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Cold‐air pooling is an important topoclimatic process that creates temperature inversions with the coldest air at the lowest elevations. Incomplete understanding of sub‐canopy spatiotemporal cold‐air pooling dynamics and associated ecological impacts hinders predictions and conservation actions related to climate change and cold‐dependent species and functions. To determine if and how cold‐air pooling influences forest composition, we characterized the frequency, strength, and temporal dynamics of cold‐air pooling in the sub‐canopy at local to regional scales in New England, USA. We established a network of 48 plots along elevational transects and continuously measured sub‐canopy air temperatures for 6–10 months (depending on site). We then estimated overstory and understory community temperature preferences by surveying tree composition in each plot and combining these data with known species temperature preferences. We found that cold‐air pooling was frequent (19–43% seasonal occurrences) and that sites with the most frequent inversions displayed inverted forest composition patterns across slopes with more cold‐adapted species, namely conifers, at low instead of high elevations. We also observed both local and regional variability in cold‐air pooling dynamics, revealing that while cold‐air pooling is common, it is also spatially complex. Our study, which uniquely focused on broad spatial and temporal scales, has revealed some rarely reported cold‐air pooling dynamics. For instance, we discovered frequent and strong temperature inversions that occurred across seasons and in some locations were most frequent during the daytime, likely affecting forest composition. Together, our results show that cold‐air pooling is a fundamental ecological process that requires integration into modeling efforts predicting future forest vegetation patterns under climate change, as well as greater consideration for conservation strategies identifying potential climate refugia for cold‐adapted species. [ABSTRACT FROM AUTHOR]

Abstract Cold‐air pooling is an important topoclimatic process that creates temperature inversions with the coldest air at the lowest elevations. Incomplete understanding of sub‐canopy spatiotemporal cold‐air pooling dynamics and associated ecological impacts hinders predictions and conservation actions related to climate change and cold‐dependent species and functions. To determine if and how cold‐air pooling influences forest composition, we characterized the frequency, strength, and temporal dynamics of cold‐air pooling in the sub‐canopy at local to regional scales in New England, USA. We established a network of 48 plots along elevational transects and continuously measured sub‐canopy air temperatures for 6–10 months (depending on site). We then estimated overstory and understory community temperature preferences by surveying tree composition in each plot and combining these data with known species temperature preferences. We found that cold‐air pooling was frequent (19–43% seasonal occurrences) and that sites with the most frequent inversions displayed inverted forest composition patterns across slopes with more cold‐adapted species, namely conifers, at low instead of high elevations. We also observed both local and regional variability in cold‐air pooling dynamics, revealing that while cold‐air pooling is common, it is also spatially complex. Our study, which uniquely focused on broad spatial and temporal scales, has revealed some rarely reported cold‐air pooling dynamics. For instance, we discovered frequent and strong temperature inversions that occurred across seasons and in some locations were most frequent during the daytime, likely affecting forest composition. Together, our results show that cold‐air pooling is a fundamental ecological process that requires integration into modeling efforts predicting future forest vegetation patterns under climate change, as well as greater consideration for conservation strategies identifying potential climate refugia for cold‐adapted species.

The vertical structure of the observed stable boundary layer often deviates substantially from textbook profiles. Even over flat homogeneous surfaces, the turbulence may not be completely related to the surface conditions and instead generated by elevated sources of turbulence such as low-level jets and transient modes. In stable conditions, even modest surface heterogeneity can alter the vertical structure of the stable boundary layer. With clear skies and low wind speeds, cold-air drainage is sometimes generated by very weak slopes and induces a variety of different vertical structures. Our study examines the vertical structure of the boundary layer at three contrasting tower sites. We emphasize low wind speeds with strong stratification. At a given site, the vertical structure may be sensitive to the surface wind direction. Classification of vertical structures is posed primarily in terms of the profile of the heat flux. The nocturnal boundary layer assumes a variety of vertical structures, which can often be roughly viewed as layering of the heat-flux divergence (convergence). The correlation coefficient between the temperature and vertical velocity fluctuations provides valuable additional information for classification of the vertical structure. [ABSTRACT FROM AUTHOR]

