Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer

Abstract

Using the unprecedented observational capabilities deployed during the Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99), we found three distinct turbulence events on the night of 18 October 1999, each of which was associated with different phenomena: a density current, solitary waves, and downward propagating waves from a low-level jet. In this study, we focus on the first event, the density current and its associated intermittent turbulence. As the cold density current propagated through the CASES-99 site, eddy motions in the upper part of the density current led to periodic overturning of the stratified flow, local thermal instability and a downward diffusion of turbulent mixing. Propagation of the density current induced a secondary circulation. The descending motion following the head of the density current resulted in strong stratification, a sharp reduction in the turbulence, and a sudden increase in the wind speed. As the wind surge propagated toward the surface, shear instability generated upward diffusion of turbulent mixing. We demonstrate in detail that the height and sequence of the local thermal and shear instabilities associated with the dynamics of the density current are responsible for the apparent intermittent turbulence.

Cloud and aerosol radiative effects as key players for anthropogenic changes in atmospheric dynamics over southernWest Africa

“…As described above, the AI front (vertical dashed line) is identified by using the 302 K isentrope (bold solid line). Several general characteristics can be concluded from Figure lead to the formation of an overhanging nose and a head that can extend to higher altitudes than the tail (Simpson, 1969;Sun et al, 2002). Vertical extensions of the head of about 1 km are found for atmospheric density currents (Simpson, 1969), which agrees with the flow in Figure 6a.…”

“…Vertical extensions of the head of about 1 km are found for atmospheric density currents (Simpson, 1969), which agrees with the flow in Figure 6a. Sun et al (2002) emphasize that the wind surge behind the nose, that propagates close to the ground, leads to strong turbulent mixing. This can also be observed in this process study when focusing on the vertical transect of TKE (Fig.…”

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“…Observations in CASES-99 (Cooperative Atmospheric Surface Exchange Study) (Poulos et al 2001;Sun et al 2002) were collected over grassland in south central Kansas, USA. (Table 1) based on CSAT-3 sonic anemometers deployed at 6.1 m above a 15-m mature ponderosa pine forest, 11 m above the surface of a 3-m sparse juniper forest and 10 m over a largely burned area (Table 1).…”

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