On the Mass-Flux Representation of Vertical Transport in Moist Convection

Abstract This study investigates to what extent the convective fluxes formulated within the mass-flux framework can represent the total vertical transport of heat and moisture in the cloud layer and whether the same approach can be extended to represent the vertical momentum transport using large-eddy simulations (LESs) of six well-documented cloud cases, including both deep and shallow convection. Two methods are used to decompose the LES-resolved vertical fluxes: decompositions based on the coherent convective features using the mass-flux top-hat profile and by two-dimensional fast Fourier transform (2D-FFT) in terms of wavenumbers. The analyses show that the convective fluxes computed using the mass-flux formula can account for most of the total fluxes of conservative thermodynamic variables in the cloud layer of both deep and shallow convection for an appropriately defined convective updraft fraction, a result consistent with the mass-flux dynamic view of moist convection and previous studies. However, the mass-flux approach fails to represent the vertical momentum transport in the cloud layer of both deep and shallow convection. The 2D-FFT and other analyses suggest that such a failure results from a number of reasons: 1) the complicated momentum distribution in the cloud layer cannot be well described by the simple top-hat profile; 2) shear-driven small-scale eddies are more efficient momentum carriers than coherent convective plumes; 3) the phase relationship between vertical velocity and horizontal momentum components is substantially different from that between vertical velocity and conservative thermodynamic variables; and 4) the structure of horizontal momentum can change substantially from case to case even in the same climate regime.

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Convectively driven wind variability in connection to wind biases in the ECMWF operational weather model

10.5194/egusphere-egu2020-21249 ◽

2020 ◽

Author(s):

Louise Nuijens ◽

Irina Sandu ◽

Beatrice Saggiorato ◽

Hauke Schulz ◽

Mariska Koning ◽

...

Keyword(s):

Model Development ◽

Meridional Overturning Circulation ◽

Momentum Transport ◽

Cloud Base ◽

Moist Convection ◽

Shallow Convection ◽

Wind Speeds ◽

Wind Variability ◽

Large Eddy

Despite playing a key role in the atmospheric circulation, the representation of momentum transport by moist convection (cumulus clouds) has been largely overlooked by the model development community over the past decade, at least compared with diabatic and radiative effects of clouds. In particular, how shallow convection may influence surface and boundary layer winds is not thoroughly investigated. In this talk, we discuss the role of convective momentum transport (CMT) in setting low-level wind speed and its variability and evaluate its role in long-standing wind biases in the ECMWF IFS model.We use high-frequency wind profiling measurements and high-resolution large-eddy simulations to inform our understanding of convectively driven wind variability. We do this at two locations: in the trades, using wind lidar and radiosonde measurements from the Barbados Cloud Observatory and the intensive EUREC4A field campaign, and over the Netherlands, using an observationally constrained reanalysis wind dataset and large-eddy simulation hindcasts.At both locations we use the data and model output to investigate whether CMT can be responsible for a missing drag near the surface in the IFS model. Namely, at short leadtimes, the model produces stronger than observed easterly/westerly flow near the surface, while &#8220;a missing drag&#8221; produces weaker than observed wind turning. Consequently, the meridional overturning circulation in both the tropics and midlatitudes is weaker in the IFS and in ERA-Interim and ERA5 reanalysis products.Comparing simulated and IFS wind tendencies at selected grid points at the above locations, and by turning off the process of CMT by shallow convection in the model, we gain insight in the role of CMT in explaining wind biases. We find that CMT alone does not explain a missing drag near the surface. CMT often acts to accelerate winds near the surface. But CMT plays a role in communicating biases in cloud base wind speeds towards the surface. In the trades, a strong jet near cloud base is determined by thermal wind and a strong flux of zonal momentum through cloud base, where &#8220;cumulus friction&#8221; minimizes. Near this jet, the presence of (counter-gradient) turbulent momentum fluxes produces most of the drag. Implications of these findings for CMT parameterization are discussed.

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On the Factors Controlling the Development of Shallow Convection in Eddy-Diffusivity/Mass-Flux Models

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0121.1 ◽

2019 ◽

Vol 76 (2) ◽

pp. 433-456 ◽

Cited By ~ 9

Author(s):

Kay Suselj ◽

Marcin J. Kurowski ◽

João Teixeira

Keyword(s):

Mass Flux ◽

Large Scale ◽

Vertical Mixing ◽

Eddy Diffusivity ◽

Moist Convection ◽

Shallow Convection ◽

Surface Conditions ◽

Large Scale Field ◽

Low Sensitivity ◽

Key Aspects

Abstract This study addresses key aspects of shallow moist convection, as simulated by a multiplume eddy-diffusivity/mass-flux (EDMF) model. Two factors suggested in the literature to be essential for the development of convective plumes are investigated: surface conditions and lateral entrainment. The model consistently decomposes the subgrid vertical mixing into convective plumes and the nonconvective environment. The modeled convection shows low sensitivity to the surface plume area. The results indicate that plume development in the subcloud layer is controlled by both surface conditions and lateral entrainment. Their impact significantly changes in the cloud layer where the surface conditions are no longer important. The development of shallow convection is dominated by the interactions between the plumes and the large-scale field and is sensitive to the representation of the variability of thermodynamic properties between the plumes. A simple two-layer model of steady-state convection is proposed to help understand the role of these processes in shaping the properties of moist convection.