In mountain landscapes, surface temperatures vary over short distances due to interacting influences of topography and overstory vegetation on local energy and water balances. At two study landscapes in the Sierra Nevada of California, characterized by foothill oak savanna at 276–481 m elevation and montane conifer forest at 1977–2135 m, we deployed 288 near-surface (5 cm above the surface) temperature sensors to sample site-scale (30 m) temperature variation related to hillslope orientation and vegetation structure and microsite-scale (2–10 m) variation related to microtopography and tree overstory. Daily near-surface maximum and minimum temperatures for the 2013 calendar year were related to topographic factors and vegetation overstory characterized using small footprint LiDAR imagery acquired by the National Ecological Observatory Network (NEON) Airborne Observation Platform (AOP). At both landscapes we recorded large site and microsite spatial variation in daily maximum temperatures, and less absolute variation in daily minimum temperatures. Generalized boosted regression trees were estimated to measure the influence of tree canopy density, understory solar radiation, cold-air drainage and pooling, ground cover and microtopography on daily maximum and minimum temperatures at site and microsite scales. Site-scale models based on indices of understory solar radiation and landscape position explained an average of 61–65% of daily variation in maximum temperature; site-scale models based on tree canopy density and landscape position explained 65–83% of variation in minimum temperatures. Models explained

Multiple fixed-wing and multirotor uncrewed aircraft systems were deployed to measure the early morning katabatic flow along a valley as part of the lower atmosphere profiling studies at elevation a remotely-piloted aircraft team experiment (LAPSE-RATE) campaign. The valley's topography was that of a narrow canyon emerging into a broader shallow-sloped valley, allowing for an assessment of the suitability of one-dimensional approximations for the broad, flat part of the valley. The one-dimensional integral model predicts growth in the katabatic layer with downslope distance, which was not observed in the broader portions of the valley. Instead, observations revealed thinning of the katabatic layer at the valley centreline, coinciding with oscillatory behavior with a period between 30 and 60 min. These features were attributed to strong asymmetry and three-dimensional features initiating in the narrow part of the valley. These features produced initial conditions upstream of the broad slope flow that were not captured by the one-dimensional model. [ABSTRACT FROM AUTHOR]

Cold‐air pooling is a global phenomenon that frequently sustains low temperatures in sheltered, low‐lying depressions and valleys and drives other key environmental conditions, such as soil temperature, soil moisture, vapor pressure deficit, frost frequency, and winter dynamics. Local climate patterns in areas prone to cold‐air pooling are partly decoupled from regional climates and thus may be buffered from macroscale climate change. There is compelling evidence from studies across the globe that cold‐air pooling impacts plant communities and species distributions, making these decoupled microclimate areas potentially important microrefugia for species under climate warming. Despite interest in the potential for cold‐air pools to enable species persistence under warming, studies investigating the effects of cold‐air pooling on ecosystem processes are scarce. Because local temperatures and vegetation composition are critical drivers of ecosystem processes like carbon cycling and storage, cold‐air pooling may also act to preserve ecosystem functions. We review research exploring the ecological impacts of cold‐air pooling with a focus on vegetation, and then present a new conceptual framework in which cold‐air pooling creates feedbacks between species and ecosystem properties that generate unique hotspots for carbon accrual in some systems relative to areas more vulnerable to regional climate change impacts. Finally, we describe key steps to motivate future research investigating the potential for cold‐air pools to serve as microrefugia for ecosystem functions under climate change. [ABSTRACT FROM AUTHOR]

Cold‐air pooling and associated air temperature inversions are important features of mountain landscapes, but incomplete understanding of their controlling factors hinders prediction of how they may mediate potential future climate changes at local scales. We evaluated how topographic and forest canopy effects on insolation and local winds altered the expression of synoptic‐scale meteorological forcing on near‐surface air temperature inversions and how these effects varied by time of day, season, and spatial scale. Using ~13 years of hourly temperature measurements in forest canopy openings and under the forest canopy at the H.J. Andrews Experimental Forest in the western Cascade Range of Oregon (USA), we calculated air temperature gradients at the basin scale (high vs. low elevation) and at the cross‐valley scale for two transects that differed in topography and forest canopy cover. ERA5 and NCEP NCAR R1 reanalysis data were used to evaluate regional‐scale conditions. Basin and cross‐valley temperature inversions were frequent, particularly in winter and often persisted for several days. Nighttime inversions were more frequent at the cross‐valley scale but displayed the same intra‐annual pattern at the basin and regional scales, becoming most frequent in summer. Nighttime temperature gradients at basin and cross‐valley scales responded similarly to regional‐scale controls, particularly free‐air temperature gradients, despite differences in topography and forest cover. In contrast, the intra‐annual pattern of daytime inversions differed between the basin and cross‐valley scales and between the two cross‐valley transects, implying that topographic and canopy effects on insolation and local winds were key controls at these scales. Plain Language Summary: In mountains, the air is often colder at higher elevations than at lower elevations. However, under certain environmental conditions, cold air can pool in valleys, making the air colder at lower elevations than at higher elevations, creating an air temperature inversion. We used measurements of air temperature, sunlight, wind, and rain to understand how cold‐air pools form, break up, and change throughout the day and from season to season in a forested basin in the Cascade Range of western Oregon (USA). We identified cold‐air pools much more often in valleys ~1 to 2 km wide than previously estimated from basin‐wide (~10‐km scale) measurements. The seasonal variability of nighttime inversion frequency was similar at the basin and cross‐valley scales, indicating that regional climate influences nighttime cold‐air pooling at all scales. The seasonal variability of daytime inversion frequency was not the same across spatial scales, indicating that local factors, such as shading by steep valley walls, were important in the persistence of cold‐air pools during the day. Understanding how and when cold‐air pools form is important because they may be able to reduce the effects of potential future climate warming on plants and animals in mountain ecosystems. Key Points: Vertical temperature gradients in a mountain valley range from −10°C km−1 to >60°C km−1 with strong diel and seasonal variabilityCold‐air pooling is the norm, and not the exception, in this landscapeSynoptic‐scale weather regulates nighttime inversions; topography and vegetation further regulate fine‐scale daytime inversions [ABSTRACT FROM AUTHOR]

