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