An Application of Airborne Global Positioning System (GPS) Measurements to Studies of Atmospheric Dynamics

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The most fundamental forcing term in the equation of motion that governs atmospheric dynamics is the horizontal component of the pressure gradient force. It represents the primary forcing mechanism for atmospheric motion. Knowledge of the horizontal pressure gradient force (PGF) also allows the atmospheric flow to be separated in terms of geostrophic and ageostrophic components. The ageostrophic component, while generally much smaller than the geostrophic component, represents the divergent component of motion and is critical to the forcing of vertical motion and hence development of significant weather events. The scientific objective of this research is to employ the University of Wyoming King Air research aircraft (KA) as a means to evaluate the potential of Global Positioning System (GPS) derived measurements in making an accurate determination of the PGF. Research flight legs will consist of straight paths using the autopilot to remain at constant pressure. Small deviations of the aircraft from the isobaric surface will be corrected assuming hydrostatic conditions since the pressure and temperature are known with high precision. Prior to May 2000 the most significant bias error in the GPS position estimate was due to Selective Availability, an intentional degradation of the GPS signals that significantly reduced the accuracy of the navigation solution. Selective Availability was inactivated in May 2000, thereby providing for nearly an order of magnitude improvement in position determination. It is estimated that vertical position estimates within 3-m are possible with the current GPS instruments. Refinements in navigation solution techniques will be explored using differential corrections from stationary GPS measurements and from the L1 and L2 GPS codes that allow for precise positioning. Radar altimetry will be used to provide independent checks on the GPS-derived PGF calculations.A field experiment will be conducted during June 2004 to study the lower atmosphere offshore from Cape Mendocino along the north central California coast as a means to test the PGF strategy and to elucidate key features of the summertime marine boundary layer. The Cape Mendocino coastal region exhibits a persistent coastal low-level jet during summer. Local enhancements to the coastal jet are frequently observed to the lee of Cape Mendocino. An examination as to the diurnal variations in the wind and temperature profiles and associated PGF surrounding Cape Mendocino during episodes of the coastal jet will be made. The dynamics of the coastal environment will be studied and the role of hydraulic features such as expansion fans and hydraulic jumps will be evaluated.If successful, GPS detection of the PGF will become a routine measurement on airborne platforms and become standard for the KA platform to be used by the atmospheric science community. This will provide a significant enhancement for future studies in atmospheric dynamics since the fundamental forcing term can be directly measured. Currently only the kinematics of atmospheric motions can be studied. Successful application of this GPS technology will also enable a host of new measurement opportunities, such as refinements in airborne wind measurements, understanding the temporal and spatial variations in dynamics associated with mesoscale circulations, and dynamical processes over irregular terrain, to name just a few.