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.

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.

Observed Boundary Layer Controls on Shallow Cumulus at the ARM Southern Great Plains Site

2018 ◽  
Vol 75 (7) ◽  
pp. 2235-2255 ◽  
Author(s):  
Neil P. Lareau ◽  
Yunyan Zhang ◽  
Stephen A. Klein
Keyword(s):  
Boundary Layer ◽  
Great Plains ◽  
Vertical Velocity ◽  
Mass Flux ◽  
Cloud Base ◽  
Liquid Water Path ◽  
Southern Great Plains ◽  
Clear Sky ◽  
Shallow Cumulus ◽  
In Situ Sensors

Abstract The boundary layer controls on shallow cumulus (ShCu) convection are examined using a suite of remote and in situ sensors at ARM Southern Great Plains (SGP). A key instrument in the study is a Doppler lidar that measures vertical velocity in the CBL and along cloud base. Using a sample of 138 ShCu days, the composite structure of the ShCu CBL is examined, revealing increased vertical velocity (VV) variance during periods of medium cloud cover and higher VV skewness on ShCu days than on clear-sky days. The subcloud circulations of 1791 individual cumuli are also examined. From these data, we show that cloud-base updrafts, normalized by convective velocity, vary as a function of updraft width normalized by CBL depth. It is also found that 63% of clouds have positive cloud-base mass flux and are linked to coherent updrafts extending over the depth of the CBL. In contrast, negative mass flux clouds lack coherent subcloud updrafts. Both sets of clouds possess narrow downdrafts extending from the cloud edges into the subcloud layer. These downdrafts are also present adjacent to cloud-free updrafts, suggesting they are mechanical in origin. The cloud-base updraft data are subsequently combined with observations of convective inhibition to form dimensionless “cloud inhibition” (CI) parameters. Updraft fraction and liquid water path are shown to vary inversely with CI, a finding consistent with CIN-based closures used in convective parameterizations. However, we also demonstrate a limited link between CBL vertical velocity variance and cloud-base updrafts, suggesting that additional factors, including updraft width, are necessary predictors for cloud-base updrafts.

scholarly journals A Stochastic Parameterization for Deep Convection Based on Equilibrium Statistics

2008 ◽  
Vol 65 (1) ◽  
pp. 87-105 ◽  
Author(s):  
R. S. Plant ◽  
G. C. Craig
Keyword(s):  
Mass Flux ◽  
Large Scale ◽  
Deep Convection ◽  
Cloud Base ◽  
Local Fluctuations ◽  
Single Column ◽  
Ensemble Mean ◽  
The Mean ◽  
Closure Method ◽  
Deterministic Limit

Abstract A stochastic parameterization scheme for deep convection is described, suitable for use in both climate and NWP models. Theoretical arguments and the results of cloud-resolving models are discussed in order to motivate the form of the scheme. In the deterministic limit, it tends to a spectrum of entraining/detraining plumes and is similar to other current parameterizations. The stochastic variability describes the local fluctuations about a large-scale equilibrium state. Plumes are drawn at random from a probability distribution function (PDF) that defines the chance of finding a plume of given cloud-base mass flux within each model grid box. The normalization of the PDF is given by the ensemble-mean mass flux, and this is computed with a CAPE closure method. The characteristics of each plume produced are determined using an adaptation of the plume model from the Kain–Fritsch parameterization. Initial tests in the single-column version of the Unified Model verify that the scheme is effective in producing the desired distributions of convective variability without adversely affecting the mean state.

scholarly journals A study of the effect of overshooting deep convection on the water content of the TTL and lower stratosphere from Cloud Resolving Model simulations

2007 ◽  
Vol 7 (18) ◽  
pp. 4977-5002 ◽  
Author(s):  
D. P. Grosvenor ◽  
T. W. Choularton ◽  
H. Coe ◽  
G. Held
Keyword(s):  
Boundary Layer ◽  
Water Content ◽  
Lower Stratosphere ◽  
Deep Convection ◽  
Cold Trap ◽  
Cloud Base ◽  
Anthropogenic Aerosols ◽  
Height Range ◽  
Cloud Resolving Model ◽  
The Impact

