Interaction between a cumulus boundary layer and atmospheric environments

本研究利用理想化的高分辨率数值模拟研究陆地上空积云边界层的发展，重点是“活动积云”的发生和以下外部因素的影响：初始水汽量、静稳定度和表面能量分配。在控制实验中，积云边界层从仅存在强制积云的条件（强制积云边界层）转变为也存在活动积云的条件（活动积云边界层）。活跃积云的出现几乎与自由对流（LFC）水平的突然下降相一致。LFC的下降是由于在逆温层底部的饱和等效位温的局部最小值的减少，除了在陆面附近的等效位温的增加.系统的敏感性实验表明，LFC的开始时间变得更早的初始水汽值，较小的静态稳定度，和更大的蒸发效率。由于活动积云边界层不断增长，活动积云出现越早，逆温高度越高，湿层越厚。

This study investigates the development of a cumulus boundary layer over land using idealized high-resolution numerical simulations with a focus on the onset of "active cumulus" and the effects of the following external factors : the initial amount of water vapor, static stability, and surface energy partition. In the control experiment, the cumulus boundary layer transitions from a condition in which only forced cumulus exists (forced cumulus boundary layer) to a condition in which active cumulus also exists (active cumulus boundary layer). The active cumulus onset almost coincides with an abrupt drop in the level of free convection (LFC). The drop in the LFC is attributed to a decrease in the local minimum value of saturated equivalent potential temperature at the bottom of the inversion layer in addition to an increase in equivalent potential temperature near the land surface.Systematic sensitivity experiments show that the onset time becomes earlier with greater values of initial water vapor, smaller static stability, and greater evaporative efficiency. Because the active cumulus boundary layer keeps growing, the earlier active cumulus onset leads to higher inversion height and subsequently a deeper moist layer.