Search Results
You are looking at 1 - 10 of 51 items for
- Author or Editor: Thomas R. Parish x
- Refine by Access: All Content x
Abstract
Certain coastal sections of Antarctica, most notably Adelie Land and Terra Nova Bay, experience anomalously intense, persistent katabatic winds. The forcing of such katabatic outflow is believed to originate several hundred kilometers upslope in the interior of the continent where cold air drainage currents from a large area converge into a relatively narrow zone focused on the steeply-sloping ice terrain near the coastline. Numerical simulations with a three-dimensional hydrostatic model incorporating terrain features representative of Adelie Land reveal a significant topographical channeling of the surface airflow. Katabatic wind speeds as depicted by the model are greatly enhanced downslope of the convergence channel. These results emphasize the importance of topography in the continental interior in shaping the character of coastal katabatic flow.
Abstract
Certain coastal sections of Antarctica, most notably Adelie Land and Terra Nova Bay, experience anomalously intense, persistent katabatic winds. The forcing of such katabatic outflow is believed to originate several hundred kilometers upslope in the interior of the continent where cold air drainage currents from a large area converge into a relatively narrow zone focused on the steeply-sloping ice terrain near the coastline. Numerical simulations with a three-dimensional hydrostatic model incorporating terrain features representative of Adelie Land reveal a significant topographical channeling of the surface airflow. Katabatic wind speeds as depicted by the model are greatly enhanced downslope of the convergence channel. These results emphasize the importance of topography in the continental interior in shaping the character of coastal katabatic flow.
Abstract
Surface winds over the Antarctic interior occur mainly due to the strong radiational cooling of the ice slopes. As a consequence, such winds exhibit a high degree of persistence with a predominant direction closely related to the terrain orientation. Using detailed contour maps of the interior ice topography and representative values of the mean wintertime strength of the temperature inversion, it is possible to infer the terrain-induced accelerations. A simple diagnostic equation system is formulated, from which a time-averaged surface airflow pattern of East Antarctica is generated. The results appear consistent with observations. The occurrence of localized, anomalously strong katabatic winds is explained as a result of typographically forced patterns of cold-air convergence depicted in the airflow analysis.
Abstract
Surface winds over the Antarctic interior occur mainly due to the strong radiational cooling of the ice slopes. As a consequence, such winds exhibit a high degree of persistence with a predominant direction closely related to the terrain orientation. Using detailed contour maps of the interior ice topography and representative values of the mean wintertime strength of the temperature inversion, it is possible to infer the terrain-induced accelerations. A simple diagnostic equation system is formulated, from which a time-averaged surface airflow pattern of East Antarctica is generated. The results appear consistent with observations. The occurrence of localized, anomalously strong katabatic winds is explained as a result of typographically forced patterns of cold-air convergence depicted in the airflow analysis.
Abstract
Observational evidence from instrumented aircraft, Doppler radar and rawinsondes suggest low-level, mountain-parallel jets are a common wintertime feature along the western slope of the Sierra Nevada Range and extending into the California Valley. It is proposed that the formation and maintenance of the low-level jet is a result of the pressure field created by the damming of stable air as it is forced up against the steep mountain barrier. Numerical experiments, using a two-dimensional (x, z) primitive equation model incorporating terrain representative of the Sierra Nevada Mountains, are carried out to test this assertion.
Abstract
Observational evidence from instrumented aircraft, Doppler radar and rawinsondes suggest low-level, mountain-parallel jets are a common wintertime feature along the western slope of the Sierra Nevada Range and extending into the California Valley. It is proposed that the formation and maintenance of the low-level jet is a result of the pressure field created by the damming of stable air as it is forced up against the steep mountain barrier. Numerical experiments, using a two-dimensional (x, z) primitive equation model incorporating terrain representative of the Sierra Nevada Mountains, are carried out to test this assertion.
Abstract
Coast-parallel low-level jets are commonplace in the offshore environment along the west coast of the United States during summer. The jet often has wind speeds in excess of 30 m s−1 and is typically situated near the top of the marine boundary layer. A field study was conducted in early summer of 1997 to study the kinematics and dynamics of the low-level jet off the California coast. The University of Wyoming King Air research aircraft was the primary observation platform. Measurement of the horizontal pressure gradient force was fundamental to understanding the dynamics of the jet. By flying at constant pressure, the height of an isobaric surface could be determined by the radar altimeter. The slope of a constant pressure surface is proportional to the pressure gradient force and hence provides an estimate of the geostrophic wind.
