Abstract
Abstract. Within the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) there is a need for an assessment of the uncertainty in the integrated water vapour (IWV) in the atmosphere estimated from ground-based global navigation satellite system (GNSS) observations. All relevant error sources in GNSS-derived IWV are therefore essential to be investigated. We present two approaches, a statistical and a theoretical analysis, for the assessment of the uncertainty of the IWV. The method is valuable for all applications of GNSS IWV data in atmospheric research and weather forecast. It will be implemented to the GNSS IWV data stream for GRUAN in order to assign a specific uncertainty to each data point. In addition, specific recommendations are made to GRUAN on hardware, software, and data processing practices to minimise the IWV uncertainty. By combining the uncertainties associated with the input variables in the estimations of the IWV, we calculated the IWV uncertainties for several GRUAN sites with different weather conditions. The results show a similar relative importance of all uncertainty contributions where the uncertainties in the zenith total delay (ZTD) dominate the error budget of the IWV, contributing over 75 % of the total IWV uncertainty. The impact of the uncertainty associated with the conversion factor between the IWV and the zenith wet delay (ZWD) is proportional to the amount of water vapour and increases slightly for moist weather conditions. The GRUAN GNSS IWV uncertainty data will provide a quantified confidence to be used for the validation of other measurement techniques.
Highlights
In the hydrological cycle, water vapour is an important variable for transferring heat energy from the Earth’s surface to its atmosphere and in moving heat around the Earth
The zenith hydrostatic delay (ZHD) can be determined from surface pressure measurements (Davis et al, 1985), and the zenith wet delay (ZWD) depends on the amount of water vapour in the column of air, i.e. the integrated water vapour (IWV), through which the signal passes and can be estimated from the data themselves
Two methods were discussed in order to determine the total uncertainty of the global navigation satellite system (GNSS)-derived IWV
Summary
Water vapour is an important variable for transferring heat energy from the Earth’s surface to its atmosphere and in moving heat around the Earth. The ZHD can be determined from surface pressure measurements (Davis et al, 1985), and the ZWD depends on the amount of water vapour in the column of air, i.e. the IWV, through which the signal passes and can be estimated from the data themselves. It is crucial to have a high accuracy with the smallest biases possible in order to obtain the absolute value over a long timescale In this case, the final GNSS orbit estimation with the highest accuracy is necessary. The focus of this study is to discuss and assess the accuracy of the IWV derived from ground-based GNSS measurements obtained from post-processed data and mainly used for climate research. The uncertainty method will be implemented to the GNSS IWV data stream for the GCOS (Global Climate Observing System) Reference Upper-Air Network (GRUAN).
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