Highlights • Local variation in daily maximum surface temperatures in montane conifer forests exceeds that in foothill savanna landscapes. • Understory radiation, relative elevation and tree canopy explain >60% of variation in daily surface temperature extremes. • Topographic and canopy controls on surface temperature extremes vary daily and seasonally. • NEON LiDAR data are useful for modeling site and microsite variation in surface temperatures. Abstract In mountain landscapes, surface temperatures vary over short distances due to interacting influences of topography and overstory vegetation on local energy and water balances. At two study landscapes in the Sierra Nevada of California, characterized by foothill oak savanna at 276–481 m elevation and montane conifer forest at 1977–2135 m, we deployed 288 near-surface (5 cm above the surface) temperature sensors to sample site-scale (30 m) temperature variation related to hillslope orientation and vegetation structure and microsite-scale (2–10 m) variation related to microtopography and tree overstory. Daily near-surface maximum and minimum temperatures for the 2013 calendar year were related to topographic factors and vegetation overstory characterized using small footprint LiDAR imagery acquired by the National Ecological Observatory Network (NEON) Airborne Observation Platform (AOP). At both landscapes we recorded large site and microsite spatial variation in daily maximum temperatures, and less absolute variation in daily minimum temperatures. Generalized boosted regression trees were estimated to measure the influence of tree canopy density, understory solar radiation, cold-air drainage and pooling, ground cover and microtopography on daily maximum and minimum temperatures at site and microsite scales. Site-scale models based on indices of understory solar radiation and landscape position explained an average of 61–65% of daily variation in maximum temperature; site-scale models based on tree canopy density and landscape position explained 65–83% of variation in minimum temperatures. Models explained <15% of variation in microsite-scale maximum temperatures but within-site heterogeneity was significantly correlated with within-site heterogeneity in modeled understory radiation at both landscapes. Tree canopy density and slope explained 33% of microsite-scale variation in minimum temperatures at savanna sites. Our results demonstrate that it is feasible to model site-scale variation in daily surface temperature extremes and within-site heterogeneity in surface temperatures using LiDAR-derived variables, supporting efforts to understand cross-scale relationships between surface microclimates and regional climate change. Improved understanding of topographic and vegetative buffering of thermal microclimates across mountain landscapes is key to projecting microclimate heterogeneity and potential species' range dynamics under future climate change. [ABSTRACT FROM AUTHOR]

Mountainous terrain creates variability in microclimate, including nocturnal cold air drainage and resultant temperature inversions. Driven by the elevational temperature gradient, vapor pressure deficit (VPD) also varies with elevation. Soil depth and moisture availability often increase from ridgetop to valley bottom. These variations complicate predictions of forest productivity and other biological responses. We analyzed spatiotemporal air temperature (T) and VPD variations in a forested, 27-km 2 catchment that varied from 1000 to 1650 m in elevation. Temperature inversions occurred on 76% of mornings in the growing season. The inversion had a clear upper boundary at midslope (∼1370 m a.s.l.). Vapor pressure was relatively constant across elevations, therefore VPD was mainly controlled by T in the watershed. We assessed the impact of microclimate and soil moisture on tree height, forest productivity, and carbon stable isotopes (δ 13 C) using a physiological forest growth model (3-PG). Simulated productivity and tree height were tested against observations derived from lidar data. The effects on photosynthetic gas-exchange of dramatic elevational variations in T and VPD largely cancelled as higher temperature (increasing productivity) accompanies higher VPD (reducing productivity). Although it was not measured, the simulations suggested that realistic elevational variations in soil moisture predicted the observed decline in productivity with elevation. Therefore, in this watershed, the model parameterization should have emphasized soil moisture rather than precise descriptions of temperature inversions. [ABSTRACT FROM AUTHOR]

