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Bart Geerts

Abstract

A regional climatology of strong wind gusts associated with thunderstorms is presented, and the ability to estimate gust strength from ambient conditions is tested. Strong wind events were selected for 10 stations in New South Wales, Australia, from anemograph records and coincident thunderstorm reports. Most events took place between midafternoon and late evening and during the warmer months of the year, which is broadly consistent with the occurrence of severe thunderstorms in general. One sounding-based index, designed to predict the strength of microbursts, proves to be of limited value in predicting the magnitude of strong convective gusts, even of short-lived gusts. A modified index that combines the microburst index with upper-level wind speed is more useful.

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Bart Geerts

A survey of 22 atmospheric science journals shows that the number of published articles tripled in 30 years during 1965–95, so that it has become increasingly difficult to keep abreast of the literature. A total of 1642 peer-reviewed articles in the journals were categorized numerically in terms of features of the abstracts and the conclusions. Consistent differences were found between journals. Most journals are mediocre in terms of their reader-friendliness, with little or no improvement over recent decades. The abstract and/or the conclusions in many papers have become too long and too discursive, preventing the reader from making a rapid assessment of the papers' usefulness. These trends may retard atmospheric research. Therefore journal editors are urged to insist on some easy improvements.

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Bart Geerts

Abstract

Two months of Lubbock, Texas, radar reflectivity data and West Texas Mesonet data are examined to detect dryline finelines and to describe their thermodynamic and propagation characteristics. Before sunset the moist air mass east of the dryline was consistently denser than the dry air mass. This air density difference waned and even reversed after sunset, because of more rapid cooling on the dry side.

This study provides further evidence that the formation and propagation of the dryline convergence zone is driven by the daytime air density difference, that is, that the dryline behaves as a density current. The implication for forecasters is that the air density (or virtual potential temperature) difference across the dryline should be monitored, as a measure of dryline strength and as an additional indicator for the likelihood of convective initiation along the dryline.

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Bart Geerts

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A preliminary survey of mesoscale convective systems (MCSs) in the southeastern United States is presented. MCSs are identified and characterized by means of high-resolution, digital, composite radar reflectivity data. Surveys of this kind are needed to give forecasters better guidance in their real-time assessment of MCS evolution, severe weather potential, and quantitative precipitation. The average lifetime and maximum length of the nearly 400 MCSs included in this survey are 9 h and 350 km, respectively. MCSs are more common in the summer months, when small and short-lived MCSs dominate. In winter larger and longer-lived systems occur more frequently. Because cold-season MCSs, which are about half as numerous as warm-season MCSs, are larger in size and duration, the MCS probability at any location is about constant throughout the year. In summer MCSs occur more commonly in the afternoon, approximately in phase with thunderstorm activity, but the amplitude of the diurnal cycle is small compared to that of observed thunderstorms. Some characteristic echo patterns are discussed.

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Bart Geerts and Yu Dawei

Abstract

Airborne measurements of vertical incidence radar reflectivity and radial velocity are analyzed for some 21 231 km of high-altitude flight tracks over tropical precipitation systems, in order to describe their characteristic vertical structure. The strength of the radar dataset lies in its superb vertical resolution, sufficient to detect unambiguously a bright band and the coincident Doppler velocity change, which identify the melting layer in stratiform precipitation. In this first of a two-part study, a technique based on the detection of this stratiform precipitation signature is developed to classify hydrometer profiles as convective, stratiform, or shallow. Even though the profiles are classified individually, stratiform and convective regions emerge, whose characteristics are described. The hydrometeor vertical velocity variability is smaller in stratiform profiles, which is consistent with the physical concept of a stratiform region. The purpose of the classification is to describe, in , the composite vertical structure of the various rain types in hurricanes, as well as in isolated to organized precipitating convection sampled in Florida and Brazil.

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Xiaoqin Jing and Bart Geerts

Abstract

This second paper of a two-part series aims to explore the ground-based glaciogenic seeding impact on wintertime orographic clouds using an X-band dual-polarization radar. It focuses on three cases with shallow to moderately deep orographic convection that were observed in January–February of 2012 as part of the AgI Seeding Cloud Impact Investigation (ASCII) project over the Sierra Madre in Wyoming. In each of the storms the bulk upstream Froude number exceeded 1, suggesting unblocked flow. Low-level potential instability was present, explaining orographic convection. The clouds contained little supercooled liquid water on account of the low cloud-base temperature. Ice-crystal photography shows that snow mainly grew by diffusion and aggregation. To examine the seeding effect of silver iodide (AgI), five study areas are defined: two target areas and three control areas. Comparisons are made between the control and target areas as well as between a treated, or seeded, period and an untreated period. Low-level reflectivity tends to increase in the target areas relative to the control. This increase is larger in the lee target area than in the upwind target area, suggesting that precipitation enhancement is delayed in the presence of convection. The echo tops of the convective cells are not higher during seeding, relative to simultaneous changes in the control regions. This result suggests that the dynamic-seeding mechanism does not apply for the cold-base convective clouds that are studied here. An analysis of differential reflectivity and snow photography suggests that static seeding is the more likely snow-enhancement mechanism in these clouds.

