A numerical model study of the stratocumulus-topped marine boundary layer

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.

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