scholarly journals Environmental sensitivities of shallow-cumulus dilution – Part 1: Selected thermodynamic conditions

2020 ◽  
Vol 20 (21) ◽  
pp. 13217-13239
Author(s):  
Sonja Drueke ◽  
Daniel J. Kirshbaum ◽  
Pavlos Kollias
Keyword(s):  
Large Scale ◽  
Similarity Theory ◽  
Cloud Layer ◽  
Cloud Base ◽  
Shallow Cumulus ◽  
Thermodynamic Conditions ◽  
Cloud Cores ◽  
Environmental Air ◽  
Experimental Findings ◽  
Cloud Core

Abstract. Cumulus entrainment, and its consequent dilution of buoyant cloud cores, strongly regulates the life cycle of shallow cumuli yet remains poorly understood. Herein, new insights into this problem are obtained through large-eddy simulations that systematically investigate the sensitivity of shallow-cumulus dilution to cloud-layer relative humidity (RH), cloud- and subcloud-layer depths, and continentality (i.e., the land–ocean contrast). The simulated cloud-core dilution is found to be strongly sensitive to continentality, with fractional dilution rates twice as large over the ocean as over land. Using a similarity theory based on the turbulent-kinetic-energy (TKE) budget, the reduced cloud-core dilution over land is attributed to larger cloud-base mass flux (mb), driven by stronger surface heating and subcloud turbulence. As mb increases, the fractional dilution rate must decrease to maintain energetic equilibrium. A positive sensitivity is also found to cloud-layer RH, with the core dilution increasing by 25 %–50 % for a 10 % enhancement in RH. This sensitivity is interpreted using the buoyancy-sorting hypothesis, in that mixtures of cloud and environmental air are more likely to become negatively buoyant and detrain (rather than diluting the cloud core) in drier cloud layers. By contrast, the sensitivities of (marine) shallow-cumulus dilution to cloud- and subcloud-layer depths are weak, with a 3 % decrease for a doubling for the former and a 4 % reduction in dilution for a 50 % deeper subcloud layer. These surprisingly weak sensitivities are readily explained by offsetting effects in the TKE similarity theory. Altogether, these experimental findings provide useful, though still incomplete, guidance for flow-dependent shallow-cumulus entrainment formulations in large-scale models.

scholarly journals Environmental sensitivities of shallow-cumulus dilution. Part I: Selected thermodynamic conditions

2020 ◽  
Author(s):  
Sonja Drueke ◽  
Daniel J. Kirshbaum ◽  
Pavlos Kollias
Keyword(s):  
Large Scale ◽  
Similarity Theory ◽  
Cloud Layer ◽  
Cloud Base ◽  
Shallow Cumulus ◽  
Thermodynamic Conditions ◽  
Cloud Cores ◽  
Environmental Air ◽  
Experimental Findings ◽  
Cloud Core

Abstract. Cumulus entrainment, and its consequent dilution of buoyant cloud cores, strongly regulates the life cycle of shallow cumuli yet remains poorly understood. Herein, new insights into this problem are obtained through large-eddy simulations that systematically investigate the sensitivity of shallow-cumulus dilution to cloud-layer relative humidity (RH), cloud- and subcloud-layer depths, and continentality (i.e., the land-ocean contrast). The simulated cloud-core dilution is found to be strongly sensitive to continentality, with fractional dilution rates twice as large over the ocean as over land. Using a similarity theory based on the turbulent-kinetic-energy (TKE) budget, the reduced cloud-core dilution over land is attributed to larger cloud-base mass flux (mb), driven by stronger surface heating and subcloud turbulence. As mb increasies, the fractional dilution rate must decrease to maintain energetic equilibrium. A positive sensitivity is also found to cloud-layer RH, with the core dilution increasing by 25–50 % for a 10 % enhancement in RH. This sensitivity is interpreted using the buoyancy-sorting hypothesis, in that mixtures of cloud and environmental air are more likely to become negatively buoyant and detrain (rather than diluting the cloud core) in drier cloud layers. By contrast, the sensitivities of (marine) shallow-cumulus dilution to cloud- and subcloud-layer depths are weak, with virtually no sensitivity to the former and only a 15 % reduction in dilution for a tripling of the latter (from 250 m to 750 m). These surprisingly weak sensitivities are readily explained by offsetting effects in the TKE similarity theory. Altogether, these experimental findings provide useful, though still incomplete, guidance for flow-dependent shallow-cumulus entrainment formulations in large-scale models.

scholarly journals Transport and chemical transformations influenced by shallow cumulus over land

