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.
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We gratefully acknowledge the helpful detailed comments of the reviewers. This project was supported by NSF Grant AGS-1115011. The SCP measurements were provided by the Integrated Surface Flux System of the Earth Observing Laboratory of the National Center for Atmospheric Research.
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Geiss, A., Mahrt, L. Decomposition of Spatial Structure of Nocturnal Flow over Gentle Terrain. Boundary-Layer Meteorol 156, 337–347 (2015). https://doi.org/10.1007/s10546-015-0043-7