A DATA QUALITY-CONTROL METHOD USING FORECASTED HORIZONTAL GRADIENT AND TENDENCY IN A NWP SYSTEM - DYNAMIC QC

A method of quality control (QC) to reject erroneous observational dat a has been developed in the Numerical Weather Prediction System of the Japan Meteorological Agency (JMA). Each observed Value is processed i n the QC, by evaluating its deviation from the first guess (FG) value given by the forecasted field, comparing it with a threshold value def ined by statistics and rejecting it if it falls outside the threshold range. In the previous method (Static QC), the threshold value was def ined as a constant regardless of the weather conditions.In the new met hod (Dynamic QC, in this paper), the threshold value is defined as a V ariable linearly depending on the local horizontal gradient and three- hour tendency of the FG. With a statistical study, we found a relation ship between the deviations, and both the horizontal gradient and the tendency of FG were approximately proportional. Then this relationship was applied to the Dynamic QC. In almost all regions, the Dynamic QC reduces erroneous judgements given in the Static QC where wrong observ ed values were accepted in less disturbed weather conditions and right values were rejected in more disturbed conditions. Forecast experimen ts using the Dynamic QC revealed that the forecast scores and the fit in root mean square error (RMSE) to rawinsonde observations were impro ved in the tropics and the Southern Hemisphere in winter. For some cas es, the improvement of the scores in Anomaly Correlation Coefficient ( ACC) was equivalent to more than one days extension of the forecast te rm, and the ratios of fit in RMSE were improved by more than 10 %. The forecast results for the Northern Hemisphere were neutral. The Dynami c QC method was put into operations at JMA on March 17, 1997. After th at, the scores of the operational global forecasts have significantly improved.