Long-time variations of precipitable water vapour estimated from GPS, MODIS and radiosonde observations in Turkey

Water vapour and its variations in the Earth's atmosphere are related to atmospheric activities and climate changes. However, it is difficult to obtain high-resolution and high-accuracy precipitable water vapour (PWV) and its variations using traditional techniques, particularly in Turkey with the complex weather variability caused by the proximity of the Black Sea and the Mediterranean Sea. Satellite observations provide unique ways to observe PWV variations at regional or global scale, e.g. global positioning system (GPS). In this study, long-time PWV variations and trends are investigated and obtained from 6-year continuous GPS observations in Turkey (January 2010-January 2016), which are compared with nearly co-located radiosonde and moderate resolution imaging spectroradiometer (MODIS) observations to check its accuracy. The root mean square error (RMSE) of PWV differences is about 1-3mm between radiosonde and GPS, and 3-7mm between MODIS and GPS. Furthermore, the linear trend and seasonal amplitudes, and phase of the GPS-estimated PWV signals are computed. Statistically significant trends are found at all stations. While stations near the Mediterranean Sea have increasing trends with about 0.30mmyear(-1), the stations at inland have increasing trends of about 0.10mmyear(-1). The annual phases between radiosonde, GPS and MODIS PWV at all the stations are almost close to each other with differences in 1 degrees-2 degrees. Finally, the relation between north Atlantic oscillation (NAO) and PWV trends is investigated. Results show that PWV trends agree with NAO behaviours especially for the stations near the Mediterranean Sea. The recent human activities may have impacts on long-term PWV variation trends.