Abstract. Simulations of overshooting, tropical deep convection using a Cloud Resolving Model with bulk microphysics are presented in order to examine the effect on the water content of the TTL (Tropical Tropopause Layer) and lower stratosphere. This case study is a subproject of the HIBISCUS (Impact of tropical convection on the upper troposphere and lower stratosphere at global scale) campaign, which took place in Bauru, Brazil (22° S, 49° W), from the end of January to early March 2004. Comparisons between 2-D and 3-D simulations suggest that the use of 3-D dynamics is vital in order to capture the mixing between the overshoot and the stratospheric air, which caused evaporation of ice and resulted in an overall moistening of the lower stratosphere. In contrast, a dehydrating effect was predicted by the 2-D simulation due to the extra time, allowed by the lack of mixing, for the ice transported to the region to precipitate out of the overshoot air. Three different strengths of convection are simulated in 3-D by applying successively lower heating rates (used to initiate the convection) in the boundary layer. Moistening is produced in all cases, indicating that convective vigour is not a factor in whether moistening or dehydration is produced by clouds that penetrate the tropopause, since the weakest case only just did so. An estimate of the moistening effect of these clouds on an air parcel traversing a convective region is made based on the domain mean simulated moistening and the frequency of convective events observed by the IPMet (Instituto de Pesquisas Meteorológicas, Universidade Estadual Paulista) radar (S-band type at 2.8 Ghz) to have the same 10 dBZ echo top height as those simulated. These suggest a fairly significant mean moistening of 0.26, 0.13 and 0.05 ppmv in the strongest, medium and weakest cases, respectively, for heights between 16 and 17 km. Since the cold point and WMO (World Meteorological Organization) tropopause in this region lies at ~15.9 km, this is likely to represent direct stratospheric moistening. Much more moistening is predicted for the 15–16 km height range with increases of 0.85–2.8 ppmv predicted. However, it would be required that this air is lofted through the tropopause via the Brewer Dobson circulation in order for it to have a stratospheric effect. Whether this is likely is uncertain and, in addition, the dehydration of air as it passes through the cold trap and the number of times that trajectories sample convective regions needs to be taken into account to gauge the overall stratospheric effect. Nevertheless, the results suggest a potentially significant role for convection in determining the stratospheric water content. Sensitivity tests exploring the impact of increased aerosol numbers in the boundary layer suggest that a corresponding rise in cloud droplet numbers at cloud base would increase the number concentrations of the ice crystals transported to the TTL, which had the effect of reducing the fall speeds of the ice and causing a ~13% rise in the mean vapour increase in both the 15–16 and 16–17 km height ranges, respectively, when compared to the control case. Increases in the total water were much larger, being 34% and 132% higher for the same height ranges, but it is unclear whether the extra ice will be able to evaporate before precipitating from the region. These results suggest a possible impact of natural and anthropogenic aerosols on how convective clouds affect stratospheric moisture levels.

An Evaluation of Mass Flux Closures for Diurnal Cycles of Shallow Cumulus

2004 ◽  
Vol 132 (11) ◽  
pp. 2525-2538 ◽  
Author(s):  
R. A. J. Neggers ◽  
A. P. Siebesma ◽  
G. Lenderink ◽  
A. A. M. Holtslag
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Climate Model ◽  
Convective Available Potential Energy ◽  
Cloud Base ◽  
Quasi Equilibrium ◽  
Difficult Case ◽  
Diurnal Cycles ◽  
Shallow Cumulus ◽  
The Impact

Abstract Three closure methods for the mass flux at cloud base in shallow cumulus convection are critically examined for the difficult case of a diurnal cycle over land. The closure methods are first evaluated against large-eddy simulations (LESs) by diagnosing all parameters appearing in the closure equations during simulations of two different observed diurnal cycles of shallow cumulus. This reveals the characteristic behavior of each closure mechanism purely as a result of its core structure. With these results in hand the impact of each closure on the development of the cloudy boundary layer is then studied by its implementation in an offline single-column model of a regional atmospheric climate model. The LES results show that the boundary layer quasi-equilibrium closure typically overestimates the cloud-base mass flux after cloud onset, due to the neglect of significant moisture and temperature tendencies in the subcloud layer. The convective available potential energy (CAPE) adjustment closure is compromised by its limitation to compensating subsidence as the only CAPE breakdown mechanism and the use of a constant adjustment time scale. The closure method using the subcloud convective vertical velocity scale gives the best results, as it catches the time development of the cloud-base mass flux as diagnosed in LES.

