Modelling Mean and Turbulence Fields in the Dry Convective Boundary Layer with the Eddy-Diffusivity/Mass-Flux Approach

Abstract A better conceptual understanding and more realistic parameterizations of convective boundary layers in climate and weather prediction models have been major challenges in meteorological research. In particular, parameterizations of the dry convective boundary layer, in spite of the absence of water phase-changes and its consequent simplicity as compared to moist convection, typically suffer from problems in attempting to represent realistically the boundary layer growth and what is often referred to as countergradient fluxes. The eddy-diffusivity (ED) approach has been relatively successful in representing some characteristics of neutral boundary layers and surface layers in general. The mass-flux (MF) approach, on the other hand, has been used for the parameterization of shallow and deep moist convection. In this paper, a new approach that relies on a combination of the ED and MF parameterizations (EDMF) is proposed for the dry convective boundary layer. It is shown that the EDMF approach follows naturally from a decomposition of the turbulent fluxes into 1) a part that includes strong organized updrafts, and 2) a remaining turbulent field. At the basis of the EDMF approach is the concept that nonlocal subgrid transport due to the strong updrafts is taken into account by the MF approach, while the remaining transport is taken into account by an ED closure. Large-eddy simulation (LES) results of the dry convective boundary layer are used to support the theoretical framework of this new approach and to determine the parameters of the EDMF model. The performance of the new formulation is evaluated against LES results, and it is shown that the EDMF closure is able to reproduce the main properties of dry convective boundary layers in a realistic manner. Furthermore, it will be shown that this approach has strong advantages over the more traditional countergradient approach, especially in the entrainment layer. As a result, this EDMF approach opens the way to parameterize the clear and cumulus-topped boundary layer in a simple and unified way.

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A Thermal Plume Model for the Convective Boundary Layer: Representation of Cumulus Clouds

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2256.1 ◽

2008 ◽

Vol 65 (2) ◽

pp. 407-425 ◽

Cited By ~ 73

Author(s):

Catherine Rio ◽

Frédéric Hourdin

Keyword(s):

Boundary Layer ◽

Mass Flux ◽

Convective Boundary Layer ◽

Turbulent Transport ◽

Thermal Plume ◽

Cumulus Clouds ◽

Convective Boundary ◽

Near Surface ◽

Plume Model ◽

Surface Drying

Abstract The “thermal plume model,” a mass-flux scheme combined with a classical diffusive approach, originally developed to represent turbulent transport in the dry convective boundary layer, is extended here to the representation of cloud processes. The modified parameterization is validated in a 1D configuration against results of large eddy simulations (LES), as well as in a 3D configuration against in situ measurements, for a series of cases of dry and cloudy convective boundary layers. Accounting for coherent structures of the mixed layer with the mass-flux scheme improves the representation of the diurnal cycle of the boundary layer, particularly its progressive deepening during the day and the associated near-surface drying. Results also underline the role of the prescription of the mixing of air between the plume and its environment, and of submean-plume fluctuations.

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Parameterization of the Dry Convective Boundary Layer Based on a Mass Flux Representation of Thermals

Journal of the Atmospheric Sciences ◽

10.1175/1520-0469(2002)059<1105:potdcb>2.0.co;2 ◽

2002 ◽

Vol 59 (6) ◽

pp. 1105-1123 ◽

Cited By ~ 76

Author(s):

Frédéric Hourdin ◽

Fleur Couvreux ◽

Laurent Menut

Keyword(s):

Boundary Layer ◽

Mass Flux ◽

Convective Boundary Layer ◽

Convective Boundary

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Evaluation of PBL Parameterizations in WRF at Subkilometer Grid Spacings: Turbulence Statistics in the Dry Convective Boundary Layer

Monthly Weather Review ◽

10.1175/mwr-d-15-0208.1 ◽

2016 ◽

Vol 144 (3) ◽

pp. 1161-1177 ◽

Cited By ~ 36

Author(s):

Hyeyum Hailey Shin ◽

Jimy Dudhia

Keyword(s):

