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Author: Keith L. Seitter
Publisher:
ISBN:
Category : Atmospheric turbulence
Languages : en
Pages : 34
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Book Description
The thunderstorm gust front is an important feature for both the maintenance and initiation of storms. Previous studies have shown that the thunderstorm outflow producing the gust front can be treated as an atmospheric density current to a good approximation. In this study, a new version of the density current speed equation, based on the surface pressure rise, is derived. This equation is shown to give much better results than other commonly used forms when applied to twenty previously reported gust front observations. A two-dimensional numerical model is used to investigate the dynamics of atmospheric density currents. Simulations with this model show the effects of the environmental wind relative to the storm and the wind shear on the propagation of the gust front and the depth of the thunderstorm outflow. The results of these simulations are discussed in terms of the conditions necessary for the gust front to remain in a position that is beneficial for the maintenance of the storm. Moist processes are included in the model and simulations are made to investigate atmospheric density current propagation through a moist atmosphere. The lifting that occurs during gust front passage is calculated and it is found that even when this lifting is sufficient to bring moist parcels above their level of free convection; deep convection is prohibited by other aspects of the circulation. The generation of an arc cloud by the gust front is found to result in a less intense and slower gust front compared to an outflow that did not produce an arc cloud. This result is interpreted in terms of the gust front speed equation based on the surface pressure rise. (Author).
Author: Keith L. Seitter
Publisher:
ISBN:
Category : Atmospheric turbulence
Languages : en
Pages : 34
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Book Description
The thunderstorm gust front is an important feature for both the maintenance and initiation of storms. Previous studies have shown that the thunderstorm outflow producing the gust front can be treated as an atmospheric density current to a good approximation. In this study, a new version of the density current speed equation, based on the surface pressure rise, is derived. This equation is shown to give much better results than other commonly used forms when applied to twenty previously reported gust front observations. A two-dimensional numerical model is used to investigate the dynamics of atmospheric density currents. Simulations with this model show the effects of the environmental wind relative to the storm and the wind shear on the propagation of the gust front and the depth of the thunderstorm outflow. The results of these simulations are discussed in terms of the conditions necessary for the gust front to remain in a position that is beneficial for the maintenance of the storm. Moist processes are included in the model and simulations are made to investigate atmospheric density current propagation through a moist atmosphere. The lifting that occurs during gust front passage is calculated and it is found that even when this lifting is sufficient to bring moist parcels above their level of free convection; deep convection is prohibited by other aspects of the circulation. The generation of an arc cloud by the gust front is found to result in a less intense and slower gust front compared to an outflow that did not produce an arc cloud. This result is interpreted in terms of the gust front speed equation based on the surface pressure rise. (Author).
Author: Kenneth E. Mitchell
Publisher:
ISBN:
Category : Fronts (Meteorology)
Languages : en
Pages : 124
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Author: Roger M. Wakimoto
Publisher:
ISBN:
Category : Aerodynamics
Languages : en
Pages : 258
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This dissertation investigates the thunderstorm gust front with the aid of Project NIMROD data. The main goal is to present the time dependent solution of the gust front. RHI and VAD cross sections of reflectivity and Doppler velocity were constructed for different three minute radar scans to determine the vertical structure. These analyses revealed that the life cycle of the gust front could be divided into four stages: 1) the formative stage; 2) the early mature stage; 3) the late mature stage; and 4) the dissipation stage. A new finding was a horizontal roll detected in the reflectivity patterns during stages II and III. This phenomenon was called a "precipitation roll." Rawinsonde launchings revealed the thermodynamic structure of the gust front. The cold air layer was found to correspond closely to the observed surface pressure rise. Calculations confirmed that the collision of two fluids produced nonhydrostatic pressure forces at the leading edge of the outflow. A dry, cool layer at the top of the gust front was a result of the low level outflow that was lifted vertically at the frontal boundary and advected back by the circulation within the bead of the gust front. The equation for the propagation speed of a density current was confirmed to accurately predict the movement of the gust front, thus disproving the theory that the gust front is an advecting phenomenon. Comparisons are made between the gust front and the "downburst" to emphasize their different nature.
Author: United States. Federal Aviation Administration. Systems Research and Development Service
Publisher:
ISBN:
Category : Thunderstorms
Languages : en
Pages : 122
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Author:
Publisher:
ISBN:
Category : Gusts
Languages : en
Pages : 100
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Author: R. W. Sinclair
Publisher:
ISBN:
Category : Meteorology in aeronautics
Languages : en
Pages : 84
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Author: A. J. Bedard
Publisher:
ISBN:
Category : Fronts (Meteorology)
Languages : en
Pages : 28
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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 147
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The Dulles International Airport wind and wind shear detection system recorded more than 160 events from November 1976 through March 1978. Eighty-two percent of 113 significant events occurred in conjunction with thunderstorms, squall-lines and frontal passages. The chief source of false alarms for anemometers was boundary layer disturbances representing 10% of the total; while gravity shear waves related to the 500mb winds caused the most false alarms for pressure sensors (4% of the total). Using complementary arrays of wind and pressure sensors, total system false alarms can be greatly reduced. These data demonstrate the importance of stable surface layers in determining how well surface temperature and wind measurements represent flow at higher levels and explain underestimates of system severity. Moreover, because we demonstrate the value of wind vector information and since temperature is a scaler quantity, there seems little operational value in using temperature sensors as part of a detection system. Conversely, pressure sensors will not reliably detect relatively thin outflows (100-200 M thick) occurring at a distance from downflows having small dimensions. Again, the wind and pressure sensors combine to provide a total system offering good reliability for detection of thunderstorm outflows. The results suggest an array configuration (composed of wind and pressure jump sensors) for use in airport warning systems. This configuration extending about 10km from the airport provides a warning for Thunderstorm gust-fronts.
Author:
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ISBN:
Category :
Languages : en
Pages :
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Author: Eric Dale Skyllingstad
Publisher:
ISBN:
Category : Thunderstorms
Languages : en
Pages : 194
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