The magnitude of the entrainment buoyancy flux, and hence the growth rate of the convective boundary layer, does not increase monotonically with wind shear. Explanations for this have previously been based on wind-shear effects on the turbulence kinetic energy. By distinguishing between turbulent and non-turbulent regions, we provide an alternative explanation based on two competing wind-shear effects: the initial decrease in the correlation between buoyancy and vertical velocity fluctuations, and the increase in the turbulent area fraction. The former is determined by the change in the dominant forcing; without wind shear, buoyancy fluctuations drive vertical velocity fluctuations and the two are thus highly correlated; with wind shear, vertical velocity fluctuations are partly determined by horizontal velocity fluctuations via the transfer of kinetic energy through the pressure–strain correlation, thus reducing their correlation with the buoyancy field. The increasing turbulent area fraction, on the other hand, is determined by the increasing shear production of turbulence kinetic energy inside the entrainment zone. We also show that the dependence of these conditional statistics on the boundary-layer depth and on the magnitude of the wind shear can be captured by a single non-dimensional variable, which can be interpreted as an entrainment-zone Froude number.

夹带浮力通量的大小，以及因此对流边界层的增长率，不会随着风切变而单调增加。对此的解释以前是基于风切变对湍流动能的影响。通过区分湍流和非湍流区域，我们提供了基于两种相互竞争的风切变效应的替代解释：浮力和垂直速度波动之间相关性的初始降低，以及湍流面积分数的增加。前者由主导力的变化决定；在没有风切变的情况下，浮力波动驱动垂直速度波动，因此两者高度相关；随着风切变，垂直速度波动部分由水平速度波动决定，通过压力-应变相关性传递动能，从而降低了它们与浮力场的相关性。另一方面，增加的湍流面积分数是由夹带区内湍流动能的剪切产生增加决定的。我们还表明，这些条件统计对边界层深度和风切变大小的依赖性可以由单个无量纲变量捕获，该变量可以解释为夹带区弗劳德数。由夹带区内湍流动能的剪切产生增加决定。我们还表明，这些条件统计对边界层深度和风切变大小的依赖性可以由单个无量纲变量捕获，该变量可以解释为夹带区弗劳德数。由夹带区内湍流动能的剪切产生增加决定。我们还表明，这些条件统计对边界层深度和风切变大小的依赖性可以由单个无量纲变量捕获，该变量可以解释为夹带区弗劳德数。