Mesocyclone Research Articles

The Cause of Internal Outflow Surges in a High-Resolution Simulation of the 8 May 2003 Oklahoma City Tornadic Supercell

Abstract A high-resolution simulation of the 8 May 2003 Oklahoma tornadic supercell is analyzed to determine the origin of internal outflow surges within the low-level cold pool. The analyzed simulation has 50-m horizontal grid spacing and is quadruply nested within larger, lower-resolution domains that were initialized via three-dimensional variational data assimilation (3DVAR) of radar and other observations. The high-resolution simulation produces two tornadoes that track in close proximity to the observed tornado on 8 May 2003. The authors’ previous study determined that an internal outflow surge instigated tornadogenesis for the first tornado in this simulation but the cause of this internal outflow surge was unclear. In this study, the vertical momentum equation is analyzed along backward trajectories that are initialized within the tornado-triggering internal outflow surge. The analysis reveals that the internal outflow surge is forced by the dynamic part of the vertical pressure gradient. Further examination reveals that the dynamic forcing is the result of a high pressure perturbation in an area of stagnating flow on the west and northwest sides of the low-level (below ~3 km AGL) mesocyclone. This region of high perturbation pressure is unsteady and forces several other warm internal outflow surges on the west side of the tornado. Cold internal outflow surges also occur later in the simulation and are shown to be buoyantly forced by evaporation and water loading in heavy precipitation.

New areas of polar lows over the Arctic as a result of the decrease in sea ice extent

Three mesocyclones (MCs) over the Russian (Eastern) Arctic are investigated using multispectral satellite remote sensing data, surface analysis maps, and reanalysis data. Advanced retrieval algorithms are used for estimating the geophysical parameter from satellite passive microwave measurements. These methods allow reconstructing in full the geophysical parameter fields characterizing polar lows. Synoptic analysis along with cloud image, atmospheric water vapor content, cloud liquid water content, and sea surface wind speed field analysis show that, while the Arctic sea ice retreats, new areas of open water appear where MCs can arise. A detailed study of several polar low cases reveals the typical conditions of their formation and development. Further studies are in demand due to the danger of MC extreme events for navigation, transport, and fishery operations in these unexplored regions.

Forcing Mechanisms for an Internal Rear-Flank Downdraft Momentum Surge in the 18 May 2010 Dumas, Texas, Supercell

AbstractThe forcing and origins of an internal rear-flank downdraft (RFD) momentum surge observed by the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2) within a supercell occurring near Dumas, Texas, on 18 May 2010 is assessed through ensemble Kalman filter (EnKF) storm-scale analyses. EnKF analyses are produced every 2 min from mobile Doppler velocity data collected by the Doppler on Wheels and Shared Mobile Atmospheric Research and Teaching radars, as well as radial velocity and reflectivity data from the KAMA (Amarillo, Texas) WSR-88D. EnKF analyses are found to reproduce the structure and evolution of an internal RFD momentum surge observed in independent mobile Doppler radar observations.Pressure retrievals of EnKF analyses reveal that the low-level RFD outflow structure is primarily determined through nonlinear dynamic perturbation pressure gradient forcing. Horizontal acceleration into a trough of low perturbation pressure between the low-level mesocyclone and mesoanticyclone and trailing the primary RFD gust front is followed by an abrupt deceleration of air parcels crossing the trough axis. This deceleration and associated strong convergence downstream of the pressure trough and horizontal velocity maximum are indicative of an internal RFD momentum surge. Backward trajectory analyses reveal that air parcels within the RFD surge originate from two source regions: near the surface to the north of the low-level mesocyclone, and in the ambient flow outside of the storm environment at a height of approximately 2 km.

