Continuous treatment of convection: from dry thermals to deep precipitating convection

J.F. GuérémyCNRM/GMGEC

The association of turbulence and convection schemes have to represent sub-grid convective processes with and without condensation,

the turbulence scheme (K-diffusion) dealing with horizontally quasi-homogeneous processes (having a rather weak vertical extension),

and the convection scheme (mass-flux) dealing with horizontally heterogeneous processes (having a larger vertical extension).

Aim: to go beyond version 4 of ARPEGE-Climat, trying to achieve a better association of the two schemes, and improving them at the same time.

Process representation:

The turbulence scheme - represents turbulence with and without condensation until shallow convection, taking into account an overestimated PBL mixing length.The convection scheme - represents convection with condensation and precipitations (all condensed water), while cancelling the sub-grid transport produced by the turbulence scheme.

The turbulence scheme - represents turbulence with and without condensation until very shallow convection (cumulus humilissimus, 1 layer).The convection scheme - represents convection with and without condensation (PBL dry thermals), precipitating or not.The time tendencies of both schemes are added.

Version 4: Version 4+:

- Version 4:

- Turbulence: Ricard-Royer 1993, TKE (production= dissipation, Mellor-Yamada 1982), turbulent sub-grid scale cloud scheme (Deardorff, Mellor 1977), without prognostic condensate; precipitations (Smith 1990).- Convection: Bougeault 1985, mass flux.

- Version 4 +:

- Turbulence: Ricard-Royer 1993 modified by Guérémy-Grenier 2005 (mixing length and top PBL entrainment), TKE (production= dissipation, Mellor-Yamada 1982), turbulent sub-grid scale cloud scheme (Deardorff, Mellor 1977), with prognostic condensate; precipitations (Smith 1990).- Convection: Guérémy 2005, mass flux; precipitations (Smith 1990).

Schemes:

- Version 5:

Process representation:Idem version 4+

Schemes:- Turbulence: Cuxart-Bougeault-Redelsperger 2000, pronostic TKE, turbulent sub-grid scale cloud sheme (Deardorff, Mellor 1977), with prognostic condensate; precipitations (Lopez 2002).- Convection: Guérémy 2005, mass flux; precipitations (Smith 1990).

Convection scheme

Key elements for a continuous treatment of convection: compensating subsidence term and detrainment term, M mass flux

- Cloud Profile: Dry adiabat until the lifting condensation level, then moist adiabat, including entrainment process.

- Mass flux formulation: Product of the grid fraction affected by convective ascents (equal to the bottom quantity -to be determined by the closure condition- times a height decreasing function -computed from the convective cloud mass budget-) by the convective vertical velocity -prognostic equation-.

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- Entrainment and detrainment :Organised entrainment and detrainment: Internal computation from

the convective cloud mass budget,

including a statistical model based upon the concept of buoyancy sorting.

Turbulent entrainment and detrainment: Analytical profile depending on the convective vertical velocity (large entrainment for a weak ascent and vice versa).

- Closure condition: CAPE relaxation to zero, according to a characteristic time proportional to the ratio of the convective depth to the mean convective vertical velocity.

- Convective precipitation: Precipitation is computed with Smith’s scheme (such as the stratiform precipitation)

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Results

Validation strategy:

Validation starts with 1D simulations of different types of convective situations corresponding to well documented cases (observations and explicit simulations), in order to represent processes at best possible. [EUROCS strategy]

The tuned schemes are then assessed in 3D (annual cycles), giving possibly rise to a new set of tuned parameters; this new version is finally tested in 1D to close the cycle.

Bomex Case: Non precipitating shallow convection

V4 V4+

V4 V4+

Cloudiness 7h-16h Mass flux 7h-16h

Q1 7h-16h Q2 7h-16h

Theta 16h Humidity 16h

Entrainment-detrainment 7h-16h

Idealised ARM case: diurnal cycle of continental convection: from dry PBL to deep precipitating

convection

Q1 and Q2 averaged above the PBL (between 800 and 100 hPa)

Prospects

RICO 1D case, notably with ARPEGE-Climat Version5 physics

Validation in 3D LAM (ALADIN-Climat) on documented cases (observations and CRM simulations), as an intermediate step between traditional 1D and 3D assessments.

Intensifying 3D global validation (transects, coupled simulation, seasonal forecasts, …)