Intermittent Turbulence and Oscillations in the Stable Boundary Layer over Land. Part I: A Bulk Model

Abstract

In the stable boundary layer (SBL) it is observed often that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alternations 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. It is well known that intermittent turbulence in the SBL can be generated by various mechanisms. 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. It is shown that the essence of this mechanism can be captured by a 1D bulk model of three coupled nonlinear differential equations. In the present paper, numerical runs with the model show that intermittent turbulence is most likely to occur over land surfaces with low vegetation during clear-sky conditions in the presence of a moderate to low synoptical pressure gradient. The existence of a vegetation layer has a strong influence on intermittency dynamics. Due to its small heat capacity, the vegetation temperature is able to quickly respond to rapidly changing conditions. This in turn affects the stability of the lower atmosphere, causing an important feedback mechanism. In addition, it was found that intermittent behavior of SBL models occurs for various first-order closure schemes with different stability functions. However, stability functions that allow turbulent transport beyond the critical Richardson number effectively suppress intermittent?oscillatory behavior. Currently, the latter type of formulations is often used in numerical weather prediction to prevent excessive SBL cooling in very stable conditions. The advantage of using a simplified SBL model, as proposed in the present paper, is that it allows an analytical study of the system, which, in turn, allows theoretical predictions about the occurrence of intermittent SBL behavior (see the companion paper).