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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

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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 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

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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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Xiaoqin Jing and Bart Geerts

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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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Coltin Grasmick and Bart Geerts

Abstract

Kelvin–Helmholtz (KH) waves are remarkably common in deep stratiform precipitation systems associated with frontal disturbances, at least in the vicinity of complex terrain, as is evident from transects of vertical velocity and 2D circulation, obtained from a 3-mm airborne Doppler radar, the Wyoming Cloud Radar. The high range resolution of this radar (~40 m) allows detection and depiction of KH waves in fine detail. These waves are observed in a variety of wavelengths, depths, amplitudes, and turbulence intensities. Proximity rawinsonde data confirm that they are triggered in layers where the Richardson number is very small. Complex terrain may locally enhance wind shear, leading to KH instability. In some KH waves, the flow remains mostly laminar, while in other cases it breaks down into turbulence. KH waves are frequently locked to the terrain, and occur at various heights, including within the free troposphere, at the boundary layer top, and close to the surface. They are observed not only upwind of terrain barriers, as has been documented before, but also in the wake of steep terrain, where the waves can be highly turbulent. Vertical-plane dual-Doppler analyses of KH waves reveal the mixing of layers of differential momentum across the high-shear zone. Doppler radar data are used to explore the dynamics of KH waves, including the response of thermodynamic and kinematic variables above, below, and within the instability layer.

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Bart Geerts and Qun Miao

Abstract

A train of Kelvin–Helmholtz billows in a deep stratiform cloud over a mountain range is documented using data from a high-resolution vertically pointing airborne Doppler radar. The billows had a spacing of 2–2.5 km and a small aspect ratio. The formation and decay of the billows appear to be related to flow acceleration over a mountain.

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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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Bart Geerts and Teferi Dejene

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The Tropical Rainfall Measuring Mission (TRMM) 2A25 radar reflectivity profiles and derived surface rain rates are used to describe the vertical structure of precipitation systems in Africa. Five years of data are used in both the boreal and austral summer rainy seasons. A number of climate regions are isolated and compared. To place the composite reflectivity profiles in context, they are contrasted against TRMM 2A25 observations over the Amazon.

In all of tropical Africa, precipitation systems tend to be deeper and more intense than in the Amazon, and shallow warm-rain events are less common. In all African regions, but especially in the Sahel and northern Savanna, storms are characterized by high echo tops, high hydrometeor loading aloft, little indication of a radar brightband maximum at the freezing level, and evidence for low-level evaporation.

Storms in Africa are generally most common, and deepest, in the late afternoon, and weaker shallow systems are relatively more common around noon. The diurnal modulation is regionally variable. The amplitude of the diurnal cycle of the mean echo top height decreases from the arid margins of the zenithal rain region toward the equatorial region, and is smallest in the Amazon. A secondary predawn (0000–0600 LT) maximum occurs in the Congo, in terms of rainfall frequency, rainfall intensity, and echo tops. The storm intensity indicators generally peak a few hours later in the Sahel and northern Savanna than in other regions in Africa.

The difference between all African regions and the Amazon, and the relatively smaller differences between regions in Africa, can be understood in terms of the climatological humidity, CAPE, and low-level shear values.

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