Impacts of Surface Cooling on Turbulence and Temperature Fronts in the Stable Atmospheric Boundary Layer

In recent work [1], we performed very high resolution large eddy simulations (LESs) of a weakly stable boundary layer (SBL) modeled after the GABLS1 intercomparison case [2]. The LES used grid spacing 0.39 m on a mesh of 1024³ grid points. One of the most intriguing features in these simulations is the presence of ubiquitous temperature fronts that populate the SBL. Animations show that these sharp fronts are tilted in the downstream direction, exhibit spatial spanwise and vertical coherence and propagate in time as organized entities. A typical temperature jump across a front can be 0.5 degrees. The observed fronts are internally generated by the dynamical interaction between turbulence and a stably stratified temperature field as the surface boundary conditions in the LES are horizontally homogeneous. Based on conditional averaging [1,3], the flow fields near a temperature front appear to be controlled by pairs of upstream and downstream vortices. In the present effort, we advance our previous work by performing LES of the SBL over a range of surface cooling. The highest cooling rate considered is four times larger than in the original GABLS1 problem design and, as a result, the bulk stable stratification z_i/L increases from 1.7 to nearly 6 (z_i is the SBL height and L is the Monin-Obukhov length). For the highest cooling rate considered continuous turbulence is maintained, but the SBL appears to split vertically with different dynamics above and below the height of the low-level jet (LLJ). Above the LLJ, the turbulence is very weak with the Richardson number near a critical value of 0.25. Below the LLJ, temperature fronts are still observed but their horizontal and vertical separation decreases and they tilt farther forward in the downstream direction. We find that the degree of tilt is determined by the balance between the background stratification and the amplitude of the turbulent fluctuations. Our analysis further connects the temperature fronts and vortical structures in the SBL under high surface cooling. Guided by the LES findings, we are also able to identify qualitatively similar patterns in observed temperature profiles collected from the 55 m tall tower during the CASES-99 field campaign [4].

References:

[1] Sullivan, P.P., J. C. Weil, E. G. Patton, H. J. J. Jonker, & D. V. Mirinov, 2016: Turbulent winds and temperature fronts in large eddy simulations of the stable atmospheric boundary layer, Journal of the Atmospheric Sciences, submitted.

[2] Beare, R. J., etal., 2006: An intercomparison of large-eddy simulations of the stable boundary layer. Boundary-Layer Meteorology, 118, 242–272.

[3] Adrian, R. J., 1996: Stochastic estimation of the structure of turbulent fields. Eddy Structure Identification, J. P. Bonnet, Ed., Springer Verlag, 145–196.

[4] Poulos, G. S., etal., 2002: CASES-99: A comprehensive investigation of the stable nocturnal boundary layer. Bull. Amer. Meteorol. Soc., 83, 555–581.