scholarly journals Eddy Diffusivity/Mass Flux and Shallow Cumulus Boundary Layer: An Updraft PDF Multiple Mass Flux Scheme

2012 ◽  
Vol 69 (5) ◽  
pp. 1513-1533 ◽  
Author(s):  
Kay Sušelj ◽  
João Teixeira ◽  
Georgios Matheou
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Eddy Diffusivity ◽  
Cloud Base ◽  
Model Parameters ◽  
Moist Convection ◽  
Boundary Layer Turbulence ◽  
Eddy Simulation ◽  
Shallow Cumulus ◽  
Base Closure

Abstract In this study, the eddy diffusivity/mass flux (EDMF) approach is used to combine parameterizations of nonprecipitating moist convection and boundary layer turbulence. The novel aspect of this EDMF version is the use of a probability density function (PDF) to describe the moist updraft characteristics. A single bulk dry updraft is initialized at the surface and integrated vertically. At each model level, the possibility of condensation within the updraft is considered based on the PDF of updraft moist conserved variables. If the updraft partially condenses, it is split into moist and dry updrafts, which are henceforth integrated separately. The procedure is repeated at each of the model levels above. The single bulk updraft ends up branching into numerous moist and dry updrafts. With this new approach, the need to define a cloud-base closure is circumvented. This new version of EDMF is implemented in a single-column model (SCM) and evaluated using large-eddy simulation (LES) results for the Barbados Oceanographic and Meteorological Experiment (BOMEX) representing steady-state convection over ocean and the Atmospheric Radiation Measurement (ARM) case representing time-varying convection over land. The new EDMF scheme is able to represent the properties of shallow cumulus and turbulent fluxes in cumulus-topped boundary layers realistically. The parameterized updraft properties partly account for the behavior of the tail of the PDF of moist conserved variables. It is shown that the scheme is not particularly sensitive to the vertical resolution of the SCM or the main model parameters.

scholarly journals Fluctuations in a quasi-stationary shallow cumulus cloud ensemble

2014 ◽  
Vol 1 (2) ◽  
pp. 1223-1282 ◽  
Author(s):  
M. Sakradzija ◽  
A. Seifert ◽  
T. Heus
Keyword(s):  
Weibull Distribution ◽  
Mass Flux ◽  
Flux Distribution ◽  
Deep Convection ◽  
Convective Cloud ◽  
Cloud Base ◽  
Cumulus Clouds ◽  
Large Eddy Simulation Model ◽  
Shallow Cumulus ◽  
Distribution Mode

Abstract. We propose an approach to stochastic parameterization of shallow cumulus clouds to represent the convective variability and its dependence on the model resolution. To collect the information about the individual cloud lifecycles and the cloud ensemble as a whole, we employ a Large-Eddy Simulation model (LES) and a cloud tracking algorithm, followed by conditional sampling of clouds at the cloud-base level. In the case of a shallow cumulus ensemble, the cloud-base mass flux distribution is bimodal due to the different shallow cloud subtypes. Each distribution mode can be approximated with a Weibull distribution, explaining the deviation from a single-parameter exponential shape through the diversity in cloud lifecycles. The exponential distribution of cloud mass flux previously suggested for deep convection parameterization is a special case of the Weibull distribution, which opens a way towards unification of the statistical convective ensemble formalism of shallow and deep cumulus clouds. Based on the empirical and theoretical findings, a stochastic model has been developed to simulate a shallow convective cloud ensemble. It is formulated as a compound random process, with the number of convective elements drawn from a Poisson distribution, and the cloud mass flux sampled from a mixed Weibull distribution. Convective memory is accounted for through the explicit cloud lifecycles, making the model formulation consistent with the choice of the Weibull cloud mass flux distribution function. The memory of individual shallow clouds is required to capture the correct convective variability. The resulting distribution of the subgrid convective states in the considered shallow cumulus case is scale-adaptive – the smaller the grid size, the broader the distribution.

