Research On Tomography Of Ground-Based GPS Three-Dimensional Distribution Of Water Vapor

Water vapor is the most active part in the atmosphere, and the distribution of water vapor changes dynamically, it is closely associated to the long persistent rainfalls, tropical cyclones, mid-latitude cyclonic storms, thunder storms and other meteorological phenomena. These disastrous weather phenomena may cause great loss to the society and economy. Therefore, obtaining water vapor information with high-precision, high spatial and temporal resolution is very meaningful for meteorology and weather forecasting. In this thesis, a series of experiments about three-dimensional distribution of water vapor are carried out using GPS data of the Sichuan region. The main contents are as follows:(1) Dr. Xu Shaoguang in Southwest Jiaotong University independently developed a precise location and orbit determination software (PLAOD software). The accuracy of the zenith total delay solved by PLAOD software is high enough to meet the requirement of the accuracy of the water vapor inversion, which verifies that choosing PLAOD software as the solver software for the follow-up study of this thesis is reliable and feasible.(2) The comparison of post-precise ephemeris and post-precise clock error, rapid precise ephemeris and rapid precise clock error, ultra-fast precise ephemeris and rapid precise clock error are included in the experiment. All of the three combinations program can meet the accuracy of water vapor inversion. According to the situation in this thesis, the rapid precise ephemeris and rapid precise clock error which has high accuracy and the smaller time delay is chosen to satisfy the requirements of near real-time water vapor dimensional tomography. In this thesis, combining with the actual situation, the elevation angle is set as10°, NMF mapping function is applied here, and oblique projection method is selected to calculate the GPS slant path delay, the Kalman filter algorithm is used to solve the tomographic equations.(3) The GPS network of Sichuan region which is consisted of five sites including Bazhong, Daxian, Guang’an, Nanjiang, Yingshan is selected as the study area. The area is horizontally divided into a grid of6x4=24. The water vapor density profile obtained by tomography of800m and1000m respectively uniform vertical resolution stratification is close to the sounding inversion profile. It initially showed that tomographic results obtained with the two resolutions are almost the same in the case of high correlation coefficients. In the study of Low-level moisture in troposphere, characteristics of800m vertical resolution are more obvious.(4) The effects of the ways of vertical stratification on the tomographic results have been studied, proposing a non-uniform vertical stratification method which is based on atmospheric characteristics of refractive index with elevation changes. Experimental results show that the water vapor density profiles solved by the non-uniform vertical stratification tomography is better than the uniform vertical stratification, and it is more consistent with the sounding vapor density profile. Taking into account the fact that the vapor mainly concentrated in the lower troposphere, the lower troposphere has relatively high vertical resolution while uneven vertical stratification is applied, it is better to reflect the variation of atmospheric water vapor in the lower troposphere and more detailed characterized changes in atmospheric water vapor. The results of water vapor tomographic experiments in the Sichuan region show that uneven vertical stratification method proposed in this thesis is feasible.(5) The experiment has studied the effects of the initial tomography on the tomographic results, when initial value of tomography has±50%deviation, the water vapor vertical information solved by the Kalman filter solver can still get better compliance with the sounding data information. It can obtain accurate and reliable vapor information in the vertical direction, indicated that Kalman filter algorithm for three-dimensional distribution of water vapor is not sensitive to the initial value of tomography, the result is conformed to the original observation data of GPS. Experiments analysis the impacts of the number of GPS sites on tomography, the results show that a reasonable grid division and more stations are the preconditions to get better tomographic results. Finally, the tomographic pictures of three-dimensional distribution of water vapor in Sichuan GPS network shows visually and clearly that water vapor mainly distribute in the areas3kilometers below the troposphere, the amount of low-level moisture is large and is uneven distributed, the amount of high-level water vapor is smaller with more uniform distribution, the amount of water vapor decreases woth the height increases in the area above3kilometers, but the phenomenon of inverse wet may also occur repeatedly.