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The role of convection in the momentum budget of ICON-LEM hindcasts over the North Atlantic

10.5194/egusphere-egu2020-4701 ◽

2020 ◽

Author(s):

Kevin Helfer ◽

Louise Nuijens ◽

Vishal Dixit ◽

Pier Siebesma

Keyword(s):

North Atlantic ◽

Cloud Layer ◽

Momentum Transport ◽

Cloud Base ◽

Relative Role ◽

Shallow Convection ◽

Near Surface ◽

Momentum Budget ◽

The Impact

Motivated by the uncertain role of convective momentum transport from low clouds in setting patterns of wind in the trades, we discuss the impact of shallow convection on boundary-layer winds and its role in the overall momentum budget in the trades from large-domain large-eddy simulations. To this end, we analyse ICON-LEM hindcast simulations over the (sub)tropical North Atlantic during the NARVAL1 and NARVAL2 flight campaigns.We describe that the character of the momentum flux profile differs significantly in regimes of shallow and deep convection and thus its influence on cloud-layer and near-surface winds. In particular, we establish that the momentum transport tendency is of similar importance as other terms in the momentum budget, and though the shape of the profile is remarkably insensitive to the horizontal resolution of the simulation, the relative role of subgrid and resolved fluxes changes with resolution. Furthermore, we find that counter-gradient transport occurs even in the absence of organisation, namely in the lower cloud layer, where cloudy updrafts carry slow momentum air upwards, which locally accelerates winds and may play a role at maintaining the cloud-base wind maximum.

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Rotating Shallow-Water Models with Moist Convection

10.1093/oso/9780198804338.003.0015 ◽

2018 ◽

Author(s):

Vladimir Zeitlin

Keyword(s):

Baroclinic Instability ◽

Potential Temperature ◽

Moist Convection ◽

Mathematical Properties ◽

Vertical Fluxes ◽

Equivalent Potential ◽

Shallow Water Models ◽

Wave Emission ◽

Rotating Shallow Water ◽

Convective Fluxes

It is shown how the standard RSW can be ’augmented’ to include phase transitions of water. This chapter explains how to incorporate extra (convective) vertical fluxes in the model. By using Lagrangian conservation of equivalent potential temperature condensation of the water vapour, which is otherwise a passive tracer, is included in the model and linked to convective fluxes. Simple relaxational parameterisation of condensation permits the closure of the system, and surface evaporation can be easily included. Physical and mathematical properties of thus obtained model are explained, and illustrated on the example of wave scattering on the moisture front. The model is applied to ’moist’ baroclinic instability of jets and vortices. Condensation is shown to produce a transient increase of the growth rate. Special attention is paid to the moist instabilities of hurricane-like vortices, which are shown to enhance intensification of the hurricane, increase gravity wave emission, and generate convection-coupled waves.

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Characteristic nature of vertical motions observed in Arctic mixed-phase stratocumulus

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-31079-2013 ◽

2013 ◽

Vol 13 (11) ◽

pp. 31079-31125 ◽

Cited By ~ 2

Author(s):

J. Sedlar ◽

M. D. Shupe

Keyword(s):

