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- Author or Editor: Toby N. Carlson x
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Abstract
Airflow through a developing midlatitude disturbance is analyzed in a relative-wind isentropic system in order to provide insight into how the cloud pattern evolves into the familiar comma shape. The model presented makes use of various concepts such as that of the conveyor belt and explores the relationship between the configuration of the major airstreams and such features as the jet streams and the dry tongue. The model also relates vertical motion and precipitation to the origin and vertical displacement of the airstreams and attaches special significance to airstream boundaries, which manifest themselves as sharp discontinuities in cloud and weather patterns.
Abstract
Airflow through a developing midlatitude disturbance is analyzed in a relative-wind isentropic system in order to provide insight into how the cloud pattern evolves into the familiar comma shape. The model presented makes use of various concepts such as that of the conveyor belt and explores the relationship between the configuration of the major airstreams and such features as the jet streams and the dry tongue. The model also relates vertical motion and precipitation to the origin and vertical displacement of the airstreams and attaches special significance to airstream boundaries, which manifest themselves as sharp discontinuities in cloud and weather patterns.
Abstract
An analysis of sea-surface temperatures over the tropical Atlantic for the past 5 yr shows a correlation between the number of tropical storms formed between July 10 and September 20 and the ocean temperatures over a wide area centered near 10Β°N and 35Β°W.
Abstract
An analysis of sea-surface temperatures over the tropical Atlantic for the past 5 yr shows a correlation between the number of tropical storms formed between July 10 and September 20 and the ocean temperatures over a wide area centered near 10Β°N and 35Β°W.
Abstract
A daily analysis of the 2,000- and 10,000-ft streamlines over West Africa was made for a 3Β½-mo period beginning in July 1968. With the aid of satellite photographs and auxiliary sea-level pressure data, a total of 33 synoptic scale wave perturbations were observed to move across West Africa and the tropical Atlantic Ocean during this period. Some general features of these disturbances are summarized, including facts on their origin, speed, intensity, distribution of sea-level pressure, appearance on the satellite photographs, and movement over the Atlantic Ocean. The effects of the large-scale circulation and the influence of sea-surface temperatures on the movement and intensity of disturbances are also discussed.
Abstract
A daily analysis of the 2,000- and 10,000-ft streamlines over West Africa was made for a 3Β½-mo period beginning in July 1968. With the aid of satellite photographs and auxiliary sea-level pressure data, a total of 33 synoptic scale wave perturbations were observed to move across West Africa and the tropical Atlantic Ocean during this period. Some general features of these disturbances are summarized, including facts on their origin, speed, intensity, distribution of sea-level pressure, appearance on the satellite photographs, and movement over the Atlantic Ocean. The effects of the large-scale circulation and the influence of sea-surface temperatures on the movement and intensity of disturbances are also discussed.
Abstract
Surface and upper air (700 mb) analyses along with high-quality satellite photographs are presented for a 2-week period during August and September 1967. These show, in particular, the structure and motion over the continent of Africa of four major wave disturbances, three of which later became Atlantic hurricanes. The evolution of cloudiness and convection and the intensification of the disturbance at low levels over West Africa are examined in detail and related to certain climatological features of the area. Some general characteristics of the disturbances are discussed.
Abstract
Surface and upper air (700 mb) analyses along with high-quality satellite photographs are presented for a 2-week period during August and September 1967. These show, in particular, the structure and motion over the continent of Africa of four major wave disturbances, three of which later became Atlantic hurricanes. The evolution of cloudiness and convection and the intensification of the disturbance at low levels over West Africa are examined in detail and related to certain climatological features of the area. Some general characteristics of the disturbances are discussed.
Abstract
During October 12β15, 1965, the southern part of Florida received an unusually heavy and prolonged rainstorm. Although the conventional isobaric analyses did not reveal any obvious pre-existing disturbance which would be expected to produce such a result, isentropic charts suggest that intense convection was being initiated and sustained by an organized lift of conditionally unstable air to saturation. Accompanying the heavy rainfall was the formation in the lower troposphere during the 14th of a tropical depression which is thought to have been of convective origin.
Abstract
During October 12β15, 1965, the southern part of Florida received an unusually heavy and prolonged rainstorm. Although the conventional isobaric analyses did not reveal any obvious pre-existing disturbance which would be expected to produce such a result, isentropic charts suggest that intense convection was being initiated and sustained by an organized lift of conditionally unstable air to saturation. Accompanying the heavy rainfall was the formation in the lower troposphere during the 14th of a tropical depression which is thought to have been of convective origin.
Abstract
Analyses drawn from the synoptic data gathered over the eastern Caribbean in Project ECCRO, October 1965, revealed the presence of an upper tropospheric cold Low. (With assumption of state motion), the fields of wind, height, temperature, water vapor, vertical velocity, clouds, and weather were transferred to a relative coordinate system traveling with the motion of the cold Low. This process yielded composite fields in which the 6 days of data were combined into 1. These fields, showing the structure of the cold Low at five different levels in the troposphere, are presented and the evolution of cloud and moisture patterns are discussed.
