The Regional Data Assimilation System (RDAS) of Huaihe Basin Experiment (HUBEX) is presented. The RDAS is an intermittent assimilation system in which an assimilation cycle includes four analyses in a daily period. Both the first guess fields for the cold starting of the system and the boundary conditions for the assimilation model are supplied by products of the global model system. The RDAS consists of quality control, σ-surface optimum interpolation objective analysis, nudging initialization, mesoscale assimilation model forecast and post-processing. The observational data from the Global Telecommunication System (GTS), HUBEX intensive observation period (IOP) and National Satellite Meteorological Center are assimilated. Finally, a heavy rain process in June 24—28, 1998 is tested using the RDAS.

The field experiment named "HUAMEX" was conducted in China from May to June 1998. In this experiment, a regional GPS-network that included three ground-based GPS receiving stations (Shantou, Yangjing and Haikou) was established to estimate integrated atmospheric water vapor (IWV) continuously. The comparison of IWV estimates derived from GPS and radiosonde observations for a 51-day period at Shantou station is given. Both have an average bias of 6.5 mm, and the root-mean-square difference is 4.3 mm. The IWV derived from GPS varies with time and it has an obvious period of 7.2 days in average. Moreover, the IWV derived from GPS is compared with the observed surface integrated precipitation hour by hour. The comparison of GPS-derived IWV and integrated surface precipitation indicates that there is a good coincidence between them. The variation period of GPS-IWV is similar to that of surface precipitation. From the time-series of IWV estimated from GPS, the accumulating and releasing processes of water are obvious.