The Relationship between Automated Low-Level Velocity Calculations from the WSR-88D and Maximum Tornado Intensity Determined from Damage Surveys

Abstract The relationship between automated low-level velocity derived from WSR-88D severe storm algorithms and two groups of tornado intensity were evaluated using a 4-yr climatology of 1975 tornado events spawned from 1655 supercells and 320 quasi-linear convective systems (QLCSs). A comparison of peak velocity from groups of detections from the Mesocyclone Detection Algorithm and Tornado Detection Algorithm for each tornado track found overlapping distributions when discriminating between weak [rated as category 0 or 1 on the enhanced Fujita scale (EF0 and EF1)] and strong (EF2–5) events for both rotational and delta velocities. Dataset thresholding by estimated affected population lowered the range of observed velocities, particularly for weak tornadoes while retaining a greater frequency of events for strong tornadoes. Heidke skill scores for strength discrimination were dependent on algorithm, velocity parameter, population threshold, and convective mode, and varied from 0.23 and 0.66. Bootstrapping the skill scores for each algorithm showed a wide range of low-level velocities (at least 7 m s−1 in width) providing an equivalent optimal skill at discriminating between weak and strong tornadoes. This ultimately limits identification of a single threshold for optimal strength discrimination but the results match closely with larger prior manual studies of low-level velocities.

Mesocyclones in Central Europe as seen by radar

The occurrence and characteristics of mesocyclones in Central Europe as seen by radar are analysed. A three year analysis shows an annual and diurnal cycle with a wider maximum in the late afternoon/evening compared to the diurnal cycle of general thunderstorms. Analysis of F2 tornado events and over a hundred hail storms show the characteristics of the corresponding mesocyclones as seen by radar. For all of the six F2 tornadoes in the three-year period in Germany a corresponding mesocyclone could be detected in radar data. Furthermore the analysis reveals that about half of all hail storms in Germany are associated with a mesocyclone detected in radar data within 10km and 10min. Some mesocyclone attributes, e.g. depth and maximum shear, and of the associated convective cell, e.g. reflectivity related parameters VIL, VILD and echotop, have predictive skill for indicating the occurrence of hail. The mesocyclone detection algorithm may support the analysis and nowcasting of severe weather events and thus support the warning process.

Storm-Scale Data Assimilation and Ensemble Forecasts for the 27 April 2011 Severe Weather Outbreak in Alabama

Abstract As part of NOAA’s Warn-on-Forecast (WoF) initiative, a multiscale ensemble-based assimilation and prediction system is developed using the WRF-ARW model and DART assimilation software. To evaluate the capabilities of the system, retrospective short-range probabilistic storm-scale (convection allowing) ensemble analyses and forecasts are produced for the 27 April 2011 Alabama severe weather outbreak. Results indicate that the storm-scale ensembles are able to analyze the observed storms with strong low-level rotation at approximately the correct locations and to retain the supercell structures during the 0–1-h forecasts with reasonable accuracy. The system predicts the low-level mesocyclones of significant isolated tornadic supercells that align well with the locations of radar-derived rotation. For cases with multiple interacting storms in close proximity, the system tends to produce more variability in mesocyclone forecasts from one initialization time to the next until the observations show the dominance of one of the cells. The short-range ensemble probabilistic forecasts obtained from this continuous 5-min storm-scale 6-h-long update system demonstrate the potential of a frequently updated, high-resolution NWP system that could be used to extend severe weather warning lead times. This study also demonstrates the challenges associated with developing a WoF-type system. The results motivate future work to reduce model errors associated with storm motion and spurious cells, and to design storm-scale ensembles that better represent typical 1-h forecast errors.

A Comparison of Near-Surface Buoyancy and Baroclinity across Three VORTEX2 Supercell Intercepts

Abstract Observations obtained during the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2) are analyzed for three supercell intercepts. These intercepts used a fleet of deployable “StickNet” probes, complemented by mobile radars and a mobile mesonet, to map state quantities over the expanse of target storms. Two of the deployments occurred for different stages of a supercell storm near and east of Dumas, Texas, on 18 May 2010. A comparison of the thermodynamic and kinematic characteristics of the storm provides a possible explanation for why one phase was weakly tornadic and the other nontornadic. The weakly tornadic phase features a stronger horizontal virtual temperature gradient antiparallel to the forward-flank reflectivity gradient and perpendicular to the near-surface flow direction, suggesting that air parcels could acquire more significant baroclinic vorticity as they approach the low-level mesocyclone. The strongly tornadic 10 May 2010 case near Seminole, Oklahoma, features comparatively small virtual and equivalent potential temperature deficits, suggesting the strength of baroclinic zones may be less useful than the buoyancy near the mesocyclone for assessing tornado potential. The distribution of positive pressure perturbations and backed ground-relative winds within the forward flank are consistent with the notion of a “starburst” pattern of diverging winds associated with the forward-flank downdraft. Narrow (~1 km wide) zones of intense baroclinic vorticity generation of O(~10−4) s−2 are shown to exist within precipitation on the forward and left sides of the mesocyclone in the Dumas intercepts, not dissimilar from such zones identified in recent high-resolution numerical studies.

