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Paul M. Markowski

Abstract

Nearly 50 years of observations of hook echoes and their associated rear-flank downdrafts (RFDs) are reviewed. Relevant theoretical and numerical simulation results also are discussed. For over 20 years, the hook echo and RFD have been hypothesized to be critical in the tornadogenesis process. Yet direct observations within hook echoes and RFDs have been relatively scarce. Furthermore, the role of the hook echo and RFD in tornadogenesis remains poorly understood. Despite many strong similarities between simulated and observed storms, some possibly important observations within hook echoes and RFDs have not been reproduced in three-dimensional numerical models.

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Paul M. Markowski

Abstract

Two long-lived tornadic supercells were sampled by an automobile-borne observing system on 3 May 1999. The “mobile mesonet” observed relatively warm and moist air, weak baroclinity, and small pressure excess at the surface within the rear-flank downdrafts of the storms. Furthermore, the downdraft air parcels, which have been shown to enter the tornado in past observational and modeling studies, were associated with substantial convective available potential energy and small convective inhibition. The detection of only small equivalent potential temperature deficits (1–4 K) within the downdrafts may imply that the downdrafts were driven primarily by nonhydrostatic pressure gradients and/or precipitation drag, rather than by the entrainment of potentially cold environmental air at midlevels.

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Paul M. Markowski

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A 25-member ensemble of relatively high-resolution (75-m horizontal grid spacing) numerical simulations of tornadic supercell storms is used to obtain insight on their intrinsic predictability. The storm environments contain large and directionally varying wind shear, particularly in the boundary layer, large convective available potential energy, and a low lifting condensation level. Thus, the environments are extremely favorable for tornadic supercells. Small random temperature perturbations present in the initial conditions trigger turbulence within the boundary layers. The turbulent boundary layers are given 12 h to evolve to a quasi–steady state before storms are initiated via the introduction of a warm bubble. The spatially averaged environments are identical within the ensemble; only the random number seed and/or warm bubble location is varied. All of the simulated storms are long-lived supercells with intense updrafts and strong mesocyclones extending to the lowest model level. Even the storms with the weakest near-surface rotation probably can be regarded as weakly tornadic. However, despite the statistically identical environments, there is considerable divergence in the finescale details of the simulated storms. The intensities of the tornado-like vortices that develop in the simulations range from EF0 to EF3, with large differences in formation time and duration also being exhibited. The simulation differences only can be explained by differences in how the initial warm bubbles and/or storms interact with turbulent boundary layer structures. The results suggest very limited intrinsic predictability with respect to predicting the formation time, duration, and intensity of tornadoes.

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Paul M. Markowski

Abstract

Idealized simulations are used to investigate the contributions of frictionally generated horizontal vorticity to the development of near-surface vertical vorticity in supercell storms. Of interest is the relative importance of barotropic vorticity (vorticity present in the prestorm environment), baroclinic vorticity (vorticity that is principally generated by horizontal buoyancy gradients), and viscous vorticity (vorticity that originates from the subgrid-scale turbulence parameterization, wherein the effects of surface drag reside), all of which can be advected, tilted, and stretched. Equations for the three partial vorticities are integrated in parallel with the model. The partial vorticity calculations are complemented by analyses of circulation following material circuits, which are often able to be carried out further in time because they are less susceptible to explosive error growth.

Near-surface mesocyclones that develop prior to cold-pool formation (this only happens when the environmental vorticity is crosswise near the surface) are dominated by only barotropic vertical vorticity when the lower boundary is free slip, but both barotropic and viscous vertical vorticity when surface drag is included. Baroclinic vertical vorticity grows large once a cold pool is established, regardless of the lower boundary condition and, in fact, dominates at the time the vortices are most intense in all but one simulation (a simulation dominated early by a barotropic mode of vortex genesis that may not be relevant to real convective storms).

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Abdullah Kahraman and Paul M. Markowski

Abstract

A climatology of tornadoes in Turkey is presented using records from a wide variety of sources (e.g., the Turkish State Meteorological Service, European Severe Weather Database, newspaper archives, Internet searches, etc.). The climatology includes the annual, diurnal, geographical, and intensity distributions of both mesocyclonic and nonmesocyclonic tornadoes. From 1818 to 2013, 385 tornado cases were obtained. The tornadoes range from F0 to F3, with F1 being the most frequently reported or inferred intensity. Mesocyclonic tornadoes are most likely in May and June, and a secondary maximum in frequency is present in October and November. Nonmesocyclonic tornadoes (waterspouts) are most common in the winter along the (southern) Mediterranean coast and in the fall along the Black Sea (northern) coast. Tornadoes (both mesocyclonic and nonmesocyclonic) are most likely in the afternoon and early evening hours.

