Evaluation of Surface Variables from Global Reanalysis Models and their Application in Precipitable Water Vapour Retrieval from GNSS Observations over Nigeria

Abstract

This study investigated the accuracy and suitability of using reanalysis datasets in retrieving atmospheric precipitable water vapour (PWV) from ground-based global navigation satellite system (GNSS) observations over Nigeria. To achieve the objective of the study, the research was conducted in two parts. In the first part of the study, three surface variables (temperature, pressure, and humidity) from NCEP/NCAR and ERA-Interim Reanalysis models were compared with observations at 26 automatic weather observing stations in order to ascertain the accuracy of the models. In the second part of the study the feasibility of retrieving PWV from GNSS using reanalysis data was investigated. The GPT2 empirical meteorological model, of which the climatological dataset is on fields of ERA-Interim and which is specifically recommended for GNSS and other geodetic applications, was investigated as well. Standard verification indices as recommended by the World Meteorological Organization were adopted in evaluating the models. The results established perfect agreement between the theoretically simulated precision of PWV estimated from ERA-Interim and NCEP/NCAR and that based on actual or practical formulations and data. Both estimates show that PWV can be retrieved to within a precision of about 1 mm provided GNSS-derived zenith tropospheric delay is of high precision. Finally, based on the results and findings of this study, the use of reanalysis models in the retrieval of PWV from GNSS is recommended for users in Nigeria and also to improve on the precision of PWV retrieval from GNSS. We further recommend that a regional/local model of the atmospheric mean temperature be developed. Keywords: Zenith Tropospheric Delay (ZTD), Precipitable Water Vapour (PWV), Global Pressure Temperature 2 (GPT2), Numerical Weather Prediction (NWP), Automatic Weather Observing Stations (AWOS), Global Navigation Satellite System (GNSS), Reanalysis model