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?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.