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
Clear All Modify Search
Thomas R. Parish

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.

Full access
Thomas R. Parish

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.

Full access
Thomas R. Parish

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.

Full access
Thomas R. Parish

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.

Full access
Thomas R. Parish

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.

Full access
Thomas R. Parish

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.

Full access
Thomas R. Parish

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.

Full access
Thomas R. Parish and Bart Geerts

Abstract

Airborne measurement of the horizontal pressure field using differential GPS technology has been established during the last few years. Accurate aircraft measurement of the horizontal pressure gradient force requires an independent determination of the height of the airborne platform above some reference level. Here the authors demonstrate a differential GPS technique that uses data from a fixed reference station to refine the vertical position of the aircraft. A series of research flight legs by the University of Wyoming King Air research aircraft (UWKA) were conducted during the winter seasons of 2008 and 2009 over the Medicine Bow Mountains in southern Wyoming. Flight patterns consisted of a series of geographically fixed, parallel legs along a quasi-isobaric surface above the mountainous terrain, allowing the finescale mapping of the horizontal pressure (or geopotential height) field. The removal of the large-scale gradient and tendency isolates the terrain-induced pressure perturbation field. Results obtained using differential GPS measurements of aircraft height show that the Medicine Bow Range induces pronounced horizontal pressure perturbations, with a leeside region of low pressure downwind of the crest, in two cases: on 11 February 2008 and 20 February 2009. A wind maximum is found downwind of the elevated terrain consistent with this pressure gradient. Simulations of these two cases were performed using the Weather Research and Forecasting Model (WRF). The WRF height patterns for the time of the UWKA flight matched the general isobaric height patterns observed. Simulations and observations consistently show that the cross-mountain acceleration is stronger when the perturbation pressure gradient is larger.

Full access
Thomas R. Parish and David Leon

Abstract

Vertical accelerations during the early stages of convective cloud formation are often the result of buoyancy and the perturbation vertical pressure gradient forces. Convection modifies the local pressure field surrounding the cloud. Measurement of the cloud perturbation pressure field is challenging over distance scales on the order of the convective elements, since the signals are often small and the turbulent environment complicates the measurement of static pressure. A technique is described that enables detection of the horizontal pressure perturbations associated with evolving convective clouds using global positioning system measurements on an airborne platform. Differential kinematic processing of data from dual-frequency, carrier-phase-tracking GPS receivers on research aircraft with static base station receivers enables the three-dimensional aircraft position to be resolved within decimeters. Vertical positioning and precise measurement of static pressure allow horizontal pressure perturbations to be determined to an accuracy of roughly 10 Pa. Errors in the static pressure measurement, rather than the GPS-derived altitude, are the largest source of error. A field experiment was conducted in May–June 2008 to demonstrate measurement of perturbations in the horizontal pressure field associated with summertime cumulus congestus clouds over the high plains. Observations of growing convective clouds show negative pressure perturbations on the order of 100 Pa near cloud base linked to updraft regions. Growing cumulus show a high degree of variability between subsequent passes that demonstrate that the horizontal pressure fields evolve rapidly along with attendant vertical circulations and cloud microphysical characteristics.

Full access
Kenneth R. Pomeroy and Thomas R. Parish

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.

Full access