Synoptic to Microscale Processes Affecting the Evolution of a Cold-Air Pool in a Northern New England Forested Mountain Valley

Abstract

The formation of katabatic winds and pooling of cold air in mountain valleys impact air quality, precipitation type, and local ecosystem functions. Much is still poorly understood about the multiscale interaction of processes in a mature mixed-hardwood forest that cause the formation and evolution of cold-air pools (CAPs). Processes involved in the evolution of a CAP in the Hubbard Brook Experimental Forest valley in New Hampshire were investigated during a field campaign on 4–5 November 2015. Vertical profiles of temperature and humidity were measured along a 150-m-long tethered balloon in the center of the valley and were compared with temperature and wind observations on the surrounding slopes to identify and assess the impacts of multiscale processes on a CAP. A CAP formed rapidly during the afternoon of 4 November and attained its maximum depth of ~150 m by sunset. This maximum depth is likely a result of the topography of the valley. Warm-air advection (WAA) occurred during the second half of the night at high elevations, and warm air mixed downward into the valley. As a result, the vertical thermal gradient strengthened and static stability increased, which allowed the lowest part of the CAP to continue to radiatively cool while the upper part of the CAP was warmed and eroded by the WAA. Results suggest that the canopy acts as the primary cooling surface for air at night, which causes split katabatic flow: cold and fast flow above canopy and warmer and slower flow below canopy. Understanding these processes in sloped forests has implications for eddy covariance research and montane microclimates.

Frequent and strong cold‐air pooling drives temperate forest composition

“…The maximum depth of cold-air pools is dependent on local mountain height and reservoir shape (Kelsey et al, 2019), and thus we designed our transects with the aim of meeting or exceeding fine-scale cold-air pool height. Because of the unique crater-like topography of the Shallow Basin site, we spaced plots along transects from the interior of the basin outward and generally upslope.…”

“…Cold‐air pooling is a topoclimatic process that can create inverted and nonlinear temperature gradients across elevation in areas with sheltered topographic features, such as sinkholes, depressions, gullies, basins, and hill or mountain valleys (Clements et al., 2003 ; Geiger, 1965 ; Lundquist & Cayan, 2007 ; Mahrt et al., 2001 ; Whiteman et al., 2001 ). Cold‐air pools primarily form around sunset when upslope radiative surface cooling leads to cold, downslope airflow (katabatic winds) or, less commonly, from in situ radiative surface cooling in valley bottoms (Kelsey et al., 2019 ; Kiefer & Zhong, 2013 ; Rupp et al., 2020 ). Cold, dense air settles in low‐lying areas and shifts them from being some of the warmest spots on the landscape to the coldest (Clements et al., 2003 ).…”

“…Rather, most cold‐air pooling studies focus on the air above forest canopies and in unforested areas (De Frenne & Verheyen, 2016 ). However, isolated studies suggest that above‐ versus below‐canopy cold‐air pooling dynamics differ (Kelsey et al., 2019 ; Pypker, Unsworth, Lamb, et al., 2007 ). This is particularly important because key biotic–abiotic relationships and drivers of future forest dynamics, namely tree regeneration, occur beneath the forest canopy and respond to the near‐ground microenvironment.…”

Polos fríos en el Calar de Hernán Pelea y Cabrilla (Jaén, España), factores sinópticos y de microescala

“…La importancia del estudio de las CAP ha sido demostrada recientemente por Kelsey et al (2019), quienes determinan que la formación de vientos catabáticos y la acumulación de aire frío en valles montañosos afectan a la calidad del aire, tipo de precipitación y funciones del ecosistema local (bosque maduro de frondosas mixtas). La monitorización térmica pionera en ámbitos remotos, como los calares de la Cabrilla y Hernán Pelea, ha supuesto la identificación de temperaturas anormalmente frías, que bien pueden contribuir a la modificación del ecosistema local, y que abre, por tanto, una nueva vía de estudio en la zona.…”

Temperature Gradients and Inversions in a Forested Cascade Range Basin: Synoptic‐ to Local‐Scale Controls

“…Studies focused on regional scales have often relied on data from existing meteorological stations (Dobrowski et al, 2009; Reeves & Stensrud, 2009), which may not be ideally placed for measuring cold‐air pooling. Other studies have analyzed data collected over short distances (<10 km) but frequently over relatively short time periods: a couple of days (Kelsey et al, 2019), one or two seasons (Lareau et al, 2013; Pagès et al, 2017; Vitasse et al, 2017), one to a few years (Fridley, 2009; Jemmett‐Smith et al, 2018; Joly & Richard, 2019; Miró et al, 2018; Pike et al, 2013; Reeves & Stensrud, 2009), and, rarely, as much as a decade (Whiteman et al, 2001). Our data set is unique, however, in its combination of a long temperature record (spanning 13 years and 2 months) with high spatial resolution (~0.14‐km average spacing along two transects).…”