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

在控制大气动力学的运动方程中，最基本的强迫项是压力梯度力的水平分量。它代表了大气运动的主要强迫机制。对水平压力梯度力（PGF）的了解也使大气流动可以根据地转和地转成分进行分离。地转分量虽然通常比地转分量小得多，但它代表了运动的发散分量，对垂直运动的强迫和重大天气事件的发展至关重要。本研究的科学目标是利用怀俄明大学空中国王研究飞机（KA）作为评估全球定位系统（GPS）衍生测量在精确确定PGF方面的潜力的手段。研究飞行腿将由使用自动驾驶仪保持恒定压力的直线路径组成。假设静压条件下，飞机与等压表面的小偏差将被纠正，因为压力和温度是高精度的。在2000年5月之前，GPS位置估计中最显著的偏差误差是由于选择性可用性，这是GPS信号的故意退化，大大降低了导航解决方案的精度。选择性可用性在2000年5月停用，因此在位置确定方面提供了近一个数量级的改进。据估计，目前的GPS仪器可以在3米内估计垂直位置。将探索导航解决方案技术的改进，使用来自固定GPS测量和允许精确定位的L1和L2 GPS代码的差分修正。雷达测高将用于对gps衍生的PGF计算提供独立检查。2004年6月将进行一次实地试验，研究门多西诺角附近沿加利福尼亚中北部海岸的低层大气，以检验PGF策略和阐明夏季海洋边界层的主要特征。门多西诺角沿海地区在夏季表现出持续的沿海低空急流。在门多西诺角的背风处经常观察到沿海急流的局部增强。本文将对门多西诺角沿岸急流发生期间的风廓线和温度廓线以及相关的PGF的日变化进行研究。沿海环境的动力学将被研究，水力特征的作用，如膨胀风扇和水力跳跃将被评估。如果成功，GPS对PGF的探测将成为机载平台的常规测量，并成为大气科学界使用的KA平台的标准。由于基本强迫项可以直接测量，这将为未来的大气动力学研究提供显著的增强。目前只能研究大气运动的运动学。GPS技术的成功应用还将带来许多新的测量机会，例如改进空中风测量，了解与中尺度环流相关的动力学时空变化，以及不规则地形上的动力学过程，仅举几例。

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.