Assimilation of GNSS-ZTD and radar radial velocity for the benefit of very short range regional weather forecast

Wind, humidity and temperature observations from aircraft and radiosondes are generally used to find the best initial state of the atmosphere for
numerical weather prediction (NWP). To be of use for very short range numerical weather forecasting (or numerical nowcasting), these observation
s need to be available within several minutes after observation time. Radiosondes have a typically observation latency of over 30 minutes and arr
ive too late for numerical nowcasting. Zenith Total Delay (ZTD) observations obtained from a ground-based network of Global Navigation Satellite
System (GNSS) receivers can fill this gap of lacking information. ZTD contains information on the total amount of water vapour. Other rapid avail
able observations, such as radial wind estimates from Doppler weather radars, can be exploited as well. Both observations are available with a de
lay of less than five minutes with adequate spatial resolution. In this paper, the impact of assimilation of these humidity and wind observation
in a very short range regional forecast model is assessed over a four month summer period and a six week winter period. As a reference of the imp
act, GNSS observations are assimilated also in an three hourly NWP scheme with longer observation cut-off times. The quality of the forecasts is
evaluated against radiosonde observations, radar radial wind and hourly precipitation observations. Assimilation of both GNSS ZTD and radar radia
l winds resulted in a positive impact on humidity, rainfall and wind forecasts.