2005 ◽  
Vol 5 (5) ◽  
pp. 8811-8849
Author(s):  
J. Vilà-Guerau de Arellano ◽  
S.-W. Kim ◽  
M. C. Barth ◽  
E. G. Patton
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Cloud Layer ◽  
Cloud Formation ◽  
Cloud Base ◽  
Chemical Transformations ◽  
Mixing Ratios ◽  
Shallow Cumulus ◽  
Photolysis Rates ◽  
Good Agreement

Abstract. The distribution and evolution of reactive species in a boundary layer characterized by the presence of shallow cumulus over land is studied by means of two large-eddy simulation models: the NCAR and WUR codes. The study focuses on two physical processes that can influence the chemistry: the enhancement of the vertical transport by the buoyant convection associated with cloud formation and the perturbation of the photolysis rates below, in and above the clouds. It is shown that the dilution of the reactant mixing ratio caused by the deepening of the atmospheric boundary layer is an important process and that it can decrease reactant mixing ratios by 10 to 50 percent compared to very similar conditions but with no cloud formation. Additionally, clouds transport chemical species to higher elevations in the boundary layer compared to the case with no clouds which influences the reactant mixing ratios of the nocturnal residual layers following the collapse of the daytime boundary layer. Estimates of the rate of reactant transport based on the calculation of the integrated flux divergence range from to −0.2 ppb hr−1 to −1 ppb hr−1, indicating a net loss of sub-cloud layer air transported into the cloud layer. A comparison of this flux to a parameterized mass flux shows good agreement in mid-cloud, but at cloud base the parameterization underestimates the mass flux. Scattering of radiation by cloud drops perturbs photolysis rates. It is found that these perturbed photolysis rates substantially (10–40%) affect mixing ratios locally (spatially and temporally), but have little effect on mixing ratios averaged over space and time. We find that the ultraviolet radiance perturbation becomes more important for chemical transformations that react with a similar order time scale as the turbulent transport in clouds. Finally, the detailed intercomparison of the LES results shows very good agreement between the two codes when considering the evolution of the reactant mean, flux and (co-)variance vertical profiles.

scholarly journals Investigating the Diurnal Evolution of the Cloud Size Distribution of Continental Cumulus Convection Using Multiday LES

2019 ◽  
Vol 76 (3) ◽  
pp. 729-747 ◽  
Author(s):  
Thirza W. van Laar ◽  
Vera Schemann ◽  
Roel A. J. Neggers
Keyword(s):  
Power Law ◽  
Cloud Cover ◽  
Large Scale ◽  
Correlation Coefficients ◽  
Scale Model ◽  
Cloud Base ◽  
Size Distributions ◽  
Total Cloud Cover ◽  
Surface Precipitation ◽  
Shallow Cumulus

Abstract The diurnal dependence of cumulus cloud size distributions over land is investigated by means of an ensemble of large-eddy simulations (LESs). A total of 146 days of transient continental shallow cumulus are selected and simulated, reflecting a low midday maximum of total cloud cover, weak synoptic forcing, and the absence of strong surface precipitation. The LESs are semi-idealized, forced by large-scale model output but using an interactive surface. This multitude of cases covers a large parameter space of environmental conditions, which is necessary for identifying any diurnal dependencies in cloud size distributions. A power-law exponential function is found to describe the shape of the cloud size distributions for these days well, with the exponential component capturing the departure from power-law scaling at the larger cloud sizes. To assess what controls the largest cloud size in the distribution, the correlation coefficients between the maximum cloud size and various candidate variables reflecting the boundary layer state are computed. The strongest correlation is found between total cloud cover and maximum cloud size. Studying the size density of the cloud area revealed that larger clouds contribute most to a larger total cloud cover, and not the smaller ones. Besides cloud cover, cloud-base and cloud-top height are also found to weakly correlate with the maximum cloud size, suggesting that the classic idea of deeper boundary layers accommodating larger convective thermals still holds for shallow cumulus. Sensitivity tests reveal that the results are only minimally affected by the representation of microphysics and the output resolution.

scholarly journals Theoretical Understanding of the Linear Relationship between Convective Updrafts and Cloud-Base Height for Shallow Cumulus Clouds. Part I: Maritime Conditions

2019 ◽  
Vol 76 (8) ◽  
pp. 2539-2558 ◽  
Author(s):  
Youtong Zheng
Keyword(s):  
Cooling Rate ◽  
Large Scale ◽  
Buoyancy Flux ◽  
Cloud Base ◽  
Radiative Cooling ◽  
Cumulus Clouds ◽  
Shallow Cumulus ◽  
Surface Buoyancy ◽  
Cloud Base Height ◽  
Surface Buoyancy Flux

