A brief analysis of the supercell storm in Croatia on 19 July 2023

A series of numerical experiments on an f plane are conducted using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model, version 3 (MM5) to investigate how environmental vertical wind shear affects the motion, structure, and intensity of a tropical cyclone. The results show that a tropical cyclone has a motion component perpendicular to the vertical shear vector, first to the right of the shear and then to the left. An initially axisymmetric, upright tropical cyclone vortex develops a downshear tilt and wavenumber-one asymmetry when embedded in environmental vertical wind shear.

This two-part study proposes fundamental explanations of the genesis, structure, and implications of low-level meso-γ-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such “mesovortices” are observed frequently, at times in association with tornadoes. Idealized simulations are used herein to study the structure and evolution of meso-γ-scale surface vortices within QLCSs and their dependence on the environmental vertical wind shear. Within such simulations, significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s−1 or greater over a 0–2.5- or 0–5-km-AGL layer. As similarly found in observations of QLCSs, these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger-scale bow-shaped system. They generally develop first near the surface but can build upward to 6–8 km AGL. Vortex longevity can be several hours, far longer than individual convective cells within the QLCS; during this time, vortex merger and upscale growth is common. It is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme “straight line” wind damage, as has also been observed with some QLCSs. Surface vortices are also produced for weaker shears but remain shallow, weak, and short-lived. Although similar in size and strength to mesocyclones associated with supercell storms, and also sometimes producing similar hooklike structures in the rain field, it is also shown that the present vortices are quite distinct, structurally and dynamically. Most critically, such vortices are not associated with long-lived, rotating updrafts at midlevels and the associated strong, dynamically forced vertical accelerations, as occur within supercell mesocyclones.

The theory of Rotunno et al. (“RKW” theory) addresses the behavior of squall-line cold pools in vertically sheared flows. It predicts that, within a given thermodynamic environment, a balance between baroclinic vorticity generation by the cold pool and low-level environmental vertical wind shear induces an upright updraft along the gust front that maximizes the initiation of new convective cells. Although this theory has been evaluated numerically, its applicability to observed systems remains unclear and is limited by a lack of critical measurements, including high-frequency thermodynamic and wind profiles across the gust front. Herein, observations from the Atmospheric Radiation Measurement Southern Great Plains (ARM-SGP) observatory near Lamont, Oklahoma, are used to evaluate RKW theory for 10 well-observed squall lines over a 11-yr period. For this evaluation, RKW parameters including cold-pool intensity (c), low-level ambient, line-normal vertical shear (ΔVn), subcloud and cloud-layer updraft tilts, and multiple measures of system intensity are estimated. The c estimates rely on thermodynamic retrievals from the Atmosphere Emitted Radiance Interferometer (AERI), which are uncertain but verify reasonably well against independent observations. As predicted by the theory, for c/ΔVn ≥ 1, c/ΔVn correlates positively with updraft tilt and negatively with system intensity, but these results are not always statistically significant and are also sensitive to the method by which ΔVn is evaluated. Specifically, ΔVn evaluations that extend above the cold-pool top yield greater consistency with RKW predictions. Also, some measures of intensity correlate more strongly with standard moist instability metrics than with RKW parameters. Significance Statement Squall lines are quasilinear mesoscale convection systems capable of producing extensive severe weather. The seminal theory of Rotunno et al. suggested that the intensity of these lines depends on the properties of the system evaporative cold pool and the environmental vertical wind shear. Although these theoretical insights remain relevant today, they have been inadequately verified observationally due to the difficulty of measuring relevant flow properties over the short time intervals associated with squall-line crossings. This study uses 11 years of observations from the Atmospheric Radiation Measurement Southern Great Plains observatory to provide the most comprehensive observational evaluation of RKW theory to date. The results not only support the basic contentions of the theory but also suggest other critical factors affecting squall-line properties.

Hurricane Bertha (1996) was influenced by vertical wind shear with highly variable direction and magnitude. The paper describes a unique method for determining the vertical tilt of a tropical cyclone vortex using satellite and aircraft data. Hurricane Bertha's vortex tracks at three levels are shown during a period of intensification just prior to landfall. During this period, the hurricane vortex becomes more closely aligned in the vertical. Changes in asymmetries of satellite infrared (IR) cold cloud areas are shown to be related to the vortex alignment. Environmental vertical shear measurements throughout Hurricane Bertha's life cycle are presented using IR cloud asymmetries and numerical model analyses. Intensification periods are associated with more symmetric IR cloud measurements. The directions of the IR cloud asymmetric orientations are compared with numerical-model-derived vertical shear directions. The changes in the vertical shear analyses are discussed with respect to observed intens...

