On the Parameterization of Convective Downdrafts for Marine Stratocumulus Clouds

Wu, Erik A.; Yang, Hu; Kleissl, Jan; Sušelj, K.; Kurowski, Marcin J.; Teixeira, J.

doi:10.1175/mwr-d-19-0292.1

Cited by 18 publications

(19 citation statements)

References 58 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This is in agreement with Wu et al. (2020), where the authors show that the implementation of downdrafts in an EDMF scheme does not significantly improve simulations of the STBL.…”

Section: Resultssupporting

confidence: 92%

See 1 more Smart Citation

A Generalized Mixing Length Closure for Eddy‐Diffusivity Mass‐Flux Schemes of Turbulence and Convection

Lopez‐Gomez

Cohen

et al. 2020

J Adv Model Earth Syst

View full text Add to dashboard Cite

Because of their limited spatial resolution, numerical weather prediction and climate models have to rely on parameterizations to represent atmospheric turbulence and convection. Historically, largely independent approaches have been used to represent boundary layer turbulence and convection, neglecting important interactions at the subgrid scale. Here we build on an eddy-diffusivity mass-flux (EDMF) scheme that represents all subgrid-scale mixing in a unified manner, partitioning subgrid-scale fluctuations into contributions from local diffusive mixing and coherent advective structures and allowing them to interact within a single framework. The EDMF scheme requires closures for the interaction between the turbulent environment and the plumes and for local mixing. A second-order equation for turbulence kinetic energy (TKE) provides one ingredient for the diffusive local mixing closure, leaving a mixing length to be parameterized. Here, we propose a new mixing length formulation, based on constraints derived from the TKE balance. It expresses local mixing in terms of the same physical processes in all regimes of boundary layer flow. The formulation is tested at a range of resolutions and across a wide range of boundary layer regimes, including a stably stratified boundary layer, a stratocumulus-topped marine boundary layer, and dry convection. Comparison with large eddy simulations (LES) shows that the EDMF scheme with this diffusive mixing parameterization accurately captures the structure of the boundary layer and clouds in all cases considered. Plain Language Summary Turbulence and convection transport heat and moisture in the atmosphere and are ultimately responsible for the formation of clouds. However, they act on scales far too small to be resolved in current global atmosphere models. Instead, parameterizations have to be used to approximate their average effect on the finite volumes that are resolved in a global model. These parameterizations are often tailored to specific atmospheric conditions and fail when those conditions are not met. Here we propose a parameterization that aims to reproduce the average effect of turbulent heat and moisture transport under all atmospheric conditions. Numerical simulations demonstrate the accuracy of the parameterization in simulating turbulence in atmospheric boundary layers under stable and convective conditions, including the simulation of stratocumulus clouds.

show abstract

“…This is in agreement with Wu et al. (2020), where the authors show that the implementation of downdrafts in an EDMF scheme does not significantly improve simulations of the STBL.…”

Section: Resultssupporting

confidence: 92%

“…These biases compensate each other to some extent, leading to a small bias in the buoyancy flux. Similar biases are reported for models using the EDMF scheme and different parameterizations (Wu et al., 2020).…”

Section: Resultssupporting

confidence: 78%

A Generalized Mixing Length Closure for Eddy‐Diffusivity Mass‐Flux Schemes of Turbulence and Convection

Lopez‐Gomez

Cohen

et al. 2020

J Adv Model Earth Syst

View full text Add to dashboard Cite

show abstract

Section: Journal Of Advances In Modeling Earth Systemssupporting

confidence: 73%

A Generalized Mixing Length Closure for Eddy-Diffusivity Mass-Flux Schemes of Turbulence and Convection