Abstract
Analyses drawn from the synoptic data gathered over the eastern Caribbean in Project ECCRO, October 1965, revealed the presence of an upper tropospheric cold Low. (With assumption of state motion), the fields of wind, height, temperature, water vapor, vertical velocity, clouds, and weather were transferred to a relative coordinate system traveling with the motion of the cold Low. This process yielded composite fields in which the 6 days of data were combined into 1. These fields, showing the structure of the cold Low at five different levels in the troposphere, are presented and the evolution of cloud and moisture patterns are discussed.
Abstract
Using VHRR brightness data obtained from the NOAA 3 satellite, isopleths of aerosol Optical depth for Saharan dust have been drawn for seven days during summer 1974 over a portion of the eastern equatorial North Atlantic. The large-scale patterns reveal an elongated dust plume which emerges from a narrow region along the African coast. Thereafter, the plume moves westward and spreads laterally though maintaining rather discrete boundaries associated with sharp gradients of turbidity, especially along the southern border. Exceptionally large values of optical depth (>2.0) are found near the centers of some dust outbreaks but these high values contribute Little to the total dust loading, which, in typical episodes, are estimated to represent a loss of topsoil from Africa of βΌ8 million metric tons of material in a period of 4β5 days. There appeared to be no direct intrusion of the dust plume into the ITCZ or north of 25Β°N in that region. Outbreaks of dust appear often to be no to the rear of a well-de-veloped easterly wave disturbance and inverted V-shaped cloud pattern. This paper demonstrates the feasibility of using satellite brightness data to quantitatively map dust outbreaks.
Abstract
Using VHRR brightness data obtained from the NOAA 3 satellite, isopleths of aerosol Optical depth for Saharan dust have been drawn for seven days during summer 1974 over a portion of the eastern equatorial North Atlantic. The large-scale patterns reveal an elongated dust plume which emerges from a narrow region along the African coast. Thereafter, the plume moves westward and spreads laterally though maintaining rather discrete boundaries associated with sharp gradients of turbidity, especially along the southern border. Exceptionally large values of optical depth (>2.0) are found near the centers of some dust outbreaks but these high values contribute Little to the total dust loading, which, in typical episodes, are estimated to represent a loss of topsoil from Africa of βΌ8 million metric tons of material in a period of 4β5 days. There appeared to be no direct intrusion of the dust plume into the ITCZ or north of 25Β°N in that region. Outbreaks of dust appear often to be no to the rear of a well-de-veloped easterly wave disturbance and inverted V-shaped cloud pattern. This paper demonstrates the feasibility of using satellite brightness data to quantitatively map dust outbreaks.
Abstract
The development and motion of fronts associated with lee-side troughs on large mountain barriers has been investigated. These fronts differ from ordinary cold fronts in their horizontal temperature field, which is characterized by a sinusoidal thermal ridge. The thermal ridge intensifies, while remaining stationary with respect to the mountains, and moves eastward upon the approach of a Pacific cold front.
An equation is derived, showing that changes in the thermal pattern can be described by changes in a potential thermal vorticity equation, which consists of three terms: (1) one representing an advection of the potential thermal vorticity by the 500-mb. wind; (2) one representing the advection of 500-mb. absolute vorticity by the thermal wind; and (3) a purely orographic term.
An idealized sinusoidal model of the thickness pattern is used in conjunction with the prognostic equation to explain the development and motion of lee-side thermal ridges. Actual examples from synoptic maps are chosen to corroborate the theory. The conclusions are: (1) the thermal ridge will develop when the surface flow is such as to produce large-scale descent on the lee slopes of the mountains; (2) no thermal ridging will appear when the 500-mb. ridge lies east of the lee slopes; (3) thermal ridging will appear with the approach of a 500-mb. ridge from the west; and (4) the thermal ridge will move eastward upon the passage of the 500-mb. ridge.
Abstract
The development and motion of fronts associated with lee-side troughs on large mountain barriers has been investigated. These fronts differ from ordinary cold fronts in their horizontal temperature field, which is characterized by a sinusoidal thermal ridge. The thermal ridge intensifies, while remaining stationary with respect to the mountains, and moves eastward upon the approach of a Pacific cold front.
An equation is derived, showing that changes in the thermal pattern can be described by changes in a potential thermal vorticity equation, which consists of three terms: (1) one representing an advection of the potential thermal vorticity by the 500-mb. wind; (2) one representing the advection of 500-mb. absolute vorticity by the thermal wind; and (3) a purely orographic term.
An idealized sinusoidal model of the thickness pattern is used in conjunction with the prognostic equation to explain the development and motion of lee-side thermal ridges. Actual examples from synoptic maps are chosen to corroborate the theory. The conclusions are: (1) the thermal ridge will develop when the surface flow is such as to produce large-scale descent on the lee slopes of the mountains; (2) no thermal ridging will appear when the 500-mb. ridge lies east of the lee slopes; (3) thermal ridging will appear with the approach of a 500-mb. ridge from the west; and (4) the thermal ridge will move eastward upon the passage of the 500-mb. ridge.
