Impact of GPS Zenith Total Delay assimilation on the Rainfall Forecast over the Korean Peninsula

Accurate measurements of spatial and temporal variations of water vapor and proper initialization to the numerical forecast model are essential for the simulation of convective systems leading to heavy rainfall. The Zenith Total Delay (ZTD) is a measure of the atmospheric delay in receipt of a radio signals from GPS satellite to a ground-based GPS receiver. The delay is introduced by hydrostatic pressure (hydrostatic delay) and water vapor (wet delay) along the ray path during the propagation and the wet delay is nearly proportional to the quantity of water vapor in the atmosphere. In this study, the impact of ground-based GPS-ZTD assimilation on the rainfall forecast is investigated using ZTD observations from the Korean GPS Network. A series of assimilation experiments were performed using three-dimensional data assimilation system with 1.5 km horizontal resolution and ZTD data derived from 31 GPS sites for the selected rainfall event. Bias correction is applied to ZTD data before the assimilation. In this case study, the assimilation of GPS-ZTD tends to increase the relative humidity in the background after the assimilation. As a results, the increased moisture field in the initial condition lead to a positive effect on the forecast of rainfall initiation at the right time and intensity.