Data are presented for two episodes of the low-level jet. In both cases wind speed maxima extending in excess of 100 km from the coast were observed. In contrast to previous observational studies, little evidence of hydraulic effects near the coastal margin was found. Measurements of the horizontal pressure gradient force within the marine boundary layer showed that the coastal jet is in a state of near-geostrophic balance. The observed vertical shear of the geostrophic wind components matched direct measurements of the thermal wind and confirms the importance of the sloping marine boundary layer in forcing the jet as proposed previously. It is offered that the large-scale structure of sloping marine layer and its attendant low-level jet is consistent with the geostrophic adjustment of thermally direct circulation forced by the horizontal temperature contrast between land and ocean.
Abstract
Coast-parallel low-level jets are commonplace in the offshore environment along the west coast of the United States during summer. The jet often has wind speeds in excess of 30 m s−1 and is typically situated near the top of the marine boundary layer. A field study was conducted in early summer of 1997 to study the kinematics and dynamics of the low-level jet off the California coast. The University of Wyoming King Air research aircraft was the primary observation platform. Measurement of the horizontal pressure gradient force was fundamental to understanding the dynamics of the jet. By flying at constant pressure, the height of an isobaric surface could be determined by the radar altimeter. The slope of a constant pressure surface is proportional to the pressure gradient force and hence provides an estimate of the geostrophic wind.
Data are presented for two episodes of the low-level jet. In both cases wind speed maxima extending in excess of 100 km from the coast were observed. In contrast to previous observational studies, little evidence of hydraulic effects near the coastal margin was found. Measurements of the horizontal pressure gradient force within the marine boundary layer showed that the coastal jet is in a state of near-geostrophic balance. The observed vertical shear of the geostrophic wind components matched direct measurements of the thermal wind and confirms the importance of the sloping marine boundary layer in forcing the jet as proposed previously. It is offered that the large-scale structure of sloping marine layer and its attendant low-level jet is consistent with the geostrophic adjustment of thermally direct circulation forced by the horizontal temperature contrast between land and ocean.
Abstract
Katabatic winds are a dominant feature of the lower atmosphere over Antarctica. The radial diffluence displayed by the drainage flows implies that a continental-scale subsidence is present over Antarctica. From mass continuity considerations, a thermally direct meridional circulation must become established. The upper-level convergence above the Antarctic continent acting to feed the katabatic circulation generates cyclonic vorticity in the middle and upper troposphere. Model simulations show that a robust circumpolar circulation becomes established within a time scale of about a week. The adverse horizontal pressure gradients in the upper atmosphere result in a gradual decay of the low-level katabatic circulation. The katabatic wind regime appears to be an important forcing mechanism for the circumpolar vortex about the periphery of the Antarctic continent.
Abstract
Katabatic winds are a dominant feature of the lower atmosphere over Antarctica. The radial diffluence displayed by the drainage flows implies that a continental-scale subsidence is present over Antarctica. From mass continuity considerations, a thermally direct meridional circulation must become established. The upper-level convergence above the Antarctic continent acting to feed the katabatic circulation generates cyclonic vorticity in the middle and upper troposphere. Model simulations show that a robust circumpolar circulation becomes established within a time scale of about a week. The adverse horizontal pressure gradients in the upper atmosphere result in a gradual decay of the low-level katabatic circulation. The katabatic wind regime appears to be an important forcing mechanism for the circumpolar vortex about the periphery of the Antarctic continent.
Abstract
The low-level jet (LLJ) is a ubiquitous feature of the lower atmosphere over the Great Plains during summer. The LLJ is a nocturnal phenomenon, developing during the 6–9-h period after sunset. Forcing of the LLJ has been debated for over 60 years, the focus being on two processes: decoupling of the residual layer from the surface owing to nighttime cooling and diurnal heating and cooling of the sloping Great Plains topography.
