tend to move faster than the mean wind, exhibiting a significantly larger degree of propagation (component of MCS motion in addition to the component contributed by the mean flow). Furthermore, well-organized systems that produce shorter-track swaths of damaging winds likewise tend to move faster than the mean wind with a significant propagation component along the mean wind. Therefore, propagation in the direction of the mean wind is not necessarily a characteristic that can be used to distinguish derechos from nonderechos. However, there is some indication that long-track damaging wind events that occur without large-scale or persistent bow echoes and mesoscale convective vortices (MCVs) require a strong propagation component along the mean wind direction to become long lived. Overall, however, there does not appear to be enough separation in the motion characteristics among the MCS types to warrant the inclusion of a mean-wind criterion into the definition of a derecho at this time.

Interaction of an Upper-Tropospheric Jet with a Squall Line Originating along a Cold Frontal Boundary

AbstractOn 8 June 2003, an expansive squall line along a surface cold frontal boundary was sampled during the Bow Echo and Mesoscale Convective Vortex Experiment. The Naval Research Laboratory P-3 aircraft and the National Oceanic and Atmospheric Administration P-3 aircraft simultaneously sampled the leading and trailing edge of this squall line, respectively, with X-band Doppler radars. Data from these two airborne radar systems have been synthesized to produce a pseudo-quad-Doppler analysis of the squall line, yielding a detailed three-dimensional kinematic analysis of its structure. A simulation of the squall line was carried out using the Weather Research and Forecasting Model to complement the pseudo-quad-Doppler analysis. The simulation employed a 3-km, convection-allowing, nested domain centered over the pseudo-quad-Doppler domain, along with a 9-km parent domain to capture the larger synoptic-scale cyclone.The pseudo-quad-Doppler analysis reveals that the convective line was embedded within the upper-tropospheric jet stream, causing local decelerations and deviations in the jet-level flow. The vertical transport of low momentum air from the boundary layer via convective updrafts is shown to significantly decelerate jet-level flow. Pressure perturbations associated with the intrusion of low momentum air into the jet stream–level flow led to deviation of the jet stream flow around the squall line that resulted in counter-rotating ribbons of vertical vorticity parallel to the squall line. Model results indicate that disturbances in the jet stream structure persisted downwind of the squall line for several hours.

Retrievals of ice cloud microphysical properties of deep convective systems using radar measurements

AbstractThis study presents newly developed algorithms for retrieving ice cloud microphysical properties (ice water content (IWC) and median mass diameter (Dm)) for the stratiform rain and thick anvil regions of deep convective systems (DCSs) using Next Generation Radar (NEXRAD) reflectivity and empirical relationships from aircraft in situ measurements. A typical DCS case (20 May 2011) during the Midlatitude Continental Convective Clouds Experiment (MC3E) is selected as an example to demonstrate the 4‐D retrievals. The vertical distributions of retrieved IWC are compared with previous studies and cloud‐resolving model simulations. The statistics from six selected cases during MC3E show that the aircraft in situ derived IWC and Dm are 0.47 ± 0.29 g m−3 and 2.02 ± 1.3 mm, while the mean values of retrievals have a positive bias of 0.19 g m−3 (40%) and negative bias of 0.41 mm (20%), respectively. To evaluate the new retrieval algorithms, IWC and Dm are retrieved for other DCSs observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) using NEXRAD reflectivity and compared with aircraft in situ measurements. During BAMEX, a total of 63, 1 min collocated aircraft and radar samples are available for comparisons, and the averages of radar retrieved and aircraft in situ measured IWC values are 1.52 g m−3 and 1.25 g m−3 with a correlation of 0.55, and their averaged Dm values are 2.08 and 1.77 mm. In general, the new retrieval algorithms are suitable for continental DCSs during BAMEX, especially within stratiform rain and thick anvil regions.

