Intermittent turbulence and oscillations in the stable boundary layer over land. Part II: A system dynamics approach

In the stable boundary layer it is often observed that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alterations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. This atmosphere–surface intermittency (ASI) is associated with the essential elements of the stable boundary layer (SBL): strong surface cooling, the supply of mechanical energy by the synoptic pressure gradient, and the limiting effect of stratification on mixing efficiency. In Part I it is shown that the essence of ASI can be captured by a system of three coupled nonlinear differential equations. This simplified system shows both intermittent and nonintermittent flow regimes for different circumstances. In the present paper, this system is studied analytically, following a system dynamics approach. The transition between the different flow regimes is identified as a Hopf bifurcation. This property is used to derive a dimensionless parameter, which is a function of external parameters, such as radiative forcing and pressure gradient. With this dimensionless parameter the equilibrium behavior of the system (i.e., intermittent or nonintermittent) can be predicted exactly. As such this parameter is used to classify SBL regimes. It is shown that the proposed classification parameter provides different information about the state of the SBL than other typical SBL classification parameters such as z/L and Ri.