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海南GPS网探测对流层水汽廓线的试验研究
Tropospheric Water Vapor Profiles Using GPS Network in Hainan
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摘要: 介绍了2005年11月在海南地区进行的一次地基GPS小网观测试验。试验目的是研究利用组网的GPS倾斜路径观测进行对流层水汽层析反演, 给出站点上空水汽的垂直廓线结构信息。概述了试验中GPS原始数据处理方法以及层析反演的方法。将GPS层析得到的水汽垂直廓线与海口站探空观测的水汽廓线进行了对比, 结果表明:二者一致性较好, 均方根误差在0.5g·m-3左右, 层析结果较好地反映出试验期间水汽减少、大气变干的过程。另外, 采用3种不同的先验信息方案测试分析了GPS层析的结果, 表明GPS观测量对水汽先验信息有明显的调整作用。并对GPS水汽层析中可能存在的问题进行了讨论。试验证明高时间分辨率的GPS观测有能力层析出GPS测站上空水汽的廓线信息。Abstract: Water vapor plays a key role in atmospheric processes. Water vapor field is also one of the initial conditions needed by numerical weather prediction. Its distribution remains difficult to quantify due to water vapor's high variability in time and space and the deficiency of available measurements. The GPS has proved its capacity to measure the integ rated water vapor at zenith with the same accuracy as other methods, such as radiosonde and water vapor radiometers, which has been demonstrated by many experiments and GPS precipitable water vapor has been operated in many areas. Recent studies also show that it is possible to quantify the integ rated water vapor in the direction of signals of the GPS satellite. These observations can be used to study the water vapor tomography using GPS network. That is to use the water vapor amount along slant path of GPS satellites to form observation equations and to obtain the vertical profile information over GPS sites by solving these equations.The principle of tomography is introduced in which slant path observations from ground-based GPS is used and the method of calculating the observation equations is presented. During November in 2005, a small GPS network experiment is carried out in Hainan. The purpose of this experiment is to study the tomography tropospheric water vapor profiles based on GPS slant path observations. Without a priori information of water vapor, the absolute value of water vapor is difficult to resolve. After the method of GPS raw data processing is summarized, how to use a priori information is also analyzed. The period of tomography is from Nov 17 to 21 in 2005. Three different vertical tomographic resolutions of 600 m, 800 m and 1000 m are tested and analyzed. Results show that these resolutions have no large difference. Therefore, tomographic resolution from 500 m to 1000 m can be applied in practical use. In the experiment, tomographic vertical resolution of 1000 m is selected. Three different plans using a priori information are described.In the first plan, priori information is the earliest radiosonde at 00:00 on Nov 17, 2005. In the second plan, the latest radiosonde before the retrieved time is used as priori information. The average radiosondes are used in the third plan during the experiment. In the first and the third plan, priori information is hold but in the second plan priori information is changed and regenerated. Water vapor vertical profiles derived from GPS tomography are compared with that measured by radiosonde in Haikou site. The results show that they agree well with each other and the root-mean-square error (RMSE) is about 0.5 g·m -3. The process of water vapor decreasing and atmosphere drying is well reflected by GPS tomography. The tomographic results from different priori information used in tomographic method are also analyzed. It is demonstrated that water vapor priori information is obviously adjusted by GPS observation. And some problems in tomography are also discussed. Vertical profile distribution of tropospheric water vapor can be retrieved from GPS measurements with high time resolution through tomography inversion.
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Key words:
- GPS;
- water vapor;
- tomography
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图 1 海南地基GPS观测网及水平网格划分示意图
Fig. 1 Location of the ground-based GPS network in Hainan with the horizontal grids
图 2 2005年11月20日 (年积日324) 海口站层析廓线与探空对比图
Fig. 2 Comparision of water vapor vertical profiles between GPS and radiosonde at Haikou station at 00:00-01:00 and 12:00-13:00 on Nov 20, 2005
图 3 2005年11月21日 (年积日325) 海口站层析廓线与探空对比图
Fig. 3 Comparision of water vapor vertical profiles between GPS and radiosonde at Haikou station at 00:00-01:00 and 12:00-13:00 on Nov 21, 2005
图 4 2005年11月21日 (年积日325) 采用不同先验信息的层析结果与实测探空对比
(十字形:方案一, 即采用第321天00:00探空为先验值的层析结果; 圆形:方案二, 即采用时间最近的上次探空为先验值; 正方形:方案三, 即用平均探空为先验值; 菱形:第325天的反演时刻的单次探空)
Fig. 4 Tomographic results from different a priori information versus observations by radiosonde
(line with cross-shaped symbol is the first plan using radiosonde in 321 day of year as a priori values; line with circle symbol represents the second plan using the latest radiosonde before this time as a priori information; line with square symbol is the third plan using a priori information from average radionsonde; line with diamond symbol shows the single radiosonde in day 325)
图 5 试验期间3种先验值层析方案与探空对比的散点图
(a) 采用2005年11月17日00:00探空为先验值的层析结果, (b) 采用反演时刻前一次探空为先验值的层析结果, (c) 以平均探空廓线为先验值的结果
Fig. 5 Scatterplots of results from radiosonde and tomography during the experiment
(a) tomographic results using radiosonde at 00:00 as a priori information on Nov 17, 2005, (b) using latest radiosonde before the time of retrieval, (c) using average radiosonde
图 6 2005年11月21日GPS层析、先验信息分别与探空的对比散点图
(菱形:先验信息与探空的对比; 实线:通过原点的拟合直线; 三角形: GPS层析与探空的对比; 虚线:通过原点的拟合直线)
Fig. 6 Scatterplots of GPS tomography, a priori information and radiosonde on Nov 21, 2005
(diamond symbol denotes comparison between a priori information and radiosonde, solid line is fitting line passing origin; triangle symbol denotes the results from GPS and radiosonde; dashed line is fitting line passing origin)
表 1 不同分辨率下的层析结果统计分析
Table 1 Tomographic statistics results of different resolutions
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