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Patrick C. Burke and David M. Schultz

Abstract

A search of radar mosaics and level-II Weather Surveillance Radar-1988 Doppler (WSR-88D) data revealed 51 cold-season (October–April) bow echoes that occurred in the contiguous United States from 1997–98 to 2000–01. Proximity soundings indicated mean 0–2.5-, 0–5-, and 5–10-km shear values of 14, 23, and 19 m s−1, respectively. Mean CAPE was 1366 J kg−1. Most bow echoes developed from squall lines, groups of cells, or squall lines overtaking cells that originated in the path of the squall line. Overall, cell mergers occurred just prior to the development of 34 (67%) of the 51 bow echoes, and embedded supercells were present in the mature stage of 22 (43%) bow echoes.

Nine severe, long-lived bow echoes (LBEs) were identified, and seven of these had damage paths that met derecho criteria. LBEs developed in strongly forced, dynamic synoptic patterns with low to moderate instability. As in previous observational studies, proximity soundings suggested that LBEs are possible within much wider ranges of sampled CAPE and shear than idealized numerical modeling studies have indicated.

Cold-season bow echoes formed overwhelmingly (47 of 51) in southwesterly 500-mb flow. Twenty (39%) bow echoes formed in a Gulf coast synoptic pattern that produced strong shear and moderate instability over the southeastern United States. Nineteen (37%) and seven (14%) bow echoes, respectively, formed in the plains and east synoptic patterns, which resemble classic severe weather outbreak patterns. Four (8%) bow echoes developed in a northwest flow synoptic pattern that produced strong shear and moderate instability over the southern plains.

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Kelsey J. Mulder and David M. Schultz

Abstract

A climatology is developed for tornadoes during 1980–2012 in the British Isles, defined in this article as England, Scotland, Wales, Northern Ireland, Republic of Ireland, Channel Islands, and the Isle of Man. The climatology includes parent storm type, interannual variability, annual and diurnal cycles, intensities, occurrence of outbreaks (defined as three or more tornadoes in the same day), geographic distribution, and environmental conditions derived from proximity soundings of tornadoes. Tornado reports are from the Tornado and Storm Research Organization (TORRO). Over the 33 years, there were a mean of 34.3 tornadoes and 19.5 tornado days (number of days in which at least one tornado occurred) annually. Tornadoes and tornado outbreaks were most commonly produced from linear storms, defined as radar signatures at least 75 km long and approximately 3 times as long as wide. Most (78%) tornadoes occurred in England. The probability of a tornado within 10 km of a point was highest in the south, southeast, and west of England. On average, there were 2.5 tornado outbreaks every year. Where intensity was known, 95% of tornadoes were classified as F0 or F1 with the remainder classified as F2. There were no tornadoes rated F3 or greater during this time period. Tornadoes occurred throughout the year with a maximum from May through October. Finally, tornadoes tended to occur in low-CAPE, high-shear environments. Tornadoes in the British Isles were difficult to predict using only sounding-derived parameters because there were no clear thresholds between null, tornadic, outbreak, and significant tornado cases.

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Bogdan Antonescu, Geraint Vaughan, and David M. Schultz

Abstract

A five-year (2006–10) radar-based climatology of tropopause folds and convective storms was constructed for Wales, United Kingdom, to determine how deep, moist convection is modulated by tropopause folds. Based on the continuous, high-resolution data from a very high frequency (VHF) wind-profiling radar located at Capel Dewi, Wales, 183 tropopause folds were identified. Tropopause folds were most frequent in January with a secondary maximum in July. Based on data from the U.K. weather radar network, a climatology of 685 convective storms was developed. The occurrence of convective storms was relatively high year-round except for an abrupt minimum in February–April. Multicellular lines (43.5%) were the most common morphology with a maximum in October, followed by isolated cells (33.1%) with a maximum in May–September, and nonlinear clusters (23.4%) with a maximum in November–January. Convective storms were associated with 104 (56.8%) of the tropopause folds identified in this study, with the association strongest in December. Of the 55 tropopause folds observed on the eastern side of an upper-level trough, 37 (67.3%) were associated with convective storms, most commonly in the form of multicellular lines. Of the 128 tropopause folds observed on the western side of an upper-level trough, 42 (32.8%) were associated with convective storms, most commonly isolated cells. These results suggest that more organized storms tend to form in environments favorable for synoptic-scale ascent.

