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