Deep convection and the boundary layerbreth/GCSS/Arrowhead-200506/Randall.pdf · - Thermodyn pr...

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Deep convection and the boundary layer

Transcript of Deep convection and the boundary layerbreth/GCSS/Arrowhead-200506/Randall.pdf · - Thermodyn pr...

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Deep convection and the boundary layer

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Acknowledgements:

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Interactions of the PBL with deep convection

•This is one of the most important “jobs” of the PBL.

• It has been neglected by us modelers.

•We should be ashamed of ourselves.

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Still the one

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What AS Included What AS left out

Explicit PBL sub-model Downdrafts

CIN (PBL-top inversion, etc.)Bubble variability - Thermodyn properties - Diameter

Both shallow and deep convection Unmixedness

Surface-flux enhancement

Deep convection that starts above the PBL

Orographic effects

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Bubbles

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TRMM-LBA, morning

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PBL moist static energy

t= 4h t= 6h

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Bubble variability

Tokioka et al.

Minimumλ 1 / h

Do we need an additional dependence on precipitation?

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JJA Local time of precipitation frequency maximum

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Diurnal cycle of precip in the Amazon Basin

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But wait!What about AS?

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JAS, 1985, p.656

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JAS, 1991, p. 44

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Some inferences:

•A simple PBL model that is explicitly coupled to deep convection can give a realistic diurnal cycle of precipitation over both land and sea.

•Without such a model, the results are considerably worse.

•To go further, we may need to simulate the thermodynamic variances in the PBL, which are strongly affected by precipitation.

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Unification?

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Advection or diffusion?

Diffusive flux = −K ∂A∂z

These can transport not only averages, but higher moments.

Advective flux = ρwcA

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Penetrative convection advects.That’s how it penetrates.

Shallow convection both advects and diffuses.

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A unified model must both advect and diffuse.

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Randall, D. A., Q. Shao, and C.-H. Moeng 1992: A Second-Order Bulk Boundary-Layer Model. J. Atmos. Sci., 49, 1903-1923.

Lappen, C.-L, and D. A. Randall, 2001: Towards a unified parameterization of the boundary layer and moist convection. Part I. A new type of mass-flux model. J. Atmos. Sci., 58, 2021-2036.

•Mass flux methods can be married with higher-order closure.

•With this approach, the triple-moment terms of the second moment equations can either advect or diffuse, depending on the regime.

These papers show that:

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ASTEX

BOMEX

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LR did only shallow convection.

Variability within the cloud field

Both convective and broken but stratiform clouds

Simplified vertical structure -- explicit PBL top?

Precipitation, etc.

To do deep, we need:

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What AS Included What AS left out

Explicit PBL sub-model Downdrafts

CIN (PBL-top inversion, etc.)Bubble variability - Thermodyn properties - Diameter

Both shallow and deep convection Unmixedness

Surface-flux enhancement

Deep convection that starts above the PBL

Orographic effects