Investigation of Aerosol Influence on Operational Weather Forecasts

Sammanfattning: Low precipitation events are difficult to predict due to the many processes taking part in formation of precipitation. An additional challenge is that many of the processes act on a smaller scale than what weather models can resolve. The aerosol impact on the state of the atmosphere is an example of one of these processes. The aim of this study is to investigate the impact of sea salt aerosols on the prediction of short wave radiation flux, air temperature, and relative humidity, meteorological parameters important for precipitation formation. Including aerosols in weather prediction models might lead to an improvement of weather forecasts. The numerical weather prediction model HARMONIE is used to complete model runs with and without sea salt aerosols. This is done for two specific cases of low precipitation events, one during winter (7 December 2014) and one during summer (31 May 2015). Impact of sea salt aerosols on the three meteorological parameters was found, both on the diurnal cycle at the surface and at vertical levels in the atmosphere. For both cases, it was found that the largest impact of sea salt aerosols on air temperature and relative humidity at the surface occurred in connection with low pressure systems present in the area of study. In the vertical, the impact on air temperature was most pronounced in the planetary boundary layer. The difference in relative humidity was strongest at the level of 850-500 hPa. When comparing the two cases, it was found that the impact on short wave radiation flux was more pronounced during summer compared to winter. The impact on air temperature and relative humidity was found to be strongest during the winter.