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

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.

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Transport and chemical transformations influenced by shallow cumulus over land

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-8811-2005 ◽

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.

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

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-17-0244.1 ◽

2018 ◽

Vol 75 (7) ◽

pp. 2235-2255 ◽

Cited By ~ 15

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.

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

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-090.1 ◽

2012 ◽

Vol 69 (5) ◽

pp. 1513-1533 ◽

Cited By ~ 36

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.

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

Monthly Weather Review ◽

10.1175/2010mwr3142.1 ◽

2010 ◽

Vol 138 (7) ◽

pp. 2895-2912 ◽

Cited By ~ 75

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.

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

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3328.1 ◽

2010 ◽

Vol 67 (7) ◽

pp. 2212-2225 ◽

Cited By ~ 31

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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Transport and chemical transformations influenced by shallow cumulus over land

Atmospheric Chemistry and Physics ◽

10.5194/acp-5-3219-2005 ◽

2005 ◽

Vol 5 (12) ◽

pp. 3219-3231 ◽

Cited By ~ 41

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.

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Evaluation of simulated aerosol properties with the aerosol-climate model ECHAM5-HAM using observations from the IMPACT field campaign

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-7709-2010 ◽

2010 ◽

Vol 10 (16) ◽

pp. 7709-7722 ◽

Cited By ~ 20

Author(s):

G.-J. Roelofs ◽

H. ten Brink ◽

A. Kiendler-Scharr ◽

G. de Leeuw ◽

A. Mensah ◽

...

Keyword(s):

Boundary Layer ◽

Climate Model ◽

Lower Boundary ◽

Synoptic Scale ◽

Concentration Gradients ◽

Cloud Properties ◽

Dry Conditions ◽

Strong Exchange ◽

The Impact ◽

Lower Boundary Layer

Abstract. In May 2008, the measurement campaign IMPACT for observation of atmospheric aerosol and cloud properties was conducted in Cabauw, The Netherlands. With a nudged version of the coupled aerosol-climate model ECHAM5-HAM we simulate the size distribution and chemical composition of the aerosol and the associated aerosol optical thickness (AOT) for the campaign period. Synoptic scale meteorology is represented realistically through nudging of the vorticity, the divergence, the temperature and the surface pressure. Simulated concentrations of aerosol sulfate and organics at the surface are generally within a factor of two from observed values. The monthly averaged AOT from the model is 0.33, about 20% larger than observed. For selected periods of the month with relatively dry and moist conditions discrepancies are approximately −30% and +15%, respectively. Discrepancies during the dry period are partly caused by inaccurate representation of boundary layer (BL) dynamics by the model affecting the simulated AOT. The model simulates too strong exchange between the BL and the free troposphere, resulting in weaker concentration gradients at the BL top than observed for aerosol and humidity, while upward mixing from the surface layers into the BL appears to be underestimated. The results indicate that beside aerosol sulfate and organics also aerosol ammonium and nitrate significantly contribute to aerosol water uptake. The simulated day-to-day variability of AOT follows synoptic scale advection of humidity rather than particle concentration. Even for relatively dry conditions AOT appears to be strongly influenced by the diurnal cycle of RH in the lower boundary layer, further enhanced by uptake and release of nitric acid and ammonia by aerosol water.

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Investigating the Scale Adaptivity of a Size-Filtered Mass Flux Parameterization in the Gray Zone of Shallow Cumulus Convection

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-17-0231.1 ◽

2018 ◽

Vol 75 (4) ◽

pp. 1195-1214 ◽

Cited By ~ 5

Author(s):

Maren Brast ◽

Vera Schemann ◽

Roel A. J. Neggers

Keyword(s):

Mass Flux ◽

Turbulent Transport ◽

Eddy Diffusivity ◽

Coarse Grained ◽

Parameterization Scheme ◽

Quasi Equilibrium ◽

Gray Zone ◽

Cumulus Clouds ◽

Eddy Simulation ◽

Shallow Cumulus

Abstract In this study, the scale adaptivity of a new parameterization scheme for shallow cumulus clouds in the gray zone is investigated. The eddy diffusivity/multiple mass flux [ED(MF)n] scheme is a bin-macrophysics scheme in which subgrid transport is formulated in terms of discretized size densities. While scale adaptivity in the ED component is achieved using a pragmatic blending approach, the MF component is filtered such that only the transport by plumes smaller than the grid size is maintained. For testing, ED(MF)n is implemented into a large-eddy simulation (LES) model, replacing the original subgrid scheme for turbulent transport. LES thus plays the role of a nonhydrostatic testing ground, which can be run at different resolutions to study the behavior of the parameterization scheme in the boundary layer gray zone. In this range, convective cumulus clouds are partially resolved. The authors find that for quasi-equilibrium marine subtropical conditions at high resolutions, the clouds and the turbulent transport are predominantly resolved by the LES. This partitioning changes toward coarser resolutions, with the representation of shallow cumulus clouds gradually becoming completely carried by the ED(MF)n. The way the partitioning changes with grid spacing matches the behavior diagnosed in coarse-grained LES fields, suggesting that some scale adaptivity is captured. Sensitivity studies show that the scale adaptivity of the ED closure is important and that the location of the gray zone is found to be moderately sensitive to some model constants.

