Interaction between a cumulus boundary layer and atmospheric environments

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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.