Abstract Zheng and Rosenfeld found linear relationships between the convective updrafts and cloud-base height zb using ground-based observations over both land and ocean. The empirical relationships allow for a novel satellite remote sensing technique of inferring the cloud-base updrafts and cloud condensation nuclei concentration, both of which are important for understanding aerosol–cloud–climate interactions but have been notoriously difficult to retrieve from space. In Part I of a two-part study, a theoretical framework is established for understanding this empirical relationship over the ocean. Part II deals with continental cumulus clouds. Using the bulk concept of mixed-layer (ML) model for shallow cumulus, I found that this relationship arises from the conservation law of energetics that requires the radiative flux divergence of an ML to balance surface buoyancy flux. Given a certain ML radiative cooling rate per unit mass Q, a deeper ML (higher zb) undergoes more radiative cooling and requires stronger surface buoyancy flux to balance it, leading to stronger updrafts. The rate with which the updrafts vary with zb is modulated by Q. The cooling rate Q manifests strong resilience to external large-scale forcing that spans a wide range of climatology, allowing the slope of the updrafts–zb relationship to remain nearly invariant. This causes the relationship to manifest linearity. The physical mechanism underlying the resilience of Q to large-scale forcing, such as free-tropospheric moisture and sea surface temperature, is investigated through the lens of the radiative transfer theory (two-stream Schwarzschild equations) and an ML model for shallow cumulus.

scholarly journals Performance of an Eddy Diffusivity–Mass Flux Scheme for Shallow Cumulus Boundary Layers

2010 ◽  
Vol 138 (7) ◽  
pp. 2895-2912 ◽  
Author(s):  
Wayne M. Angevine ◽  
Hongli Jiang ◽  
Thorsten Mauritsen
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Eddy Diffusivity ◽  
Cloud Layer ◽  
Atmospheric Composition ◽  
Cloud Base ◽  
Convective Boundary ◽  
Shallow Cumulus ◽  
Lower Cloud ◽  
Boundary Layer Scheme

Abstract Comparisons between single-column (SCM) simulations with the total energy–mass flux boundary layer scheme (TEMF) and large-eddy simulations (LES) are shown for four cases from the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) 2006 field experiment in the vicinity of Houston, Texas. The SCM simulations were run with initial soundings and surface forcing identical to those in the LES, providing a clean comparison with the boundary layer scheme isolated from any other influences. Good agreement is found in the simulated vertical transport and resulting moisture profiles. Notable differences are seen in the cloud base and in the distribution of moisture between the lower and upper cloud layer. By the end of the simulations, TEMF has dried the subcloud layer and moistened the lower cloud layer more than LES. TEMF gives more realistic profiles for shallow cumulus conditions than traditional boundary layer schemes, which have no transport above the dry convective boundary layer. Changes to the formulation and its parameters from previous publications are discussed.

scholarly journals Shallow Cumulus Representation and Its Interaction with Radiation and Surface at the Convection Gray Zone

2019 ◽  
Vol 147 (7) ◽  
pp. 2467-2483 ◽  
Author(s):  
Xabier Pedruzo-Bagazgoitia ◽  
Pedro A. Jiménez ◽  
Jimy Dudhia ◽  
Jordi Vilà-Guerau de Arellano
Keyword(s):  
Late Onset ◽  
Horizontal Resolution ◽  
Cloud Layer ◽  
Shortwave Radiation ◽  
Cloud Base ◽  
Gray Zone ◽  
Shallow Convection ◽  
Systematic Analysis ◽  
Eddy Simulation ◽  
Shallow Cumulus

Abstract This study presents a systematic analysis of convective parameterizations performance with interactive radiation, microphysics, and surface on an idealized day with shallow convection. To this end, we analyze a suite of mesoscale numerical experiments (i.e., with parameterized turbulence). In the first set, two different convection schemes represent shallow convection at a 9-km resolution. These experiments are then compared with model results omitting convective parameterizations at 9- and 3-km horizontal resolution (gray zone). Relevant in our approach is to compare the results against two simulations by different large-eddy simulation (LES) models. Results show that the mesoscale experiments, including the 3-km resolution, are unable to adequately represent the timing, intensity, height, and extension of the shallow cumulus field. The main differences with LES experiments are the following: a too late onset, too high cloud base, and a too early transport of moisture too high, overestimating the second cloud layer. Related to this, both convective parameterizations produce warm and dry biases of up to 2 K and 2 g kg−1, respectively, in the cloud layer. This misrepresentation of the cloud dynamics leads to overestimated shortwave radiation variability, both spacewise and timewise. Domain-averaged shortwave radiation at the surface, however, compares satisfactorily with LES. The shortwave direct and diffuse partition is misrepresented by the convective parameterizations with an underestimation (overestimation) of diffuse (direct) radiation both locally and, by a relative 40% (10%), of the domain average.