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David M. Schultz and Charles A. Doswell III

Abstract

Since numerical forecast models often err in predicting the timing and location of lee cyclogenesis, a physically based method to diagnose such errors is sought. A case of Rocky Mountain lee cyclogenesis associated with strong winds is examined to explore the transformation from a stationary lee trough to a mobile midlatitude cyclone (hereafter, departure). Up to 12 h before departure, a pronounced surface pressure trough travels eastward across western North America at an average speed of 22 m s−1. Several methods are employed to examine the structure and evolution of the pressure field: total sea level pressure, time series at individual stations, isallobars, and bandpass filtering. Bandpass filtering of the observed sea level pressure data is useful for clarifying the movement of the mobile trough through the complex terrain. Quasigeostrophic height-tendency diagnostics show that the mobile pressure trough is related to the traveling mid- to upper-tropospheric vorticity maximum that is responsible for departure. At many stations, surface temperature changes associated with this pressure trough are not consistent with those commonly associated with surface frontal passages. To test the hypothesis that mobile pressure troughs are associated with departure, a five-winter climatology of 111 southern Alberta lee cyclones is constructed. Sixty-two percent of these events feature an upstream pressure minimum 3–9 h prior to departure, in a manner resembling the case study. Seventy-six percent of these 111 events are associated with reports listed in Storm Data, indicating the potential severity of these storms.

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David M. Schultz and Philip N. Schumacher

Abstract

A commonly employed explanation for single- and multiple-banded clouds and precipitation in the extratropics is slantwise convection due to the release of moist symmetric instability (MSI), of which one type is conditional symmetric instability (CSI). This article presents a review of CSI with the intent of synthesizing the results from previous observational, theoretical, and modeling studies. This review contends that CSI as a diagnostic tool to assess slantwise convection has been, and continues to be, misused and overused. Drawing parallels to an ingredients-based methodology for forecasting deep, moist convection that requires the simultaneous presence of instability, moisture, and lift, some of the misapplications of CSI can be clarified. Many of these pitfalls have been noted by earlier authors, but are, nevertheless, often understated, misinterpreted, or neglected by later researchers and forecasters. Topics include the evaluation of the potential for slantwise convection, the relationship between frontogenesis and MSI, the coexistence of moist gravitational instability and MSI, the nature of banding associated with slantwise convection, and the diagnosis of slantwise convection using mesoscale numerical models. The review concludes with suggested directions for future observational, theoretical, and diagnostic investigation.

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David M. Schultz and John A. Knox

Abstract

Several east–west-oriented bands of clouds and light rain formed on 20 July 2005 over eastern Montana and the Dakotas. The cloud bands were spaced about 150 km apart, and the most intense band was about 20 km wide and 300 km long, featuring areas of maximum radar reflectivity factor of about 50 dBZ. The cloud bands formed poleward of an area of lower-tropospheric frontogenesis, where air of modest convective available potential energy was being lifted. During initiation and maintenance of the bands, mesoscale regions of dry symmetric and inertial instability were present in the region of the bands, suggesting a possible mechanism for the banding. Interpretation of the extant instabilities in the region of the bands was sensitive to the methodology to assess the instability. The release of these instabilities produced circulations with enough vertical motion to lift parcels to their lifting condensation level, resulting in the observed cloud bands. A high-resolution, numerical weather prediction model demonstrated that forecasting these types of events in such real-time models is possible, although the timing, evolution, and spacing of the bands were not faithfully reproduced. This case is compared to two previous cases in the literature where banded convection was associated with a combination of conditional, symmetric, and inertial instability.

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David M. Schultz and W. James Steenburgh

Abstract

A mesoscale-model simulation is used to examine the evolution of the cold front and accompanying cloud bands in eastern Mexico associated with Superstorm 1993 (12–14 March). The simulated cold front differed in structure and evolution from a classical cold front, in agreement with evidence from observations and European Centre for Medium-Range Weather Forecasts analyses. The surface cold front, as defined by the leading edge of strong northerlies and cold advection, initially possessed a rearward tilt with height over southern Texas. Within 6 h, the leading edge of the front moved equatorward and developed a large-scale forward tilt of greater than 200 km in the horizontal from the surface to 700 hPa. This forward tilt occurred as a mid- to upper-tropospheric baroclinic zone arrived from over the Sierra Madre, descended into eastern Mexico, and interacted with the surface cold front. Embedded within this large-scale forward tilt was a locally enhanced horizontal potential temperature gradient that also tilted forward ∼100 km from the surface to 850 hPa. Tilting frontogenesis associated with ascent at the leading edge of the surface front was responsible for the smaller-scale forward-tilting structure. This surface-based ascent is believed to have caused the primary cloud band observed from satellite imagery that is coincident with the leading edge of the front, whereas a second region of ascent, elevated at the leading edge of the mid- to upper-tropospheric baroclinic zone, is believed to have caused the prefrontal cloud band revealed by satellite imagery. Subsidence behind the forward-tilting cold-frontal structure at and above 850 hPa (and concomitant divergence underneath) resulted in frontolysis of the surface front, and eventually the dissipation of the primary cloud band, leading to the dominance of the prefrontal cloud band. Finally, the Superstorm 1993 cold front is compared and contrasted to nonclassical cold-frontal structures found in the literature and a general context for frontal interaction is discussed.