On Contrasting Ensemble Simulations of Two Great Plains Bow Echoes

AbstractBow echo structures, a subset of mesoscale convective systems (MCSs), are often poorly forecast within deterministic numerical weather prediction model simulations. Among other things, this may be due to the inherent low predictability associated with bow echoes, deficient initial conditions (ICs), and inadequate parameterization schemes. Four different ensemble configurations assessed the sensitivity of the MCSs’ simulated reflectivity and radius of curvature to the following: perturbations in initial and lateral boundary conditions using a global dataset, different microphysical schemes, a stochastic kinetic energy backscatter (SKEB) scheme, and a mix of the previous two. One case is poorly simulated no matter which IC dataset or microphysical parameterization is used. In the other case, almost all simulations reproduce a bow echo. When the IC dataset and microphysical parameterization is fixed within a SKEB ensemble, ensemble uncertainty is smaller. However, while differences in the location and timing of the MCS are reduced, variations in convective mode remain substantial. Results suggest the MCS’s positioning is influenced primarily by ICs, but its mode is most sensitive to the model error uncertainty. Hence, correct estimation of model error uncertainty on the storm scale is crucial for adequate spread and the probabilistic forecast of convective events.

Wind estimation around the shipwreck of Oriental Star based on field damage surveys and radar observations

Based on observational analyses and on-site ground and aerial damage surveys, this work aims to reveal the weather phenomena—especially the wind situation—when Oriental Star capsized in the Yangtze River on June 1, 2015. Results demonstrate that the cruise ship capsized when it encountered strong winds at speeds of at least 31m s −1 near the apex of a bow echo embedded in a squall line. As suggested by the fallen trees within a 2-km radius around the wreck location, such strong winds were likely caused by microburst straight-line wind and/or embedded small vortices, rather than tornadoes.

Long-Term Morphological Changes in Simulated Supercells Following Mergers with Nascent Supercells in Directionally Varying Shear

Abstract Mergers involving supercells remain a challenge for severe thunderstorm forecasting. In this study, mergers between supercells and ordinary cells (e.g., cells forming in a similar environment but too young to be fully developed supercells) are investigated. A series of numerical experiments are performed using an idealized, homogenous environment supportive of cyclonically rotating, right-moving supercells. Warm bubbles are introduced at different times, resulting in two storms of different maturity; their placement is used to control the location of the merger and the relative maturity of the second storm. Simplified conceptual models for the long-term outcomes of mergers are developed. In the simplest mode of merger, outflow from the new cell cuts off inflow to the original. If the new cell’s cold pool is not sufficiently strong to cut off the inflow to the original cell, the minimum separation of the updraft maxima during the merger becomes a key controlling factor in the outcome. If it is less than 10 km, an updraft collision occurs, resulting in a classic supercell. If it is greater than 20 km and the new cell merges into the original cell’s forward flank, a dual-cell system results. If it is between 10 and 20 km, the enhanced precipitation produced during the merger leads to a cold pool surge and an updraft bridge, joining the original updrafts and developing into either a small bow echo (with forward-flank mergers) or a supercell on the classic high-precipitation spectrum (with rear-flank mergers), depending on the distribution of precipitation in the merging system.

Storm-Scale Data Assimilation and Ensemble Forecasting with the NSSL Experimental Warn-on-Forecast System. Part I: Radar Data Experiments

AbstractThis first part of a two-part study on storm-scale radar and satellite data assimilation provides an overview of a multicase study conducted as part of the NOAA Warn-on-Forecast (WoF) project. The NSSL Experimental WoF System for ensembles (NEWS-e) is used to produce storm-scale analyses and forecasts of six diverse severe weather events from spring 2013 and 2014. In this study, only Doppler reflectivity and radial velocity observations (and, when available, surface mesonet data) are assimilated into a 36-member, storm-scale ensemble using an ensemble Kalman filter (EnKF) approach. A series of 1-h ensemble forecasts are then initialized from storm-scale analyses during the 1-h period preceding the onset of storm reports. Of particular interest is the ability of these 0–1-h ensemble forecasts to reproduce the low-level rotational characteristics of supercell thunderstorms, as well as other convective hazards. For the tornado-producing thunderstorms considered in this study, ensemble probabilistic forecasts of low-level rotation generally indicated a rotating thunderstorm approximately 30 min before the time of first observed tornado. Displacement errors (often to the north of tornado-affected areas) associated with vorticity swaths were greatest in those forecasts launched 30–60 min before the time of first tornado. Similar forecasts were produced for a tornadic mesovortex along the leading edge of a bow echo and, again, highlighted a well-defined vorticity swath as much as 30 min prior to the first tornado.

