GPS Meteorology: With Focus On Climate Application

Doktorsavhandling, 2012

The vital role of water vapour in the Earth’s climate system requires measurements of the atmospheric Integrated Water Vapour (IWV) with a long-term stability and a high accuracy. This work focuses on using the Global Positioning System (GPS) to provide IWV estimates for climate applications. The advantages of the GPS measurements are that they can be performed independently on the weather and have a high temporal resolution (a few minutes) as well as a continuously improving spatial resolution (a few km for some local networks). The uncertainty of the GPS-derived IWV highly depends on the accuracy of the estimated Zenith Total Delay (ZTD), which is determined by many parameters, i.e. satellite orbit errors, ionospheric delay, signal multipath, antenna related errors (e.g. phase centre variations), and mapping functions. We demonstrated that the uncertainty of the GPS-derived IWV below 1 kg/m^2 is achievable. The long-term change of the IWV can be an independent data source to detect climate changes. Using a global GPS IWV data set covering a 15-year-long time period, we found estimated IWV trends in a range from -1.65 to +2.32 kg/(m^2*decade) which, however, are comparable to the trend uncertainties varying from 0.21 to 1.52 kg/(m^2*decade). The trend uncertainty is mainly caused by the short-term variations of the IWV which cannot be modelled accurately. The uncertainty is also due to the errors in IWV estimates, which are random and/or elevation-dependent systematic errors. A higher elevation cutoff angle used in the GPS data analysis (a 25 degree was revealed for the time period investigated and for the region of Fennoscandia) can be an advantage to reduce the impact of such systematic errors. The GPS-derived IWV can also be used for the evaluation of climate models. The IWV derived from the GPS measurements acquired at 99 European sites, each with a maximum time series of 14 years, were compared to the IWV simulated by a regional climate model. Overall, a monthly mean difference of ~0.5 kg/m^2 (model-GPS) is obtained where a significant seasonal variation is seen in the difference. The model is too dry in the summer. Study of the diurnal cycle of the IWV using both the GPS data and the model simulation shows a good agreement for the phase while a smaller amplitude is seen in the results from the model.