Space-time structure of mesoscale motions in the stable boundary layer

摘要

In this article, we examine the stochastic characteristics of non-turbulent motions in the stable boundary layer. The behaviour of mesoscale motions in the atmospheric stable boundary layer is poorly understood beyond case studies of unusually well-defined events or monotonic waves. Eight networks of wind observations are analysed in order to examine stochastically the space-time characteristics of such motions in the stable boundary layer. The networks include a variety of surface conditions and network configurations, although none of the networks is optimal for our studies. Collectively, the networks allow examination of scales from tens of metres to tens of kilometres. A variety of analysis tools reveal the complexity of mesoscale motions near the surface in stratified flow. For the data as a whole, the horizontal scale of coherence increases systematically with time-scale, albeit with large scatter between records. The relationship between spatial coherence and time-scale is posed in terms ofa velocity scale, which is found to be unrelated to the wind speed. The time variability of the flow decreases systematically with increasing spatial averaging, suggesting that comparison of model winds (with implied spatial averaging over the grid area)with observations requires suitable spatial averaging of the observations. Additional characteristics are discussed in terms of non-dimensional ratios of velocity scales. Necessary improvements of network measurements are also noted.