Q. Wang et Ba. Albrecht, OBSERVATIONS OF CLOUD-TOP ENTRAINMENT IN MARINE STRATOCUMULUS CLOUDS, Journal of the atmospheric sciences, 51(11), 1994, pp. 1530-1547
Measurements of the thermodynamic and dynamic properties of entrainmen
t events in marine stratocumulus are used to explain why cloud-top ent
rainment instability may not lead to the breakup of the clouds and to
define the role of cloud-top entrainment on the turbulent mixing proce
sses when buoyancy reversal due to mixing is released. The measurement
s were made off the coast of California during the First ISCCP Regiona
l Experiment (FIRE 1987) by the NCAR Electra research aircraft. The da
ta used in this study were collected on a day when the cloud-top jump
conditions indicate possible buoyancy reversal for the entrained parce
ls that mix with cloudy air. The entrainment events are identified usi
ng a conditional sampling method. Ozone concentration is used as a tra
cer of inversion air to define the entrainment mixing fraction. It is
found that cloud-top entrainment ceases to be a mere interfacial pheno
menon when buoyancy reversal of the entrainment parcel occurs. Strong
entrainment preferentially occurs in the downdraft branch of the bound
ary-layer circulation, and its effect is not confined to a region near
the cloud top. In the case studied here, the contribution to the nega
tive buoyancy in the entrainment downdrafts through evaporative coolin
g is comparable with that from radiative cooling. The buoyancy deficit
as the result of evaporation of cloud droplets is found to be insuffi
cient to promote enhanced entrainment that leads to the breakup of the
cloud deck, as suggested by the simple application of cloud-top entra
inment instability (CTEI). A conceptual model for cloud-top entrainmen
t that results in buoyancy reversal is proposed. This model emphasizes
the interaction between entrainment and the boundary-layer circulatio
n. According to this conceptual model, while buoyancy reversal tends t
o maintain a well-mixed boundary layer by providing deficit negative b
uoyancy to drive turbulent mixing, it may also accelerate the thinning
arid dissipation of a cloud deck once the boundary layer is decoupled
by other processes such as solar absorption or drizzle. It is suggest
ed here that a simple criterion for CTEI based solely on the cloud-top
discontinuities is unlikely to exist since the dynamics of the entire
boundary layer are involved in the entrainment process.