Impacts of a Storm Merger on the 24 May 2011 El Reno, Oklahoma, Tornadic Supercell

Abstract On 24 May 2011, a tornadic supercell (the El Reno, Oklahoma, storm) produced tornadoes rated as category 3 and 5 events on the enhanced Fujita scale (EF3 and EF5, respectively) during a severe weather outbreak. The transition (“handoff”) between the two tornadoes occurred as the El Reno storm merged with a weaker, ancillary storm. To examine the impacts of the merger on the dynamics of these storms, a series of three-dimensional cloud-scale analyses are created by assimilating 1-min volumetric observations from the National Weather Radar Testbed’s phased array radar into a numerical cloud model using the local ensemble transform Kalman filter technique. The El Reno storm, its updrafts, and vortices in the analyzed fields are objectively identified, and the changes in these objects before, during, and after the merger are examined. It is found that the merger did not cause the tornado handoff, which preceded the updraft merger by about 5 min. Instead, the handoff likely resulted from midlevel mesocyclone occlusion, in which the midlevel mesocyclone split and a portion is shed rearward with respect to storm motion. During the merger process, the midlevel mesocyclone and updraft structure in the El Reno storm became relatively disorganized. New updraft pulses that formed above colliding outflow boundaries between the two storms tilted environmental vorticity from low levels to generate an additional midlevel vortex that later merged with the El Reno storm’s midlevel mesocyclone. Once the ~10-min merger process was complete, the El Reno storm and its mesocyclone rapidly reintensified, as access to buoyant inflow sector air was restored.

A Multiscale Overview of the El Reno, Oklahoma, Tornadic Supercell of 31 May 2013

Abstract On 31 May 2013 a broad, intense, cyclonic tornado and a narrower, weaker companion anticyclonic tornado formed in a supercell in central Oklahoma. This paper discusses the synoptic- and mesoscale environment in which the parent storm formed, based on data from the operational network of surface stations, rawinsondes, and WSR-88D radars, and from the Oklahoma Mesonet, a Doppler radar wind profiler, Rapid Refresh (RAP) analyses, and photographs. It also documents the overall behavior of the tornadoes and their relationships to features in their parent supercell based on data from a nearby, rapid-scan, polarimetric, mobile Doppler radar. The supercell formed near the intersection of a cold front and a dryline in an environment of moderately strong vertical shear and high CAPE, at the southern end of a line of multicell convective storms. The tornado damage path was as wide as 4.2 km according to the NWS damage assessment and ground-relative Doppler velocities of at least 135 m s−1 were found at the theoretical beam height of <20 m AGL. The tornado debris signature in the copolar cross-correlation coefficient ρhv was as wide as ~4–5 km. After the strong tornado formed, at least one additional cyclonic tornado formed and rotated cyclonically around the main tornado; it was then absorbed by it and the main tornado broadened. Smaller subvortices, which rotated cyclonically around a common axis of rotation, were subsequently observed. The tornado then weakened but remained broad, while the anticyclonic tornado formed to the southeast along the rear-flank gust front.

A statistical analysis of the association between tropical cyclone intensity change and tornado frequency

Tropical cyclones often produce tornadoes that have the potential to compound the injury and fatality counts and the economic losses associated with tropical cyclones. These tornadoes do not occur uniformly through time or across space. Multiple statistical methods were used in this study to analyze the association between tropical cyclone intensity change and tornado frequency. Results indicate that there is an association between the two and that tropical cyclones tend to produce more tornadoes when they are weakening, but the association is weak. Tropical cyclones can also produce a substantial number of tornadoes when they are relatively stable or strengthening.

