Nonstationary and Intermittent Turbulence in the Stable Surface Layer: a Perspective from the Hilbert-Huang Transform

In the atmospheric surface layer under the stable conditions, the primary source of turbulence is shear-generated turbulence which is semidetached from the surface. This shear-generated turbulence behaves as intermittent and nonstationary burst events. In addition, low-frequency motions may modulate and interact with shear-generated turbulence. In this study, we use the Hilbert-Huang transform (HHT) to analyze intermittent and nonstationary events that influence turbulence and its structures by analyzing the data from the Energy Balance Experiment (EBEX) over a cotton field. Our results indicate that the intermittent events can be extracted by HHT to be the amplitude-varying events and substantially contribute to flux exchange. The local frequencies and amplitudes for different bursts are also determined, which shows multi-scale events that affect turbulence. These burst events are believed to be cooler air mass invasions. However, the low frequency motions are relatively week in terms of their contributions to fluxes.