3D-Var Assimilation of GPS data in RAMS NWP model: Impact Studies over Italy

The purpose of the data assimilation is to optimally use all the available information, to determine as accurately as possible the state of the atmosphere. We show the assimilation of the GPS-ZTD (Global Positioning System - Zenith Total Delay) by a 3D-Var data assimilation system that can be used in cycling mode with the Regional Atmospheric Modeling System (RAMS; Federico, 2013). The water vapour mixing ratio (q) and temperature (T) given by the RAMS model (background) are modified according to the assimilation of the GPS-ZTD with the purpose of improving the representation of the humidity and temperature fields and eventually decrease the model spin-up time. To verify the impact of the GPS-ZTD data assimilation on the representation of the atmospheric humidity field, especially at the local scale, a numerical experiment is performed over the Lazio region, in Central Italy, for summer 2017. The ZTD values used were obtained by processing the data coming from the Italpos, Netgeo and Rete Lazio databases. Processing was carried out in PPP (Precise Point Positioning) using RTKLIB, an Open Source Program Package for GNSS Positioning (http://www.rtklib.com/). The background error matrix is computed by the NMC method (Parrish and Derber, 1992) and is invariant for the whole period. The observation error is computed by the RMSE of the GPS data and varies form few millimetres to centimetres. The numerical experiment can be divided in two stages: a) verification of the 3D-Var system; b) quantification of the improvement of the IWV forecast at the short range (< 3h). For the verification of 3D-Var system, the whole dataset is divided in two parts: a set of receivers is used for the data assimilation, while the remaining stations are used for verification.Results show a substantial decrease of the RMSE of the GPS-ZTD and IWV for the stations used for the verification of the methodology; also and the absolute value of the bias is decreased, showing the potential of the assimilation of the GPS-ZTD. For the quantification of the improvement of the IWV forecast, short range (< 3h) prediction of the RAMS model were taken at 4 km horizontal resolution, starting from the 3D-Var analyses. Preliminary results show the improvement of the IWV forecast of the order of few percents. The results of this study are promising and will be further developed in future work with the aim to evaluate the impact of the GPS-ZTD data assimilation on the precipitation forecast. References Bock, O., Bosser, P., Pacione, R., Nuret, M., Fourrie?, N., and Parracho, A.: A high-quality reprocessed ground-based GPS dataset for atmospheric process studies, radiosonde and model evaluation, and reanalysis of HyMeX Special Observing Period, Q. J. Roy. Meteor. Soc., 142, 56-71, https://doi.org/10.1002/qj.2701, 2016. Federico, S.: Implementation of a 3D-Var system for atmospheric profiling data assimilation into the RAMS model: initial results, Atmos. Meas. Tech., 6, 3563-3576, doi:10.5194/amt-6-3563-2013, 2013. Parrish, D. F. and Derber, J. C.: The National Meteorological Center's Spectral Statistical Interpolation analysis system, Mon. Weather Rev., 120, 1747-1763, 1992.