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Measuring the variability of the shallow convective mass flux profiles in the tropical trade wind region

10.5194/egusphere-egu2020-4780 ◽

2020 ◽

Author(s):

Marcus Klingebiel ◽

Heike Konow ◽

Bjorn Stevens

Keyword(s):

Boundary Layer ◽

Vertical Velocity ◽

Mass Flux ◽

Doppler Radar ◽

Cloud Fraction ◽

Cloud Base ◽

Trade Wind ◽

Geophysical Research ◽

Wind Region ◽

Convective Mass Flux

Mass flux is a key parameter to represent shallow convection in global circulation models. To estimate the shallow convective mass flux as accurately as possible, observations of this parameter are necessary. Prior studies from Ghate et al. (2011) and Lamer et al. (2015) used Doppler radar measurements over a few months to identify a typical shallow convective mass flux profile based on cloud fraction and vertical velocity. In this study, we extend their observations by using long term remote sensing measurements at the Barbados Cloud Observatory (13&#176; 09&#8217; N, 59&#176; 25&#8217; W) over a time period of 30 months and check a hypothesis by Grant (2001), who proposed that the cloud base mass flux is just proportional to the sub-cloud convective velocity scale. Therefore, we analyze Doppler radar and Doppler lidar measurements to identify the variation of the vertical velocity in the cloud and sub-cloud layer, respectively. Furthermore, we show that the in-cloud mass flux is mainly influenced by the cloud fraction and provide a linear equation, which can be used to roughly calculate the mass flux in the trade wind region based on the cloud fraction.&#160;References: Ghate,&#160; V.&#160; P.,&#160; M.&#160; A.&#160; Miller,&#160; and&#160; L.&#160; DiPretore,&#160; 2011:&#160;&#160; Vertical&#160; velocity structure of marine boundary layer trade wind cumulus clouds. Journal&#160; of&#160; Geophysical&#160; Research: Atmospheres, 116&#160; (D16), doi:10.1029/2010JD015344.Grant,&#160; A.&#160; L.&#160; M.,&#160; 2001:&#160;&#160; Cloud-base&#160; fluxes&#160; in&#160; the&#160; cumulus-capped boundary layer. Quarterly Journal of the Royal Meteorological Society, 127 (572), 407&#8211;421, doi:10.1002/qj.49712757209.Lamer, K., P. Kollias, and L. Nuijens, 2015:&#160; Observations of the variability&#160; of&#160; shallow&#160; trade&#160; wind&#160; cumulus&#160; cloudiness&#160; and&#160; mass&#160; flux. Journal of Geophysical Research: Atmospheres, 120&#160; (12), 6161&#8211;6178, doi:10.1002/2014JD022950.

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Regional modeling of urban climate: the impact of physical process representation

10.5194/egusphere-egu2020-7001 ◽

2020 ◽

Author(s):

Peter Huszár ◽

Jan Karlický ◽

jana Ďoubalová ◽

Tereza Nováková ◽

Filip Švábik ◽

...

Keyword(s):

Boundary Layer ◽

Urban Areas ◽

Climate Model ◽

Numerical Models ◽

Absolute Humidity ◽

Average Wind Speed ◽

Substantial Impact ◽

Mitigation And Adaptation ◽

Urban Rural ◽

The Impact

The urban heat island (UHI) is a relaively old concept and has been widely studied using both observational and modeling approches. However, urban canopies impact the meteorological conditions in the planetary boundary layer (PBL) and above in many other ways, e.g. urban breeze circulation can form, enhanced drag causes intensification of the turbulent diffusion leading to elevated PBL height, reduced evaporation results in decreased absolute humidity, changes in cloudiness etc. A well established regional model representation of these phenomena is crucial for both mitigation and adaptation in areas affected by intense urbanization and climate change. There are however large uncertainities how the underlying physical processes are represented in numerical models, i.e. what models are used along with which parameterizations and parameters.Here we perform a regional multi-model analysis over central Europe using the Regional Climate Model (RegCM4) and Weather Research and Forecast (WRF) regional models with different configurations representing different PBL treatment, convection parameterization, surface layer physics, microphysics and urban canopy models. Model results are extensively compared to surface measurements as well as satellite observation of surface temperatures. We analyse the model results mainly in terms of the urban-rural contrast which is a measure of the difference between the urban core value and the vicitinity (with respect to the particular city) for selected meteorological parameters. Our results show substantial impact of the choice of the model as well as the choice of parameterization on the intensity of UHI and other meteorological effects. The urban-rural difference of PBL height and average wind speed between urban areas and their vicinity is affected the most, controlled by the boundary layer physics parameterization. Our simulations confirm the large uncertainity in how models resolve the meteorological features specific to urbanized areas and this has to be taken into account when designing different strategies for urban planning and multimodel approaches should be preferred.

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