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

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 Ϫ1 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.

Stratospheric Gravity Wave Fluxes and Scales during DEEPWAVE

“…As the autopilot is trying to maintain a constant pressure altitude, the variations in P C along a leg, especially for longer disturbances, come primarily from altitude (z) variations. This kind of correction has been made for several decades over water using a radar altimeter, but only recently could it made over rough terrain using differential GPS (Parish et al 2007). With estimated OmniStar GPS altitude errors of 10 cm or less, and a typical air density of 0.3 kg m 23 , the potential error in the second term in (7) is only 0.3 Pa. We have confirmed this altitude accuracy during DEEPWAVE by comparing OmniStar with New Zealand ground GPS stations.…”

Cessation of the 22–25 June 2006 Coastally Trapped Wind Reversal

“…A coastal jet at the top of the marine boundary layer (MBL) is often embedded in this flow (e.g., Zemba and Friehe 1987;Burk and Thompson 1996;Rogers et al 1998;Parish 2000;Rahn and Parish 2007). This northerly wind regime is interrupted several times each summer by weak southerly flow adjacent to the coast that is often accompanied by fog or low stratus that moves northward along the coast.…”

Correction of static pressure on a research aircraft in accelerated flight using differential pressure measurements

“…LeMone and Tarleton (1986) and LeMone et al (1988) used altitude derived from accelerometer measurements instead of radar altitude in perturbation pressure studies around clouds suggesting that accuracies of 20 Pa can be obtained, but only with substantial empirical corrections and carefully flown legs. More recently, Parish et al (2007) and Parish and Leon (2012) demonstrated that Global Navigation Satellite System (GNSS, hereafter referred to as global positioning system -GPS) data can resolve both pressure gradients and perturbations associated with mesoscale and cloud-scale systems.…”

“…Differential global positioning system (dGPS) techniques use data from one or more stationary reference or base GPS stations which have precisely determined location to refine position estimates for the receiver on the aircraft. dGPS processing techniques using dual-frequency (L1/L2) carrier phase data can eliminate errors caused by ionospheric and tropospheric delays entirely, resulting in position estimates with accuracy of centimeters under optimal conditions (Parish et al, 2007).…”

“…Periods of turns were not considered in the analysis. The aircraft was flown at nominally constant pressure with deviations corrected hydrostatically to that pressure using the method described by Parish et al (2007). Pressurederived altitude changes were determined from integration of the hydrostatic equation: …”