Long-term trends in convective environments suggest a decline in environmental vertical wind shear, however instability (i.e. CAPE) is projected to increase. This may be particularly important in the Southeastern U.S. cool season, where instability is characteristically limited but vertical shear is generally very large and can still be favorable for convection given an overall decline. An increase in cool-season instability could result in more frequent hazardous convective weather (HCW) as well as a change in storm mode, resulting in a different distribution of hazards that can impact a new subset of the population that would not currently expect HCW. We exploit 10-year present and future global time-slice MPAS simulations from Michaelis et. al (2019) as a baseline for studying the change in frequency, mode, and intensity of HCW in the Southeastern U.S. The MPAS model uses a global-scale, 60 km grid which is reduced to a high resolution 15 km grid over the Northern Hemisphere to incorporate the effects of large-scale processes. We identify convective windows using the MPAS convective precipitation, CAPE, and 0-6 km vertical shear parameters at 6-hour output intervals. First, we address whether the location, frequency, and character of these windows is changing over time in the non-convection-allowing MPAS simulations. Subsequently, we use a downscaling approach to study whether the mode and severity of resolved convection will change within the convective environments extracted from MPAS.

This paper describes results from an improvement to the objective deviation angle variance technique to estimate the intensity of tropical cyclones from satellite infrared imagery in the North Atlantic basin. The technique quantifies the level of organization of the infrared cloud signature of a tropical cyclone as an indirect measurement of its maximum wind speed. The major change described here is to use the National Hurricane Center’s best-track database to constrain the technique. Results are shown for the 2004–10 North Atlantic hurricane seasons and include an overall root-mean-square intensity error of 12.9 kt (6.6 m s−1, where 1 kt = 0.514 m s−1) and annual root-mean-square intensity errors ranging from 10.3 to 14.1 kt. A direct comparison between the previous version and the one reported here shows root-mean-square intensity error improvements in all years with a best improvement in 2009 from 17.9 to 10.6 kt and an overall improvement from 14.8 to 12.9 kt. In addition, samples from the 7-yr period are binned based on level of intensity and on the strength of environmental vertical wind shear as extracted from Statistical Hurricane Intensity Prediction Scheme (SHIPS) data. Preliminary results suggest that the deviation angle variance technique performs best at the weakest intensity categories of tropical storm through hurricane category 3, representing 90% of the samples, and then degrades in performance for hurricane categories 4 and 5. For environmental vertical wind shear, there is far less spread in the results with the technique performing better with increasing vertical wind shear.

This study uses nine classification machine learning algorithms to examine their skill in making short-fused, storm-based predictions of significant or nonsignificant tornado damage intensity, conditioned upon tornadogenesis, using pretornadic mesocyclone characteristics and the near-storm environment. Radar predictors are from approximately 30 min before tornadogenesis, while environmental predictors are from the model-analysis hour nearest but before the time of tornadogenesis. The most-skilled classifiers are logistic regression, random forests, and gradient boosting as measured by each model’s cross-validated accuracy (≈89%), precision (≈93%), and recall (≈73%) and other binary classification metrics. Learning curves indicate adequate training of models, and calibration curves reveal the reliability of predicted probabilities, with random forests being the most reliable. Also, permutation tests demonstrate the statistical significance of the cross-validated model accuracy. Out of the four radar predictors included in this study, radar-derived pretornadic mesocyclone width and differential velocity are the most important over convective mode and distance from the radar, followed by environmental vertical wind shear and composite parameters. Specifically, wider and stronger pretornadic mesocyclones in environments characterized by larger values of vertical wind shear and composite parameters increase the likelihood of significant tornadoes. The model results could build forecaster confidence in the anticipation of tornado damage intensity and aid forecasters in making informed impact-based warning tag decisions. This could better protect life and property by providing a summary of data relevant to potential tornado damage rating before tornado formation. Important future work includes the addition of other radar-based predictors and the development of a more diverse and realistic sample of tornadic events. Significance Statement Given that the majority of tornado damage and fatalities are due to strong-to-violent tornadoes (EF2+), the purpose of this study is to explore tornado damage intensity prediction for ongoing thunderstorms using binary classification machine learning applied to pretornadic radar data and the near-storm environment. Our results show a skilled prediction of potential tornado damage intensity, conditioned upon tornadogenesis, across multiple models demonstrated through the correct prediction of 73% of EF2+ tornadoes included in this study from the top-performing model. Applying this study in an operational setting may aid in better short-fused anticipation of significant tornado events.

