Multi-platform Observations of Intermittent Nocturnal Drainage Flows in Shallow Terrain

Mesoscale atmospheric flows in nocturnal stable planetary boundary layers (SPBL) can lead to periods of shallow vertical mixing, changes in horizontal transport of gases, particles and pollens, and abrupt shifts in wind velocity. A common flow pattern in these environments is drainage winds, whereby relatively dense air cooled by contact with terrain features flows in a downslope direction. While drainage flows are common in areas of high terrain relief, much less is understood about such flows in shallow topography, which is typical of croplands in much of the Midwestern US. Observations taken during the Stable Atmospheric Variability and Transport (SAVANT) field project in east-central Illinois.

SAVANT observations from multiple instrumented towers, Doppler and Raman lidars, revealed that drainage flows tended to be shallower and more variable than those reported for mountainous regions. The observed drainage flows (thermally-direct circulations) frequently occurred in pulses, interspersed with periods of calm or upslope winds, and often drifted horizontally side-to-side, relative to the drainage flow direction. Cases of drainage flows during SAVANT were generally limited to depths of less than 10 m. Regions between and above drainage flows occasionally exhibited thermally-indirect circulations (i.e., cold air moving upslope, warm air moving downslope). During one case to be discussed, thermally-indirect circulations were surprisingly stronger than the drainage flow, possibly reflecting interactions between terrain-driven flows driven by the downslope flows and air motions well above the surface.

This presentation will discuss efforts to understand the characteristics and causes of highly variable drainage flows in shallow terrain. Observations of cases of variable nocturnal drainage flows will be discussed, particularly conditions coincident with initiation and termination of downslope flows.