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

在支配大气动力学的运动方程中，最基本的强迫项是气压梯度力的水平分量。它代表了大气运动的主要强迫机制。水平气压梯度力(PGF)的知识也可以根据地转和非地转分量来区分大气流动。虽然非地转分量一般比地转分量小得多，但它代表着运动的发散分量，对垂直运动的强迫和重大天气事件的发展至关重要。这项研究的科学目标是利用怀俄明大学国王航空研究飞机(KA)作为一种手段，评估全球定位系统(GPS)衍生测量在准确确定PGF方面的潜力。研究飞行段将由直线路径组成，使用自动驾驶仪保持恒定压力。假设静压条件下，飞机与等压面的微小偏差将被修正，因为压力和温度是高精度已知的。在2000年5月之前，全球定位系统位置估计中最大的偏差误差是由于选择性可用性，即故意降低全球定位系统信号的质量，大大降低了导航解决方案的精度。2000年5月停用了选择性供应，从而在确定位置方面有了近一个数量级的改进。据估计，利用目前的GPS仪器可以在3米内估计垂直位置。将使用来自静止GPS测量和允许精确定位的L1和L2 GPS代码的差分校正来探索导航解决方案技术的改进。2004年6月将进行一项实地试验，研究加利福尼亚州中北部海岸门多西诺角附近的低层大气，以测试PGF战略并阐明夏季海洋边界层的主要特征。门多西诺角沿海地区夏季呈现持续的沿海低空急流。在门多西诺角的背风处经常可以观察到沿海急流的局地增强。将检查沿海急流期间门多西诺角周围的风和温度分布以及相关的PGF的日变化情况。将研究海岸环境的动态，评估膨胀扇和水力跳跃等水力特性的作用。如果成功，对PGF的GPS探测将成为机载平台的常规测量，并成为KA平台的标准，供大气科学界使用。这将大大加强未来大气动力学研究，因为基本强迫项可以直接测量。目前只能研究大气运动的运动学。这项全球定位系统技术的成功应用还将带来一系列新的测量机会，例如改进空中风速测量，了解与中尺度环流相关的动力学的时间和空间变化，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.