Abstract
To elucidate the role of each fluid motion in the transport of momentum and heat fluxes in an unstable atmospheric surface layer (ASL) flow from single point measurements on a micrometeorological tower, we develop a novel method based on quadrant analysis where the contour maps of the turbulent statistics (fluxes, temperature variances and triple order moments between vertical velocity and temperature) are plotted on the quadrant planes between streamwise (u)–vertical (w) velocities, and vertical velocity (w)–temperature (T). We find that the dissimilarities in the heat and momentum transport with atmospheric stability are closely linked to the non-Gaussian nature of the joint probability density function (JPDF) between w and T. To highlight the changes in the fluid motions which cause this dissimilarity, we plot the contour maps of the third order moments between w and T on the u − w quadrant plane, and also of the streamwise momentum flux conditioned on every quadrant of u − w plane onto the T − w plane, referred to as octant analysis. The results indicate that in a highly-convective ASL, the cold downdrafts interspersed with strong ejections of hot fluid, carry a significant amount of both down-gradient and counter-gradient momentum flux, thus making the momentum transport inefficient. However, in a near-neutral ASL, the heat and momentum both are carried by the ejection and sweep quadrants of u − w quadrant plane, which indicates the temperature fluctuations are highly correlated with the high-speed and low-speed streaks commonly found in pure shear flows in the laboratory experiments.
Published Version
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