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- Author or Editor: Stephen S. Parker x
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Abstract
Patterns of the formation of isolated, severe convective storms along the dryline in the southern plains of the United States during the spring over a 16-year period were determined from an examination of the evolution of radar echoes as depicted by WSR-57 microfilm data. It was found that in the first 30 min after the first echo, more than half of the radar echoes evolved into isolated storms as isolated cells from the start; others developed either from a pair of cells, from a line segment, from a cluster of cells, from the merger of mature cells, or from a squall line. Proximity soundings were constructed from both standard and special soundings, and from standard surface data. It was found that the estimated convective available potential energy and vertical shear are characteristic of the environment of supercell storms. The average time lag between the first echo and the first occurrence of severe weather of any type, or tornadoes alone, was approximately 2 h. There were no significant differences in the environmental parameters for the different modes of storm formation.
Abstract
Patterns of the formation of isolated, severe convective storms along the dryline in the southern plains of the United States during the spring over a 16-year period were determined from an examination of the evolution of radar echoes as depicted by WSR-57 microfilm data. It was found that in the first 30 min after the first echo, more than half of the radar echoes evolved into isolated storms as isolated cells from the start; others developed either from a pair of cells, from a line segment, from a cluster of cells, from the merger of mature cells, or from a squall line. Proximity soundings were constructed from both standard and special soundings, and from standard surface data. It was found that the estimated convective available potential energy and vertical shear are characteristic of the environment of supercell storms. The average time lag between the first echo and the first occurrence of severe weather of any type, or tornadoes alone, was approximately 2 h. There were no significant differences in the environmental parameters for the different modes of storm formation.
Abstract
A 54-yr climatology (1950–2003) of synoptic conditions associated with significant (F2 or greater) tornado events in the southern Appalachian region was compiled to 1) investigate the observed relative minimum of tornadoes in the Great Tennessee Valley, 2) test a hypothesis concerning northwest versus southwest 500-hPa flow events across the Great Tennessee Valley and Cumberland Plateau, 3) examine common operational forecasting techniques often used with synoptic-scale data to determine potentially tornadic environments, and 4) compare the patterns associated with significant, outbreak, and weak tornado events. Individual surface and upper-air charts along with composite charts closest to the time of significant tornado occurrences were used in this investigation. It was found that significant tornado events that occurred with prefrontal troughs (the most common surface boundary in the study) produced significant tornadoes almost exclusively over the Cumberland Plateau and southern Appalachian Mountains, with very few prefrontal trough events producing significant tornadoes in the Great Tennessee Valley of eastern Tennessee. Only four northwest 500-hPa flow events (which produced significant tornadoes almost exclusively in the southern Appalachian Mountains and during the summer) were found in this study, which refuted the initial hypothesis that northwest 500-hPa flow may produce tornadoes mainly in the Great Tennessee Valley with southwest 500-hPa flow producing tornadoes mainly across the Cumberland Plateau. Most significant tornado events in this study were associated with southwest 500-hPa flow ahead of a neutral-tilted trough, which revealed that the particular tilt of a 500-hPa trough does not necessarily enhance the formation of significant tornadoes in the southern Appalachian region. However, outbreak events (with five or more significant tornadoes) in the southern Appalachian region were typically associated with positive-tilted troughs. At 300 or 250 hPa, the southern Appalachian region was frequently located on the right side of a jet streak, with an even split between the entrance and exit regions. This finding indicated that significant tornado events in the southern Appalachian region did not necessarily favor the right-entrance or left-exit regions of a jet streak where rising motion is expected to be most intense (with straight jet streaks). A comparison of the composites of weak, significant, and outbreak tornado events revealed that wind dynamics were more important than instability in the distinction between weak and significant tornado events across the southern Appalachian region.
