scholarly journals Aircraft Measurements and Numerical Simulations of an Expansion Fan off the California Coast

2016 ◽  
Vol 55 (9) ◽  
pp. 2053-2062 ◽  
Author(s):  
Thomas R. Parish ◽  
David A. Rahn ◽  
David C. Leon
Keyword(s):  
Pressure Gradient ◽  
Cross Sections ◽  
The United States ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Differential Gps ◽  
Horizontal Pressure Gradient ◽  
Point Conception ◽  
Expansion Fan ◽  
Horizontal Pressure

AbstractMountains along the California coastline play a critical role in the dynamics of marine atmospheric boundary layer (MBL) airflow in the vicinity of the shoreline. Large changes in the MBL topology have been known to occur downwind of points and capes along the western coast of the United States. Large spatial gradients in wind and temperature become established that can cause anomalous electromagnetic wave propagation. Detailed airborne measurements using the University of Wyoming King Air were conducted to study the adjustment of the MBL to the Point Arguello and Point Conception headlands. Pronounced thinning of the MBL consistent with an expansion fan occurred to the south of Point Conception on 13 June 2012. A sharp cloud edge was collocated with the near collapse of the MBL. D-value cross sections derived from differential GPS altitude measurements allow assessment of the vertical profile of the horizontal pressure gradient force and hence thermal wind forcing in response to the near collapse of the MBL. The Weather Research and Forecasting Model was run with a 1-km grid spacing to examine the atmospheric adjustment around Point Conception during this period. Results from the simulations including the vertical cross sections of the horizontal pressure gradient force were consistent with the aircraft observations. Model results suggest that divergence occurs as the flow rounds Point Conception, characteristic of an expansion fan. Wind speeds in the MBL increase coincident with the decrease in MBL thickness, and subsiding flow associated with the near collapse of the MBL is responsible for the sharp cloud edge.

scholarly journals Research Aircraft Determination of D-Value Cross Sections

2016 ◽  
Vol 33 (2) ◽  
pp. 391-396 ◽  
Author(s):  
Thomas R. Parish ◽  
David A. Rahn ◽  
Dave Leon
Keyword(s):  
Pressure Gradient ◽  
Cross Sections ◽  
Vertical Structure ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Horizontal Pressure Gradient ◽  
Airborne Measurements ◽  
Horizontal Pressure ◽  
D Values

AbstractUse of an airborne platform to determine the dynamics of atmospheric motion has been ongoing for over three decades. Much of the effort has been centered on the determination of the horizontal pressure gradient force along an isobaric surface, and with wind measurements the nongeostrophic components of motion can be obtained. Recent advances using differential GPS-based altitude measurements allow accurate assessment of the geostrophic wind. Porpoise or sawtooth maneuvers are used to determine the vertical cross section of the horizontal pressure gradient force. D-values, the difference of the height of a given pressure level from that in a reference atmosphere, are used to isolate the vertical structure of the horizontal component of the pressure gradient force from the vastly larger hydrostatic pressure gradient. Comparison of measured D-value cross sections with airborne measurements of the horizontal pressure gradient is shown. Comparison of D-values with output from the WRF Model demonstrates that the airborne measurements are consistent with finescale numerical simulations. This technique provides a means of inferring the thermal wind, thereby enabling a detailed examination of the vertical structure of the forcing of mesoscale and synoptic-scale wind regimes.

Isobaric Height Perturbations Associated with Mountain Waves Measured by Aircraft during the Terrain-Induced Rotor Experiment

2012 ◽  
Vol 29 (12) ◽  
pp. 1825-1834 ◽  
Author(s):  
Thomas R. Parish ◽  
Larry D. Oolman
Keyword(s):  
Pressure Gradient ◽  
Sierra Nevada ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Differential Gps ◽  
Mountain Waves ◽  
Horizontal Pressure Gradient ◽  
Rotor Experiment ◽  
Horizontal Pressure ◽  
Forcing Mechanisms

Abstract It has only been in the last few years that accurate measurement of the horizontal pressure gradient has been possible over complex terrain using an airborne platform. To infer forcing mechanisms for the wind, an independent measure of the height of an isobaric surface is required. Differential GPS analyses have enabled determination of the aircraft height with sufficient accuracy to infer isobaric heights. When coupled with an accurate measurement of static pressure, the horizontal pressure field can be determined. To demonstrate this measurement technique, research flight legs by the University of Wyoming King Air (UWKA) conducted in support of the Terrain-Induced Rotor Experiment (T-REX) in March and April 2006 are examined. UWKA flights conducted on 14 and 25 March and 16 April 2006 encountered strong mountain waves in response to winds directed primarily normal to the Sierra Nevada ridgeline. Winds at flight level showed pronounced variation that suggested topographic influence. The magnitude of isobaric height perturbations along UWKA flight tracks obtained using differential GPS during case study days of 14 and 25 March and 16 April are shown to exceed 70 m, corresponding to horizontal pressure perturbations greater than 4 hPa. Measurements suggest that changes in wind speed are linked primarily to the perturbation height field and that the flow can be classified as Eulerian, implying that Coriolis accelerations are negligible and flows respond to the horizontal pressure gradient force.