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 A Mass-Flux Scheme View of a High-Resolution Simulation of a Transition from Shallow to Deep Cumulus Convection

2006 ◽  
Vol 63 (7) ◽  
pp. 1895-1909 ◽  
Author(s):  
Zhiming Kuang ◽  
Christopher S. Bretherton
Keyword(s):  
High Resolution ◽  
Mass Flux ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Cumulus Parameterization ◽  
Cloud Base ◽  
Cumulus Convection ◽  
Thermodynamic Structure ◽  
Cumulus Schemes ◽  
Resolution Simulation

Abstract In this paper, an idealized, high-resolution simulation of a gradually forced transition from shallow, nonprecipitating to deep, precipitating cumulus convection is described; how the cloud and transport statistics evolve as the convection deepens is explored; and the collected statistics are used to evaluate assumptions in current cumulus schemes. The statistical analysis methodologies that are used do not require tracing the history of individual clouds or air parcels; instead they rely on probing the ensemble characteristics of cumulus convection in the large model dataset. They appear to be an attractive way for analyzing outputs from cloud-resolving numerical experiments. Throughout the simulation, it is found that 1) the initial thermodynamic properties of the updrafts at the cloud base have rather tight distributions; 2) contrary to the assumption made in many cumulus schemes, nearly undiluted air parcels are too infrequent to be relevant to any stage of the simulated convection; and 3) a simple model with a spectrum of entraining plumes appears to reproduce most features of the cloudy updrafts, but significantly overpredicts the mass flux as the updrafts approach their levels of zero buoyancy. A buoyancy-sorting model was suggested as a potential remedy. The organized circulations of cold pools seem to create clouds with larger-sized bases and may correspondingly contribute to their smaller lateral entrainment rates. Our results do not support a mass-flux closure based solely on convective available potential energy (CAPE), and are in general agreement with a convective inhibition (CIN)-based closure. The general similarity in the ensemble characteristics of shallow and deep convection and the continuous evolution of the thermodynamic structure during the transition provide justification for developing a single unified cumulus parameterization that encompasses both shallow and deep convection.

scholarly journals Boundary Layer and Shallow Cumulus Clouds in a Medium-Range Forecast of a Large-Scale Weather System

2005 ◽  
Vol 133 (7) ◽  
pp. 1938-1960 ◽  
Author(s):  
Stéphane Bélair ◽  
Jocelyn Mailhot ◽  
Claude Girard ◽  
Paul Vaillancourt
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Deep Convection ◽  
Cumulus Clouds ◽  
Weather System ◽  
Low Level ◽  
Boundary Layer Clouds ◽  
Shallow Cumulus ◽  
Medium Range Forecast ◽  
Medium Range

Abstract The role and impact that boundary layer and shallow cumulus clouds have on the medium-range forecast of a large-scale weather system is discussed in this study. A mesoscale version of the Global Environmental Multiscale (GEM) atmospheric model is used to produce a 5-day numerical forecast of a midlatitude large-scale weather system that occurred over the Pacific Ocean during February 2003. In this version of GEM, four different schemes are used to represent (i) boundary layer clouds (including stratus, stratocumulus, and small cumulus clouds), (ii) shallow cumulus clouds (overshooting cumulus), (iii) deep convection, and (iv) nonconvective clouds. Two of these schemes, that is, the so-called MoisTKE and Kuo Transient schemes for boundary layer and overshooting cumulus clouds, respectively, have been recently introduced in GEM and are described in more detail. The results show that GEM, with this new cloud package, is able to represent the wide variety of clouds observed in association with the large-scale weather system. In particular, it is found that the Kuo Transient scheme is mostly responsible for the shallow/intermediate cumulus clouds in the rear portion of the large-scale system, whereas MoisTKE produces the low-level stratocumulus clouds ahead of the system. Several diagnostics for the rear portion of the system reveal that the role of MoisTKE is mainly to increase the vertical transport (diffusion) associated with the boundary layer clouds, while Kuo Transient is acting in a manner more consistent with convective stabilization. As a consequence, MoisTKE is not able to remove the low-level shallow cloud layer that is incorrectly produced by the GEM nonconvective condensation scheme. Kuo Transient, in contrast, led to a significant reduction of these nonconvective clouds, in better agreement with satellite observations. This improved representation of stratocumulus and cumulus clouds does not have a large impact on the overall sea level pressure patterns of the large-scale weather system. Precipitation in the rear portion of the system, however, is found to be smoother when MoisTKE is used, and significantly less when the Kuo Transient scheme is switched on.

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