To examine characteristics and forcing mechanisms for the LLJ, composite grids were compiled from the North American Mesoscale Forecast System for the summertime months of June and July over a 5-yr period (2008–12). One composite set was assembled from well-developed LLJ episodes during which the maximum nocturnal jet magnitude at 0900 UTC over northwestern Oklahoma exceeded 20 m s−1. A second set consists of nonjet conditions for which the maximum nighttime wind magnitude in the lowest 3 km did not exceed 10 m s−1.
The intensity of the horizontal pressure gradient and hence background geostrophic flow at jet level was the dominant difference between composite cases. The horizontal pressure gradient forms in response to the thermal wind above jet level that results primarily from seasonal heating of the sloping Great Plains. Thermal wind forcing is thus the key link between the Great Plains and the high frequency of LLJ occurrence. The nocturnal wind maximum develops primarily because of the inertial oscillation of the ageostrophic wind occurring after decoupling of the lower atmosphere from the surface owing to radiational cooling in the early evening.
Abstract
The low-level jet (LLJ) is a ubiquitous feature of the lower atmosphere over the Great Plains during summer. The LLJ is a nocturnal phenomenon, developing during the 6–9-h period after sunset. Forcing of the LLJ has been debated for over 60 years, the focus being on two processes: decoupling of the residual layer from the surface owing to nighttime cooling and diurnal heating and cooling of the sloping Great Plains topography.
To examine characteristics and forcing mechanisms for the LLJ, composite grids were compiled from the North American Mesoscale Forecast System for the summertime months of June and July over a 5-yr period (2008–12). One composite set was assembled from well-developed LLJ episodes during which the maximum nocturnal jet magnitude at 0900 UTC over northwestern Oklahoma exceeded 20 m s−1. A second set consists of nonjet conditions for which the maximum nighttime wind magnitude in the lowest 3 km did not exceed 10 m s−1.
The intensity of the horizontal pressure gradient and hence background geostrophic flow at jet level was the dominant difference between composite cases. The horizontal pressure gradient forms in response to the thermal wind above jet level that results primarily from seasonal heating of the sloping Great Plains. Thermal wind forcing is thus the key link between the Great Plains and the high frequency of LLJ occurrence. The nocturnal wind maximum develops primarily because of the inertial oscillation of the ageostrophic wind occurring after decoupling of the lower atmosphere from the surface owing to radiational cooling in the early evening.
Abstract
Detailed ground-based and airborne measurements were conducted of the summertime Great Plains low-level jet (LLJ) in central Kansas during the Plains Elevated Convection at Night (PECAN) campaign. Airborne measurements using the University of Wyoming King Air were made to document the vertical wind profile and the forcing of the jet during the nighttime hours on 3 June 2015. Two flights were conducted that document the evolution of the LLJ from sunset to dawn. Each flight included a series of vertical sawtooth and isobaric legs along a fixed track at 38.7°N between longitudes 98.9° and 100°W.
Comparison of the 3 June 2015 LLJ was made with a composite LLJ case obtained from gridded output from the North American Mesoscale Forecast System for June and July of 2008 and 2009. Forcing of the LLJ was detected using cross sections of D values that allow measurement of the vertical profile of the horizontal pressure gradient force and the thermal wind. Combined with observations of the actual wind, ageostrophic components normal to the flight track can be detected. Observations show that the 3 June 2015 LLJ displayed classic features of the LLJ, including an inertial oscillation of the ageostrophic wind. Oscillations in the geostrophic wind as a result of diurnal heating and cooling of the sloping terrain are not responsible for the nocturnal wind maximum. Net daytime heating of the sloping Great Plains, however, is responsible for the development of a strong background geostrophic wind that is critical to formation of the LLJ.
Abstract
Detailed ground-based and airborne measurements were conducted of the summertime Great Plains low-level jet (LLJ) in central Kansas during the Plains Elevated Convection at Night (PECAN) campaign. Airborne measurements using the University of Wyoming King Air were made to document the vertical wind profile and the forcing of the jet during the nighttime hours on 3 June 2015. Two flights were conducted that document the evolution of the LLJ from sunset to dawn. Each flight included a series of vertical sawtooth and isobaric legs along a fixed track at 38.7°N between longitudes 98.9° and 100°W.
