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| Titel |
Wind Shear Effects within the Entrainment Zone of Stratocumulus |
| VerfasserIn |
Bernhard Schulz, Juan-Pedro Mellado |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250148545
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| Publikation (Nr.) |
 EGU/EGU2017-12809.pdf |
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| Zusammenfassung |
| Stratocumulus clouds are crucial for the Earth’s radiative budget and are hence thought to be
important for understanding climate change. Still, atmospheric models suffer from order-one
uncertainties associated with these clouds. Cloud-top entrainment is particularly
challenging because of the small-scales associated with it. Convective instabilities
driven by evaporative and radiative cooling of the stratocumulus cloud-top set a
continuous encroachment of the cloud layer into the entrainment interfacial layer (EIL), a
process defining the entrainment velocity. Wind shear might play an important role in
enhancing the entrainment velocity, but has been largely overlooked in the past decades.
Therefore, direct numerical simulations focusing on meter and sub-meter scales are used
to investigate the interaction between a mean vertical shear and the entrainment
velocity.
Our main findings are as follows. First, wind shear effects stay localized within the EIL,
whose thickness is proportional to the shear layer thickness. This implies that the
in-cloud turbulent state is independent of the imposed wind shear as long as the EIL
is much thinner than the cloud layer. Therefore, a strong mean wind shear does
not necessarily weaken the in-cloud turbulent state by depleting the cloud, which
contradicts conjectures based on previous large eddy simulations. Second, a critical
nondimensional shear number Scrit exits, such that no significant additional cloud-top
cooling is created for S < Scrit, showing that wind shear effects are negligible in
this regime. In contrast, a strong wind shear with S > Scrit enhances cloud-top
cooling significantly by amplifying radiative and evaporative cooling. For typical
atmospheric conditions with a strong capping inversion, Scrit corresponds to a shear
velocity of 1 − 2ms−1. Consequently, large scale convective motions inside the cloud
layer, associated with velocities of ∼ 1ms−1, are unable to significantly enhance
cloud-top forcing of the in-cloud turbulence. Third, we show that choosing different
inversion points introduces order one deviations in the shear enhancement of the single
contributions of the entrainment velocity, as for example radiative and evaporative
cooling. Likewise, deformations of the cloud top introduce order one deviations
among the different entrainment velocity definitions. Even for a quasi-steady state,
where the entrainment velocity is equal for all inversion points, the partitioning
between the single contributions depends strongly on the choice of the inversion
point.
In sum we find that a strong mean wind shear enhances cloud-top cooling significantly and
that the choice of the inversion point is nontrivial. Hence parametrizations of the different
contributions of the entrainment velocity should be done in a consistent way (assume the
same inversion point) and should consider shear effects. However, large scale convective
motions within the cloud layer do not enhance cloud-top cooling, implying that their effects
need not be retained in parametrizations. |
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