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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.
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.
Abstract
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.
Abstract
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.
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.
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.
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.
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.
Abstract
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.
Abstract
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.
Abstract
Aircraft and airborne cloud radar data are used to describe the vertical structure of the convective boundary layer (CBL) during cold-air outbreaks over Lake Michigan in January 2004. Two days with mesoscale cloud street structure and a day with cellular organization are contrasted. The radar reflectivity and vertical velocity structure of the CBL, as well as the radar-inferred topography of the CBL inversion, are collected along flight legs normal to the cloud streets. High-resolution horizontal and vertical transects of the dual-Doppler airflow field capture horizontal convective roll circulations on one day.
Coherent structures within the CBL are analyzed as echo plumes, updraft plumes, and CBL domes. Only updraft plumes have the characteristics of buoyant thermals. Updrafts are narrower, weaker, and less buoyant on the no-roll day, but the differences in characteristics between two cloud street days are larger than those between the no-roll day and the two cloud street days. The lack of a clear buoyancy signal in echo plumes and under CBL domes is attributed to a temporal phase shift between maximum buoyancy, maximum ice particle size, and maximum overshooting in thermals, and the transience of convective updrafts.
Abstract
Aircraft and airborne cloud radar data are used to describe the vertical structure of the convective boundary layer (CBL) during cold-air outbreaks over Lake Michigan in January 2004. Two days with mesoscale cloud street structure and a day with cellular organization are contrasted. The radar reflectivity and vertical velocity structure of the CBL, as well as the radar-inferred topography of the CBL inversion, are collected along flight legs normal to the cloud streets. High-resolution horizontal and vertical transects of the dual-Doppler airflow field capture horizontal convective roll circulations on one day.
Coherent structures within the CBL are analyzed as echo plumes, updraft plumes, and CBL domes. Only updraft plumes have the characteristics of buoyant thermals. Updrafts are narrower, weaker, and less buoyant on the no-roll day, but the differences in characteristics between two cloud street days are larger than those between the no-roll day and the two cloud street days. The lack of a clear buoyancy signal in echo plumes and under CBL domes is attributed to a temporal phase shift between maximum buoyancy, maximum ice particle size, and maximum overshooting in thermals, and the transience of convective updrafts.
Abstract
Data collected around the Santa Catalina Mountains in Arizona as part of the Cumulus Photogrammetric, In Situ and Doppler Observations (CuPIDO) experiment during the 2006 summer monsoon season are used to investigate the effect of soil moisture on the surface energy balance, boundary layer (BL) characteristics, thermally forced orographic circulations, and orographic cumulus convection. An unusual wet spell allows separation of the two-month campaign in a wet and a dry soil period. Days in the wet soil period tend to have a higher surface latent heat flux, lower soil and air temperatures, a more stable and shallower BL, and weaker solenoidal forcing resulting in weaker anabatic flow, in comparison with days in the dry soil period. The wet soil period is also characterized by higher humidity and moist static energy in the BL, implying a lower cumulus cloud base and higher convective available potential energy. Therefore, this period witnesses rather early growth of orographic cumulus convection, growing rapidly to the cumulonimbus stage, often before noon, and producing precipitation rather efficiently, with relatively little lightning. Data alone do not allow discrimination between soil moisture and advected airmass characteristics in explaining these differences. Hence, the need for a numerical sensitivity experiment, in Part II of this study.
Abstract
Data collected around the Santa Catalina Mountains in Arizona as part of the Cumulus Photogrammetric, In Situ and Doppler Observations (CuPIDO) experiment during the 2006 summer monsoon season are used to investigate the effect of soil moisture on the surface energy balance, boundary layer (BL) characteristics, thermally forced orographic circulations, and orographic cumulus convection. An unusual wet spell allows separation of the two-month campaign in a wet and a dry soil period. Days in the wet soil period tend to have a higher surface latent heat flux, lower soil and air temperatures, a more stable and shallower BL, and weaker solenoidal forcing resulting in weaker anabatic flow, in comparison with days in the dry soil period. The wet soil period is also characterized by higher humidity and moist static energy in the BL, implying a lower cumulus cloud base and higher convective available potential energy. Therefore, this period witnesses rather early growth of orographic cumulus convection, growing rapidly to the cumulonimbus stage, often before noon, and producing precipitation rather efficiently, with relatively little lightning. Data alone do not allow discrimination between soil moisture and advected airmass characteristics in explaining these differences. Hence, the need for a numerical sensitivity experiment, in Part II of this study.
Abstract
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.
Abstract
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.
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.
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.
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.
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.
