Twenty-One Years of Verification from the HIRLAM NWP System

Abstract

he High-Resolution Limited-Area Model (HIRLAM) international research program maintains a synoptic-scale NWP system. At the Finnish Meteorological Institute, the HIRLAM system has been run operationally since 1990. The HIRLAM forecasts from 1990 to 2012 have been verified against the numerical analysis. In 2-day forecasts, the monthly rms error of the mean sea level pressure has decreased from about 4 to about 2 hPa; that is, the error is now about half of the value it was in the early 1990s. Similar reduction is seen in the 500-hPa height. The negative bias has decreased significantly. In addition, the dependence on the weather regime, measured as the correlation between the North Atlantic Oscillation (NAO) index and rms error, has decreased. The reason for these improvements can often be attributed to changes in the HIRLAM system. A single improvement, improving most significantly the forecast skill, is the rerun concept, which improves the HIRLAM first guess by utilizing the high-quality ECMWF analysis. Verifying against observations or against the initial analysis gives similar results for a 48-h forecast. For a 6-h forecast, however, the field verification gives lower rms error values and lower bias values. In summary, the results indicate that the goal of the HIRLAM program has been fulfilled: to develop and maintain an up-to-date NWP system for 1- and 2-day forecasts on a limited domain.