Abstract
To examine spatial and temporal scales of katabatic flow, a distributed temperature sensing (DTS) optical fiber was deployed 2 km down a mild slope irregularly interrupted by small-scale drainage features as part of the Mountain Terrain Atmospheric Modeling and Observation (MATERHORN) experiment conducted at the U.S. Army Dugway Proving Ground, Utah. The fiber was suspended at two heights near the surface, enabling measurement of variations in lapse rate near the surface at meter-scale spatial resolution with 1-min temporal resolution. Experimental results derived from the DTS and tower-mounted instrumentation indicate that airflow through small-scale drainage features regulated the local cooling rate whereas topographic slope and distance along the drainage strongly influenced the larger-scale cooling rate. Empirical results indicate that local cooling rate decays exponentially after local sunset and basin-wide cooling rate decreases linearly with time. The difference in the functional form for cooling rate between local and basin-wide scales suggests that small-scale features have faster timescales that manifests most strongly shortly after local sunset. More generally, partitioning drainage flow by scale provides insight and a methodology for improved understanding of drainage flow in complex terrain.
Highlights
During the clear sky transition from day to night, longwave surface cooling begins to exceed warming by solar insolation and the earth’s surface temperature decreases
Radiant cooling on sloped terrain cools the air just above it, initiating downslope flow, referred to as a katabatic wind or “drainage flow” [1,2]
We examine, in detail, how the temperature field of katabatic flow evolves as a function of relevant spatial measures, such as slope, distance downslope, and proximity to drainage features
Summary
During the clear sky transition from day to night, longwave surface cooling begins to exceed warming by solar insolation and the earth’s surface temperature decreases. Air cooled near the surface obtains sufficient density to become negatively buoyant and air flows downhill despite warming by adiabatic compression. Laboratory and field-based experiments have identified numerous environmental factors that influence the expression of katabatic flow across both idealized and complex landscapes. These environmental factors include spatial measures, such as slope, elevation, aspect, and surface roughness, as well as spatio-temporal measures of ambient atmospheric conditions and surface energy balance (SEB) [11]. A common shortcoming of these experiments, has been that instrument spacing has been inadequate to resolve the broad range of time scales of the evolving near-surface temperature field that drives katabatic flows
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