Analysis of Predictability on a High-Altitude Hail/Graupel Disaster Weather in Nanjing Airspace

Synoptic analysis,radar observation and numerical simulation are used to analyze a convection weather process occurred in Nanjing airspace on 10 August 2011 w hich causing aero damage. All analysis support a deduction that this damage caused by hail strike,but there w as no hail observed on the ground. Since hail generates on high level of about thousands of meters,it may melt in the falling process and rain on the ground. Nanjing w as northw estw ard to the subtropical high pressure zone and in the southw est flow before the trough of Hetao low pressure area at that day. It w as near a low level shear line and had sufficient convective potential condition.Dry cold air distributed on upper level and w arm w et air distributed on low er level. This structure is benefit to the generation of hail. Analysis of radar observation show s that this convective system developed very high. Reflectivity in the core w as very strong. VIL and hail index all had high value. And there w ere meso-cyclone and middle altitude radial convergence in this system. The dynamic and flow structure of this system w ere corresponding to typical hail storm. This paper indicates that this high-altitude hail process w as obviously predicted.

The Relationship Between the Madden–Julian Oscillation and Tropical Cyclone Tornado Clusters

Tropical cyclones (TCs) such as tropical depressions, tropical storms, hurricanes, and their extratropical remnant stages frequently produce tornadoes (TCTORs). These tornadoes are often in clusters and are responsible for considerable property damage and mortality. Recent work has centered on identifying the synoptic-scale features associated with these events. This study is the first to attempt to link the synoptic scale to the intraseasonal scale by examining the impact of the Madden–Julian oscillation (MJO) on the occurrence of tropical cyclone tornado clusters. Results indicate a contributing but not controlling link between MJO phase and TCTOR occurrence. This work aligns with previous work done on TCs and tornadoes as individual events, but indicates additional research is needed on conditions during MJO phase transitions and the interaction with interannual climate variability on the production of TCTORs.

Supercell Low-Level Mesocyclones in Simulations with a Sheared Convective Boundary Layer

Abstract Simulations of supercell thunderstorms in a sheared convective boundary layer (CBL), characterized by quasi-two-dimensional rolls, are compared with simulations having horizontally homogeneous environments. The effects of boundary layer convection on the general characteristics and the low-level mesocyclones of the simulated supercells are investigated for rolls oriented either perpendicular or parallel to storm motion, as well as with and without the effects of cloud shading. Bulk measures of storm strength are not greatly affected by the presence of rolls in the near-storm environment. Though boundary layer convection diminishes with time under the anvil shadow of the supercells when cloud shading is allowed, simulations without cloud shading suggest that rolls affect the morphology and evolution of supercell low-level mesocyclones. Initially, CBL vertical vorticity perturbations are enhanced along the supercell outflow boundary, resulting in nonnegligible near-ground vertical vorticity regardless of roll orientation. At later times, supercells that move perpendicular to the axes of rolls in their environment have low-level mesocyclones with weaker, less persistent circulation compared to those in a similar horizontally homogeneous environment. For storms moving parallel to rolls, the opposite result is found: that is, low-level mesocyclone circulation is often enhanced relative to that in the corresponding horizontally homogeneous environment.

A Parameter for Forecasting Tornadoes Associated with Landfalling Tropical Cyclones

Abstract The authors develop a statistical guidance product, the tropical cyclone tornado parameter (TCTP), for forecasting the probability of one or more tornadoes during a 6-h period that are associated with landfalling tropical cyclones affecting the coastal Gulf of Mexico and the southern Atlantic coast. TCTP is designed to aid forecasters in a time-limited environment. TCTP provides a “quick look” at regions where forecasters can then conduct detailed analyses. The pool of potential predictors included tornado reports and tropical cyclone data between 2000 and 2008, as well as storm environmental parameters. The original pool of 28 potential predictors is reduced to six using stepwise regression and logistic regression. These six predictors are 0–3-km wind shear, 0–3-km storm relative helicity, azimuth angle of the tornado report from the tropical cyclone, distance from the cyclone’s center, time of day, and 950–1000-hPa convective available potential energy. Mean Brier scores and Brier skill scores are computed for the entire TCTP-dependent dataset and for corresponding forecasts produced by the Storm Prediction Center (SPC). TCTP then is applied to four individual cyclone cases to qualitatively and quantitatively assess the parameter and compare its performance with SPC forecasts. Results show that TCTP has skill at identifying regions of tornado potential. However, tornadoes in some tropical systems are overpredicted, but underpredicted in others. TCTP 6-h forecast periods provide slightly poorer statistical performance than the 1-day tornado probability forecasts from SPC, probably because the SPC product includes forecaster guidance and because their forecasts are valid for longer periods (24 h).

