Observational validation of the compensating mass flux through the shell around cumulus clouds

Heus, Thijs; Pols, Freek; Jonker, Harm J. J.; Akker, H.E.A. van den; Lenschow, Donald H.

doi:10.1002/qj.358

Cited by 56 publications

(68 citation statements)

References 22 publications

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“…The skewness is significantly higher for the day with fair-weather cumuli development (5 April) than for the cloud-free day (5 May) in agreement with the discussion given by Heus et al (2009) and Verzijlbergh et al (2009).…”

Section: Meteorological and Turbulence Characteristicssupporting

confidence: 88%

Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer

2010

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Abstract. For the first time, a comprehensive, heightresolved Doppler lidar study of updrafts and downdrafts in the mixing layer is presented. The Doppler lidar measurements were performed at Leipzig, Germany, in the summer half year of 2006. The conditional sampling method is applied to the measured vertical velocities to identify, count, and analyze significant updraft and downdraft events. Three cases of atmospheric boundary-layer (ABL) evolution with and without fair-weather cumuli formation are discussed. Updrafts occur with an average frequency of 1-2 per unit length z i (boundary-layer depth z i ), downdrafts 20-30% more frequently. In the case with cumuli formation, the draft occurrence frequency is enhanced by about 50% at cloud level or near cloud base. The counted updraft events cover 30-34%, downdrafts 53-57% of the velocity time series in the central part of the ABL (subcloud layer) during the main period of convective activity. By considering all drafts with horizontal extent >36 m in the analysis, the updraft mean horizontal extent ranges here from 200-420 m and is about 0.16 z i -0.18 z i in all three cases disregarding the occurrence of cumulus clouds. Downdraft extents are a factor of 1.3-1.5 larger. The average value of the updraft mean vertical velocities is 0.5-0.7 m/s or 0.40 w * -0.45 w * (convective velocity scale w * ), and the negative downdraft mean vertical velocities are weaker by roughly 10-20%. The analysis of the relationship between the size (horizontal extent) of the updrafts and downdrafts and their mean vertical velocity reveals a pronounced increase of the average vertical velocity in updrafts from 0.4-0.5 m/s for small thermals (100-200 m) to about 1.5 m/s for large updrafts (>600 m) in the subcloud layer in the case with fair-weather cumuli. At cloudless conditions, the updraft velocities were found to be 20% smaller for the large thermals.

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Section: Meteorological and Turbulence Characteristicssupporting

confidence: 88%

Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer

2010

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“…The plume is positively skewed, also observed in the bottom graph of the cloud layer. The value of the skewness is about 3 times higher than in the dry CBL, which is due to the rare but very strong in-cloud updrafts in the cumulus layer (Heus et al, 2009). Nonetheless, looking at the height of the maximum concentration, we observe that it descends only very slowly, unlike we would expect from the analogy with the CBL.…”

Section: Velocity Statisticscontrasting

confidence: 63%

“…This configuration of the cloud layer results in a top-hat velocity distribution that is strongly skewed ( Fig. 1d; Heus et al, 2009). On the basis of this velocity distribution, one would expect dispersion characteristics similar to those in the CBL.…”

Section: Resultsmentioning

confidence: 99%

Turbulent dispersion in cloud-topped boundary layers

et al. 2009

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Abstract. Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer.We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion -a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance.The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.

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“…For example, Heus et al (2009) showed that to obtain correct values for the in-cloud vertical mass flux in cumulus clouds, much finer resolutions are necessary than are commonly used in for instance intercomparison studies. When the correct spatial mass flux distribution is desired, DALES shows convergence at a horizontal resolution of 25 m (see Fig.…”

Section: Resolution Dependencies and Convergencymentioning

confidence: 99%

Formulation of the Dutch Atmospheric Large-Eddy Simulation (DALES) and overview of its applications

et al. 2010

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Abstract. The current version of the Dutch AtmosphericLarge-Eddy Simulation (DALES) is presented. DALES is a large-eddy simulation code designed for studies of the physics of the atmospheric boundary layer, including convective and stable boundary layers as well as cloudy boundary layers. In addition, DALES can be used for studies of more specific cases, such as flow over sloping or heterogeneous terrain, and dispersion of inert and chemically active species. This paper contains an extensive description of the physical and numerical formulation of the code, and gives an overview of its applications and accomplishments in recent years.

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Observational validation of the compensating mass flux through the shell around cumulus clouds

Cited by 56 publications

References 22 publications

Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer

Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer

Turbulent dispersion in cloud-topped boundary layers

Formulation of the Dutch Atmospheric Large-Eddy Simulation (DALES) and overview of its applications

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