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PBL stratus
cloud is often found to be decoupled from the surface,
i.e., a thin stably-stratified layer
exists between the cloud layer and the surface.
Two mechanisms for decoupling have been suggested:
heating inside the cloud layer due to solar absorption and
cooling beneath the cloud layer due to drizzle evaporation.
Such decoupling can reduce the turbulent vertical transport
and hence suppress moisture supply from the ocean surface.
A number of studies support this view.
Hignett [1991], using tethered balloon data from San Nicolas
Island during FIRE, documented a marked diurnal variation of the
cloud-capped marine PBL, and showed how solar heating inside
the cloud results in decoupling. Based on their observation of FIRE
data, Paluch and Lenschow [1991] developed
a conceptual model of the life cycle of a uniform stratiform
cloud that evolves into a field of cumulus.
The transition begins with a stabilization of the subcloud air, from either
drizzle evaporation or passage over colder water. This stablized
layer prevents surface moisture from reaching the stratus cloud, but
enables the accumulation of moisture in the layer beneath.
Under proper conditions (e.g., passing over warmer water),
the lower layer becomes unstable, cumulus clouds may form there,
and then grow into the stratus layer.
Higher-order turbulence closure modeling has also been used to study
such decoupling mechanism, e.g.,
Rogers and Koracin [1992] who showed the importance of
solar absorption inside the cloud; and
Wang and Wang [1994] who presented the effect of drizzle.
Wai [1991] also used a second-order closure model to suggest
the role of solar absorption,
large-scale subsidence, surface flux,
mesoscale advection, and cloud-top jump conditions in dissipating cloud.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union