The use of GPS to validate NWP systems: The HIRLAM model

A meteorological synoptic situation using Global Positioning System (GPS) o bservations and a numerical weather prediction (NWP) model in the vicinity of the Madrid Sierra, Spain, between 2 and 15 December 1996 has been studie d. The experiment was characterized by high precipitable water (PW) values associated to rainfall events. The PW was estimated at the level of 1 mm wi th 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 i n 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-mean-square (rms) difference of 2 mm (relative agreement of 85%), which is in agreemen t with the standard deviation of each method. A similar comparison between GPS and the high-resolution HIRLAM/F results in a bias and rms that increas e when extending the forecast range up to a bias of -1.2 mm and an rms of 3 mm (relative agreement of 78%) for the longest forecast range studied, whi ch is 24 h. Radiosonde profiles from a location near one of the sites of the GPS networ k have also been used to estimate PW. The PW bias and rms that result from comparing this data to the previous two methods are -1 and 1.6 mm (relative agreement of 88%) between GPS and radiosondes, and -1.2 and 1.3 mm (relati ve agreement of 90%) between radiosonde and HIRLAM/A. The PW estimated from GPS is probed to be an accurate measurement to valida te NWP models. The study also shows that GPS measurements can detect small- scale fluctuations and therefore can be used to evaluate NWP models with fi ner resolution.