In this study, observational data and WRF model simulations are used to investigate the characteristics and predictability of wind reversals during the morning and evening transition periods in a small valley with gently complex terrain in the Blue Ridge Mountains of Virginia, USA. Focusing on “valley wind days”, the analysis examines the timing of slope wind reversals relative to surface heating and cooling and the influence of a small hill located near the observation site. WRF simulations reproduce the observed flow conditions and demonstrate the utility of a stability index, defined as the vertical temperature gradient between 2 and 10 m AGL, which may be used as a proxy for surface heating and cooling. A complete wind direction reversal occurs only when the stability index exceeds a consistent threshold. Terrain sensitivity experiments reveal that especially the down-slope wind over the valley floor including at the observation site is influenced by the combined effects of the small hill and downslope slope winds from the western sidewall mountain. The study highlights how numerical simulations can clarify the mechanisms controlling observed wind directions and their reversal times during morning and evening transition in a small, topographically complex valley. Furthermore, the identified stability threshold and the influence of local micro-topography provide a critical physical framework for predicting the stagnation and ventilation of air pollutants. Understanding these transition dynamics is essential for assessing local environmental risks and developing mitigation strategies for atmospheric hazards in complex mountain-valley regions.   

thermally driven winds,wind reversals,complex terrain,WRF Model