The Akio Arakawa Symposium


Large-eddy simulation of evaporatively driven entrainment in cloud-topped mixed layers

Takanobu Yamaguchi, Colorado State Univ., Fort Collins, CO

As a part of atmospheric general circulation model development, the progress of modeling the boundary layer is slow because of inadequacies in our physical understanding of boundary layer processes.

The cloud-top entrainment instability (CTEI) is a hypothesized rapid cloud evaporation due to a positive feedback between cloud-top entrainment and enhanced turbulence associated with buoyancy reversal. Numerous studies have been devoted to investigating the possible role of CTEI in cloud breakup, with ambiguous results.

CTEI is extensively investigated using large-eddy simulations (LESs). Idealized experiment of buoyancy reversal (BR) is designed so as not to have any source of turbulent kinetic energy production except for entrainment due to evaporative cooling. A large number of LESs are performed.

The results of the BR experiment do show the hypothesized positive feedback when the Randall-Deardorff CTEI criterion is met. When CTEI takes place in the BR experiment, entrainment develops spontaneously and leads to cloud dissipation. Rapid cloud dissipation, i.e., within several hours, is simulated in some cases, but some clouds subject to CTEI do not fully evaporate in a ten-hour simulation. A hypothesized dependence of the results on the liquid water mixing ratio is also confirmed. As expected, with a typical stratocumulus liquid water mixing ratio, the CTEI feedback is weak.

Additional simulations with either surface latent heat flux, long wave radiation or both suggest that negative feedback of radiative cooling can compete with cloud warming and evaporation by CTEI. Thus, CTEI is not necessarily linked to and does not always result in cloud dissipation.

Poster Session 1, Poster Session
Tuesday, 16 January 2007, 9:45 AM-11:00 AM, Exhibit Hall C

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