Lopez‐Gomez

Cohen

et al. 2020

Preprint

View full text Add to dashboard Cite

Because of their limited spatial resolution, numerical weather prediction and climate models have to rely on parameterizations to represent atmospheric turbulence and convection. Historically, largely independent approaches have been used to represent boundary layer turbulence and convection, neglecting important interactions at the subgrid scale. Here we build on an eddy-diffusivity mass-flux (EDMF) scheme that represents all subgrid-scale mixing in a unified manner, partitioning subgrid-scale fluctuations into contributions from local diffusive mixing and coherent advective structures and allowing them to interact within a single framework. The EDMF scheme requires closures for the interaction between the turbulent environment and the plumes and for local mixing. A second-order equation for turbulence kinetic energy (TKE) provides one ingredient for the diffusive local mixing closure, leaving a mixing length to be parameterized. Here, we propose a new mixing length formulation, based on constraints derived from the TKE balance. It expresses local mixing in terms of the same physical processes in all regimes of boundary layer flow. The formulation is tested at a range of resolutions and across a wide range of boundary layer regimes, including a stably stratified boundary layer, a stratocumulus-topped marine boundary layer, and dry convection. Comparison with large eddy simulations (LES) shows that the EDMF scheme with this diffusive mixing parameterization accurately captures the structure of the boundary layer and clouds in all cases considered.Plain Language Summary Turbulence and convection transport heat and moisture in the atmosphere and are ultimately responsible for the formation of clouds. However, they act on scales far too small to be resolved in current global atmosphere models. Instead, parameterizations have to be used to approximate their average effect on the finite volumes that are resolved in a global model. These parameterizations are often tailored to specific atmospheric conditions and fail when those conditions are not met. Here we propose a parameterization that aims to reproduce the average effect of turbulent heat and moisture transport under all atmospheric conditions. Numerical simulations demonstrate the accuracy of the parameterization in simulating turbulence in atmospheric boundary layers under stable and convective conditions, including the simulation of stratocumulus clouds.

show abstract

“…The strong mixing by updrafts and downdrafts promotes the well‐mixed profiles that are characteristic of STBLs (Lilly, 1968). Their importance in non‐local mixing has led to the development of mass flux parameterizations with consideration of downdrafts to improve turbulent parameterizations in NWP models (Han & Bretherton, 2019; Wu et al., 2020). Recent studies have focused more closely on the identification of updrafts and downdrafts in the STBL (Brient et al., 2019; Chinita et al., 2018; Davini et al., 2017), finding that these structures are responsible for nearly 80% of the heat and moisture fluxes in the STBL (Brient et al., 2019).…”

Section: Introductionmentioning

confidence: 99%

Effects of Surface and Top Wind Shear on the Spatial Organization of Marine Stratocumulus‐Topped Boundary Layers

2021

Self Cite

View full text Add to dashboard Cite

The convective nature of Stratocumulus topped boundary layers (STBL) involves the motion of updrafts and downdrafts, driven by surface fluxes and radiative cooling, respectively. The balance between shear and buoyant forcings at the surface can determine the organization of updrafts between cellular and roll structures. We investigate the effect of varying shear at the surface and top of the STBL using Large Eddy Simulations, taking DYCOMS II RF01 as a base case. We focus on spatial identification of the following features: coherent updrafts and downdrafts, and observe how they are affected by varying shear. Stronger surface shear organizes the updrafts in rolls, causes less well‐mixed thermodynamic profiles, and decreases cloud fraction and liquid water path (LWP). Stronger top shear also decreases cloud fraction and LWP more than surface shear, by thinning the cloud from the top. Features with stronger top than surface shear are associated with a net downward momentum transport and show early signs of decoupling. Classifying updrafts and downdrafts based on their vertical span and horizontal size confirms the dominance of tall objects spanning the whole STBL. Tall objects occupy 30% of the volume in the STBL, while short ones occupy less than 1%. For updraft and downdraft fluxes, these tall objects explain 65% of the vertical velocity variance and 83% of the buoyancy flux, on average. Stronger top shear also weakens the contribution of downdrafts to the turbulent fluxes and tilts the otherwise vertical development of updrafts.

show abstract

On the Parameterization of Convective Downdrafts for Marine Stratocumulus Clouds

Cited by 18 publications

References 58 publications

A Generalized Mixing Length Closure for Eddy‐Diffusivity Mass‐Flux Schemes of Turbulence and Convection

A Generalized Mixing Length Closure for Eddy‐Diffusivity Mass‐Flux Schemes of Turbulence and Convection

A Generalized Mixing Length Closure for Eddy-Diffusivity Mass-Flux Schemes of Turbulence and Convection

Effects of Surface and Top Wind Shear on the Spatial Organization of Marine Stratocumulus‐Topped Boundary Layers

Contact Info

Product

Resources

About