Wind Shear and Buoyancy Reversal at the Top of Stratocumulus

Mellado, Juan Pedro; Stevens, Björn; Schmidt, Heiko

doi:10.1175/jas-d-13-0189.1

Cited by 33 publications

(43 citation statements)

References 48 publications

(52 reference statements)

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“…16 therein), who argued that thickness of the CTMSL diminishes with growing CTEI. Similar structure of "classical" non-POST stratocumulus was also reported in numerical simulations of CTEI permitting in the DYCOMS RF01 case by Mellado et al (2014), who demonstrated a "peeling off" of the negatively buoyant volumes from the shear layer at the cloud top.…”

Section: Thickness Of the Sublayerssupporting

confidence: 75%

“…The above relation is equivalent to Eq. (6) in Mellado et al (2014), who analyze the results of numerical simulations of stratocumulus top mixing and adopted estimates of the asymptotic thickness of shear layers in oceanic flows (Smyth and Moum, 2000;Brucker and Sarkar, 2007) and in the cloud-free atmospheric boundary layer (Conzemius and Fedorovich, 2007).…”

Section: Bulk Richardson Numbermentioning

confidence: 99%

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Physics of Stratocumulus Top (POST): turbulence characteristics

Plante

Nurowska

et al. 2016

Atmos. Chem. Phys.

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Abstract. Turbulence observed during the Physics of Stratocumulus Top (POST) research campaign is analyzed. Using in-flight measurements of dynamic and thermodynamic variables at the interface between the stratocumulus cloud top and free troposphere, the cloud top region is classified into sublayers, and the thicknesses of these sublayers are estimated. The data are used to calculate turbulence characteristics, including the bulk Richardson number, mean-square velocity fluctuations, turbulence kinetic energy (TKE), TKE dissipation rate, and Corrsin, Ozmidov and Kolmogorov scales. A comparison of these properties among different sublayers indicates that the entrainment interfacial layer consists of two significantly different sublayers: the turbulent inversion sublayer (TISL) and the moist, yet hydrostatically stable, cloud top mixing sublayer (CTMSL). Both sublayers are marginally turbulent, i.e., the bulk Richardson number across the layers is critical. This means that turbulence is produced by shear and damped by buoyancy such that the sublayer thicknesses adapt to temperature and wind variations across them. Turbulence in both sublayers is anisotropic, with Corrsin and Ozmidov scales as small as ∼ 0.3 and ∼ 3 m in the TISL and CTMSL, respectively. These values are ∼ 60 and ∼ 15 times smaller than typical layer depths, indicating flattened large eddies and suggesting no direct mixing of cloud top and free-tropospheric air. Also, small scales of turbulence are different in sublayers as indicated by the corresponding values of Kolmogorov scales and buoyant and shear Reynolds numbers.

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Section: Thickness Of the Sublayerssupporting

confidence: 75%

Section: Bulk Richardson Numbermentioning

confidence: 99%

Physics of Stratocumulus Top (POST): turbulence characteristics

Plante

Nurowska

et al. 2016

Atmos. Chem. Phys.

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“…This analysis does not distinguish between the effects of higher shear production of TKE at higher wind speed at the surface and at the inversion (Fig. 11d), and therefore does not determine whether shear production of TKE at the inversion due to geostrophic wind supports entrainment, as locally generated shear does (Wang et al, , 2012Katzwinkel et al, 2012;Mellado et al, 2014).…”

Section: Buoyancy-and Shear-driven Dynamicsmentioning

confidence: 99%

“…A number of studies (e.g., Wang et al, 2008Wang et al, , 2012Katzwinkel et al, 2012;Mellado et al, 2014) have investigated the effect of strong wind shear at the inversion on stratocumulus clouds. Such shear is often caused by a jump in large-scale wind speed and direction across the inversion, and differs qualitatively and quantitatively from shear that arises from the interaction of a constant large-scale wind speed with the surface and with the potential temperature gradient at the inversion.…”

mentioning

confidence: 99%

Wind speed response of marine non-precipitating stratocumulus clouds over a diurnal cycle in cloud-system resolving simulations

