Non-stationary drainage flows and motions in the cold pool

Abstract

The initial formation of drainage flows and subsequent interaction with the cold pool are examined by contrasting winds measured with 2‐sonic anemometers at three stations along a gentle slope for a 45‐d observational period in late summer and early fall. On clear nights with weak winds, the station at the bottom of the slope experiences downslope flow only at the beginning of the evening, which quickly yields to light winds of variable direction after formation of a cold pool on the valley floor. A second station, several hundred metres up the slope, experiences drainage flow in the first part of the evening, which diminishes and yields to light and variable winds in the middle of the night as the influence of the cold pool deepens and engulfs the station. Drainage flow continues throughout the night at the third station located still farther up the slope and above the cold pool throughout the night. The interplay between the drainage flow, large‐scale flow and submeso motions leads to frequent large shifts of wind direction. The character of the wind‐direction variability differs substantially between the three stations. Large remaining uncertainties are noted.

Cold‐pool formation in a narrow valley

“…In the lower part of the valley (east of Duffryn), however, they do not appear to transport cold air down to the valley bottom consistently. This is consistent with recent observations of drainage flow and cold-pool development by Mahrt et al (2010) and Bodine et al (2009), but the role of advection in different parts of the valley will be examined in greater detail in section 5.…”

Seasonal variation of local atmospheric circulations and boundary layer structure in the Beijing‐Tianjin‐Hebei region and implications for air quality

“…This pool of cool air is characterized by very stable stratification and weak turbulent mixing, favoring the accumulation of air pollutants. Similar cool air pools have been reported to form frequently near other mountains and plateaus due to similar mechanisms [e.g., Mahrt et al ., ]. The cool air pool persists until the early morning (Figure d) and slows the growth of convective BL (Figure a) in the western plains of the BTH region.…”

Surface layer response to topographic solar shading in Antarctica's dry valleys

“…Winds in case 5 appear to approach stagnation earlier than the other cases for only about 15 min (see between the red lines in Figure a) before the flow starts to oscillate and eventually reverse to down valley. Previous work in mountainous terrain has noted a period of “early evening calm” after sunset and before the onset of downslope flow [ Acevedo and Fitzjarrald , ; Mahrt et al ., ]. This period has been reported to be around 15 min and is associated with weak winds and variable wind directions.…”

“…Mahrt et al . [] examined the interactions between drainage flows along a gentle long slope with the cold air pool developing at the bottom of the valley. Their analysis showed that at different locations up valley, the EET onset will depend on the dynamics of the cold air pool, large‐scale flow, and submesoscale (< 2 km) motions registering different wind directional shifts within the valley.…”