Mesoscale Convection and Bimodal Cyclogenesis over the Bay of Bengal

AbstractMesoscale convective systems (MCSs) are an essential component of cyclogenesis, and their structure and characteristics determine the intensity and severity of associated cyclones. Case studies were performed by simulating tropical cyclones that formed during the pre- and postmonsoon periods in 2007 and 2010 over the Bay of Bengal (BoB). The pre- (post) monsoon environment was characterized by the coupling of northwesterly (southwesterly) wind to the early advance southwesterly (northeasterly) monsoonal wind in the BoB. The surges of low-level warm southwesterlies with clockwise-rotating vertical shear in the premonsoon period and moderately cool northeasterlies with anticlockwise-rotating vertical shear in the postmonsoon period transported moisture and triggered MCSs within preexisting disturbances near the monsoon trough over the BoB. Mature MCSs associated with bimodal cyclone formations were quasi linear, and they featured leading-edge deep convection and a trailing stratiform precipitation region, which was very narrow in the postmonsoon cases.In the premonsoon cases, the MCSs became severe bow echoes when intense and moist southwesterlies were imposed along the dryline convergence zone in the northern and northwestern BoB. However, the development formed a nonsevere and nonorganized linear system when the convergence zone was farther south of the dryline. In the postmonsoon cases, cyclogenesis was favored by squall-line MCSs with a north–south orientation over the BoB. All convective systems moved quickly, persisted for a long time, and contained suitable environments for developing low-level cyclonic mesovortices at their leading edges, which played an additional role in forming mesoscale convective vortices during cyclogenesis in the BoB.

Mesovortices within the 8 May 2009 Bow Echo over the Central United States: Analyses of the Characteristics and Evolution Based on Doppler Radar Observations and a High-Resolution Model Simulation

Abstract A derecho-producing bow-echo event over the central United States on 8 May 2009 is analyzed based on radar observations and a successful real-data WRF simulation at 0.8-km grid spacing. Emphasis is placed on documenting the existence, evolution, and characteristics of low-level mesovortices (MVs) that form along the leading edge of the bowing system. The genesis of near-surface high winds within the system is also investigated. Significant MVs are detected from the radar radial velocity using a linear least squares derivatives (LLSD) method, and from the model simulation based on calculated vorticity. Both the observed and simulated bow-echo MVs predominantly form north of the bow apex. MVs that develop on the southern bow tend to be weaker and shorter-lived than their northern counterparts. Vortex mergers occur between MVs during their forward movement, which causes redevelopment of some MVs in the decaying stage of the bow echo. MVs located at (or near) the bow apex are found to persist for a notably longer lifetime than the other MVs. Moreover, the model results show that these bow-apex MVs are accompanied with damaging straight-line winds near the surface. These high winds are mainly caused by the descent of the rear-inflow jet at the bow apex, but the MV-induced vortical flow also has a considerable contribution. The locally enhanced descent of the rear-inflow jet near the mesovortex is forced primarily by the dynamically induced downward vertical pressure gradient force while the buoyancy force only plays a minor role there.

The Genesis of Mesovortices within a Real-Data Simulation of a Bow Echo System

Abstract The genesis of two mesovortices (MVs) within a real-data, convection-resolving simulation of the 8 May 2009 central U.S. bow echo system is studied. Both MVs form near the bow apex but differ distinctively in intensity, lifetime, and damage potential. The stronger and longer-lived mesovortex, MVa, stays near the bow apex where the system-scale rear-inflow jet (RIJ) is present. The descending RIJ produces strong downdrafts and surface convergence, which in turn induce strong vertical stretching and intensification of MVa into an intense mesovortex. In contrast, the weaker and shorter-lived mesovortex, MVb, gradually moves away from the bow apex, accompanied by localized convective-scale downdrafts. Lagrangian circulation and vorticity budget analyses reveal that the vertical vorticity of MVs in general originate from the tilting of near-surface horizontal vorticity, which is mainly created via surface friction. The circulation of the material circuit that ends up to be a horizontal circuit at the foot of the MVs increases as the frictionally generated horizontal vortex tubes pass through the tilted material circuit (tilted following backward trajectories defining the material circuit) surface, especially in the final few minutes prior to mesovortex genesis. The tilted material circuit becomes horizontal at the MV foot, turning associated horizontal vorticity into vertical. The results show at least qualitatively that, in addition to baroclinicity, surface friction can also have significant contributions to the generation of low-level MVs, which was not considered in previous MV studies.