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Bogdan Antonescu, Hugo M. A. M. Ricketts, and David M. Schultz

Abstract

Alfred Wegener (1880–1930) was a leading geophysicist, atmospheric scientist, and an Arctic explorer who is mainly remembered today for his contributions to the theory of continental drift. Less well known are his contributions to research on tornadoes in Europe. Published 100 years ago, book Wind- und Wasserhosen in Europa (Tornadoes and Waterspouts in Europe) is an impressive synthesis of knowledge on tornadoes and is considered the first modern pan-European tornado climatology, with 258 reports from 1456 to 1913. Unfortunately, Wegener’s book was overlooked after the 1950s amid declining interest in tornadoes by European researchers and meteorologists. The recent revival of tornado studies in Europe invites a reflection on Wegener’s book. Using a relatively small dataset, Wegener was able to describe characteristics of tornadoes (e.g., direction of movement, speed, rotation, formation mechanism), as well as their frequency of occurrence and climatology, comparable with the results from modern tornado climatologies. Wegener’s lasting scientific contributions to tornado research are presented in the context of European research on this topic. Specifically, his book showed the utility of reports from citizen scientists and inspired other researchers, namely, Johannes Letzmann, who continued to study European tornadoes after Wegener’s death.

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Katharine M. Kanak, Jerry M. Straka, and David M. Schultz

Abstract

Mammatus are hanging lobes on the underside of clouds. Although many different mechanisms have been proposed for their formation, none have been rigorously tested. In this study, three-dimensional numerical simulations of mammatus on a portion of a cumulonimbus cirruslike anvil are performed to explore some of the dynamic and microphysical factors that affect mammatus formation and evolution. Initial conditions for the simulations are derived from observed thermodynamic soundings. Five observed soundings are chosen—four were associated with visually observed mammatus and one was not. Initial microphysical conditions in the simulations are consistent with in situ observations of cumulonimbus anvil and mammatus. Mammatus form in the four model simulations initialized with the soundings for which mammatus were observed, whereas mammatus do not form in the model simulation initialized with the no-mammatus sounding. Characteristics of the modeled mammatus compare favorably to previously published mammatus observations.

Three hypothesized formation mechanisms for mammatus are tested: cloud-base detrainment instability, fallout of hydrometeors from cloud base, and sublimation of ice hydrometeors below cloud base. For the parameters considered, cloud-base detrainment instability is a necessary, but not sufficient, condition for mammatus formation. Mammatus can form without fallout, but not without sublimation. All the observed soundings for which mammatus were observed feature a dry-adiabatic subcloud layer of varying depth with low relative humidity, which supports the importance of sublimation to mammatus formation.

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David B. Mechem, Yefim L. Kogan, and David M. Schultz

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More studies on the dynamics of marine stratus and stratocumulus clouds have been performed than comparable studies on continental stratocumulus. Therefore, to increase the number of observations of continental stratocumulus and to compare marine and continental stratocumulus to each other, the approach of large-eddy observation (LEO) was applied to a case of nocturnal continental stratocumulus observed over the Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) in the central United States on 8 April 2006. The stratocumulus occurred in cold-air and dry-air advection behind a surface cold front. LEOs were obtained from millimeter-wavelength cloud radar and micropulse lidar, whereas traditional meteorological observations described the synoptic environment. This study focuses on a 9-h period of a predominantly nonprecipitating stratocumulus layer 250–400 m thick. A slight thinning of the cloud layer over time is consistent with dry-air advection. A deep layer of descent overlaid a shallower layer of ascent from the surface up to 800 mb, providing a mechanism for strengthening the inversion at cloud top. Time series of Doppler velocity indicate vertically coherent structures identifiable throughout much of the cloud layer. The magnitude of turbulence, as indicated by the variance of the vertical velocity, was weak relative to typical marine stratocumulus and to the one other case of continental stratocumulus in the literature. Conditional sampling of the eddy structures indicate that strong downdrafts were more prevalent than strong updrafts, and negative skewness of vertical velocity in the cloud implies an in-cloud circulation driven by longwave cooling at cloud top, similar to that in marine stratocumulus.

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