Transport and Chemistry in the TTL

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Vertical Structure of the Tropical
Troposphere (including the TTL)
Ian Folkins Department of Physics and
Atmospheric Science Dalhousie University
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ubiquitous shallow convection (cumulus
congestus), 28 of rainfall during
TOGA/COARE (Johnson et al., JAS, 1999)
Deep Convection
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Trimodal cloud top distribution (lidar obs)
Land
Ocean
Deep
Shallow
Shallow
Boundary Layer
Boundary Layer
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Mapes and Houze, JAS, 1995
Deep Outflow Layer
Inflow to feed downdrafts (mainly)
Johnson and Cieslinski, 2000
Rawinsonde wind measurements from the TOGA/COARE
IFA when deep convection present
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Convective Outflow can be estimated from clear
sky mass fluxes (radiative evaporative).
Clear Column Radiative Descent
Cloudy Column
Deep Outflow
evaporative moistening (downdrafts)
Shallow Outflow
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Mass Flux Divergence
Mass Flux
Deep
shallow
Folkins and Martin, JAS, 2005
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Two Distinct Circulations?

• Tropical-scale Hadley/Walker circulation deep
• condensational heating balances radiative cooling.

2) Regional scale downdraft/shallow
convection circulations shallow convective
heating balances Evaporative cooling.
heating
radiative cooling
cooling
heating
?----- 1000 km ----?

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Outflow Layers related to changes in stability
Deep Outflow Layer
Shallow Outflow Layer
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Ozone is rapidly destroyed in the tropical marine
boundary layer. Deep convection pumps this low
ozone air to higher altitudes.
Ozone is chemically produced at a rate of 1-2
ppbv/day above 6 km in the background atmosphere
Low O3
Low O3
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Convective Tracers O3 and CO
At deep convective marine locations, there is an
ozone minimum near 12 km, probably associated
with deep convective outflow
Deep convective outflow maintains high CO mixing
ratios till 15 km, presumably the height at which
the convective replacement time is similar To the
chemical lifetime.
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Tropical mean cloud mass flux and
divergence profiles from 3 convective schemes
Emanuel, Zhang and McFarlane (GEOS-4), Relaxed
Arakawa Schubert (GEOS-3)
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Dry Mixing
Emanuel and Bister JAS, 1996
dT 1 C M 2 kg
dT 0 C M 1 kg
dT 2 C M 1 kg
Mixing
Entrainment of dry air reduces the buoyancy B of
a rising air parcel, but has no effect on the
buoyancy flux MB. (MB mass flux buoyancy)
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Moist Mixing
Moist Mixing
Evaporation of cloud droplets
Dry Air
More rapid decrease in buoyancy, and a decrease
in MB
Buoyancy Reversal
higher condensate loading
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PDF of Updraft Buoyancy (850 mb 600 mb)
Moist mixing is very effective at reducing
updraft buoyancies. (at least in the lower
troposphere.)
Wei et al, JAS, 1998
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PDF of Downdraft Buoyancy (850 mb 600 mb)
Wei et al, JAS, 1998
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Physics of Dry Mixing (constant buoyancy
flux) Deep Convection
colder temperatures (higher altitudes)
reduced condensate loading (rapid rainout)
Higher background RH (water vapor feedback)
Physics of Moist Mixing (strongly damped buoyancy
flux) Shallow Convection
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Brewer Dobson Circulation
TTL
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What is the Tropical Tropopause?
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What is the TTL?
Brewer Dobson Circulation
Convective Outflow
Top Level of Convective outflow
17.5 km
TTL
Level of Mean Ascent
15.5 km
Hadley Circulation
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17 km
T192 K
TTL uplift moistening need dehydration mechanism
RH gt 100
15.5 km
T198 K
LZH level of zero radiative heating
RH 80
Detrainment Moistening
RH 60
10 km
Subsidence Drying
RH should increase as you approach the TTL from
below
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Aircraft measurements show high RH in the TTL
TTL
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Harvard Group
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Positive heating rates at the cold point
tropopause are due to LW heating from Ozone
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Ozone has a seasonal cycle At the tropical
tropopause (probably cause by a
seasonal variation in convective outflow)
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Ozone affects seasonal cycle in radiative heating
rates
Ozone high
Ozone and Water Vapor Budgets Coupled
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Start Here
good BD mass flux
good convective mass fluxes
good convective outflow profile

chemistry STE

TTL Virtuous Circle
better climate/ozone depletion forecasts
good ozone profile

good strat H2O entry mixing ratio
cloud radiative effects
good temperature profile

good dehydration mechanism
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Summary
1. Moist convection has a rich vertical
structure. 2. Accurate modelling of the cold
point temperature, and of chemical species
profiles in the TTL, requires convective schemes
which can accurately simulate the shape of the
deep outflow layer. 3. There are significant
variations in convective outflow between
convective parameterizations (at least when run
in assimilated modes). 4. The water vapor
budget of the TTL is unique it appears to
require an in situ irreversible
dehydration mechanism to prevent large scale
supersaturation. 5. Ozone-Temperature coupling
in the TTL