驗證GPS衛星訊號反演之大氣可降水量

Verification on the precipitable water vapor of GPS satellite signals

水氣對於地球上來說是原本存在的物理現象，水循環是地球上的水經由蒸發作用轉換成水氣傳送到大氣，再藉由降雨回到地球上，是地球上的重要物理現象。相較於其他氣體，大氣中的水氣屬於變動氣體，對於氣候變化有著重要的影響。因此，若能有效的獲取大氣中可降水量（Precipitable Water Vapor, PWV）資訊，對於天氣狀態的研究與分析以及氣象的預報會有相當之幫助。本研究以地面GPS接收訊號來求解對流層天頂向的水氣含量，並藉由地面的雨量觀測資料來釐清兩者之間的關係。GPS衛星資料是以Bernese 5.0求解水氣含量，並利用水氣微波輻射儀（Water Vapor Radiometer, WVR）所觀測到的濕延遲量來驗證本研究所計算之成果。成果顯示，WVR觀測值及GPS計算值呈現一致的趨勢，相關係數都在0.9以上，平均誤差則介於-1至-2mm之間。若將計算成果與美國大氣研究中心（National Center for Atmospheric Research, NCAR）相對應，由數據顯示NCAR的估計值有5~6mm的低估現象。換言之若以WVR的觀測值為標準，本研究計算的成果低估PWV約1~2mm，NCAR的計算成果則低估PWV約4~6mm。PWV較高的情況下通常即有降雨的跡象，平地的PWV在60mm、山區的PWV在40mm就容易有降雨的現象，本研究藉由數據上的統計分析了解天氣的狀態，希望對氣象的預報上提供參考的數據。 Water vapor is part of the evaporation process – a long existing physical phenomenon on earth. It transfers energy in the Nature as the weather changes, and is therefore less stable than other types of gasses in the atmosphere. Because of its instability, it affects people’s lives in both good and bad ways. If we can more effectively obtain information on Precipitable Water Vapor (PWV) in the atmosphere, we will be able to conduct more detailed researches and analyses about the weather and give more accurate forecasts. This research aims to measure water vapor in tropospheric zenith delay through signals received from the GPS. It also seeks to understand the relations between GPS and PWV through precipitation data. Bernese 5.0 is used to calculate the water vapor, and the wet delay data obtained through Water Vapor Radiometer (WVR) is used to verify such calculation. Results show that, in terms of measurement accuracy, WVR and GPS works just as well as one another, with their correlation coefficients standing over 0.9 and average errors between -1 and -2 mm. If comparing GPS measurement with that of the National Center for Atmospheric Research (NCAR), however, the NCAR statistics appear to have an underestimation of 5 to 6 mm. In other words, based on the WVR measurement, GPS appears to underestimate PWV by only 1 to 2 mm, while NCAR appears to underestimate PWV by 4 to 6 mm. In addition, there is often rain when PWV is high, such as 60 mm in plain areas and 40 mm in the mountains. We hope the statistical results of this research can not only help researchers understand more about the weather, but help for improving the quality of weather forecasting in the future.

Water vapor is part of the evaporation process – a long existing physical phenomenon on earth. It transfers energy in the Nature as the weather changes, and is therefore less stable than other types of gasses in the atmosphere. Because of its instability, it affects people’s lives in both good and bad ways. If we can more effectively obtain information on Precipitable Water Vapor (PWV) in the atmosphere, we will be able to conduct more detailed researches and analyses about the weather and give more accurate forecasts. This research aims to measure water vapor in tropospheric zenith delay through signals received from the GPS. It also seeks to understand the relations between GPS and PWV through precipitation data. Bernese 5.0 is used to calculate the water vapor, and the wet delay data obtained through Water Vapor Radiometer (WVR) is used to verify such calculation. Results show that, in terms of measurement accuracy, WVR and GPS works just as well as one another, with their correlation coefficients standing over 0.9 and average errors between -1 and -2 mm. If comparing GPS measurement with that of the National Center for Atmospheric Research (NCAR), however, the NCAR statistics appear to have an underestimation of 5 to 6 mm. In other words, based on the WVR measurement, GPS appears to underestimate PWV by only 1 to 2 mm, while NCAR appears to underestimate PWV by 4 to 6 mm. In addition, there is often rain when PWV is high, such as 60 mm in plain areas and 40 mm in the mountains. We hope the statistical results of this research can not only help researchers understand more about the weather, but help for improving the quality of weather forecasting in the future.