Increasing Daytime Stability Enhances Downslope Moisture Transport in the Subcanopy of an Even‐Aged Conifer Forest in Western Oregon, USA

Mountain breezes, including katabatic and anabatic flows, and temperature inversions are common features of forested mountain landscapes. However, the effects of mountain breezes on moisture transport in forests and implications for regional climate change are not well understood. A detailed, instrumented study was conducted from July to September 2012 in an even‐aged conifer forest in the Oregon Cascade Range to investigate how temperature profiles within the forest canopy influenced atmospheric surface layer processes that ventilate the forest. Subcanopy inversion strength has a bimodal relationship to subcanopy wind speed and moisture flux from the forest. On days with relatively modest heating of the top of the canopy and weak subcanopy inversions, above canopy winds more efficiently mix subcanopy air, leading to greater than average vertical moisture flux and weaker than average along‐slope, subcanopy water vapor advection. On days with strong heating of the top of the canopy and a strong subcanopy inversion, vertical moisture flux is suppressed, and daytime downslope winds are stronger than average under the canopy. Increased downslope winds lead to increased downslope transport of water vapor, carbon dioxide, and other scalars under the canopy. Increasing summer vapor pressure deficit in the Pacific Northwest will enhance both processes: vertical moisture transport by mountain breezes when subcanopy inversions are weak and downslope water vapor transport when subcanopy inversions are strong. These mountain breeze dynamics have implications for climate refugia in forested mountains, forest plantations, and other forested regions with a similar canopy structure and regional atmospheric forcings. Plain Language Summary The summer and fall seasons in the Pacific Northwest are typically warm and dry, and solar radiation and locally generated breezes affect temperature and moisture content of air under the forest canopy. In forest plantations, which have uniform height, the sun heats the canopy and creates an inversion—an increase in temperature with height under the forest canopy. On days with strong canopy heating, this inversion limits moisture loss through the top of the canopy and enhances winds that flow downslope below the canopy, carrying moisture out of the system. On days with less canopy heating, winds mix air above and within the canopy and promote moisture loss to the air above the forest canopy. Regional models of future climate simulate dec