Abstract
Surface latent heat flux (LHF) has been considered as the determinant driver of the stratocumulus-to-cumulus transition (SCT). The distinct signature of the LHF in driving the SCT, however, has not been found in observations. This motivates us to ask, How determinant is the LHF to SCT? To answer this question, we conduct large-eddy simulations in a Lagrangian setup in which the sea surface temperature increases over time to mimic a low-level cold-air advection. To isolate the role of LHF, we conduct a mechanism-denial experiment in which the LHF adjustment is turned off. The simulations confirm the indispensable roles of LHF in sustaining (although not initiating) the boundary layer decoupling (first stage of SCT) and driving the cloud regime transition (second stage of SCT). However, using theoretical arguments and LES results, we show that decoupling can happen without the need for LHF to increase as long as the capping inversion is weak enough to ensure high entrainment efficiency. The high entrainment efficiency alone cannot sustain the decoupled state without the help of LHF adjustment, leading to the recoupling of the boundary layer that eventually becomes cloud-free. Interestingly, the stratocumulus sheet is sustained longer without LHF adjustment. The mechanisms underlying the findings are explained from the perspectives of cloud-layer budgets of energy (first stage) and liquid water path (second stage).
Significance Statement
An important but poorly understood phenomenon about the stratocumulus (low-lying blanket-like clouds) is its tendency to transition to cumulus clouds (cauliflower-like clouds) as the sea surface warms, called the stratocumulus-to-cumulus transition (SCT). We confirmed an existing hypothesis that an increase in the evaporation of seawater [latent heat flux (LHF)] is the key driver of the SCT. However, we found the role of LHF depends on environmental conditions. For example, if the temperature jump above the cloud is weak, the overlying warm air can sink more effectively into the cloud, initiating the boundary layer decoupling, the first phase of SCT. These results advance our understanding of the conditions under which SCT happens, allowing better quantification of its role in climate change.
Zheng’s current affiliation: Atmospheric and Oceanic Sciences, Princeton University, and NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey.
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