A Multiple-Vortex Tornado in Southeastern Brazil

AbstractDuring the late afternoon hours of 24 May 2005 a severe weather outbreak occurred in the state of São Paulo, southeastern Brazil. Severe thunderstorms were observed ahead of a surface cold front, including a (Southern Hemisphere) cyclonic left-moving supercell that produced a multiple-vortex tornado in the outskirts of the town of Indaiatuba, Brazil (23.1°S, 47.2°W). A documentation of the multivortex structure of the tornado and of the cloud-base features is performed using still images from a video that recorded the event. Characteristics of the tornadic thunderstorm and the synoptic-scale environment in which it developed are examined using Doppler radar data, geostationary satellite imagery, surface and upper-air observations, and data from the National Centers for Environmental Prediction’s Climate Forecast System Reanalysis. The cloud base of the thunderstorm displayed morphological features associated with midlatitude tornadic supercells, including a low-level mesocyclone and a “clear slot”; however, the rear-flank downdraft did not obscure the view of the tornado from the western flank of the storm. The tornadic storm developed in a moist prefrontal environment with a low-level jet. Limited mesoscale observations hampered the quantitative analysis of the local thermodynamic forcing, but the available data suggest that the supercell developed under moderate conditional instability. Strong speed and directional vertical wind shear were observed, while the local boundary layer displayed very high relative humidity and low surface-based lifting condensation level.

VORTEX2 Observations of a Low-Level Mesocyclone with Multiple Internal Rear-Flank Downdraft Momentum Surges in the 18 May 2010 Dumas, Texas, Supercell*

Abstract Observations collected in the second Verification of the Origins of Rotation in Tornadoes Experiment during a 15-min period of a supercell occurring on 18 May 2010 near Dumas, Texas, are presented. The primary data collection platforms include two Ka-band mobile Doppler radars, which collected a near-surface, short-baseline dual-Doppler dataset within the rear-flank outflow of the Dumas supercell; an X-band, phased-array mobile Doppler radar, which collected volumetric single-Doppler data with high temporal resolution; and in situ thermodynamic and wind observations of a six-probe mobile mesonet. Rapid evolution of the Dumas supercell was observed, including the development and decay of a low-level mesocyclone and four internal rear-flank downdraft (RFD) momentum surges. Intensification and upward growth of the low-level mesocyclone were observed during periods when the midlevel mesocyclone was minimally displaced from the low-level circulation, suggesting an upward-directed perturbation pressure gradient force aided in the intensification of low-level rotation. The final three internal RFD momentum surges evolved in a manner consistent with the expected behavior of a dynamically forced occlusion downdraft, developing at the periphery of the low-level mesocyclone during periods when values of low-level cyclonic azimuthal wind shear exceeded values higher aloft. Failure of the low-level mesocyclone to acquire significant vertical depth suggests that dynamic forcing above internal RFD momentum surge gust fronts was insufficient to lift the negatively buoyant air parcels comprising the RFD surges to significant heights. As a result, vertical acceleration and the stretching of vertical vorticity in surge parcels were limited, which likely contributed to tornadogenesis failure.

