Verification of retrieval accuracy of PWV based on ground-based GPS signal
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Abstract:
Precipitable water vapor(PWV) is an important parameter for the study of regional water vapor distribution and formation mechanism of precipitation,which is a quantified indicator to describe water vapor.In QinghaiTibet Plateau,the PWV is hard to be acquired for the low spatial and temporal resolution of water vapor detection due to the large variation in altitude as well as the scarcity and uneven distribution of surface radiosonde stations.While the water vapor data with high precision and high spatiotemporal resolution is the premise to improve accuracy of regional precipitation prediction,herewith the PWV with high precision is needed.Water vapor detection by groundbased Global Positioning System (GPS) is an effective supplement to radiosonde.To analyze the accuracy and influencing factors of PWV detected by GPS,5 typical GPS and radiosonde stations were selected in QinghaiTibet Plateau as the source of the data for further research. GPS data of the Chinese constructed environmental monitoring network,and sounding data of the website of Wyoming University (http://weather.uwyo.edu/wyoming/)were applied.The international highprecision GPS data processing software GAMIT based on the double difference method has been used to calculate the total zenith delay,which contributes to the obtainment of the millimeter scale zenith wet delay.And then the PWV in zenith direction is accordingly gained by the proportional relationship between zenith wet delay and PWV.The GPT2 tropospheric delay model is utilized for the calculation with cutoff height angle setting at 10 degrees.In order to eliminate the systematic error caused by correlation,3 IGS reference stations with a distance of more than 500 km from the point to be measured are introduced.Retrival calculation is an integration of total zenith delay calculation,tropospheric delay calculation as well as PWV calculation.The GPS retrival accuracy evaluated and verified by the near radiosonde data (approximate true value) is used to the analysis on the daily and monthly accuracy of PWV measured by GPS as well as on the impact of the retrival accuracy exerted by PWV and surface temperature. The results show that the daily variation of PWV retrieved by GPS is consistent with radiosonde data with the correlation coefficients from 0.87 to 0.93.The RMSE and ARE of each station at UTC0 are less than UTC12,and the correlation coefficient at UTC0 time is higher than the value at UTC12 time.By analyzing the monthly deviation of the stations,it is found that the MAE from June to September is relatively large,about 2.0-5.5 mm;the value in November,January and February is relatively small,about 0.5~2.0 mm.Among the 5 stations,XNIN Station has the largest MAD,while QHDL Station and QHGE Station are relatively stable.The RMSE and MAD of all stations have the same trend.The RMSE of all stations reaches the maximum in August,and the value of XNIN Station is the largest,which is 5.05 mm at UTC0 and 6.04 mm at UTC12,respectively.The minimum value of RMSE appears in January or February,which area about 1.1 mm for each station.The MAD,RMSE,and ARE at UTC12 are significantly higher than those at UTC0,and the MAD and RMSE at UTC0 and UTC12 from May to September are larger than those in other months.In addition,the influence of PWV and surface temperature was analyzed on the accuracy of PWV retrival.It is found that the PWV and the MAD,the ground temperature and the MAD all have a moderate correlation.Apparently,both the PWV and the surface temperature have a certain effect on the accuracy of GPS detection of PWV. In conclusion,the detection accuracy of each GPS station is equivalent to that of nearby radiosonde stations with RMSE of 2~3 mm and the correlation coefficient of 0.88~0.93,which satisfies the accuracy requirements.In this circumstance,the retrival results can be put into use on the study of PWV on representative areas.In the future,the research on water vapor retrival may dependent on the information combined radiosonde with GPS,and thus provides data support on water vapor conditions and precipitation forecasts in QinghaiTibet Plateau.Based on PWV retrival of GPS,the detection accuracy at UTC0 is more exquisite than that at UTC0,and more exquisite in winter than in summer.It is found that PWV detected by GPS is susceptible to the change of surface temperature and PWV.Hence,the errors caused by the variation should be considered on the application of GPS detection data in QinghaiTibet Plateau.
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