scholarly journals Parameterizations for convective transport in various cloud-topped boundary layers

2015 ◽  
Vol 15 (18) ◽  
pp. 10399-10410 ◽  
Author(s):  
M. Sikma ◽  
H. G. Ouwersloot
Keyword(s):  
Boundary Layer ◽  
Convective Transport ◽  
Area Fraction ◽  
Cloud Base ◽  
Accurate Estimation ◽  
Shallow Cumulus ◽  
Wide Range ◽  
Upward Velocity ◽  
Independent Variable ◽  
Cloud Cores

Abstract. We investigate the representation of convective transport of atmospheric compounds by boundary layer clouds. We focus on three key parameterizations that, when combined, express this transport: the area fraction of transporting clouds, the upward velocity in the cloud cores and the chemical concentrations at cloud base. The first two parameterizations combined represent the kinematic mass flux by clouds. To investigate the key parameterizations under a wide range of conditions, we use large-eddy simulation model data for 10 meteorological situations, characterized by either shallow cumulus or stratocumulus clouds. The parameterizations have not been previously tested with such large data sets. In the analysis, we show that the parameterization of the area fraction of clouds currently used in mixed-layer models is affected by boundary layer dynamics. Therefore, we (i) simplify the independent variable used for this parameterization, Q1, by considering the variability in moisture rather than in the saturation deficit and update the parameters in the parameterization to account for this simplification. We (ii) next demonstrate that the independent variable has to be evaluated locally to capture cloud presence. Furthermore, we (iii) show that the area fraction of transporting clouds is not represented by the parameterization for the total cloud area fraction, as is currently assumed in literature. To capture cloud transport, a novel active cloud area fraction parameterization is proposed. Subsequently, the scaling of the upward velocity in cloud cores by the Deardorff convective velocity scale and the parameterization for the concentration of atmospheric reactants at cloud base from literature are verified and improved by analysing six shallow cumulus cases. For the latter, we additionally discuss how the parameterization is affected by wind conditions. This study contributes to a more accurate estimation of convective transport, which occurs at sub-grid scales.

scholarly journals Environmental sensitivities of shallow-cumulus dilution – Part 2: Vertical wind profile

2021 ◽  
Author(s):  
Sonja Drueke ◽  
Daniel J. Kirshbaum ◽  
Pavlos Kollias
Keyword(s):  
Wind Speed ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Dilution Effect ◽  
Cloud Layer ◽  
Vertical Wind ◽  
Exposure Effect ◽  
The Core ◽  
Shallow Cumulus ◽  
Cloud Core

Abstract. This second part of a numerical study on shallow-cumulus dilution focuses on the sensitivity of cloud dilution to changes in the vertical wind profile. Insights are obtained through large-eddy simulations of maritime and continental cloud fields. In these simulations, the speed of the initially uniform geostrophic wind, and the strength of geostrophic vertical wind shear in the cloud and subcloud layer are varied. Increases in the cloud-layer vertical wind shear (up to 9 m/s/km) lead to 40–50 % larger cloud-core dilution rates compared to their respective unsheared counterparts. When the background wind speed, on the other hand, is enhanced by up to 10 m/s and subcloud-layer vertical wind shear develops or is initially prescribed, the dilution rate decreases by up to 25 %. The sensitivities of the dilution rate are linked to the updraft strength and the properties of the entrained air. Increases in the wind speed or vertical wind shear result in lower vertical velocities across all sets of experiments with stronger reductions in the cloud-layer wind shear simulation (27–47 %). Weaker updrafts are exposed to mixing with the drier surrounding air for a longer time period, allowing more entrainment to occur (i.e., the "core exposure effect"). However, reduced vertical velocities, in concert with increased cloud-layer turbulence, also assist in widening the humid shell surrounding the cloud cores, leading to entrainment of more humid air (i.e., the "core-shell dilution effect"). In the experiments with cloud-layer vertical wind shear, the core exposure effect dominates and the cloud-core dilution increases with increasing shear. Conversely, when the wind speed is increased and subcloud-layer vertical wind shear develops or is imposed, the core-shell dilution effect dominates to induce a purifying effect. The sensitivities are generally stronger in the maritime simulations, where weaker sensible heat fluxes lead to narrower, more tilted, and, therefore, more suppressed cumuli when cloud-layer shear is imposed. Moreover, in the experiments with subcloud wind shear, the weaker baseline turbulence in the maritime case allows for a larger turbulence enhancement, resulting in a widening of the transition zones between the cores and their environment, leading to the entrainment of more humid air.