Abstract
A 54-yr climatology (1950–2003) of synoptic conditions associated with significant (F2 or greater) tornado events in the southern Appalachian region was compiled to 1) investigate the observed relative minimum of tornadoes in the Great Tennessee Valley, 2) test a hypothesis concerning northwest versus southwest 500-hPa flow events across the Great Tennessee Valley and Cumberland Plateau, 3) examine common operational forecasting techniques often used with synoptic-scale data to determine potentially tornadic environments, and 4) compare the patterns associated with significant, outbreak, and weak tornado events. Individual surface and upper-air charts along with composite charts closest to the time of significant tornado occurrences were used in this investigation. It was found that significant tornado events that occurred with prefrontal troughs (the most common surface boundary in the study) produced significant tornadoes almost exclusively over the Cumberland Plateau and southern Appalachian Mountains, with very few prefrontal trough events producing significant tornadoes in the Great Tennessee Valley of eastern Tennessee. Only four northwest 500-hPa flow events (which produced significant tornadoes almost exclusively in the southern Appalachian Mountains and during the summer) were found in this study, which refuted the initial hypothesis that northwest 500-hPa flow may produce tornadoes mainly in the Great Tennessee Valley with southwest 500-hPa flow producing tornadoes mainly across the Cumberland Plateau. Most significant tornado events in this study were associated with southwest 500-hPa flow ahead of a neutral-tilted trough, which revealed that the particular tilt of a 500-hPa trough does not necessarily enhance the formation of significant tornadoes in the southern Appalachian region. However, outbreak events (with five or more significant tornadoes) in the southern Appalachian region were typically associated with positive-tilted troughs. At 300 or 250 hPa, the southern Appalachian region was frequently located on the right side of a jet streak, with an even split between the entrance and exit regions. This finding indicated that significant tornado events in the southern Appalachian region did not necessarily favor the right-entrance or left-exit regions of a jet streak where rising motion is expected to be most intense (with straight jet streaks). A comparison of the composites of weak, significant, and outbreak tornado events revealed that wind dynamics were more important than instability in the distinction between weak and significant tornado events across the southern Appalachian region.
Abstract
On 26 March 1999, an unexpectedly heavy and complex snowfall event occurred across the southern Appalachian region. This event produced 20–30 cm (8–12 in.) of snow across the Smoky Mountains and 10–15 cm (4–6 in.) across other portions of southwest North Carolina, northeast Tennessee, and southwest Virginia. This snowfall event was complex in that several different lifting mechanisms combined to produce unexpectedly heavy amounts, especially in a narrow band across the Great Tennessee Valley. Lift from frontogenesis, orography, cold air damming, and mesoscale waves contributed to the snowfall amounts across the entire region. An interesting aspect of this snowfall was the banded enhancements observed during the initial stage of the event. These banded enhancements, observed by both satellite and radar, were determined to be the result of mesoscale waves. These waves developed around 0900 UTC in the lee of the Smoky Mountains as a strengthening southerly flow above 850 hPa became nearly perpendicular to the Smokies. A moist stable layer just above the mountain ridges (between 850 and 650 hPa) provided a sufficient duct for mountain waves to form across northeast Tennessee. Convective activity later developed around 1200 UTC across northeast Georgia along an inverted surface trough. This convective activity appeared to have helped trigger additional waves across western North Carolina. It appeared that the waves contributed to the heavy snowfall amounts by providing additional lift to the larger-scale lift present, which together maximized the release of the conditional instability across the region. After 1400 UTC, wave activity appeared to diminish across the southern Appalachian region as the larger-scale lift overwhelmed the waves.
