Abstract
A realistic representation of mixed-phase clouds in weather and climate models is essential to accurately simulate the model’s radiative balance and water cycle. In addition, it is important for providing downstream applications with physically realistic model data for computation of, for instance, atmospheric icing on societal infrastructure and aircraft. An important quantity for forecasts of atmospheric icing is to model accurately supercooled liquid water (SLW). In this study, we implement elements from the Thompson cloud microphysics scheme into the numerical weather prediction model HARMONIE-AROME, with the aim to improve its ability to predict SLW. We conduct an idealised process-level evaluation of microphysical processes, and analyse the water phase budget of clouds and precipitation to compare the modified and original schemes, and also identify the processes with the most impact to form SLW. Two idealised cases representing orographic lift and freezing drizzle, both known to generate significant amounts of SLW, are setup in a 1 D column version of HARMONIE-AROME. The experiments show that the amount of SLW is largely sensitive to the ice initiation processes, snow and graupel collection of cloud water, and the rain size distribution. There is a doubling of the cloud water maximum mixing ratio, in addition to a prolonged existence of SLW, with the modified scheme compared with the original scheme. The spatial and temporal extent of cloud ice and snow are reduced, due to stricter conditions for ice nucleation. The findings are important as the HARMONIE-AROME models is used for operational forecasting in many countries in northern Europe having a colder climate, as well as for climate assessments over the Arctic region.
Acknowledgements
The authors would like to thank Bjørn Egil Nygaard (Kjeller Vindteknikk), Trude Storelvmo, and Terje Berntsen (both from University of Oslo) for valuable discussions and comments. We would also like to thank Eric Bazile at Météo-France and Karl-Ivar Ivarsson at SMHI for help with the experimental setup and discussions of the model code. Lastly, we would like to dedicate this work to the late Professor Jón-Egill Kristjánsson whom suffered a tragic accident after initiating this study. Professor Kristjánsson was an advocate for basic research, but was also keen to emphasise that the results should be used for the public good and often collaborated with the operational weather service in Norway, this study is an example of the legacy he left behind.
Disclosure statement
No potential conflict of interest was reported by the authors.