Synoptic and Mesoscale Processes Affecting the Evolution of a Cold-Air Pool in a Forested Mountain Valley

The formation of katabatic winds and pooling of cold air in mountain valleys impact air quality, forest carbon, water and energy budgets, and local ecosystem functions. Much is still poorly understood about the multiscale interaction of processes that allow for the formation of cold-air pools (CAPs) and how they respond to external forcings. The synoptic and mesoscale processes involved in the evolution of a CAP in the Hubbard Brook Experimental Forest (HBEF) valley in New Hampshire were investigated during a field campaign on 4-5 November 2015.

HBEF is a west-east elongated mountain valley approximately 9 km long and 5 km wide. The forest is a mix of coniferous and deciduous trees with a canopy height of 20-35 m. Vertical profiles of free-air temperature and humidity were measured along a 150-m long tethered balloon in the center of the valley. Surface temperature and wind observations on the adjacent north- and south-facing slopes were used to assess the impacts of large-scale and local processes on a CAP during the field campaign. The CAP formation and evolution, intensity and depth, static stability, and influence of forest canopy on temperature and wind were evaluated.

On the afternoon of 4 November 2015, a CAP formed rapidly and attained its maximum depth of 140-150 m by sunset. A steady CAP depth through the first half of the night likely resulted from the shape and size of the valley that regulates the balance of katabatic inflow to the CAP and outflow into the larger Pemigewasset River valley to the east. Synoptic-scale warm-air advection mixed warm air downward into the HBEF valley causing an increase in static stability by eroding the upper layer of the CAP and increasing the vertical thermal gradient. Evolution of the vertical profile of temperature suggests variability in wind speed above the CAP may have created a seiche at the top of the CAP. We hypothesize that heat retention in the biomass and ground, and trapped longwave radiation below the canopy maintained an isentropic profile through the 15-20 m deep sub-canopy layer for most of the night. Along the slopes above the CAP, the katabatic air flow was colder and faster above canopy than below the canopy, with the dense canopy layer allowing intermittent mixing of the two katabatic flows. After sunrise, insolation slowly diminished the katabatic flow that acted to maintain the CAP. Most of the remaining cold air drained out of the valley or warmed from sensible heating by 1630 UTC while a weak inversion persisted below 20 m under the protection of the canopy.