Comparison of the 3 June 2015 LLJ was made with a composite LLJ case obtained from gridded output from the North American Mesoscale Forecast System for June and July of 2008 and 2009. Forcing of the LLJ was detected using cross sections of D values that allow measurement of the vertical profile of the horizontal pressure gradient force and the thermal wind. Combined with observations of the actual wind, ageostrophic components normal to the flight track can be detected. Observations show that the 3 June 2015 LLJ displayed classic features of the LLJ, including an inertial oscillation of the ageostrophic wind. Oscillations in the geostrophic wind as a result of diurnal heating and cooling of the sloping terrain are not responsible for the nocturnal wind maximum. Net daytime heating of the sloping Great Plains, however, is responsible for the development of a strong background geostrophic wind that is critical to formation of the LLJ.
Abstract
Coast-parallel low-level jets are commonplace in the marine boundary layer off the west coast of the United States during summer. A field study was conducted in early summer of 1997 to document the forcing of boundary layer winds in the near-coastal environment off California. On 8 June 1997 the Wyoming King Air collected data along a 350-km stretch of coastal margin from Cape Mendocino to San Francisco in order to examine the interaction between the coastal topography and the low-level jet. During the course of the flight, 32 soundings were conducted. The maximum speed of the coastal jet was found near the top of the marine boundary layer at altitudes from 200 to 600 m. Analysis of the data revealed a westward increase in the height of the marine boundary layer and maximum jet wind speeds. Strongest jet winds were observed southwest of Cape Mendocino with a maximum speed of 28 m s−1. The coastal jet was characterized by a broad horizontal extent. Wind maxima were found at distances approximately 30 km to more than 100 km offshore.
Hydraulic features such as jumps and expansion fans have previously been observed downwind of coastal capes and points along the California coast. The flow upwind of Cape Mendocino and Point Arena was found to be supercritical, but the King Air data showed that accelerations associated with possible expansion fan phenomena were minimal. It is proposed that the sloping inversion at the top of the marine boundary layer and attendant coastal jet are fundamentally the result of a geostrophic adjustment process arising because of the horizontal temperature contrast between the cool ocean and warm continent. This view emphasizes that the coastal jet is a ubiquitous, large-scale feature of the summertime coastal environment. Terrain-induced wind speed variations associated with expansion fans and hydraulic jumps only modulate the primary jet structure.
Abstract
Coast-parallel low-level jets are commonplace in the marine boundary layer off the west coast of the United States during summer. A field study was conducted in early summer of 1997 to document the forcing of boundary layer winds in the near-coastal environment off California. On 8 June 1997 the Wyoming King Air collected data along a 350-km stretch of coastal margin from Cape Mendocino to San Francisco in order to examine the interaction between the coastal topography and the low-level jet. During the course of the flight, 32 soundings were conducted. The maximum speed of the coastal jet was found near the top of the marine boundary layer at altitudes from 200 to 600 m. Analysis of the data revealed a westward increase in the height of the marine boundary layer and maximum jet wind speeds. Strongest jet winds were observed southwest of Cape Mendocino with a maximum speed of 28 m s−1. The coastal jet was characterized by a broad horizontal extent. Wind maxima were found at distances approximately 30 km to more than 100 km offshore.
Hydraulic features such as jumps and expansion fans have previously been observed downwind of coastal capes and points along the California coast. The flow upwind of Cape Mendocino and Point Arena was found to be supercritical, but the King Air data showed that accelerations associated with possible expansion fan phenomena were minimal. It is proposed that the sloping inversion at the top of the marine boundary layer and attendant coastal jet are fundamentally the result of a geostrophic adjustment process arising because of the horizontal temperature contrast between the cool ocean and warm continent. This view emphasizes that the coastal jet is a ubiquitous, large-scale feature of the summertime coastal environment. Terrain-induced wind speed variations associated with expansion fans and hydraulic jumps only modulate the primary jet structure.
