Intermittent bursting of turbulence in a stable boundary layer with low-level jet

1. Introduction and Experimental Method

The atmospheric boundary layer under stable stratification (Stable Boundary Layer, here called SBL) is difficult to describe and model. Sometimes the turbulence in SBLs with strong stability is intermittent and patchy, allowing the upper portions of the boundary layer to decouple from surface forcing [1]. We have been investigating the turbulence phenomena in various SBLs with a wide range of stability experimentally in a wind tunnel [2,3]. From field observations, it is often reported that SBLs are accompanied by a low-level jet (called LLJ). Namely, stable stratification and a strong wind shear near the ground coexist.

In the present study, we have developed a simulation method for SBLs with LLJ by using a specially designed thermally stratified wind tunnel. To produce thermally stratified flows, the tunnel is equipped with two independent temperature systems, which consist of an airflow heating unit and a floor temperature-controlling unit. To simulate a LLJ, we have adopted a non-uniform vertical screen placed a downstream section in a stable boundary layer. We have investigated the turbulence phenomena of SBLs with LLJ for a wide range of stability, particularly focusing on the intermittent turbulence that occurs in a certain critical condition.

2. Results

A type of stably stratified flows, in which the mean temperature increases upward in a polynomial manner, is successfully created by heating the wind tunnel airflow and by cooling the test-section floor. The vertical U-velocity profiles simulated as LLJ, in which a maximum of air speed appears near the ground, are also successfully simulated using a vertical screen. The Reynolds number, Red, based on the LLJ height, d, ranges from (1.3 – 1.6)x104 and the bulk Richardson number, Rid(= (g/Θo)・(Θ∞ -Θs)d/Ｕmax2 ), ranges from 0.07 to 0.13. Here, Ｕmax is the maximum air speed of LLJ and Θ∞ is the temperature at the height. And the temperatures, Θs and Θo are the surface and reference temperatures.

We have measured the velocity and temperature fluctuations of SBL flows with various stability using a hot-wire and cold-wire anemometers with X-type probe. Focusing on the time histories of vertical velocity fluctuation, we have found intermittent turbulence bursting in a certain condition of SBL flows. Corresponding to the intermittent vertical velocity fluctuations, the horizontal velocity and temperature fluctuations also show peculiar fluctuations.

To investigate the bursting phenomena in detail, we have measured the velocity fluctuations simultaneously at two different heights. Comparing the two time histories of velocity fluctuations, we have found that the turbulence bursting occurs in the lower part of the boundary layer but detached from the ground. Using a smoke-wire method, we have visualized the bursting. It is clear that the bursting appears detached from the ground.

Furthermore, we have investigated the relationship between the occurrence of turbulence bursting and the local gradient Richardson number. We have found that there is a strong correlation between them. Namely, when the turbulence bursting occurs, the Ri number is low less than the critical Ri number of 0.25, while the bursting ceases, the Ri number becomes high.

References 1) Mahrt, L., 1999 : Stratified Atmospheric Boundary Layers, Boundary-Layer Meteorol., 90, 375-396. 2) Ohya, Y., 2001 : Wind tunnel study of atmospheric stable boundary layers over a rough surface, Boundary- Layer Meteorol., 98, 57-82. 3) Ohya, Y. and Uchida, T. 2003: Turbulence structure of stable boundary layers with a near-linear temperature profile, Boundary-Layer Meteorol., 108, 19-38