Abstract
On the last day of May 1985, a series of tornadic thunderstorms devastated pans of Ohio. Pennsylvania, New York, and Ontario, Canada. Analysis of this outbreak reveals that a lid, which moved from the southwestern United States the previous day, played a major role in enabling the latent instability to achieve large values. Severe convection began along the edge of the lid (underrunning) in a region experiencing baroclinically forced ascent associated with a migrating jet streak. This paper discusses the role of the lid in the development of the latent instability, the relationship between underrunning and the migrating jet streak, and the effects of terrain on the evolution of latent instability and on the formation of the lid. Finally, we present a climatological study of the lid in order to show why the convection was so severe, yet so unusual in this part of the country.
Abstract
On the last day of May 1985, a series of tornadic thunderstorms devastated pans of Ohio. Pennsylvania, New York, and Ontario, Canada. Analysis of this outbreak reveals that a lid, which moved from the southwestern United States the previous day, played a major role in enabling the latent instability to achieve large values. Severe convection began along the edge of the lid (underrunning) in a region experiencing baroclinically forced ascent associated with a migrating jet streak. This paper discusses the role of the lid in the development of the latent instability, the relationship between underrunning and the migrating jet streak, and the effects of terrain on the evolution of latent instability and on the formation of the lid. Finally, we present a climatological study of the lid in order to show why the convection was so severe, yet so unusual in this part of the country.
Abstract
Model experiments have isolated several effects of surface heating and topography which may act in concert to focus the potential for severe thunderstorms in certain areas downstream of dry elevated terrain. These effects are examined primarily in results from 12 h and 36 h three-dimensional model simulations of the 9β11 April 1979 (SESAME I) case, but are also found in simulations of the 9β10 May 1979 (SESAME IV) case. The experiments are performed using the Pennsylvania State University mesoscale model, which includes a sophisticated boundary-layer component with horizontally varying surface characteristics and cloud effects on the surface radiation budget.
Comparisons of simulations with and without surface fluxes of heat and moisture confirm that differential surface heating, topography, and differential advection may combine to produce a stabilizing effect. In certain synoptic situations, air strongly heated over the arid Mexican plateau rides over moist potentially cooler air downstream, thereby forming a strong restraining inversion or βlidβ over Texas. This elevated mixed layer inversion prevents the occurrence of thunderstorms over a large area; instead, storms are focused in the region where a southeasterly low-level flow underruns the northern and western boundary of the inversion. Model trajectories also support the hypothesis that this lateral boundary of the lid, a baroclinic zone approximately between 800 and 500 mb, is formed by confluence between the Mexican airstream and a subsided polar airstream which has traversed the upper-level trough approaching from the western United States.
Model experiments also reveal that the strength of the low-level flow toward the severe storm region may itself be enhanced by effects related to surface heating and topography. First, the lee trough which forms with flow aloft across the southern Rockies and Mexican plateau intensifies as a result of daytime surface heating. This effect, which is related to differential heating and differential mixing of momentum, is significant in that the low-level flow east of the lee trough from the Gulf of Mexico toward the severe storm region is also strengthened by several meters per second. Secondly, transverse circulations associated with jet streaks, which were crucial in inducing low-level flow in both the SESAME I and IV cases, were also found to intensify in the presence of surface heating.
Abstract
Model experiments have isolated several effects of surface heating and topography which may act in concert to focus the potential for severe thunderstorms in certain areas downstream of dry elevated terrain. These effects are examined primarily in results from 12 h and 36 h three-dimensional model simulations of the 9β11 April 1979 (SESAME I) case, but are also found in simulations of the 9β10 May 1979 (SESAME IV) case. The experiments are performed using the Pennsylvania State University mesoscale model, which includes a sophisticated boundary-layer component with horizontally varying surface characteristics and cloud effects on the surface radiation budget.
Comparisons of simulations with and without surface fluxes of heat and moisture confirm that differential surface heating, topography, and differential advection may combine to produce a stabilizing effect. In certain synoptic situations, air strongly heated over the arid Mexican plateau rides over moist potentially cooler air downstream, thereby forming a strong restraining inversion or βlidβ over Texas. This elevated mixed layer inversion prevents the occurrence of thunderstorms over a large area; instead, storms are focused in the region where a southeasterly low-level flow underruns the northern and western boundary of the inversion. Model trajectories also support the hypothesis that this lateral boundary of the lid, a baroclinic zone approximately between 800 and 500 mb, is formed by confluence between the Mexican airstream and a subsided polar airstream which has traversed the upper-level trough approaching from the western United States.
Model experiments also reveal that the strength of the low-level flow toward the severe storm region may itself be enhanced by effects related to surface heating and topography. First, the lee trough which forms with flow aloft across the southern Rockies and Mexican plateau intensifies as a result of daytime surface heating. This effect, which is related to differential heating and differential mixing of momentum, is significant in that the low-level flow east of the lee trough from the Gulf of Mexico toward the severe storm region is also strengthened by several meters per second. Secondly, transverse circulations associated with jet streaks, which were crucial in inducing low-level flow in both the SESAME I and IV cases, were also found to intensify in the presence of surface heating.
