To test the performance and study the impact of assimilation of slant delays in NWP.
At KNMI, the NWP model HIRLAM (High Resolution Limited Area Model) will be used for the impact studies with a resolution of at least 10km x 10km over a 10 day period, using a three-dimensional variational data assimilation system of HIRLAM, adjusted by KNMI for assimilation of (simulated) slant delays. The performance of assimilation of these slant delays will be investigated by conducting observation system simulation experiments (OSSE):Simulated slant delay observations can be retrieved from a ECMWF nature run. The basic elements of an OSSE are a state-of-the-art data assimilation system, a nature run "truth" and a database of simulated observations. The later includes both simulated observations of conventional meteorological systems, comparable to the operational network, and simulated slant delay observations. All simulated observations, including the simulated slant delay observations, will be constructed such that the error characteritics are realistic. Assimilation of these simulated observations will be conducted with HIRLAM 3DVAR. These observations are added to the commonly used observations to asses the impact of the extra moisture information. The nature run and observation database are available for ECMWF members.
Impact studies will be performed with the analysed water vapour fields, obtained from the GPS data of a dense GPS network (Observation System Experiment, OSE). In addition, GPS data can be combined with wind profiler data to investigate the impact of the combination of these observations.