Climate refugia are locations where decoupled climate processes enable species to persist despite unfavorable climate changes in surrounding landscapes. Despite theoretic bases and paleo-ecological evidence, refugia have not been widely characterized under modern conditions in mountain regions. Conifers in the Great Basin, USA, provide an opportunity to evaluate the potential of low-elevation ravine and riparian (LERR) contexts to function as climate refugia. We provide evidence for significantly higher than expected occurrence of limber pine ( Pinus flexilis E. James) in LERR contexts (mean 64%) across 43 mountain ranges. We document with observed and modeled data that LERR contexts are cooler and wetter than expected for their elevations, have low solar radiation, and produce larger (more positive) lapse rates relative to upland slopes. Together these findings suggest that LERR contexts generate decoupled microclimates that provide climate refugia for limber pine. In that refugia management has been promoted as a contemporary climate adaptation strategy, our findings suggest that LERR contexts be further evaluated for their conservation potential. [ABSTRACT FROM AUTHOR]

We examine the potential importance of wind-directional shear in the surface layer of the stable nocturnal boundary layer by analyzing two tower datasets with eddy-correlation measurements at multiple levels. Directional shear is a major contributor to the total vector shear for weak winds due primarily to frequent shallow drainage flows at one site and due primarily to non-stationary modes at the second site. For weak winds, the turbulence intensity is more related to the wind-directional shear than to the wind speed or stratification, at least for these two datasets. [ABSTRACT FROM AUTHOR]

Understanding of the evening boundary-layer transition has increased substantially in the past few years although such progress has led to new questions particularly regarding the evolution of the wind field. Results from the literature and analyses of measurements from three contrasting sites in our study suggest that the near-surface temperature evolution of the evening transition begins with a late afternoon period of slow cooling. Subsequent reversal from upward heat flux to downward heat flux is followed by a period of rapid cooling. This rapid cooling typically occurs over a 60-90 min period, mostly before sunset, and accounts for much of the total 'nocturnal cooling'. The rapid cooling subsequently transitions into slow cooling that can last for the rest of the night. The details of the temperature evolution vary substantially between sites and vary between nights at a given site. The evolution of the wind and temperature fields during the evening transition are coupled, but both are characterized by unique features. The near-surface wind usually decelerates in the late afternoon sometimes reaching temporary near-calm conditions. The wind-directional shear close to the surface can account for most of the total vector shear for part of the evening transition. Even weak slopes can generate local drainage flows that are sometimes eliminated by formation of regional drainage flows. Our study also examines other aspects of the evening transition that do not necessarily fit into this simple framework. For example, at one site, an intermediate flow develops between the elimination of a larger-scale upslope flow and the formation of local cold-air drainage. [ABSTRACT FROM AUTHOR]

Stratified nocturnal flow above and within a small valley of approximately 12-m depth and a few hundred metres width is examined as a case study, based on a network of 20 sonic anemometers and a central 20-m tower with eight levels of sonic anemometers. Several regimes of stratified flow over gentle topography are conceptually defined for organizing the data analysis and comparing with the existing literature. In our case study, a marginal cold pool forms within the shallow valley in the early evening but yields to larger ambient wind speeds after a few hours, corresponding to stratified terrain-following flow where the flow outside the valley descends to the valley floor. The terrain-following flow lasts about 10 h and then undergoes transition to an intermittent marginal cold pool towards the end of the night when the larger-scale flow collapses. During this 10-h period, the stratified terrain-following flow is characterized by a three-layer structure, consisting of a thin surface boundary layer of a few metres depth on the valley floor, a deeper boundary layer corresponding to the larger-scale flow, and an intermediate transition layer with significant wind-directional shear and possible advection of lee turbulence that is generated even for the gentle topography of our study. The flow in the valley is often modulated by oscillations with a typical period of 10 min. Cold events with smaller turbulent intensity and duration of tens of minutes move through the observational domain throughout the terrain-following period. One of these events is examined in detail. [ABSTRACT FROM AUTHOR]

