The Use of GPS to Validate NWP Systems: The HIRLAM Model

Abstract A meteorological synoptic situation using Global Positioning System (GPS) observations and a numerical weather prediction (NWP) model in the vicinity of the Madrid Sierra, Spain, between 2 and 15 December 1996 has been studied. The experiment was characterized by high precipitable water (PW) values associated to rainfall events. The PW was estimated at the level of 1 mm with five GPS receivers to study the passage of a winter frontal system. The GPS network had baselines ranging from 5 to 50 km. These observations have been used to study the spatial and temporal variations of PW. For this same location and time period, PW calculations were carried out by HIRLAM (High-Resolution Limited Area Modeling), the hydrostatic NWP system operational at the Spanish National Weather Service. HIRLAM has been run in two modes: analysis (HIRLAM/A) and forecast (HIRLAM/F). The comparison of PW values obtained using GPS and high-resolution HIRLAM/A shows a PW bias of −0.4 mm (GPS-derived PW higher), and a root-me...