October 19, 2018 - 2:00 pm
October 19, 2018 - 3:00 pm
Address120 David L. Boren Blvd, Room 5600, Norman, OK 73072 View map
Large-Scale Motions in Convective Boundary Layers: Effects of Buoyancy and Implications for Land-Atmosphere Interactions
Recent studies of neutrally-stratified turbulent boundary layers have revealed the existence of large scale motions (LSMs), regions of high- and low-momentum fluid with significant coherence in the streamwise direction. LSMs have been found to populate the logarithmic region of turbulent boundary layers and are responsible for a large fraction of the Reynolds stress and scalar flux, which has generated a great deal of interest within the engineering community. In particular, LSMs have been shown to modulate the amplitude and frequency of small-scale turbulent fluctuations near the ground, which has led to a new model to predict near-wall turbulence [e.g. Marusic et al, Science 329(5988), 2010]. However, the extent to which LSMs occur in the convective atmospheric boundary layer (CBL) and their implications for land-atmosphere interactions and deviations from classical scaling laws (i.e. Monin-Obukhov similarity theory) are not well understood.
In this talk, we will consider how buoyancy effects in the CBL modify the structure and dynamics of LSMs using a suite of large eddy simulations spanning weakly to highly convective conditions. We find that, as the CBL becomes increasingly unstable, the inclination angle of structures near the ground increases from 12-15° to nearly vertical and the vertical velocity field transitions from horizontal convective rolls to open cells. Furthermore, we will demonstrate that amplitude modulation of small-scale turbulent fluctuations occurs in the CBL due to both the large-scale streamwise and vertical velocity components. Our results indicate that significant spatial and temporal variability of surface fluxes (e.g. of momentum, heat, water vapor, etc.) occurs to due amplitude modulation by LSMs, causing these quantities to deviate from their ensemble mean values. Implications for deviations from Monin-Obukhov similarity theory and for turbulence modeling will be discussed.