Assimilation of ground-based GNSS Zenith Total Delays and tropospheric gradients for Severe Weather Prediction
Creators: name: Rohith Thundathil
Publisher: name: Zenodo
Year: 2024

description: The Global Navigation Satellite System (GNSS) comprises a dense ground-based system that provides moisture observations through measurements such as Zenith Total Delays (ZTDs) and tropospheric gradients. ZTDs offer data on column water vapor, while tropospheric gradients indicate moisture distribution. A tropospheric gradient operator (Zus et al., 2023) was recently developed and implemented in the Weather Research and Forecasting (WRF) model (Thundathil et al., 2024).

We conducted experiments on ZTD and tropospheric gradient assimilation with our newly developed operator in WRF over the Benchmark Campaign organized within the European COST Action, which took place in May and June 2013. More than 250 stations in central Europe, covering Germany, the Czech Republic, and part of Poland and Austria, were involved in this campaign. Our experiments involved configuring each test period on a 0.1 x 0.1-degree mesh in the WRF model with 50 vertical levels up to 50 hPa for central Europe. Model forcing was done using the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analysis data. We conducted three main runs with each case, including conventional data assimilation from ECMWF (or control run), ZTD alone on top, and ZTD and gradients on top of the control run. 

From the two months of statistics, we have observed a 42% reduction in Zenith Total Delays (ZTDs) and a 16% reduction in gradients in the ZTD assimilation run. Furthermore, these reductions improved to 43% and 21% for ZTDs and gradients, respectively, in the ZTD and gradient assimilation. We validated these improvements with ERA5 and independent GNSS stations and radiosondes, and the results were promising. In addition to the three experiments, we also conducted a gradient-only assimilation, which showed promising improvements in the analyses.

We also conducted a sensitivity experiment where we de-densified the GNSS network to 100 stations distributed at approximately 0.5- and 1-degree distances from each other. The results showed that assimilating ZTD led to an improvement in humidity. Moreover, when we assimilated gradients on top of ZTDs from a sparsely distributed network, we passed on more information to correct the analysis state. This allowed us to maximize the humidity field improvement at places with fewer GNSS stations. Additionally, we observed that gradients performed at a 1-degree network were roughly the same as the assimilation of ZTD alone at 0.5 degrees.

We are currently testing the effectiveness of our gradient operator on a new area to simulate Typhoon activity over Taiwan. We aim to leverage the region's GNSS stations for more accurate results. We will be sharing the initial outcomes from our high-resolution simulations.

ReferencesZus, Florian, Rohith Thundathil, Galina Dick, and Jens Wickert. "Fast Observation Operator for Global Navigation Satellite System Tropospheric Gradients." Remote Sensing 15, no. 21 (2023): 5114.Thundathil, Rohith, Florian Zus, Galina Dick, and Jens Wickert. "Assimilation of GNSS tropospheric gradients into the Weather Research and Forecasting (WRF) model version 4.4. 1." Geoscientific Model Development 17, no. 9 (2024): 3599-3616.