A numerical model study of the stratocumulus-topped marine boundary layer
A numerical model study of the stratocumulus-topped marine boundary layer
A one-dimensional model with second order turbulence closure has been developed and used
to investigate processes in the cloud-topped marine atmospheric boundary layer. Model
developments were required to correctly apply surface flux terms near the sea surface, poor
representation of which is common to several models from the recent literature. The improved
surface forcing is shown to affect the predicted boundary layer structure. Other developments
included the implementation of a fully implicit numerical code, which generated less numerical
noise than that originally used in the model, and an improved initialisation procedure. The
new model code was then shown to quantitatively reproduce processes in the stratocumulustopped
boundary layer using measurements of atmospheric turbulence from aircraft from the
North Sea and the subtropical North Atlantic and North Pacific. The model is robust to
changes in the mixing length coefficients used in the turbulence closure and to perturbations in
the initial profiles.
The model is used to simulate conditions that occur as winds circulate from the subtropics
towards the tradewind regions. The observed transition from a shallow stratocumulus layer to
a deeper stratocumulus layer interacting with cumulus clouds beneath is simulated in response
to realistic external forcing. The final stages of transition, from cumulus under stratocumulus
to shallow cumulus is however not observed in the simulation; possible reasons for this are
discussed. The model shows in detail the interaction between the stratocumulus layer and
cumulus clouds beneath. The cumulus clouds thicken, moisten and cool the stratocumulus
layer and therefore act to maintain the layer, but can also drive entrainment. The peaks in
turbulent kinetic energy in the stratocumulus layer which follow cumulus penetrations of the
stratocumulus layer can be large enough to directly cause the boundary layer to entrain air from
above the boundary layer and grow in height. The entrained air is warmer and drier than the
boundary layer air and tends to dissipate the stratocumulus layer. The model is then used to
show how the imposed environmental conditions affect processes within the boundary layer.
An important model prediction is that cloud top entrainment instability may act to promote
mixing between the surface and cloud in deep-decoupled boundary layers. The mixing acts to
replenish the cloud liquid water and sustain the cloud. Cloud top entrainment instability has
previously been thought to have the capacity to lead to rapid erosion of the cloud, although this
has not been observed in practice. This mechanism could help to explain the observed
persistence of stratocumulus clouds under these conditions.
Kent, E.C.
ea23f6f0-ccf6-4702-a5c9-184e9c5d4427
September 1999
Kent, E.C.
ea23f6f0-ccf6-4702-a5c9-184e9c5d4427
Kent, E.C.
(1999)
A numerical model study of the stratocumulus-topped marine boundary layer.
University of Southampton, Faculty of Science, School of Ocean and Earth Science, Doctoral Thesis, 211pp.
Record type:
Thesis
(Doctoral)
Abstract
A one-dimensional model with second order turbulence closure has been developed and used
to investigate processes in the cloud-topped marine atmospheric boundary layer. Model
developments were required to correctly apply surface flux terms near the sea surface, poor
representation of which is common to several models from the recent literature. The improved
surface forcing is shown to affect the predicted boundary layer structure. Other developments
included the implementation of a fully implicit numerical code, which generated less numerical
noise than that originally used in the model, and an improved initialisation procedure. The
new model code was then shown to quantitatively reproduce processes in the stratocumulustopped
boundary layer using measurements of atmospheric turbulence from aircraft from the
North Sea and the subtropical North Atlantic and North Pacific. The model is robust to
changes in the mixing length coefficients used in the turbulence closure and to perturbations in
the initial profiles.
The model is used to simulate conditions that occur as winds circulate from the subtropics
towards the tradewind regions. The observed transition from a shallow stratocumulus layer to
a deeper stratocumulus layer interacting with cumulus clouds beneath is simulated in response
to realistic external forcing. The final stages of transition, from cumulus under stratocumulus
to shallow cumulus is however not observed in the simulation; possible reasons for this are
discussed. The model shows in detail the interaction between the stratocumulus layer and
cumulus clouds beneath. The cumulus clouds thicken, moisten and cool the stratocumulus
layer and therefore act to maintain the layer, but can also drive entrainment. The peaks in
turbulent kinetic energy in the stratocumulus layer which follow cumulus penetrations of the
stratocumulus layer can be large enough to directly cause the boundary layer to entrain air from
above the boundary layer and grow in height. The entrained air is warmer and drier than the
boundary layer air and tends to dissipate the stratocumulus layer. The model is then used to
show how the imposed environmental conditions affect processes within the boundary layer.
An important model prediction is that cloud top entrainment instability may act to promote
mixing between the surface and cloud in deep-decoupled boundary layers. The mixing acts to
replenish the cloud liquid water and sustain the cloud. Cloud top entrainment instability has
previously been thought to have the capacity to lead to rapid erosion of the cloud, although this
has not been observed in practice. This mechanism could help to explain the observed
persistence of stratocumulus clouds under these conditions.
More information
Published date: September 1999
Additional Information:
Digitized via the E-THOS exercise.
Organisations:
University of Southampton
Identifiers
Local EPrints ID: 42126
URI: http://eprints.soton.ac.uk/id/eprint/42126
PURE UUID: 0665b258-facb-4bf7-a8b5-00c42633cf92
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Date deposited: 22 Nov 2006
Last modified: 13 Mar 2019 21:13
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Author:
E.C. Kent
University divisions
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