Large‐eddy simulation of the diurnal cycle of shallow cumulus convection over land

Brown, Andrew R.; Cederwall, R.T.; Chlond, Andreas; Duynkerke, Peter G.; Golaz, Jean‐Christophe; Khairoutdinov, Marat; Lewellen, D. C.; Lock, A. P.; MacVean, M. K.; Moeng, Chin‐Hoh; Neggers, Roel; Siebesma, A. Pier; Stevens, Björn

doi:10.1256/003590002320373210

Cited by 288 publications

(409 citation statements)

References 24 publications

(19 reference statements)

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“…The initial jumps from the mixed layer to the free troposphere for θ ( θ ) and q ( q) are 1 K and −3 × 10 −6 kg kg −1 , respectively. In accordance with Brown (2002), the tropospheric θ gradient is 3 × 10 −3 K m −1 up to 4 km and 6 × 10 −3 K m −1 above 4 km . The specific humidity gradient in the troposphere is −3 × 10 −6 m −1 until the specific humidity becomes zero.…”

Section: Design Of Numerical Experimentssupporting

confidence: 80%

Cloud Shading Effects on Characteristic Boundary-Layer Length Scales

Horn

Ouwersloot

Arellano

et al. 2015

Boundary-Layer Meteorol

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We studied the effects of shading by shallow cumulus (shallow Cu) and the subsequent effect of inducing heterogeneous conditions at the surface on boundary-layer characteristics. We placed special emphasis on quantifying the changes in the characteristic length and time scales associated with thermals, shallow Cu and induced thermal circulation structures. A series of systematic numerical experiments, inspired by Amazonian thermodynamic conditions, was performed using a large-eddy simulation model coupled to a land-surface model. We used four different experiments to disentangle the effects of shallow Cu on the surface and the response of clouds to these surface changes. The experiments include a 'clear case', 'transparent clouds', 'shading clouds' and a case with a prescribed uniform domain and reduced surface heat flux. We also performed a sensitivity study on the effect of introducing a weak background flow. Length and time scales were calculated using autocorrelation and two-dimensional spectral analysis, and we found that shading controlled by shallow Cu locally lowers surface temperatures and consequently reduces the sensible and latent heat fluxes, thus inducing spatial and temporal variability in these fluxes. The length scale of this surface heterogeneity is not sufficiently large to generate circulations that are superimposed on the boundary-layer scale, but the heterogeneity does disturb boundary-layer dynamics and generates a flow opposite to the normal thermal circulation. Besides this effect, shallow Cu shading reduces turbulent kinetic energy and lowers the convective velocity scale, thus reducing the mass flux. This hampers the thermal lifetime, resulting in a decrease in the shallow Cu residence time (from 11 to 7 min). This reduction in lifetime, combined with a decrease in mass flux, leads to smaller clouds. This is partially compensated for by a decrease in thermal cell size due to a reduction in turbulent kinetic energy. As a result, inter-cloud distance is reduced, leading to a larger population of smaller clouds, while maintaining cloud cover similar to the non-shading clouds experiment. Introducing a 1 m s −1 background wind speed increases the thermal size in the sub-cloud layer, but the diagnosed surface-cloud coupling, quantified by characteristic time and length scales, remains.

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Section: Design Of Numerical Experimentssupporting

confidence: 80%

Cloud Shading Effects on Characteristic Boundary-Layer Length Scales

Horn

Ouwersloot

Arellano

et al. 2015

Boundary-Layer Meteorol

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“…[19] Finally, a third case of shallow (liquid only) continental cumulus [Brown et al, 2002] complements Figures 7 and 8 by providing a view of S o in individual clouds in a cloud field with approximately 15% cloud cover. Cloud bases are at about 1 km and maximum cloud tops at 3 km, cloud depths are highly variable.…”

Section: Parcel Ensemblementioning

confidence: 84%

On the relationship between cloud contact time and precipitation susceptibility to aerosol

et al. 2013

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[1] The extent to which the rain rate from shallow, liquid-phase clouds is microphysically influenced by aerosol, and therefore drop concentration N d perturbations, is addressed through analysis of the precipitation susceptibility, S o . Previously published work, based on both models and observations, disagrees on the qualitative behavior of S o with respect to variables such as liquid water path L or the ratio between accretion and autoconversion rates. Two primary responses have emerged: (i) S o decreases monotonically with increasing L and (ii) S o increases with L, reaches a maximum, and decreases thereafter. Here we use a variety of modeling frameworks ranging from box models of (size-resolved) collision-coalescence, to trajectory ensembles based on large eddy simulation to explore the role of time available for collision-coalescence t c in determining the S o response. The analysis shows that an increase in t c shifts the balance of rain production from autoconversion (a N d -dependent process) to accretion (roughly independent of N d ), all else (e.g., L) equal. Thus, with increasing cloud contact time, warm rain production becomes progressively less sensitive to aerosol, all else equal. When the time available for collision-coalescence is a limiting factor, S o increases with increasing L whereas when there is ample time available, S o decreases with increasing L. The analysis therefore explains the differences between extant studies in terms of an important precipitation-controlling parameter, namely the integrated liquid water history over the course of an air parcel's contact with a cloud.

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“…In that case many LES studies showed that the cloud fraction varies strongly with height (e.g. Siebesma and Cuijpers, 1995;Stevens et al, 2001;Brown et al, 2002). The turbulent flux across the cloud interface can be well approximated by an eddy diffusivity approach (Asai and Kasahara, 1967;Kuo, 1962):…”

Section: Basicsmentioning

confidence: 99%

“…The main differences between these two cases concern the cloud depth (∼ 1000 m for BOMEX and ∼ 1700 m for RICO) and the mass flux profiles (more variable in RICO). The third case is based on an idealization of observations made at the Southern Great Plains ARM (Atmospheric Radiation Measurement Program) site on 21 June 1997 (Brown et al, 2002). The ARM case describes the development of daytime shallow cumulus convection over land.…”

Section: Validation Set-upmentioning

confidence: 99%

Analytical expressions for entrainment and detrainment in cumulus convection

Rooy

Siebesma

2010

Quart J Royal Meteoro Soc

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vertical velocity, new formulae for entrainment and detrainment rates can be expressed in terms of buoyancy, vertical velocity and cloud fraction. For a variety of shallow convection cases, results from large-eddy simulations show a good correspondence of these new formulae with more traditional methods to diagnose entrainment and detrainment rates. Moreover, the formulae give insight into the behaviour and the physical nature of the mixing coefficients. They explain the observed large variability of the detrainment. The formulae cannot be directly applied as a parametrization. However, it is demonstrated how they can be used to evaluate existing parametrization approaches and as a sound physical base for future parametrization developments.

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Large‐eddy simulation of the diurnal cycle of shallow cumulus convection over land

Cited by 288 publications

References 24 publications

Cloud Shading Effects on Characteristic Boundary-Layer Length Scales

Cloud Shading Effects on Characteristic Boundary-Layer Length Scales

On the relationship between cloud contact time and precipitation susceptibility to aerosol

Analytical expressions for entrainment and detrainment in cumulus convection

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