Abstract
On 26 March 1999, an unexpectedly heavy and complex snowfall event occurred across the southern Appalachian region. This event produced 20–30 cm (8–12 in.) of snow across the Smoky Mountains and 10–15 cm (4–6 in.) across other portions of southwest North Carolina, northeast Tennessee, and southwest Virginia. This snowfall event was complex in that several different lifting mechanisms combined to produce unexpectedly heavy amounts, especially in a narrow band across the Great Tennessee Valley. Lift from frontogenesis, orography, cold air damming, and mesoscale waves contributed to the snowfall amounts across the entire region. An interesting aspect of this snowfall was the banded enhancements observed during the initial stage of the event. These banded enhancements, observed by both satellite and radar, were determined to be the result of mesoscale waves. These waves developed around 0900 UTC in the lee of the Smoky Mountains as a strengthening southerly flow above 850 hPa became nearly perpendicular to the Smokies. A moist stable layer just above the mountain ridges (between 850 and 650 hPa) provided a sufficient duct for mountain waves to form across northeast Tennessee. Convective activity later developed around 1200 UTC across northeast Georgia along an inverted surface trough. This convective activity appeared to have helped trigger additional waves across western North Carolina. It appeared that the waves contributed to the heavy snowfall amounts by providing additional lift to the larger-scale lift present, which together maximized the release of the conditional instability across the region. After 1400 UTC, wave activity appeared to diminish across the southern Appalachian region as the larger-scale lift overwhelmed the waves.
Abstract
The geographical and monthly frequencies of 500 mb cyclones and anticyclones in the National Meteorological Center analyses over the western half of the Northern Hemisphere are investigated for the period 1950–85. These cyclones and anticyclones, defined by the appearance of at least one closed (approximately) 6-dekameter contour around relatively low or high heights in the 500 mb height field, are generally observed less than ten percent of the time in any 10° by 10° latitude-longitude quadrangle, with cyclones being more numerous than anticyclones. The 500 mb cyclones are found primarily at middle and high latitudes, while anticyclones are observed most frequently over the subtropics. Cyclone frequency increases over the northern oceanic regions during summer, while anticyclone frequency increases throughout the subtropics during summer, especially over southwestern North America. Exceptions to these rules are observed; relatively high springtime 500 mb anticyclone frequency is found over the northeastern Atlantic Ocean while relatively high 500 mb cyclone frequency is found over the central subtropics Pacific Ocean and near Alaska during summer, southwestern North America during winter, and near southwestern Europe throughout the year. Abnormally strong diffluent flow over southwestern North America is suggested as an antecedent condition for 500 mb cyclogenesis in this same region. The correlation between 500 mb cyclone frequencies and 300 mb westerly momentum transports is also investigated, revealing that 500 mb cyclones may be associated with the convergence of westerly momentum into the 300 mb westerly jet. Finally, temporal trends in the frequencies indicate that 500 mb cyclone frequencies declined from 1950 through 1970 but increased from 1971 through 1985, while 500 mb anticyclone frequencies declined from 1950 through 1985.
Abstract
The geographical and monthly frequencies of 500 mb cyclones and anticyclones in the National Meteorological Center analyses over the western half of the Northern Hemisphere are investigated for the period 1950–85. These cyclones and anticyclones, defined by the appearance of at least one closed (approximately) 6-dekameter contour around relatively low or high heights in the 500 mb height field, are generally observed less than ten percent of the time in any 10° by 10° latitude-longitude quadrangle, with cyclones being more numerous than anticyclones. The 500 mb cyclones are found primarily at middle and high latitudes, while anticyclones are observed most frequently over the subtropics. Cyclone frequency increases over the northern oceanic regions during summer, while anticyclone frequency increases throughout the subtropics during summer, especially over southwestern North America. Exceptions to these rules are observed; relatively high springtime 500 mb anticyclone frequency is found over the northeastern Atlantic Ocean while relatively high 500 mb cyclone frequency is found over the central subtropics Pacific Ocean and near Alaska during summer, southwestern North America during winter, and near southwestern Europe throughout the year. Abnormally strong diffluent flow over southwestern North America is suggested as an antecedent condition for 500 mb cyclogenesis in this same region. The correlation between 500 mb cyclone frequencies and 300 mb westerly momentum transports is also investigated, revealing that 500 mb cyclones may be associated with the convergence of westerly momentum into the 300 mb westerly jet. Finally, temporal trends in the frequencies indicate that 500 mb cyclone frequencies declined from 1950 through 1970 but increased from 1971 through 1985, while 500 mb anticyclone frequencies declined from 1950 through 1985.
