Cellular structures in the atmospheric convective boundary layer (CBL) play a critical role in mass and energy exchange. In this study, large-eddy simulation data of a typical CBL were utilized to investigate the dynamical equilibrium and maintenance mechanisms of the cellular structures near the surface. A novel geometry-based conditional sampling method was proposed to identify and track near-surface downdrafts, which constitute a prominent component of the cellular structures. Dynamical and thermal fields of these structures and their temporal evolution were analysed. Statistics show that smaller-scale downdrafts exhibit lower pressure and weaker horizontal velocities throughout their life cycles. Based on this finding, “expanding-squeezing” and “self-limiting” mechanisms were proposed to explain the scale-selection process: pressure differences between adjacent cells drive the squeezing and dissipation of smaller cells, while continuous surface heating restricts the further growth of larger cells. The surface heating process is pivotal to the self-limiting mechanism. A simplified thermal dispersion model was utilized to quantify the surface heating process. It revealed a simple truth: thermal heterogeneity can be generated continuously by homogeneous surface heating, in the presence of horizontal motions. This thermal heterogeneity, in combination with the pressure forcing in the downdrafts, limited the scale of larger cells. These findings provide new insights into the turbulent coherent structures in the CBL.
原文链接
分享
引用