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Paul M. Markowski and Jerry M. Straka

Abstract

The authors document some of the unusual rotating updrafts (one of which produced a tornado) that developed over central Oklahoma on 28 October 1998 in an environment of strong (1.8 × 10−2 s−1) low-level (0–3 km) mean shear. The maximum convective available potential energy (including virtual temperature effects) a “storm” could have realized was approximately 300 J kg−1; however, most of the storms probably realized less than 100 J kg−1. Average (maximum) parcel virtual temperature excesses were estimated to be 0.4–1.2 K (1.8–2.8 K). Echo tops were measured from less than 5 km to 11.2 km above ground level (AGL), although visual observations and radar data suggested echoes that extended above approximately 5–6 km AGL were not associated with significantly buoyant cloud elements. Radar characteristics of many of the storms were similar to supercell storms (e.g., weak echo regions, echo overhang, velocity couplets, hook echoes), as were some of the visual characteristics near cloud base (e.g., wall clouds, rain-free bases, and striated low-level updrafts); however, visual characteristics in middle to upper portions of the storms were not characteristic of typical severe storms, supercells, or previously documented “minisupercells.” Furthermore, the buoyancy realized by the updrafts was estimated to be considerably less than environments associated with the aforementioned minisupercells.

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Richard P. James and Paul M. Markowski

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A three-dimensional cloud model was used to investigate the sensitivity of deep convective storms to dry air above the cloud base. In simulations of both quasi-linear convective systems and supercells, dry air aloft was found to reduce the intensity of the convection, as measured by updraft mass flux and total condensation and rainfall. In high-CAPE line-type simulations, the downdraft mass flux and cold pool strength were enhanced at the rear of the trailing stratiform region in a drier environment. However, the downdraft and cold pool strengths were unchanged in the convective region, and were also unchanged or reduced in simulations of supercells and of line-type systems at lower CAPE. This result contrasts with previous interpretations of the role of dry air aloft in the development of severe low-level outflow winds.

The buoyancy-sorting framework is used to interpret the influence of environmental humidity on the updraft entrainment process and the observed strong dependence on the environmental CAPE. The reduction in convective vigor caused by dry air is relatively inconsequential at very high CAPE, but low-CAPE convection requires a humid environment in order to grow by entrainment.

The simulated responses of the downdraft and cold pool intensities to dry air aloft reflected the changes in diabatic cooling rates within the downdraft formation regions. When dry air was present, the decline in hydrometeor mass exerted a negative tendency on the diabatic cooling rates and acted to offset the favorable effects of dry air for cooling by evaporation. Thus, with the exception of the rearward portions of the high-CAPE line-type simulations, dry air was unable to strengthen the downdrafts and cold pool.

A review of the literature demonstrates that observational evidence does not unambiguously support the concept that dry air aloft favors downdraft and outflow strength. It is also shown that the use of warm rain microphysics in previous modeling studies may have reinforced the tendency to overemphasize the role of dry air aloft.

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Abdullah Kahraman, Mikdat Kadioglu, and Paul M. Markowski

Abstract

Severe convective storms occasionally result in loss of life and property in Turkey, a country not known for its severe convective weather. However, relatively little is known about the characteristics of Turkish severe weather environments. This paper documents these characteristics using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data on tornado and severe hail days in Turkey from 1979 to 2013. Severe storm environments are characterized by larger convective available potential energy (CAPE) in Turkey compared to the rest of Europe, but the CAPE values are less than those in typical U.S. severe storm environments. Severe hail is associated with large CAPE and vertical wind shear. Nonmesocyclonic tornadoes are associated with less CAPE compared with the other forms of severe weather. Deep-layer vertical wind shear is slightly weaker in Turkish supercell environments than in U.S. supercell environments, and Turkish tornadic supercell environments are characterized by much weaker low-level shear than in the United States and Europe, at least in the ECMWF reanalysis data. Composite parameters such as the supercell composite parameter (SCP) and energy–helicity index (EHI) can discriminate between very large hail and large hail environments.

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Paul M. Markowski and George H. Bryan

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In idealized simulations of convective storms, which are almost always run as large-eddy simulations (LES), the planetary boundary layers (PBLs) are typically laminar (i.e., they lack turbulent eddies). When compared with turbulent simulations, theory, or simulations with PBL schemes, the typically laminar LES used in the severe-storms community produce unrealistic near-surface vertical wind profiles containing excessive vertical wind shear when the lower boundary condition is nonfree slip. Such simulations are potentially problematic given the recent interest within the severe storms community in the influence of friction on vorticity generation within tornadic storms. Simulations run as LES that include surface friction but lack well-resolved turbulent eddies thus probably overestimate friction’s effects on storms.

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