Synoptically-forced downslope wind induced by low mountain over lake surface

Downslope windstorm is a synoptically forced wind forming on the lee side of mountains. In Hungary this phenomenon is frequently observed at Lake Balaton, where the Hungarian Meteorological Service operates a lake storm warning system. North to Lake Balaton lies the Bakony Mountains with highest peak of around 700 m. The axis of this hilly area is perpendicular to the most frequent, northwest wind direction. This pattern allows the frequent formation of downslope windstorms. Although these downslope winds are significantly weaker than that can form on the lee side of the higher mountains, accurate forecast of them is crucial for the operation of the lake warning system. Our experience suggests that the lake due to its lower surface roughness and its effect on surface temperature can significantly impact the large scale wind locally. In this study the WRF (Weather Research and Forecasting) numerical model was used, and the model results were validated by data provided by dense wind measurements network. Our results reveal: (i) even the relatively low Bakony Mountains can generate downslope windstorms and the wind speed is amplified by the lake effects over Lake Balaton; (ii)  the backside of a cyclon with a north, northwest, nearly geostrophic wind in the Bakony-Balaton region is the most preferable synoptic condition for the formation of the severe downslope wind;  (iii)  the presence of lee trough, stable layer over the mountain, self-induced critical layer in the wind cross sections can also promote the formation the severe downslope wind; (iv) in the case of weaker downslope wind events the turbulence has considerable role in the formation of strong wind gusts; (v) the wind field observed by the surface network and the cloud structure can be also used to justify impact of the mountain waves on the formation of downslope wind. The comparison of the observation data and the model results of NWP (WRF) model shows that the forecast of strong downslope wind events is reliable.  However, in the case of weaker downslope winds the WRF frequently overestimates the peak of the gusts. The results of this study can be taken into consideration for the improvement of the reliability of the lake storm warning system.