Vertical Velocity ◽

Vertical Mixing ◽

Vertical Motion ◽

Temperature Inversion ◽

Arctic Sea Ice ◽

Cloud Layer ◽

Mixed Phase ◽

The Arctic ◽

Cloud Base ◽

Mixed Phase Clouds

Abstract. Over the Arctic Ocean, little is known, observationally, on cloud-generated buoyant overturning vertical motions within mixed-phase stratocumulus clouds. Characteristics of such motions are important for understanding the diabatic processes associated with the vertical motions, the lifetime of the cloud layer and its micro- and macrophysical characteristics. In this study, we exploit a suite of surface-based remote sensors over the high Arctic sea ice during a week-long period of persistent stratocumulus in August 2008 to derive the in-cloud vertical motion characteristics. In-cloud vertical velocity skewness and variance profiles are found to be strikingly different from observations within lower-latiatude stratocumulus, suggesting these Arctic mixed-phase clouds interact differently with the atmospheric thermodynamics (cloud tops extending above a stable temperature inversion base) and with a different coupling state between surface and cloud. We find evidence of cloud-generated vertical mixing below cloud base, regardless of surface-cloud coupling state, although a decoupled surface-cloud state occurred most frequently. Detailed case studies are examined focusing on 3 levels within the cloud layer, where wavelet and power spectral analyses are applied to characterize the dominant temporal and horizontal scales associated with cloud-generated vertical motions. In general, we find a positively-correlated vertical motion signal across the full cloud layer depth. The coherency is dependent upon other non-cloud controlled factors, such as larger, mesoscale weather passages and radiative shielding of low-level stratocumulus by multiple cloud layers above. Despite the coherency in vertical velocity across the cloud, the velocity variances were always weaker near cloud top, relative to cloud mid and base. Taken in combination with the skewness, variance and thermodynamic profile characteristics, we observe vertical motions near cloud-top that behave differently than those from lower within the cloud layer. Spectral analysis indicates peak cloud-generated w variance timescales slowed only modestly during decoupled cases relative to coupled; horizontal wavelengths only slightly increased when transitioning from coupling to decoupling. The similarities in scales suggests that perhaps the dominant forcing for all cases is generated from the cloud layer, and it is not the surface forcing that characterizes the time and space scales of in-cloud vertical velocity variance. This points toward the resilient nature of Arctic mixed-phase clouds to persist when characterized by thermodynamic regimes unique to the Arctic.

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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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Representation of Boundary Layer Moisture Transport in Cloud-Resolving Models

Monthly Weather Review ◽

10.1175/mwr-d-12-00046.1 ◽

2012 ◽

Vol 140 (11) ◽

pp. 3682-3698 ◽

Cited By ~ 8

Author(s):

Chin-Hoh Moeng ◽

Akio Arakawa

Keyword(s):

Vertical Velocity ◽

Moisture Transport ◽

Deep Convection ◽

Cloud Layer ◽

Cloud System ◽

Small Scale ◽

Closure Model ◽

Eddy Simulation ◽

Gradient Model ◽

Local Gradient

Abstract One of the important roles of the PBL is to transport moisture from the surface to the cloud layer. However, how this transport process can be accounted for in cloud-resolving models (CRMs) is not sufficiently clear and has rarely been examined. A typical CRM can resolve the bulk feature of large convection systems but not the small-scale convection and turbulence motions that carry a large portion of the moisture fluxes. This study uses a large-eddy simulation of a tropical deep-convection system as a benchmark to examine the subgrid-scale (SGS) moisture transport into a cloud system. It is shown that most of the PBL moisture transport to the cloud layer occurs in the areas under low-level updrafts, with rain, or under cloudy skies, although these PBL regimes may cover only a small fraction of the entire cloud-system domain. To develop SGS parameterizations to represent the spatial distribution of this moisture transport in CRMs, three models are proposed and tested. An updraft–downdraft model performs exceptionally well, while a statistical-closure model and a local-gradient model are less satisfactory but still perform adequately. Each of these models, however, has its own closure issues to be addressed. The updraft–downdraft model requires a scheme to estimate the mean SGS updraft–downdraft properties, the statistical-closure model needs a scheme to predict both SGS vertical-velocity and moisture variances, while the local-gradient model requires estimation of the SGS vertical-velocity variance.

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Characterization of Momentum Transport Associated with Organized Moist Convection and Gravity Waves

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3418.1 ◽

2010 ◽

Vol 67 (10) ◽

pp. 3208-3225 ◽

Cited By ~ 28

Author(s):

Todd P. Lane ◽

Mitchell W. Moncrieff

Keyword(s):

Gravity Wave ◽

Gravity Waves ◽

Momentum Flux ◽

Tropical Convection ◽

Momentum Transport ◽

Moist Convection ◽

Convective Systems ◽

Convective Momentum Transport ◽

Convective Organization ◽

Wave Momentum

Abstract Tropical convection is inherently multiscalar, involving complex fields of clouds and various regimes of convective organization ranging from small disorganized cumulus up to large organized convective clusters. In addition to being a crucial component of the atmospheric water cycle and the global heat budget, tropical convection induces vertical fluxes of horizontal momentum. There are two main contributions to the momentum transport. The first resides entirely in the troposphere and is due to ascent, descent, and organized circulations associated with precipitating convective systems. The second resides in the troposphere, stratosphere, and farther aloft and is caused by vertically propagating gravity waves. Both the convective momentum transport and the gravity wave momentum flux must be parameterized in general circulation models; yet in existing parameterizations, these two processes are treated independently. This paper examines the relationship between the convective momentum transport and convectively generated gravity wave momentum flux by utilizing idealized simulations of multiscale tropical convection in different wind shear conditions. The simulations produce convective systems with a variety of regimes of convective organization and therefore different convective momentum transport properties and gravity wave spectra. A number of important connections are identified, including a consistency in the sign of the momentum transports in the lower troposphere and stratosphere that is linked to the generation of gravity waves by tilted convective structures. These results elucidate important relationships between the convective momentum transport and the gravity wave momentum flux that will be useful for interlinking their parameterization in the future.

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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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