Stochastic method to determine the scale and anomalous diffusion of gusts in a windy atmospheric boundary layer

Abstract

In the atmospheric boundary layer, especially during strong wind period, the coherent structures are obvious and related to the direct interaction of the air masses with the ground. In this paper, we used the observation data during dust weather in Northwest Gansu to study the coherent structure and their “anomalous diffusion”. The structures in the atmospheric boundary layer included turbulent fluctuations and gusty wind disturbances, and could be denoted as “critical events”. Their fractal dimensions were expressed by the complex index μ of waiting times. Although the complex index can indicate the ability of the system to generate coherent structures, it has a strong dependence on the threshold marking the “critical events”. Hence, the continuous time random walk method was used to analyze the coherent structures. The scaling law of anomalous diffusion of coherent structures was obtained, and the diffusion scaling exponent H that indicated the ability of diffusion of different structures was analyzed. The exponents changed with structure scales which were affected by velocities and heights. At small scales, it was almost isotropic, and at large scales, the coherent structures were obvious and the diffusion was anomalous.