GNSS-GPS derived integrated water vapor and performance assessment of ERA-5 data over India

Abstract

Integrated Water Vapor (IWV) is an important parameter that influences a wide variety of atmospheric processes such as the earth's radiation budget, cloud formation, and the hydrological cycle. IWV derived from 18 GNSS-based GPS locations in India at hourly intervals for a period of one year are analyzed for diurnal, seasonal, and spatial variability. Diurnal variation in IWV anomaly range from −0.25 – 0.3 cm with a peak in the afternoon hours and has seasonal dependence at the majority of the stations. Seasonal variation of IWV suggests a maximum in summer monsoon in the range 3.88–6.29 cm and a minimum in winter in the range 0.94–3.76 cm. This is due to the transport of the high amount of moisture from the Arabian Sea during the summer monsoon, and in winter low temperature (low wind speeds) inhibits the moisture convection (advection). IWV displays significant spatial variability at inter-and intra-seasonal timescales. Evaluation of ECMWF reanalysis (ERA5) IWV using GPS IWV, carried out first time over India using hourly comparison data sets, reveals a high correlation coefficient within the range 0.93–0.99, indicating its ability to capture the variation in IWV very well. RMSE is observed to be in the range 0.21–0.38 cm and mean RMSE is observed to be 0.28 ± 0.04 cm. Bias (GPS IWV – ERA-5 IWV) is observed to be in the range −0.49 to 0.24 cm, and the mean bias is observed to be - 0.15 ± 0.21 cm. GPS IWV registers a sharp rise before the rainfall event and indicates a strong correlation between them. Evaluation of ERA-5 IWV data suggests its great application prospects in the tropical Indian region, including its use in the improvement of rainfall prediction in the absence of a dense GPS network.