Evaluation of atmospheric precipitable water vapour distribution and trend over India

Integrated Precipitable Water Vapor (IPWV) wields significant influence over atmospheric processes, the climate system, and the hydrological cycle. Spatial and temporal variability characterizes water vapor distribution in the atmosphere, with equatorial regions registering elevated water vapor percentages. There are various types of instruments and methods to assess the quantity of moisture in the air. Global Navigation Satellite System (GNSS) and radiosonde techniques have been widely used to estimate IPWV in the atmosphere. European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-5) is the latest reanalysis IPWV dataset. This study aims to assess the congruence of ERA-5-derived IPWV with GNSS-derived IPWV and study the spatial and temporal variability of IPWV over Indian region. The IPWV data from 16 monitoring stations of GNSS Atmosphere Water Vapor Watch Network of the India Meteorological Department (IMD) have been compared with ERA-5 data. The IPWV data from GNSS and ERA-5 are in excellent agreement as corroborated by correlation coefficients spanning 0.97 to 1.00 and the Root Mean Square Error (RMSE) values varying between 1.5 mm and 5.6 mm. IPWV values exhibit prominent seasonal variations, with minimum values during the winter months and peak appears between June and September, aligning with warm and moist monsoon season of India. The ERA5 data from 1981 to 2020 were used to study variability and trend over Indian region. Strong positive correlations are observed between rainfall and IPWV. The results indicated IPWV trends are moistening especially over Indian landmass, the Indian Ocean, Arabian Sea and Bay of Bengal during all the seasons except winter.