Monday, 3 June 2002
Looking for evidences of drizzle-induced decoupling in the stratocumulus-topped boundary layer
A number of physical processes act over stratocumulus clouds (Sc), including longwave radiative cooling at the cloud top, which drives turbulence; shortwave absorption throughout the cloud layer; entrainment of warm, dry air from the free atmosphere and microphysical processes (condensation / evaporation, drizzle formation). Due to their complex nature, Sc dynamics is probably influenced by several environmental characteristics. Some researchers proposed that entrainment can be a factor causing Sc to breakup, or at least a necessary condition to their dissipation. On the other hand, drizzle was addressed as one of the mechanisms resulting in boundary-layer decoupling (used in a "weak sense", i.e., as a local reduction of turbulent fluxes) by other scientists. Aircraft measurements from the 2nd Aerosol Characterization Experiment (ACE2) analyzed by Durand and Bourcy (2001) showed that non-polluted STBL were typically decoupled whereas coupling between the mixed layer and the cloud layer was found in a continental/polluted environment. The influence of drizzle was also investigated via numerical simulations by Wang and Wang (1994) and Stevens et al. (1998), who concluded that Sc driven by radiative cooling cannot persist in the presence of heavy drizzle. In the present paper, turbulence and microphysics data from ACE2 CLOUDYCOLUMN are presented and a possible connection between drizzle formation and boundary-layer decoupling is investigated. It was found that, in general, the stratocumulus-topped boundary-layer showed two basic states: one, characterized by a shallow mixed-layer with mostly solid stratocumuli with no precipitation on top, and a second, in which a broken, drizzling stratocumuli layer was separated from the near-surface mixed layer by a region of minimum turbulent kinetic energy.