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 instrumental study conducted from July to September 2012 in an
even-aged conifer forest in the Oregon Cascade Range was investigated to
determine how temperature profiles within the forest canopy influenced
atmospheric surface layer processes that ventilate the forest.
Within-canopy inversion strength has a bi-modal relationship to
sub-canopy wind speed and resulting moisture flux from the forest. On
days with relatively modest heating of the top of the canopy and weak
within-canopy inversions, above canopy winds more efficiently mix
subcanopy air, leading to greater than average vertical moisture flux
and weaker than average along-slope, sub-canopy water vapor advection.
On days with strong heating of the top of the canopy and a strong
within-canopy 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
within-canopy inversions are weak, and downslope water vapor transport
when within-canopy inversions are strong. These mountain breeze dynamics
have implications for climate refugia in forested mountains, forest
plantations, and other forested regions with similar canopy structure
and regional atmospheric forcings.