Cloud-to-ground lightning signatures of long-lived tornadic supercells on 27–28 April 2011

This study integrates past research methodologies, data from the National Lightning Detection Network (NLDN), and geographic information system techniques to assess the lightning and severe weather hazard relationship for the 27–28 April 2011 United States tornado outbreak. NLDN and Doppler radar data are used to examine the cloud-to-ground (CG) lightning characteristics associated with seven supercell thunderstorms that produced long-track, significant and/or violent tornadoes. Analyses indicate that CG lightning flashes alone do not provide enough information for the detection of a lightning jump prior to tornadogenesis. All seven supercells were dominated by negative-polarity CG lightning flashes; which is expected due to the geographic location and elevated low-level moisture found in the outbreak environment. The correlation between low-level mesocyclone strength and total CG lightning flash rate was varied and inconsistent among all storms despite their formation and sustenance in similar environmental and geographic space. Additional case studies, as well as climatological approaches, are required to discover if the varying lightning–tornado relationships found in this study are consistent with other tornadic environments.

Patterns in 500 hPa geopotential height associated with temporal clusters of tropical cyclone tornadoes

Tropical cyclones are known to produce tornadoes, but the total number of tornadoes a tropical cyclone will produce, or the rate at which it will produce tornadoes throughout its lifespan, is not well understood. To understand better the link between the large-scale environment in which a tropical cyclone is located and tornado production, this study analyses and classifies 500 hPa geopotential height plots that correspond to temporal clusters of tornadoes. The primary 500 hPa patterns identified in this study represent various stages of interaction with the mid-latitude westerlies that a tropical cyclone may experience as it tracks from the tropics and subtropics to the mid-latitudes. Statistical analysis of the number of tornadoes per cluster suggests that tropical cyclones produce more tornadoes when they begin to interact with, or become embedded within, the mid-latitude westerlies.

The spatial distribution of tornadoes and high wind impacts associated with inland-moving tropical cyclones in the southeastern United States

The southeastern United States is routinely hit by tropical cyclones (TC). As TC track inland and dissipate their inland impacts can be substantial. This study examined the spatial patterns of societal impacts associated with tornadoes and high winds with 31 inland-moving TC that made landfall from 1985 to 2008. Hourly weather information was collected from all available first-order weather stations affected by each storm, as well as tropical cyclone preliminary reports issued from the National Hurricane Center. Societal impacts were identified through selected newspapers across the region and the National Climatic Data Center’s Storm Data. Geographic Information System (GIS) software was employed to make geometric measurements of the distance and direction of the impacts relative to storm center. From these measurements, the spatial distribution of the societal and meteorological impacts was plotted relative to the track (e.g., left vs. right) and location (forward vs. rear sector) of the cyclone center. Various tropical cyclone attributes, including size, strength, and forward speed of movement were then related to the occurrence of different impacts and their location relative to the cyclone track. The majority of tropical cyclone tornado and high wind impacts occur in the right-forward sector of the tropical cyclone. However, many TC produce impacts that occur in other sectors far from the center of circulation. These abnormalities are associated in many cases with interactions between the tropical cyclone circulation, topography, peripheral dry air, and extratropical synoptic weather features.

Sensitivities of tornadogenesis to drop size distribution in a simulated subtropical supercell over eastern China

Numerical simulations with the Advanced Regional Prediction System (ARPS) model were performed to investigate the impact of microphysical drop size distribution (DSD) on tornadogenesis in a subtropical supercell thunderstorm over Anhui Province, eastern China. Sensitivity experiments with different intercept parameters of rain, hail and snow DSDs in a Lin-type microphysics scheme were conducted. Results showed that rain and hail DSDs have a significant impact on the simulated storm both microphysically and dynamically. DSDs characterized by larger (smaller) intercepts have a smaller (larger) particle size and a lower (higher) mass-weighted mean fall velocity, and produce relatively stronger (weaker) and wider (narrower) cold pools through enhanced (reduced) rain evaporation and hail melting processes, which are then less favorable (favorable) for tornadogenesis. However, tornadogenesis will also be suppressed by the weakened mid-level mesocyclone when the cold pool is too weak. When compared to a U.S. Great Plain case, the two microphysical processes are more sensitive to DSD variations in the present case with a higher melting level and deeper warm layer. This suggests that DSD-related cloud microphysics has a stronger influence on tornadogenesis in supercells over the subtropics than the U.S. Great Plains.