Observation And Analysis Of The Variation Features Of The Precipitable Water In Beijing By The Ground-based GPS Technique

The water vapor, which affects the climate by means of atmospheric and solar radiation, plays an important role in the synoptic and climatic changes. Since the emergence of the vapor convergence has the strong relationship with the development of atmospheric convection, it is meaningful to study the water vapor by the remote sensing technology, either for researching or operational interest. The GPS vapor detection and quantification is based on the signal delay caused by the refraction when the GPS signals passes through the atmosphere. In contrast with the traditional approach, this technology has the advantage of accuracy, all-weather, high resolution, observational consistence and self-emendation.In this paper, the present methods for the calculation of weight-averaged temperature have been reviewed and summarized. Based on the sounding data and the regression analysis, the formula for the weight-averaged temperature calculation in Beijing area has been set up.Using the GAMIT/GLOBK software, the GPS data has been processed and, in combination with the surface observation of temperature and pressure, the Precipitable Water Vapor with the temporal resolution of two hours has been retrieved from the GPS observation in Beijing from July 2004 to July 2005 . Compared with the records of the radiometer and the water vapor radiometer ,the results show a good agreement with the rms 3.05mm and 3.29mm respectively.Based on these data, the seasonal characteristics of the water vapor over the Beijing area have been analyzed and it is found that the total amount of the vapor in Beijing declines from summer to winter during the study period. The diurnal change of vapor amount is more obvious in summer than that in winter. In addition, it has been found that the varying cycles are 11-day, 5-day and 3-day, respectively. The key factors affecting the vapor variety have also been discussed. Finally, it has also been found that the vapor usually increases sharply 1~2 days before the precipitation. When the vapor amount increases above , the precipitation can beexpected. This threshold varies from month to month, and it is generally greater than the monthly averaged precipitation. However, the relation between the amount of precipitation and the vaporare not so obvious.