Abstract
Determination of the horizontal pressure gradient force over distance scales less than 100 km is possible using airborne altimetry and detailed maps of the underlying terrain. To detect the very small isobaric slopes, instrumentation must perform up to specification and aircraft position must be known within about 250 m in order to achieve an adequate matching of altimeter height and terrain height. Numerous test flights were conducted to study the stability of the technique. Results indicate the system is capable of resolving pressure gradients with equivalent geostrophic wind errors of approximately ± 1 m s−1 over a 100 km horizontal scale.
Abstract
Determination of the horizontal pressure gradient force over distance scales less than 100 km is possible using airborne altimetry and detailed maps of the underlying terrain. To detect the very small isobaric slopes, instrumentation must perform up to specification and aircraft position must be known within about 250 m in order to achieve an adequate matching of altimeter height and terrain height. Numerous test flights were conducted to study the stability of the technique. Results indicate the system is capable of resolving pressure gradients with equivalent geostrophic wind errors of approximately ± 1 m s−1 over a 100 km horizontal scale.
Abstract
Two aircraft missions to sample the boundary layer dynamics associated with the intense katabatic wind regime at Terra Nova Bay, Antarctica were flown on successive days in early November 1987. Light winds averaging 5 m s−1 were monitored at the 170 m flight level over the interior of the ice sheet. Dramatic acceleration of the airflow and abrupt 5°–7°C cooling were encountered on both days near the head of Reeves Glacier just upslope from where the terrain steepens considerably. These results suggest that much of the airflow convergence which sustains the coastal katabatic winds is forced by localized topographic channeling into Reeves Glacier, and that the descending airstream is negatively buoyant. The horizontally propagating katabatic winds were followed for 250 km directly offshore and for 200 km southward parallel to the Victoria Land coast the airstream momentum gradually decreased along both flight paths.
In conjunction with the descent of negatively buoyant air down Reeves Glacier and horizontal flow acres Nansen Ice Sheet, thermal infrared satellite images showed a warm katabatic signature along the trajectory. This paradox is explained by vigorous vertical mixing within the katabatic layer which makes the temperature of the emitting snow surface beneath the katabatic jet much warmer than that for adjacent light-wind areas. Thermal images often suggest that katabatic winds propagate for hundreds of kilometers beyond the slope break; this interpretation is strongly supported by the offshore aircraft data.
Primitive-equation model simulations for the aircraft flight level reproduced the light Dearly frictionless contour-parallel winds wen in the interior. The model also reproduced the abrupt airflow acceleration near the head of Reeves Glacier. Maximum speeds within the steeply-sloping glacier valley are underestimated, however, and it appears that a much finer rid spacing than 32 km is required to accurately simulate katabatic drainage through the complex coastal mountains.
Abstract
Two aircraft missions to sample the boundary layer dynamics associated with the intense katabatic wind regime at Terra Nova Bay, Antarctica were flown on successive days in early November 1987. Light winds averaging 5 m s−1 were monitored at the 170 m flight level over the interior of the ice sheet. Dramatic acceleration of the airflow and abrupt 5°–7°C cooling were encountered on both days near the head of Reeves Glacier just upslope from where the terrain steepens considerably. These results suggest that much of the airflow convergence which sustains the coastal katabatic winds is forced by localized topographic channeling into Reeves Glacier, and that the descending airstream is negatively buoyant. The horizontally propagating katabatic winds were followed for 250 km directly offshore and for 200 km southward parallel to the Victoria Land coast the airstream momentum gradually decreased along both flight paths.
In conjunction with the descent of negatively buoyant air down Reeves Glacier and horizontal flow acres Nansen Ice Sheet, thermal infrared satellite images showed a warm katabatic signature along the trajectory. This paradox is explained by vigorous vertical mixing within the katabatic layer which makes the temperature of the emitting snow surface beneath the katabatic jet much warmer than that for adjacent light-wind areas. Thermal images often suggest that katabatic winds propagate for hundreds of kilometers beyond the slope break; this interpretation is strongly supported by the offshore aircraft data.
Primitive-equation model simulations for the aircraft flight level reproduced the light Dearly frictionless contour-parallel winds wen in the interior. The model also reproduced the abrupt airflow acceleration near the head of Reeves Glacier. Maximum speeds within the steeply-sloping glacier valley are underestimated, however, and it appears that a much finer rid spacing than 32 km is required to accurately simulate katabatic drainage through the complex coastal mountains.