A network of sonic anemometers was deployed over gentle terrain in north-eastern Colorado, USA to observe and characterize local nocturnal circulations. Our study focuses on a small valley about 270 m wide and 12 m deep with a down-valley slope of 2-3 %. The measurements include 19 stations with sonic anemometers at 1 m and a 20-m tower that includes six sonic anemometers in the lowest 5 m. Shallow cold pools and drainage down the valley develop for weak ambient flow and relatively clear skies. However, transient modes constantly modulate or intermittently eliminate the cold pool, which makes extraction and analysis of the horizontal structure of the cold pool difficult with traditional analysis methods. Singular value decomposition successfully isolates the effects of large-scale flow from local down-valley cold-air drainage within the cold pool in spite of the intermittent nature of this local flow. Shortcomings of the method are noted. [ABSTRACT FROM AUTHOR]

Abstract: Even moderately complex topography can lead to significant horizontal and vertical advection and a consequent underestimation of nocturnal CO2 effluxes derived from eddy covariance measurements on a single tower. The standard approach to select nighttime eddy flux data uses a threshold in friction velocity to exclude periods when advection is important but this is problematic in situations where turbulence is intermittent. van Gorsel et al. [van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Suni, T., 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u *-threshold filtering technique. Tellus 59B, 397–403] have developed an alternative method that estimates the CO2 flux from the maximum of the eddy flux plus change in storage term in the period after sunset when stable stratification develops. During this time the advection terms do not contribute significantly to the mass balance of the air layer below the eddy flux instruments at the Tumbarumba flux station (SE Australia). Advection dominates only later in the night, following the development of large horizontal and vertical gradients of CO2. As net nighttime and daytime fluxes are often of similar magnitude but opposite in sign, underestimation of respiration can lead to large errors in annual ecosystem carbon budgets. Nocturnal respiration rates obtained using the new approach are systematically higher compared to previously published results that used a u * -threshold filter, and are in excellent agreement with independent chamber measurements. Revised net ecosystem exchange (NEE) calculations are therefore substantially reduced compared to those previously published (differing by 5.6±0.5tCha−1 year−1 based on a 34-month period) and compare well with results from an ecosystem model (differing by −0.2±0.7tCha−1 year−1 based on the same time period). Due to a drought period in 2003 and a subsequent insect attack NEE at Tumbarumba has been highly variable. Within 5 years the forest changed from being a carbon sink of −7.5tCha−1 year−1 to a source of +2.3tCha−1 year−1. This high variability demonstrates the importance of long-term, high-resolution time series when interpreting ecosystem data. The study further confirms the importance of using multiple measurement techniques and modelling to improve confidence in the mutually constrained NEE values. [Copyright &y& Elsevier]

Spatial variation in microclimate caused by air temperature inversions plays an important role in determining the timing and rate of many physical and biophysical processes. Such phenomena are of particular interest in mountainous regions where complex physiographic terrain can greatly complicate these processes. Recent work has demonstrated that, in some plants, stomata do not close completely at night, resulting in nocturnal transpiration. The following work was undertaken to develop a better understanding of nocturnal cold air drainage and its subsequent impact on the reliability of predawn leaf water potential (Ψpd) as a surrogate for soil water potential (Ψs). Eight temperature data loggers were installed on a transect spanning a vertical distance of 155m along a north facing slope in the Mica Creek Experimental Watershed (MCEW) in northern Idaho during July and August 2004. Results indicated strong nocturnal temperature inversions occurring from the low- to upper-mid-slope, typically spanning the lower 88 m of the vertical distance. Based on mean temperatures for both months, inversions resulted in lapse rates of 29.0, 27.0 and 25.0 °C km-1 at 0000, 0400 and 2000 h, respectively. At this scale (i.e., < 1 km), the observed lapse rates resulted in highly variable nighttime vapor pressure deficits (D) over the length of the slope, with variable impacts on modeled disequilibrium between soil and leaf water potential. As a result of cold air drainage, modeled Ψpd became consistently more negative (up to -0.3 MPa) at higher elevations during the night based on mean temperatures. Nocturnal inversions on the lower- and mid-slopes resulted in leaf water potentials that were at least 30 and 50% more negative over the lower 88m of the inversion layer, based on mean and maximum temperatures, respectively. However, on a cloudy night, with low D, the maximum decrease in Ψpd was -0.04 MPa. Our results indicate that, given persistent cold air drainage and nighttime stomatal opening, serious errors will result if Ψs is estimated from Ψpd. [ABSTRACT FROM AUTHOR]