diabatic tropics intro - climate change research centre · pdf filetropics - an introduction...
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Diabatic processes in the tropics - An introduction
Christian Jakob School of Earth, Atmosphere and Environment, Monash University
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The Tropics
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The Tropics
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The Tropics
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The Tropics
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The Tropics
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What do we mean by diabatic processes?
Let’s define the dry static energy as:s = cpT +gz
We can then rewrite the first law of thermodynamics as:dsdt
= Q
Combining this equation with the continuity equation yields the flux form as:
∂(rs)∂t
+— · (r~vs) = rQ
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We write all variables as spatial averages and deviations from that average, e.g.,
Substituting in the flux form equation, neglecting density variations and taking the average again yields:
s = s+ s0
∂(rs)∂t
= �∂(rus)∂x
� ∂(rvs)∂y
� ∂(rws)∂z
� ∂(ru
0s
0)∂x
� ∂(rv
0s
0)∂y
� ∂(rw
0s
0)∂z
+rQ
resolved terms small-scale horizontal fluxes
small-scale vertical
fluxsources
Average over an area (say 100 km or so)
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The small-scale horizontal fluxes are small compared to the vertical flux and usually neglected. This leaves us with:
small-scale vertical flux divergence
sources
�∂(rw0s0)∂z
and+rQ = +r(Qrad +C +E)
radiation evaporationcondensation
Similar small-scale flux terms appear in the moisture and momentum equations.
The “diabatic” terms
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Tropical convection
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What do diabatic processes do?
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Radiation
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Turbulence
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PBL Turbulence
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Convection
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In convective regions these terms will be dominated by convection
Large-scale effects of convection - Q1 and Q2
Averaged thermodynamic equation (in dry static energy)
“large-scale” terms “small-scale” terms
∂s∂t
+~vh—s+w∂s∂z
= Qrad +L(c� e)� 1r
∂(rw0s0)∂z
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Large-scale effects of convection - Q1 and Q2
This quantity can be derived from observations of the “large-scale” terms on the l.h.s. of the area-averaged equations and describe the influence of the “sub-grid”
processes on the atmosphere.
Define:
Apparent heat source
Analogous:
Apparent moisture sink
Note that:
with Moist static energy
Q1 = Qrad +L(c� e)� 1r
∂(rw0s0)∂z
Q1�Q2�Qrad = �1r
∂(rw0h0)∂z
Q2 = L(c − e)+Lρ∂ ρ ′w ′q( )
∂z
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Q1 and Q2 examples
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Heating and the tropical circulation
Response to idealised heating on the equator in a simple model
Gill, 1980
w and horizontal wind
Streamfunction
SLP and Wind
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Stratiform processes
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Organised convection
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Organised convection
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Convective vs stratiform processes
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Equilibria
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Shallow convectionThermodynamic structure of the trade-cumulus boundary layer
Emanuel, 1994
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Shallow convection
Basic physics of the trade-cumulus boundary layer
Emanuel, 1994
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Convective quasi-equilibrium
GARP Atlantic Tropical Experiment (1974)
Thompson et al., JAS, 1979
Wave composite vertical motion Wave composite temperature anomaly
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Convective quasi-equilibrium
GARP Atlantic Tropical Experiment (1974)
Thompson et al., JAS, 1979
v (700 hPa)
−ω (700 hPa)
Precipitation
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The importance of organised convection
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Location of convective and stratiform precipitation
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Location of organised vs “pop-corn” convection
Organised
Not organised
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Contribution to rainfall
Organised Not organised All
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Contribution to rainfall trendsTotal trend
(1998-2009)
Frequency of org. convection
Intensity of org. convection
All other convection
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Thank you