A two‐fluid single‐column model of turbulent shallow convection. Part III: Results and parameter sensitivity

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The two‐fluid single‐column model of Thuburn et al. (Quart. J. R. Meteorol. Soc., 2019, 145, 1535–1550) is extended to include moisture and horizontal wind shear. Turbulent kinetic energy is introduced as a prognostic variable, dependence on a diagnosed boundary‐layer height is removed, and subfilter fluxes are approximated using a two‐fluid version of a Mellor–Yamada scheme. Three mechanisms for entrainment and detrainment processes are introduced, which represent entrainment of unstable air at the surface, forced detrainment of air at the top of the boundary/cloud layers, and turbulent mixing that relaxes the convective fluid to a reference profile. A semi‐implicit Eulerian discretization replaces the semi‐implicit semi‐Lagrangian implementation of Thuburn et al. (Quart. J. R. Meteorol. Soc., 2019, 145, 1535–1550) to improve numerical stability and conservation. The equations for the implicit time step are solved using a quasi‐Newton method, which is shown to perform well in numerical tests for conservation and convergence. The two‐fluid single‐column model presented in this article will be applied to simulations of shallow cumulus convection in Part III.

Section: Mixing and Relaxation To Uniform Statementioning

confidence: 99%

“…The test cases discussed in Part III (McIntyre et al ., 2022) involve specified time series of surface sensible heat flux

{H}^{\mathrm{S}}

and latent heat flux

{H}^{\mathrm{L}}

. For use in our SCM, these are re‐expressed as surface fluxes of mass, water, and entropy, with

{H}^q={H}^{\mathrm{L}}/{L}_0

and

{L}_0=2.5\times 1{0}^6

.…”

Section: Parameterizationsmentioning

confidence: 99%

“…The model includes the option to use different

{b}_{ij}^{\mathrm{MIX}}

coefficients in the cloud layer (henceforth denoted by a different identifier,

{b}_{ij}^{\mathrm{MIC}}

{W}^{\mathrm{CLD}}=\frac{1}{2}\left\{1+\tanh \left[{f}^{\mathrm{CLD}}\left({q}_{\mathrm{l},2}-1{0}^{-5}\right)\right]\right\},

where

{f}^{\mathrm{CLD}}=2\times 1{0}^3

is a constant.…”

Section: Parameterizationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

A two‐fluid single‐column model of turbulent shallow convection. Part II: Single‐column model formulation and numerics

Thuburn

Efstathiou

McIntyre

2022

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“…The main developments, relative to the model described by Thuburn et al (2019), are the inclusion of moist processes and horizontal wind components, the prediction or diagnosis of various subfilter-scale turbulence quantities, which are used in the parameterization of several processes in the model, and improved numerical methods, making the model more accurate, stable, and robust. The new model formulation is detailed in Part 2 (Thuburn et al, 2022) and example results are presented in Part 3 (McIntyre et al, 2022). The present article derives the equations for subfilter-scale turbulent second moments in the multifluid framework, providing the basis for the turbulence component of the model as well as its possible future extensions and generalizations.…”

Section: Introductionmentioning

confidence: 99%

A two‐fluid single‐column model of turbulent shallow convection. Part 1: Turbulence equations in the multifluid framework

Thuburn

Efstathiou

McIntyre

2022

Self Cite

The multifluid equations are derived from the compressible Euler equations (or any of the usual approximate equation sets used in meteorology) by conditional filtering. They have the potential to provide the basis for an improved representation of cumulus convection and its coupling to the boundary layer and larger scale flow in numerical models. The present article derives the prognostic equations for subfilter-scale turbulent second moments in the multifluid framework, along with certain systematic simplifications of them, thus providing a multifluid analogue of the well-known Mellor and Yamada hierarchy of turbulence closures. As well as enabling a more accurate calculation of subfilter-scale fluxes and the effects of subfilter-scale variability on cloud fraction, liquid water, and buoyancy, the second moment information can be used to obtain a more accurate parameterization of entrainment and detrainment. A subset of the turbulence equations derived here is employed in the two-fluid single-column model described in Part 2 and applied to the simulation of shallow cumulus cases in Part 3.

The use of idealised experiments in testing a new convective parametrization: Performance of CoMorph‐A

Lavender,

Stirling,

Whitall

et al. 2024

CoMorph is a new mass‐flux convection parametrization under development at the Met Office designed for use within the Unified Model and its successor model, LFRic. Use of a three‐dimensional idealised model enables controlled tests of the performance of the scheme across different regimes. This includes the interaction between the physical parametrizations and the resolved dynamics, allowing study of the emergent organisation of convection on the resolved scale. A selection of well‐known cases is revisited here, with the purpose of documenting the extent to which CoMorph captures a range of important, but challenging, behaviour such as the diurnal cycle and sensitivity to tropospheric moisture. Simulations using CoMorph‐A, a new physics package, that has been demonstrated to perform well at numerical weather prediction (NWP) and climate scales, are compared against the current global atmosphere configuration and high‐resolution results. In addition to an entirely new convection scheme, the package of changes includes significant changes to the cloud, microphysics, and boundary‐layer parametrizations. Recognising that CoMorph‐A is the first version of a scheme that will continue to be substantially developed and to obtain good performance, compromises in tuning have had to be made. These idealised tests therefore show what works well in this configuration, and what areas will require further work. As such, it is quite a demanding testbed and could be viewed as some of the equipment required for a “convective playground”.