On The Remote Sensing Of Precipitable Water Vapor Using Ground-Based GPS Technique And Applications In Meteorology

The resolution of conventional observation for atmospheric water vapor extremely restricts the level of our understanding for the spatial and temporal distribution of water vapor, thus the detailed knowledge of local and global distribution of water vapor and cycle of water vapor is poor. The lack information of water vapor affects the level of forecast of severe weather especially for the nowcasting of heavy rainfall, and limits the accuracy of numerical prediction about amount of rainfall. Thus the water vapor is one of atmospheric composition we have to know more. It is an urgent task for meteorology today that atmospheric water content should be measured more accurately by a new technology.As an inverse problem of GPS (Global Positioning System) geodesy, the technology of detecting atmospheric water vapor based on GPS provides a new means for remote sensing atmosphere with many advantages such as real-time, continuity, high precision, low cost and unaffected by weather, which has become a powerful supplementation for routine atmospheric sounding system. In the view of the theories of remote sensing water vapor based on GPS and applications in meteorology, the applied fields of GPS technology in meteorology are analyzed comprehensively, the computational scheme and basic steps are studied, and main error resources in the computation of atmospheric water vapor by using ground-based GPS data are discussed in this thesis.The main contents of this thesis include following four parts:(1) The method, error analysis and improvement in the computation of atmospheric water vapor are studied. The accuracy of estimating atmospheric precipitable water vapor by ground-based GPS is evaluated by using radiosonde meteorological data. The suitability and improvement of the formula for calculating the mean weighted tropospheric temperature in Chengdu and North China are discussed; some methods of estimating mean weighted temperature and their influences on high precise precipitable water vapor derived from GPS are compared.The sequence of mean weighted temperature using integral method is obtained using the data of radiosonde in Chengdu and North China, and the suitability and error of Bevis's regression formula to estimate mean weighted temperature using surface air temperature are analyzed. The linear regression formulas of estimating mean weighted temperature in different types are established, and some modified Bevis's formulas available in Chengdu and North China are suggested. There is difference of Bevis's formula in different months and different areas, thus for high precise precipitable water vapor derived from ground-based GPS, it is very important for ground-based GPS meteorology to determinate local regression formula for estimating mean weighted temperature based on the long-term radiosonde data.(2) According to the meteorological sounding data from the radiosonde stations in Chengdu and North China, the precipitable water vapor and surface water-vapor pressure are computed and the relationships between these two factors are analyzed. The results indicate that the relationship between precipitable water vapor and the corresponding ground water is highly linear. From this, it is possible to calculate the precipitable water content of the entire atmosphere only by using the ground water, which can be as a simple spare method under the condition of no radiasonde data or GPS data. Therefore, this replacement and related empirical formulas have good actual values in the research and application of atmospheric water. (3) The first experiment of remote sensing local precipitable water vapor by using ground-based GPS network in Chengdu area has been finished. The estimates of total zenith delay are available using Bernese GPS Software V4.2 based on GPS data every 30 second from the first measurement experiment of a ground-based GPS network in Chengdu Plain of southwest China during the period of July to September 2004. Then estimates of 0.5 hourly precipitable water vapor derived from GPS are obtained using meteorological data from automatic weather stations. The comparison of precipitable water vapor derived from GPS and those from radiosonde observations is given at Chengdu station, with RMS (Root Mean Square) differences of 3.09 mm. The consistency of precipitable water vapor derived from GPS to those from radiosonde is very good. It is concluded that Bevis empirical formula for estimating the weighted atmospheric mean temperature can be basically applicable in Chengdu areas because the relationship of GPS PWV with Bevis' formula between GPS PWV with radiosonde method shows a high correlation. The results of this GPS measurement experiment are helpful both for accumulating the study of precipitable water vapor derived from GPS in Chengdu areas located at the eastern flank of the Tibetan Plateau and for studying spatial-temporal variations of regional atmospheric water vapor and for applying in nowcasting, usage of atmospheric water resources and weather modification through many disciplines cooperatively.(4) The temporal variation and spatial difference of GPS-derived precipitable water in different climatic regions or under the influence of different topography, the relationships among GPS PWV and surface pressure, air temperature, vapor pressure and precipitation, and the variation features of GPS PWV in rainfall processes are studied deeply. The emphasis is on the diurnal cycle of GPS PWV and the relations to the peculiarity of local circulation and rainfall.The diurnal variations of water vapor in Kanto Plain of central Japan (as the first experimental region in this study) under the background of ocean monsoon climate are investigated with GPS-derived precipitable water and surface meteorological data as classified to three kinds of locations (mountainous area, basin, and plain-coast). The GPS PWV shows a clear diurnal cycle. The daily amplitude of GPS PWV is the largest in basin and the smallest in plain. A typical feature of the diurnal variation of GPS PWV in central Japan is a maximum appearing in the evening. The results suggest that the diurnal variation of GPS PWV seems to be strongly affected by the local thermal circulations generated by the topography around these stations. The moisture transport causes the differences in phase of the diurnal cycle of GPS PWV between different locations as well as the phase difference in precipitation. The precipitation observed frequently in the evening also shows a similar diurnal variation to that of the GPS PWV, indicating the peak of precipitation appearing in late afternoon or in the evening over central Japan. Meanwhile, the GPS PWV reaches its nocturnal maximum. There is a good relationship between the diurnal cycle of observed precipitation and that of the GPS PWV.Moreover, the diurnal variations of water vapor in Chengdu Plain (as the second experimental region in this study) under the influence of subtropical monsoon humid climate are preliminarily investigated with surface meteorological data and GPS PWV from the first experiment of remote sensing local precipitable water vapor by using ground-based regional GPS network. The diurnal variations of GPS PWV and some surface meteorological elements are composite during the warm days at Chengdu and Pixian in the basin climatic region in the eastern flank of the Tibetan Plateau. The GPS PWV in the midsummer stable weather shows a clear diurnal cycle with the amplitude and changes obviously in day and little at night. A typical feature of the diurnal variation at Chengdu and Pixian is a maximum appears in late afternoon, a minimum appears in early morning. The comparison of GPS PWV and rainfall shows that there is a good correlation between them in calm warm season. The precipitating is observed frequently in the evening. After GPS PWV reaches its maximum in the late afternoon, main rainfall process will occur, which leads to a remarkable decrease of GPS PWV and a rapid rise of surface water vapor. When the drop of GPS PWV is stable, the main rainfall process will stop. The accumulating and releasing processes of GPS PWV is also well related with rainfall, its increase or decrease continuously suggests the beginning or end of rainfall, respectively.Finally, based on the data of PWV derived from ground-based GPS networks and rainfall data from AWS in Chengdu and North China, the variational features of GPS PWV in several typical cases of heavy rainfall processes are analyzed, the results are help to obtain some index which can be used in the forecast of rainy weather, especially for the short forecast and nocasting of heavy rain.