Boundary Layer ◽

Mass Flux ◽

Weather Prediction ◽

Wrf Model ◽

Eddy Diffusivity ◽

Weather Research And Forecasting ◽

Turbulence Statistics ◽

Convective Boundary ◽

Large Eddies ◽

Horizontal Grid

Abstract Planetary boundary layer (PBL) parameterizations in mesoscale models have been developed for horizontal resolutions that cannot resolve any turbulence in the PBL, and evaluation of these parameterizations has been focused on profiles of mean and parameterized flux. Meanwhile, the recent increase in computing power has been allowing numerical weather prediction (NWP) at horizontal grid spacings finer than 1 km, at which kilometer-scale large eddies in the convective PBL are partly resolvable. This study evaluates the performance of convective PBL parameterizations in the Weather Research and Forecasting (WRF) Model at subkilometer grid spacings. The evaluation focuses on resolved turbulence statistics, considering expectations for improvement in the resolved fields by using the fine meshes. The parameterizations include four nonlocal schemes—Yonsei University (YSU), asymmetric convective model 2 (ACM2), eddy diffusivity mass flux (EDMF), and total energy mass flux (TEMF)—and one local scheme, the Mellor–Yamada–Nakanishi–Niino (MYNN) level-2.5 model. Key findings are as follows: 1) None of the PBL schemes is scale-aware. Instead, each has its own best performing resolution in parameterizing subgrid-scale (SGS) vertical transport and resolving eddies, and the resolution appears to be different between heat and momentum. 2) All the selected schemes reproduce total vertical heat transport well, as resolved transport compensates differences of the parameterized SGS transport from the reference SGS transport. This interaction between the resolved and SGS parts is not found in momentum. 3) Those schemes that more accurately reproduce one feature (e.g., thermodynamic transport, momentum transport, energy spectrum, or probability density function of resolved vertical velocity) do not necessarily perform well for other aspects.

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Quantifying horizontal and vertical tracer mass fluxes in an idealized valley during daytime

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-13049-2016 ◽

2016 ◽

Vol 16 (20) ◽

pp. 13049-13066 ◽

Cited By ~ 17

Author(s):

Daniel Leukauf ◽

Alexander Gohm ◽

Mathias W. Rotach

Keyword(s):

Boundary Layer ◽

Mass Flux ◽

Convective Boundary Layer ◽

Transport Processes ◽

Surface Heating ◽

Convective Boundary ◽

Mass Fluxes ◽

Initial Stability ◽

Slope Wind ◽

Stable Core

Abstract. The transport and mixing of pollution during the daytime evolution of a valley boundary layer is studied in an idealized way. The goal is to quantify horizontal and vertical tracer mass fluxes between four different valley volumes: the convective boundary layer, the slope wind layer, the stable core, and the atmosphere above the valley. For this purpose, large eddy simulations (LES) are conducted with the Weather Research and Forecasting (WRF) model for a quasi-two-dimensional valley. The valley geometry consists of two slopes with constant slope angle and is homogeneous in the along-valley direction. The surface sensible heat flux is horizontally homogeneous and prescribed by a sine function. The initial sounding is characterized by an atmosphere at rest and a constant Brunt–Väisälä frequency. Various experiments are conducted for different combinations of surface heating amplitudes and initial stability conditions. A passive tracer is released with an arbitrary but constant rate at the valley floor and resulting tracer mass fluxes are evaluated between the aforementioned volumes.As a result of the surface heating, a convective boundary layer is established in the lower part of the valley with a stable layer on top – the so-called stable core. The height of the slope wind layer, as well as the wind speed within, decreases with height due to the vertically increasing stability. Hence, the mass flux within the slope wind layer decreases with height as well. Due to mass continuity, this along-slope mass flux convergence leads to a partial redirection of the flow from the slope wind layer towards the valley centre and the formation of a horizontal intrusion above the convective boundary layer. This intrusion is associated with a transport of tracer mass from the slope wind layer towards the valley centre. A strong static stability and/or weak forcing lead to large tracer mass fluxes associated with this phenomenon. The total export of tracer mass out of the valley atmosphere increases with decreasing stability and increasing forcing. The effects of initial stability and forcing can be combined to a single parameter, the breakup parameter B. An analytical function is presented that describes the exponential decrease of the percentage of exported tracer mass with increasing B. This study is limited by the idealization of the terrain shape, stratification, and forcing, but quantifies transport processes for a large range of forcing amplitudes and atmospheric stability.

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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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