Upload
buck-watkins
View
219
Download
0
Embed Size (px)
Citation preview
Vertical Structure of the Tropical Troposphere(including the TTL)
Ian Folkins Department of Physics and Atmospheric Science
Dalhousie University
Deep Convection
ubiquitous shallow convection (cumulus congestus), ~ 28% of
rainfall during TOGA/COARE(Johnson et al., JAS, 1999)
Trimodal cloud top distribution (lidar obs):
Shallow
BoundaryLayer
Deep
Shallow
BoundaryLayer
Land Ocean
Mapes and Houze, JAS, 1995
Rawinsonde wind measurementsfrom the TOGA/COARE IFA when
deep convection present
Johnson and Cieslinski, 2000
Deep Outflow Layer
Inflow to feed downdrafts (mainly)
Clear Column: Radiative DescentCloudy Column
Convective Outflow can be estimated from clear skymass fluxes (radiative + evaporative).
evaporativemoistening
(downdrafts)
DeepOutflow
ShallowOutflow
Folkins and Martin, JAS, 2005
Mass Flux Mass Flux Divergence
Deep
shallow
----- ~ 1000 km ----
cooling heating
heating
Two Distinct Circulations?1)Tropical-scale Hadley/Walker circulation: deep condensational heating balances radiative cooling.
2) Regional scale downdraft/shallow convectioncirculations: shallow convective heating balancesEvaporative cooling.
radiativecooling
Shallow OutflowLayer
Deep OutflowLayer
Outflow Layers related to changes in stability
Ozone is rapidly destroyed in the tropical marine boundary layer.Deep convection pumps this low ozone air to higher altitudes.
Low O3
Ozone is chemically produced at a rate of 1-2 ppbv/day above 6 km in the background atmosphere
Low O3
Convective Tracers: O3 and COAt deep convective marine locations, there is an ozone
minimum near 12 km, probably associated with deep
convective outflow
Deep convective outflow maintainshigh CO mixing ratios till 15 km,
presumably the height at which theconvective replacement time is similar
To the chemical lifetime.
Tropical mean cloud mass flux and divergenceprofiles from 3 convective schemes[Emanuel, Zhang and McFarlane (GEOS-4),
Relaxed Arakawa Schubert (GEOS-3)]
Dry Mixing
dT = 2 CM = 1 kg
dT = 0 CM = 1 kg
dT = 1 CM = 2 kg
Entrainment of dry air reduces the buoyancy B of a risingair parcel, but has no effect on the buoyancy flux MB.
(MB = mass flux *buoyancy)
Mixing
Emanuel and Bister JAS, 1996
Moist Mixing
Dry Air
Evaporation ofcloud droplets:
Moist Mixing
More rapid decreasein buoyancy, and a
decrease in MBBuoyancy Reversal
higher condensateloading
Wei et al, JAS, 1998
PDF of Updraft Buoyancy (850 mb – 600 mb)
Moist mixing isvery effective atreducing updraft
buoyancies.(at least in the
lower troposphere.)
PDF of Downdraft Buoyancy (850 mb – 600 mb)
Wei et al, JAS, 1998
Physics of Moist Mixing(strongly damped buoyancy flux)
Shallow Convection
Physics of Dry Mixing(constant buoyancy flux)
Deep Convection
colder temperatures(higher altitudes)
Higher background RH(water vapor feedback)
reduced condensateloading (rapid rainout)
Brewer Dobson Circulation
TTL
What is the Tropical Tropopause?
What is the TTL?
Convective Outflow
HadleyCirculation
Brewer Dobson Circulation
Level of Mean Ascent
Top Level of Convective outflow
TTL
17.5 km
15.5 km
RH ~ 60%
RH ~ 80%
LZH: level of zeroradiative heating
TTL: uplift moistening;need dehydration mechanism
17 km
15.5 km
Subsidence Drying
RH > 100%
T~198 K
T~192 K
RH should increase as you approach the TTL from below
Detrainment Moistening
10 km
TTL
Aircraft measurements show high RH in the TTL
HarvardGroup
Positive heating rates at the cold point tropopause are due to LW heating from Ozone
Ozone has a seasonal cycleAt the tropical tropopause
(probably cause by a seasonalvariation in convective outflow)
Ozone and Water Vapor Budgets Coupled
Ozone high
Ozone affects seasonal cycle in radiative heating rates
good BDmass flux
good convectivemass fluxes+
good convectiveoutflow profile
good ozoneprofile
+
+
+chemistry
STE
good temperatureprofile
cloud radiativeeffects
good strat H2Oentry mixing ratio
good dehydrationmechanism
better climate/ozonedepletion forecasts
TTL“Virtuous
Circle”
Start Here
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, requiresconvective schemes which can accurately simulate theshape of the deep outflow layer.
3. There are significant variations in convective outflowbetween 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 dehydrationmechanism to prevent large scale supersaturation.
5. Ozone-Temperature coupling in the TTL