Vertical wind shear and storm motion are two of the most important factors contributing to rainfall asymmetries in tropical cyclones (TCs). Global TC rainfall structure, in terms of azimuthal distribution and asymmetries relative to storm motion, has been previously described using the Tropical Rainfall Measuring Mission Microwave Imager rainfall estimates. The mean TC rainfall distribution and the wavenumber-1 asymmetry vary with storm intensity and geographical location among the six oceanic basins. This study uses a similar approach to investigate the relationship between the structure of TC rainfall and the environmental flow by computing the rainfall asymmetry relative to the vertical wind shear. The environmental vertical wind shear is defined as the difference between the mean wind vectors of the 200- and 850-hPa levels over an outer region extending from the radius of 200–800 km around the storm center. The wavenumber-1 maximum rainfall asymmetry is downshear left (right) in the Northern (Southern) Hemisphere. The rainfall asymmetry decreases (increases) with storm intensity (shear strength). The rainfall asymmetry maximum is predominantly downshear left for shear values > 7.5 m s−1. Large asymmetries are usually observed away from the TC centers. As TC intensity increases, the asymmetry maximum shifts upwind to the left. The analysis is further extended to examine the storm motion and the vertical wind shear and their collective effects on TC rainfall asymmetries. It is found that the vertical wind shear is a dominant factor for the rainfall asymmetry when shear is >5 m s−1. The storm motion–relative rainfall asymmetry in the outer rainband region is comparable to that of shear relative when the shear is <5 m s−1, suggesting that TC translation speed becomes an important factor in the low shear environment. The overall TC rainfall asymmetry depends on the juxtaposition and relative magnitude of the storm motion and environmental shear vectors in all oceanic basins.

The wind- and rainfall areas of tropical cyclones (TCs) making landfall over South Korea were examined for the period 1998–2013 by using the Modern Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) and Tropical Rainfall Measuring Mission (TRMM) 3B42 data. Here, the wind- and rainfall areas were defined as the regions where wind speeds and precipitation rates exceed 14 m s-1 and 80 mm day-1 within 1000 km from the TC center, respectively. In general, TCs show significantly asymmetric wind and rainfall structures, with strong vertical wind shear appearing over South Korea during the landfall period. The rainfall area significantly increases with environmental vertical wind shear while the wind area is not sensitive to it. Composite analyses of the cases of strong and weak vertical wind shear confirm that the increase of rainfall area is related to the asymmetric convection (rising/sinking motion in the downshear-left/upshear-right side) induced by the vertical wind shear. This work highlights the importance of local atmospheric environment in determining the area primarily affected by strong winds or heavy rainfall during TC landfalls.

The first part of the study analyzes the precipitations recorded in June over the 1961-2020 period for 22 climatological weather stations over different regions of Romania in order to determine if precipitations in June 2020 were higher than those recorded before 2020. For this, 5 precipitation indices have been used: number of heavy and very heavy precipitation days, 24-, 48- and 72-hours precipitation amounts. The second part is dedicated to precipitations recorded in June 2020, registered at 157 weather stations from Romania, divided in historical regions; it analyzes the number of heavy and very heavy precipitation days, synoptic and mesoscale conditions from different periods of time using the specific methods of investigation (charts of sea level pressure, geopotential height, temperature and humidity, atmospheric soundings, different stability indices, vertical wind shear, infrared and visible satellite images and radar images of convective storms). The main findings are: precipitations recorded in June 2020 at 22 climatological weather stations did not exceed the absolute maximum precipitation recorded between 1961-2019, and precipitations recorded in 24, 48 and 72 hours were higher only at 2 or 3 weather stations; the number of heavy and very heavy precipitation days recorded in June 2020 were higher in the Carpathians, the North and Center, and the West regions of Romania; synoptic conditions were determined by low values of atmospheric pressure at the surface and/or by the atmospheric depressions, while in the middle troposphere, an atmospheric trough or a cut-off low was present; mesoscale conditions presented low or medium values of Convective Available Potential Energy, negative values of Lifted Index, and weak or medium wind shear in the 0-3 km layer.