2016

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Abstract. Observed and projected trends in large-scale wind speed over the oceans prompt the question: how do marine stratocumulus clouds and their radiative properties respond to changes in large-scale wind speed? Wind speed drives the surface fluxes of sensible heat, moisture, and momentum and thereby acts on cloud liquid water path (LWP) and cloud radiative properties. We present an investigation of the dynamical response of non-precipitating, overcast marine stratocumulus clouds to different wind speeds over the course of a diurnal cycle, all else equal. In cloud-system resolving simulations, we find that higher wind speed leads to faster boundary layer growth and stronger entrainment. The dynamical driver is enhanced buoyant production of turbulence kinetic energy (TKE) from latent heat release in cloud updrafts. LWP is enhanced during the night and in the morning at higher wind speed, and more strongly suppressed later in the day. Wind speed hence accentuates the diurnal LWP cycle by expanding the morning-afternoon contrast. The higher LWP at higher wind speed does not, however, enhance cloud top cooling because in clouds with LWP 50 g m −2 , longwave emissions are insensitive to LWP. This leads to the general conclusion that in sufficiently thick stratocumulus clouds, additional boundary layer growth and entrainment due to a boundary layer moistening arises by stronger production of TKE from latent heat release in cloud updrafts, rather than from enhanced longwave cooling. We find that large-scale wind modulates boundary layer decoupling. At nighttime and at low wind speed during daytime, it enhances decoupling in part by faster boundary layer growth and stronger entrainment and in part because shear from large-scale wind in the sub-cloud layer hinders vertical moisture transport between the surface and cloud base. With increasing wind speed, however, in decoupled daytime conditions, shear-driven circulation due to large-scale wind takes over from buoyancy-driven circulation in transporting moisture from the surface to cloud base and thereby reduces decoupling and helps maintain LWP. The total (shortwave + longwave) cloud radiative effect (CRE) responds to changes in LWP and cloud fraction, and higher wind speed translates to a stronger diurnally averaged total CRE. However, the sensitivity of the diurnally averaged total CRE to wind speed decreases with increasing wind speed.

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“…It is modeled such that it plays a substantial part in mixing near the surface as well as in the entrainment zone. In the entrainment zone, it is designed to mix the warmer air from above the boundary layer into the layer, following for instance, the theoretical arguments of Mellado et al [2014]. The diffusivity also physically incorporates the vertical grid size, and in so doing can help minimize some of the deleterious effects of insufficient vertical resolution.…”

Section: Journal Of Advances In Modeling Earth Systems 101002/2015msmentioning

confidence: 99%

A two Turbulence Kinetic Energy model as a scale‐adaptive approach to modeling the planetary boundary layer

Bhattacharya

Stevens

2016

J Adv Model Earth Syst

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A two Turbulence Kinetic Energy (2TKE) model is developed to address the boundary layer ''grey zone'' problem. The model combines ideas from local and nonlocal models into a single energetically consistent framework. By applying the Reynolds averaging to the large eddy simulation (LES) equations that employ Deardorff's subgrid TKE, we arrive at a system of equations for the boundary layer quantities and two turbulence kinetic energies: one which encapsulates the TKE of large boundary-layer-scale eddies and another which represents the energy of eddies subgrid to the vertical grid size of a typical large-scale model. These two energies are linked via the turbulent cascade of energy from larger to smaller scales and are used to model the mixing in the boundary layer. The model is evaluated for three dry test cases and found to compare favorably to large eddy simulations. The usage of two TKEs for mixing helps reduce the dependency of the model on the vertical grid scale as well as on the free tropospheric stability and facilitates a smoother transition from convective to stable regimes. The usage of two TKEs representing two ranges of scales satisfies the prerequisite for modeling the boundary layer in the ''grey zone'': an idea that is explored further in a companion paper.

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Wind Shear and Buoyancy Reversal at the Top of Stratocumulus

Cited by 33 publications

References 48 publications

Physics of Stratocumulus Top (POST): turbulence characteristics

Physics of Stratocumulus Top (POST): turbulence characteristics

Wind speed response of marine non-precipitating stratocumulus clouds over a diurnal cycle in cloud-system resolving simulations

A two Turbulence Kinetic Energy model as a scale‐adaptive approach to modeling the planetary boundary layer

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