The mean wind speed at which the stable boundary layer (SBL) experiences a turbulence regime transition (V_r) depends on other flow characteristics, such as its thermal stability. Here, V_r variability is examined both at a single site and across sites using three multiple-level datasets: Santa Maria, Cooperative Atmosphere–Surface Exchange Study–1999 (CASES-99), and Fluxes over Snow Surfaces (FLOSS II). A method to determine V_r is introduced and validated. It is taken as the mean wind speed at which the vertical gradient of the turbulence velocity scale switches sign. Emphasis is given to the control exerted on V_r by quantities that are external to the SBL, such as radiation, roughness, and soil properties. In each of the experiments, V_r increases with net radiative loss at the surface (R_n) at a rate that is site-dependent. It also increases for smaller roughness lengths, as indicated by its wind direction dependence. No conclusive relationship has been found between V_r and downward longwave radiation observed at the surface. The across-site comparison indicates that soil heat capacity influences the rate at which V_r increases with R_n.

中文翻译：

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稳定边界层湍流状态之间过渡的外部控制

稳定边界层 (SBL) 经历湍流状态转变 ( V _r ) 时的平均风速取决于其他流动特性，例如其热稳定性。在这里，使用三个多级数据集在单个站点和跨站点检查V _r变异性：Santa Maria、Cooperative Atmosphere–Surface Exchange Study–1999 (CASES-99) 和雪面上的通量 (FLOSS II)。介绍并验证了确定V _{r 的}方法。取其作为湍流速度标度的垂直梯度切换符号时的平均风速。重点是对V _r施加的控制通过 SBL 外部的数量，例如辐射、粗糙度和土壤特性。在每个实验中，V _r随着表面的净辐射损失 ( R _n ) 以与位置有关的速率增加。如其风向依赖性所表明的那样，对于较小的粗糙度长度，它也会增加。在V _r和在地表观察到的向下长波辐射之间没有发现决定性的关系。跨站点比较表明土壤热容量影响V _r随R _n增加的速率。