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

2005 ◽  
Vol 5 (12) ◽  
pp. 3219-3231 ◽  
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 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 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.

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.

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 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.

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

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

<p>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° 09’ N, 59° 25’ 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.</p><p> </p><p>References:<br>Ghate,  V.  P.,  M.  A.  Miller,  and  L.  DiPretore,  2011:   Vertical  velocity structure of marine boundary layer trade wind cumulus clouds. Journal  of  Geophysical  Research: Atmospheres, 116  (D16), doi:10.1029/2010JD015344.</p><p>Grant,  A.  L.  M.,  2001:   Cloud-base  fluxes  in  the  cumulus-capped boundary layer. Quarterly Journal of the Royal Meteorological Society, 127 (572), 407–421, doi:10.1002/qj.49712757209.</p><p>Lamer, K., P. Kollias, and L. Nuijens, 2015:  Observations of the variability  of  shallow  trade  wind  cumulus  cloudiness  and  mass  flux. Journal of Geophysical Research: Atmospheres, 120  (12), 6161–6178, doi:10.1002/2014JD022950.</p>

scholarly journals Atmosphere Boundary Layer Height (ABLH) Determination under Multiple-Layer Conditions Using Micro-Pulse Lidar

2019 ◽  
Vol 11 (3) ◽  
pp. 263 ◽  
Author(s):  
Ruijun Dang ◽  
Yi Yang ◽  
Hong Li ◽  
Xiao-Ming Hu ◽  
Zhiting Wang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Correlation Coefficient ◽  
Cloud Layer ◽  
Horizontal Wind ◽  
Cloud Base ◽  
Accurate Estimation ◽  
Lidar Measurement ◽  
Height Distribution ◽  
Layer Height ◽  
Boundary Layer Height

Accurate estimation of the atmospheric boundary layer height (ABLH) is critically important and it mainly relies on the detection of the vertical profiles of atmosphere variables (temperature, humidity,’ and horizontal wind speed) or aerosols. Aerosol Lidar is a powerful remote sensing instrument frequently used to retrieve ABLH through the detection of the vertical distribution of aerosol concentration. A challenge is that cloud, residual layer (RL), and local signal structure seriously interfere with the lidar measurement of ABLH. A new objective technique presenting as giving a top limiter altitude is introduced to reduce the interference of RL and cloud layer on ABLH determination. Cloud layers are identified by looking for the rapid increase and sharp attenuation of the signal combined with the relative increase in the signal. The cloud layers weather overlay are classified or are decoupled from the ABL by analyzing the continuity of the signal below the cloud base. For cloud layer capping of the ABL, the limiter is determined to be the altitude where a positive signal gradient first occurs above the cloud upper edge. For a cloud that is decoupled from the ABL, the cloud base is considered to be the altitude limiter. For RL in the morning, the altitude limiter is the greatest positive gradient altitude below the RL top. The ABLH will be determined below the top limiter altitude using Haar wavelet (HM) and the curve fitting method (CFM). Besides, the interference of local signal noise is eliminated through consideration of the temporal continuity. While comparing the lidar-determined ABLH by HM (or CFM) and nearby radiosonde measurements of the ABLH, a reasonable concordance is found with a correlation coefficient of 0.94 (or 0.96) and 0.79 (or 0.74), presenting a mean of the relative absolute differences with respect to radiosonde measurements of 10.5% (or 12.3%) and 22.3% (or 17.2%) for cloud-free and cloudy situations, respectively. The diurnal variations in the ABLH determined from HM and CFM on four selected cases show good agreement with a mean correlation coefficient higher than 0.99 and a mean absolute bias of 0.22 km. Also, the determined diurnal ABLH are consistent with surface turbulent kinetic energy (TKE) combined with the time-height distribution of the equivalent potential temperature.

scholarly journals Dust in brown dwarfs and extra-solar planets

2020 ◽  
Vol 634 ◽  
pp. A23 ◽  
Author(s):  
Peter Woitke ◽  
Christiane Helling ◽  
Ophelia Gunn
Keyword(s):  
Gas Phase ◽  
Independent Solution ◽  
Cloud Layer ◽  
Diffusion Reaction ◽  
Cloud Formation ◽  
Cloud Base ◽  
Cloud Particle ◽  
Steep Decline ◽  
Cloud Particles ◽  
Kinetic Diffusion

The precipitation of cloud particles in brown dwarf and exoplanet atmospheres establishes an ongoing downward flux of condensable elements. To understand the efficiency of cloud formation, it is therefore crucial to identify and quantify the replenishment mechanism that is able to compensate for these local losses of condensable elements in the upper atmosphere, and to keep the extrasolar weather cycle running. In this paper, we introduce a new cloud formation model by combining the cloud particle moment method we described previously with a diffusive mixing approach, taking into account turbulent mixing and gas-kinetic diffusion for both gas and cloud particles. The equations are of diffusion-reaction type and are solved time-dependently for a prescribed 1D atmospheric structure, until the model has relaxed toward a time-independent solution. In comparison to our previous models, the new hot-Jupiter model results (Teff ≈ 2000 K, log g = 3) show fewer but larger cloud particles that are more concentrated towards the cloud base. The abundances of condensable elements in the gas phase are featured by a steep decline above the cloud base, followed by a shallower, monotonous decrease towards a plateau, the level of which depends on temperature. The chemical composition of the cloud particles also differs significantly from our previous models. Through the condensation of specific condensates such as Mg2SiO4[s] in deeper layers, certain elements, such as Mg, are almost entirely removed early from the gas phase. This leads to unusual (and non-solar) element ratios in higher atmospheric layers, which then favours the formation of SiO[s] and SiO2[s], for example, rather than MgSiO3[s]. These condensates are not expected in phase-equilibrium models that start from solar abundances. Above the main silicate cloud layer, which is enriched with iron and metal oxides, we find a second cloud layer made of Na2S[s] particles in cooler models (Teff ⪅ 1400 K).

scholarly journals Role of Surface Friction on Shallow Nonprecipitating Convection

2018 ◽  
Vol 75 (1) ◽  
pp. 163-178 ◽  
Author(s):  
Seung-Bu Park ◽  
Steven Böing ◽  
Pierre Gentine
Keyword(s):  
Mass Flux ◽  
Cloud Cover ◽  
Layer Structure ◽  
Topological Analysis ◽  
Surface Friction ◽  
Cloud Layer ◽  
Cloud Base ◽  
Identification Algorithm ◽  
Entrainment Rate

The role of surface friction on shallow nonprecipitating convection is investigated using a series of large-eddy simulations with varying surface friction velocity and with a cloud identification algorithm. As surface friction intensifies, convective rolls dominate over convective cells and secondary overturning circulation becomes stronger in the subcloud layer, thus transporting more moisture upward and more heat downward between the subcloud and cloud layers. Identifying individual clouds, using the identification algorithm based on a three-dimensional topological analysis, reveals that intensified surface friction increases the number of clouds and the degree of tilting in the downstream direction. Highly intensified surface friction increases wind shear across the cloud base and induces cloud tilting, which leads to a vertically parabolic profile of liquid water mixing ratio instead of the classical two-layer structure (conditionally unstable and trade inversion layers). Furthermore, cloud tilting induces more cloud cover and more cloud mass flux much above the cloud base (e.g., 0.8 < z < 1.2 km), but less cloud cover and less cloud mass flux in the upper cloud layer (e.g., z > 1.2 km) because of increased lateral entrainment rate. Similarly, profiles of directly measured entrainment and detrainment rates show that detrainment in the lower cloud layer becomes smaller with stronger surface friction.

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