Evaluation of two versions of the HIRLAM numerical weather prediction model during an air pollution episode in southern Finland

The main objective of this work is to evaluate the performance of two operational versions of the numerical weather prediction model HIRLAM (HIgh Resolution Limited Area Model) in severe wintertime air pollution conditions. The evaluation has been done by comparing the predicted temperature, wind speed and relative humidity with observational data during one specific episode in December 1995. Simulations were executed with versions 4.6.2 (operational 1999–2003) and 6.2.1 (operational since 2004) of the HIRLAM model. During 27–29 December 1995, an area of high pressure was formed over Southern Scandinavia and Finland, leading to the formation of an exceptionally strong ground-based temperature inversion. According to radiosonde measurements, the maximum strength of the inversion was 18 °C at Jokioinen. These conditions caused poor air quality for 2 days, especially in the Helsinki Metropolitan Area. The simulations with both model versions were able to predict the occurrence of the inversions, but the surface temperatures and wind speeds near the surface were generally overestimated, thus leading to underestimation of the strength of the inversion. However, the more recent model version performed better in predicting the ground-based inversions. The improved performance of the more recent model version is probably due to both the improved surface parameterisation and the better vertical resolution in the lowest atmospheric layers, compared with those of the older model version.