scholarly journals Environmental sensitivities of shallow-cumulus dilution – Part 2: Vertical wind profile

2021 ◽  
Vol 21 (18) ◽  
pp. 14039-14058
Author(s):  
Sonja Drueke ◽  
Daniel J. Kirshbaum ◽  
Pavlos Kollias
Keyword(s):  
Wind Speed ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Dilution Effect ◽  
Cloud Layer ◽  
Vertical Wind ◽  
Exposure Effect ◽  
The Core ◽  
Shallow Cumulus ◽  
Cloud Core

Abstract. This second part of a numerical study on shallow-cumulus dilution focuses on the sensitivity of cloud dilution to changes in the vertical wind profile. Insights are obtained through large-eddy simulations of maritime and continental cloud fields. In these simulations, the speed of the initially uniform geostrophic wind and the strength of geostrophic vertical wind shear in the cloud and subcloud layer are varied. Increases in the cloud-layer vertical wind shear (up to 9 ms-1km-1) lead to 40 %–50 % larger cloud-core dilution rates compared to their respective unsheared counterparts. When the background wind speed, on the other hand, is enhanced by up to 10 m s−1 and subcloud-layer vertical wind shear develops or is initially prescribed, the dilution rate decreases by up to 25 %. The sensitivities of the dilution rate are linked to the updraft strength and the properties of the entrained air. Increases in the wind speed or vertical wind shear result in lower vertical velocities across all sets of experiments with stronger reductions in the cloud-layer wind shear simulation (27 %–47 %). Weaker updrafts are exposed to mixing with the drier surrounding air for a longer time period, allowing more entrainment to occur (i.e., the “core-exposure effect”). However, reduced vertical velocities, in concert with increased cloud-layer turbulence, also assist in widening the humid shell surrounding the cloud cores, leading to entrainment of more humid air (i.e., the “core–shell dilution effect”). In the experiments with cloud-layer vertical wind shear, the core-exposure effect dominates and the cloud-core dilution increases with increasing shear. Conversely, when the wind speed is increased and subcloud-layer vertical wind shear develops or is imposed, the core–shell dilution effect dominates to induce a buffering effect. The sensitivities are generally stronger in the maritime simulations, where weaker sensible heat fluxes lead to narrower, more tilted, and, therefore, more suppressed cumuli when cloud-layer shear is imposed. Moreover, in the experiments with subcloud wind shear, the weaker baseline turbulence in the maritime case allows for a larger turbulence enhancement, resulting in a widening of the transition zones between the cores and their environment, leading to the entrainment of more humid air.

scholarly journals Evaluating Boundary Layer–Based Mass Flux Closures Using Cloud-Resolving Model Simulations of Deep Convection

2010 ◽  
Vol 67 (7) ◽  
pp. 2212-2225 ◽  
Author(s):  
Jennifer K. Fletcher ◽  
Christopher S. Bretherton
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Large Scale ◽  
Deep Convection ◽  
Cloud Base ◽  
Cumulus Convection ◽  
Model Simulations ◽  
Shallow Cumulus ◽  
Wide Range ◽  
Cloud Resolving Model

Abstract High-resolution three-dimensional cloud resolving model simulations of deep cumulus convection under a wide range of large-scale forcings are used to evaluate a mass flux closure based on boundary layer convective inhibition (CIN) that has previously been applied in parameterizations of shallow cumulus convection. With minor modifications, it is also found to perform well for deep oceanic and continental cumulus convection, and it matches simulated cloud-base mass flux much better than a closure based only on the boundary layer convective velocity scale. CIN closure maintains an important feedback among cumulus base mass flux, compensating subsidence, and CIN that keeps the boundary layer top close to cloud base. For deep convection, the proposed CIN closure requires prediction of a boundary layer mean turbulent kinetic energy (TKE) and a horizontal moisture variance, both of which are strongly correlated with precipitation. For our cases, CIN closure predicts cloud-base mass flux in deep convective environments as well as the best possible bulk entraining CAPE closure, but unlike the latter, CIN closure also works well for shallow cumulus convection without retuning of parameters.

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