Pressure Perturbations and Upslope Flow over a Heated, Isolated Mountain

2008 ◽  
Vol 136 (11) ◽  
pp. 4272-4288 ◽  
Author(s):  
Bart Geerts ◽  
Qun Miao ◽  
J. Cory Demko
Keyword(s):  
Pressure Gradient ◽  
Diurnal Variation ◽  
Monsoon Season ◽  
Potential Temperature ◽  
Surface Flow ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Thermal Forcing ◽  
Horizontal Pressure Gradient ◽  
Horizontal Pressure

Abstract Surface and upper-air data, collected as part of the Cumulus Photogrammetric, In Situ, and Doppler Observations (CuPIDO) experiment during the 2006 monsoon season around the Santa Catalina Mountains in southeast Arizona, are used to study the diurnal variation of the mountain-scale surface convergence and its thermal forcing. The thermal forcing is examined in terms of a horizontal pressure gradient force, which is derived assuming hydrostatic balance. The mountain is ∼30 km in diameter, ∼2 km high, and relatively isolated. The environment is characterized by weak winds, a deep convective boundary layer in the afternoon, and sufficient low-level moisture for orographic cumulus convection on most days. The katabatic, divergent surface flow at night and anabatic, convergent flow during the day are in phase with the diurnal variation of the horizontal pressure gradient force, which points toward the mountain during the day and away from the mountain at night. The daytime pressure deficit over the mountain of 0.5–1.0 mb is hydrostatically consistent with the observed 1–2-K virtual potential temperature excess over the mountain. The interplay between surface convergence and orographic thunderstorms is examined, and the consequence of deep convection (outflow spreading) is more apparent than its possible trigger (enhanced convergence).

Airborne Measurements of Terrain-Induced Pressure Perturbations

2013 ◽  
Vol 141 (11) ◽  
pp. 3814-3826
Author(s):  
Thomas R. Parish ◽  
Bart Geerts
Keyword(s):  
Pressure Gradient ◽  
Reference Station ◽  
Reference Level ◽  
Vertical Position ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Differential Gps ◽  
Airborne Measurements ◽  
Horizontal Pressure ◽  
Pressure Perturbations

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.

scholarly journals Determination of the Horizontal Pressure Gradient Force Using Global Positioning System on board an Instrumented Aircraft

2007 ◽  
Vol 24 (3) ◽  
pp. 521-528 ◽  
Author(s):  
Thomas R. Parish ◽  
Matthew D. Burkhart ◽  
Alfred R. Rodi
Keyword(s):  
Pressure Gradient ◽  
Global Positioning System ◽  
Base Station ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Positioning System ◽  
Isobaric Surface ◽  
Horizontal Pressure Gradient ◽  
Global Positioning ◽  
Horizontal Pressure

Abstract The horizontal pressure gradient force is the single most important dynamical term in the equation of motion that governs the forcing of the atmosphere. It is well known that the slope of an isobaric surface is a measure of the horizontal pressure gradient force. Measurement of this force over mesoscale distances using an airborne platform has been attempted for over two decades in order to understand the dynamics of various wind systems. The most common technique has been to use a radar altimeter to measure the absolute height of an isobaric surface above sea level. Typical values of the horizontal pressure gradient force in the atmosphere are quite small, amounting to an isobaric surface slope of 0.0001 for a 10 m s−1 geostrophic wind at middle latitudes. Detecting the horizontal pressure gradient over irregular terrain using an instrumented aircraft has proven to be especially difficult since correction for the underlying terrain features must be made. Use of the global positioning system (GPS) is proposed here as a means to infer the horizontal pressure gradient force without the need for altimetry and terrain registration over irregular surface topography. Differential kinematic processing of data from dual-frequency, carrier phase tracking receivers on research aircraft with similar static base station receivers enables the heights of an isobaric surface to be determined with an accuracy estimated to be a few decimeters. Comparison of results obtained by conventional altimetry-based methods over the ocean and Lake Michigan with GPS reveals the potential of the GPS method at determining the horizontal pressure gradient force, even over complex terrain.

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