Horizontal Variations of Nocturnal Temperature and Turbulence Over Microtopography

Abstract

Nocturnal spatial variation of temperature, wind, and turbulence over microtopography is generally poorly understood. Low amplitude microtopography covers much of the Earth’s surface and, with very stable conditions, can produce significant spatial variations of temperature and turbulence. We examine such variations over gentle terrain that include two shallow gullies that feed into a small valley. The gullies are covered by a sub-network of seven flux stations that is embedded within a larger network that covers the valley. The measurements indicate that gullies of only 2–5-m depth and 100-m width can often lead to spatial variations of temperature of several kelvin or more. Such variations depend on ambient wind speed and direction and the near-surface stratification. We investigate the surprising importance of microscale lee turbulence occurring over the gentle microtopography with slopes of only 5%. Near-surface stratification unexpectedly tends to increase with surface elevation on the slopes. We examine the potential causes of this puzzling behaviour of the near-surface stratification.

Microclimate complexity in temperate grasslands: implications for conservation and management under climate change

“…Regardless, our findings suggest that although grassland ecosystems are characterized as low-lying and lacking in topographic complexity (e.g. compared to montane forests), small microtopographic differences can support strong microclimate variability (Mahrt 2006(Mahrt , 2022.…”

“…Both elevation and topographic position also influenced microclimate, and low-lying areas relative to neighborhoods, as well as lower elevations in general, were associated with cooler and more humid anomalies-an effect possibly driven by the pooling of cold air and surface water run-off, as well as nighttime humidity inversion (Geiger et al 2009, Bramer et al 2018, Pastore et al 2022. Although cold air drainage is more often associated with mesoclimatic processes in mountainous systems (Dobrowski 2011, Ashcroft and Gollan 2012), even depressions with depths as little as 2 m may be sufficient to influence temperature conditions and cold air movement (Mahrt 2022). Another possibility is that lower and less exposed positions tend to receive more terrain shading and reduced wind speeds, thus preserving moisture and leading to greater evaporative cooling; this might explain the interaction of elevation and vegetation height observed in our temperature model.…”