Decomposition of Spatial Structure of Nocturnal Flow over Gentle Terrain

Abstract

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.

Boundary-Layer Flow Over Complex Topography

“…Katabatic flows can also contribute to the development of cold pools, in which cold air stagnates in basins and valleys (Gryning et al 1985;Mahrt et al 2010;Burns and Chemel 2014;Geiss and Mahrt 2015;Foster et al 2017). Subsequently in the presence of a cold pool, katabatic flows can 'peel off', or intrude, into the cold pool (Mahrt et al 2010;Whiteman et al 2010;Haiden et al 2010;Soler et al 2014), generating waves Chemel 2014, 2015) and instigating sloshing (Lehner et al 2015) or seiches (Lareau and Horel 2015).…”

The influence of the wind field and stratification on the nocturnal surface air temperature over modest topography

“…This limit is characterized by high wind speeds where both hd z ui and hs u i become small. The s u does not vanish partly because cooler air sometimes remains on the valley floor during periods of flow acceleration when the temperature lags changes of the wind field (Geiss and Mahrt 2015). Based on intercomparisons, instrumental offsets are thought to be 0.1 K or less for the SCP network and thus have little influence on hd z ui.…”

100 Years of Progress in Boundary Layer Meteorology

“…In contrast, low-lying sites sheltered from the wind are influenced more by radiative cooling. Local (of scale , 1 km) cold pools with shallow slopes are often intermittent (Geiss and Mahrt 2015). Cold pools are better defined and thus better understood in uncommon completely enclosed depressions (Whiteman et al 2004).…”