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Bart Geerts and Yu Dawei

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High-resolution airborne measurements of vertical incidence radar reflectivity and Doppler velocity, as well as coincident upwelling 85-GHz radiances, are analyzed for several Atlantic Ocean hurricanes and for numerous convection-generated systems in Florida and Amazonia. Characteristic reflectivity, hydrometeor motion, and vertical air motion profiles of convective and stratiform precipitation are compared and related to their ice-scattering signature, with an emphasis on the difference between hurricanes and convection-generated storms. Hurricanes are found to be largely and clearly stratiform, displaying a remarkably narrow echo and vertical velocity spectrum. Air currents are inferred to be rising steadily at all levels, even in stratiform regions. Land-based, convection-generated stratiform regions tend to experience low-level descent and mid- to upper-level ascent, although the vertical velocity variability is large. Florida storms produce little stratiform precipitation. Their spectrum of echo and updraft strengths is broad, including some of the highest reflectivities aloft, resulting in very low 85-GHz radiances. Amazonian storms are relatively weak and are more “maritime” in echo, vertical velocity, and ice-scattering characteristics, when compared with those in Florida, especially during a westerly low-level wind regime.

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Yonggang Wang and Bart Geerts

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High-resolution vertical-plane dual-Doppler velocity data, collected by an airborne profiling cloud radar in transects across nonprecipitating orographic cumulus clouds, are used to examine vortical circulations near cloud top. These vortices are part of a toroidal ring centered at an updraft, usually near the cloud top, and they are essential to cumulus entrainment and dynamics. A large number of transects across toroidal circulations are composited to reveal the typical kinematic structure and associated entrainment patterns. The toroidal ring circulation is ~1 km wide and about half as deep in the sampled clouds (Cu mediocris). The composite flow field shows two nearly symmetric, counterrotating vortices, with a core updraft of ~3 m s−1, consistent vortex-top divergence, two flanking downdrafts of the about same strength, and horizontal (toroidal) vorticity of ~0.03 s−1. Variations with vortex size, age, and ambient shear are examined, and the relative dilution of air in the vortex core is estimated by comparing the liquid water content, estimated from path-integrated power attenuation, with the adiabatic value.

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Yonggang Wang and Bart Geerts

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Vertical transects of Doppler vertical velocity data, obtained from an airborne profiling millimeter-wave cloud radar, are composited for a large number of cumulus clouds (Cu) at various stages of their life cycle, to examine typical circulations patterns. The Cu clouds range in depth between ~500 and 6000 m and are generally nonprecipitating. They were sampled on board the University of Wyoming King Air over a mountain in southern Arizona during the summer monsoon, and over the high plains of southeastern Wyoming. The composite analysis shows clear evidence of an updraft/downdraft dipole in the upper cloud half, consistent with a horizontal vortex ring. A single cloud-scale toroidal circulation emerges notwithstanding the complex finescale structure with multiple vortices, commonly evident in individual transects of Cu clouds. The stratification of all Cu samples as a function of their buoyancy and mean vertical velocity shows that the vortex ring pattern tends to be more pronounced in positively buoyant Cu with rising motion (presumably young clouds) than in negatively buoyant and/or sinking Cu near the end of their life cycle. Yet no reverse vortex ring is observed in the latter Cu, suggesting that the decaying phase is short lived in these dry environments. The vortex-ring circulation pattern is more intense in the shallower Cu, which are also more buoyant and have a liquid water content closer to adiabatic values. Wind shear tends to tilt Cu clouds and their vortex ring, resulting in a broadening of the upshear updraft and downshear downdraft.

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Qun Miao and Bart Geerts

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Several radar fine lines, all with a humidity contrast, were sampled in the central Great Plains during the 2002 International H2O Project (IHOP). This study primarily uses aircraft and airborne millimeter-wave radar observations to dynamically interpret the presence and vertical structure of these fine lines as they formed within the well-developed convective boundary layer. In all cases the fine line represents a boundary layer convergence zone. This convergence sustains a sharp contrast in humidity, and usually in potential temperature, across the fine line. The key question addressed herein is whether, at the scale examined here (∼10 km), the airmass contrast itself, in particular the horizontal density (virtual potential temperature) difference and resulting solenoidal circulation, is responsible for the sustained convergence and the radar fine line. For the 10 cases examined herein, the answer is affirmative.

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