Comparative analysis of heavy rainfall area between landfalling typhoon LUPIT (2109) and typhoon LISA (9610)

The precipitation pattern of a landfalling tropical cyclone (TC) with and without a weak environmental vertical wind shear (VWS) is investigated using WRF/NCAR model simulations under idealized conditions. In the simulations without VWS, results show that for the outer band (r ∼ 100–300 km), the cold and dry air originating over smooth land is advected offshore, reduces the stability and develops a band of rainfall on the eastern side of the TC, while the rough land surface tends to trigger more rainfall to the west. For the inner core (r< 100 km), there is only small rainfall asymmetry when the land surface is smooth and dry, but with a rough land surface, the rainfall asymmetry becomes evident and generally stronger rainfall is found over the land areas to the west. Further experiments are performed to compare the effects from weak environmental VWS and land–sea contrast. It is found that the storm‐scale (within 400 km from TC centre) VWS changes continuously in direction and magnitude due to asymmetric diabatic heating and accompanying upper‐level winds. The rainfall pattern in the inner‐core region follows closely the storm‐scale VWS with a downshear‐left relationship regardless of the surface properties, while in the outer‐band region, rainfall distribution is first strongly affected by the surface roughness before landfall and by the environmental VWS afterwards. Therefore, an evolving rainfall–VWS (both environmental and due to storm‐scale dynamics) relationship during TC landfall results.

Previous studies have investigated how the environmental vertical wind shear (VWS) may trigger the asymmetric structure in an initially axisymmetric tropical cyclone (TC) vortex and how TC intensity changes in response. In this study, the possible effect of the initial vortex asymmetric structure on the TC intensity change in response to an imposed environmental VWS is investigated based on idealized full‐physics model simulations. Results show that the effect of the asymmetric structure in the initial TC vortex can either enhance or suppress the initial weakening of the TC in response to the imposed environmental VWS. When the initial asymmetric structure is in phase of the VWS‐induced asymmetric structure, the TC weakening will be enhanced and vice versa. Our finding calls for realistic representation of initial TC asymmetric structure in numerical weather prediction models and observations to better resolve the asymmetric structure in TCs.

Tropical cyclone intensification is simulated with a cloud-resolving model under idealized conditions of constant SST and unidirectional environmental vertical wind shear maximized in the middle troposphere. The intensification process commonly involves a sharp transition to relatively fast spinup before the surface vortex achieves hurricane-force winds in the azimuthal mean. The vast majority of transitions fall into one of two categories labeled S and A. Type S transitions initiate quasi-symmetric modes of fast spinup. They occur in tropical cyclones after a major reduction of tilt and substantial azimuthal spreading of inner-core convection. The lead-up also entails gradual contractions of the radii of maximum wind speed rm and maximum precipitation. Type A transitions begin before an asymmetric tropical cyclone becomes vertically aligned. Instead of enabling the transition, alignment is an essential part of the initially asymmetric mode of fast spinup that follows. On average, type S transitions occur well after and type A transitions occur once the cyclonically rotating tilt vector becomes perpendicular to the shear vector. Prominent temporal peaks of lower-tropospheric CAPE and low-to-midlevel relative humidity averaged over the entire inner core of the low-level vortex characteristically coincide with type S but not with type A transitions. Prominent temporal peaks of precipitation and midlevel vertical mass flux in the meso-β-scale vicinity of the convergence center characteristically coincide with type A but not with type S transitions. Despite such differences, in both cases, the transitions tend not to begin before the distance between the low-level convergence and vortex centers divided by rm reduces to unity.

Building upon the authors’ previous work that examined the dynamics of numerically simulated cyclic mesocyclogenesis and its dependence upon model physical and computational parameters, this study likewise uses idealized numerical simulations to investigate associated dependencies upon ambient vertical wind shear. Specifically, the authors examine variations in hodograph shape, shear magnitude, and shear distribution, leading to storms with behavior ranging from steady state to varying degrees of aperiodic occluding cyclic mesocyclogenesis. However, the authors also demonstrate that a different mode of nonoccluding cyclic mesocyclogenesis may occur in certain environments. Straight hodographs (unidirectional shear) produce only nonoccluding cyclic mesocyclogenesis. Introducing some curvature by adding a quarter circle of turning at low levels results in steady, nonoccluding, and occluding modes. When a higher degree of curvature is introduced—for example, turning through half and three-quarter circles—the tendency for nonoccluding behavior is diminished. None of the full-circle hodographs exhibited cycling during 4 h of simulation. Overall, within a given storm, the preferred mode of cycling is related principally to hodograph shape and magnitude of the ambient vertical shear.