GPS (GNSS) signals can be used to probe the atmosphere. When a GPS signal travel through the atmosphere, it will get delayed due to the refraction of the atmosphere. From these delayed information, the Zenith Total Dealy (ZTD) data can be derived which contains the moisture information of the atmoshpere. Currently, there are lots of ground-based GPS sites all around the world. These sites provide un-interrupted and accurate monitoring of atmospheric moisture information. These precious data is not assimilated into the NOAA Global Forecast Model yet. This study added the ZTD data assimilation function into GSI, the NOAA operational data assimialtion package. 1-month-long Parallel experiments will be conducted to examine the impact of the ZTD data on global weather forecasts. The results will be presented at the conference.
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