Critical Processes in the Tropical Tropopause Layer

Andrew Gettelman, NCAR+ Q. Fu, P. Forster, W. J. Randel

11km, Tropical Atlantic (20N, 65W), August

TTL Importance

• TTL sets chemistry of lower stratosphere– Stratospheric H2O set in TTL

– ‘Short lived’ species set in TTL (Bromine)

– Aerosols & precursors set in TTL (Sulfur)

• Radiation from TTL clouds affects climate

• Changes to TTL over time affect climate

Outline

• Definition of the TTL• Key Processes• Process Interactions & Variability

• Conclusions: how air and water vapor enter the stratosphere

A Transition LayerAbove 12-14km

-O3 Increases

-Lapse Rate Change

Trop at 17km

Folkins et al, 1999, Fig 1

TTL & Convective OutflowLapse Rate (T) Ozone

Folkins, JAS, 2001

Definition of the TTL

Schematic

Gettelman & Forster, JMSJ 2002

Key TTL Processes

RadiationRadiation

Large Scale TransportLarge Scale Transport

Tropical WavesTropical Waves

Cloud MicrophysicsCloud Microphysics

ConvectionConvection

OHOH BrBr SOSO22 OO33PANPAN

ChemistryChemistry

HNOHNO33

ConvectionConvection

Monsoon Convection, Ganges Valley, IndiaMonsoon Convection, Ganges Valley, India

Convection (2)Convection (2)

Amazon Basin, BrazilAmazon Basin, Brazil

Clouds above the Tropopause(also see Rossow talk)

Gettelman, Sassi & Salby JGR 2002

Convective Mass Fluxes

Küpper et al, 2004

COO3

CloudsRadiationCloud ModelECMWF (BD)

JanJul

X

*

Model Upwelling

Impact of convection(also see Bretherton talk)

A cloud resolving model indicates convection matters for temperature (due to long radiative timescales)

Result depends on the circulation

Kuang and Bretherton, 2004

Base Case

SST+2

Backgroundcooling

Non-local effect of ConvectionNon-Uniform Zonal Mean

Equatorial Heating T Response (K)

Norton, JAS, 2005

Why: Tropical (Kelvin) wave response to latent heat (convection)

Radiation Balance of TTL

Gettelman et al 2004, JGR

LWSWNet

Qclear=0 (LZH): 15km, 125hPa,

200ºK, 360K () ±400m (1)Diff Radiation Models: ±300m, Diff Season/Location: ±500mDiurnal cycle: 1-2 km

(lower for low SZA)

Important for background atmosAlso Fixed Anvil Temp hypothesis(see Hartmann poster)

Gas Contributions

TTL

Notes: CO2 heating is actually critical for TTL!O3 heating important from 17km up

Net Cloud ImpactTropical mean Heating Rates

Corti et al 2005, GRL

Full Sky all ISCCPFull Sky ISCCP meanClear Sky Mean BalloonClear Sky mean H2O

Absorption in clouds dominates if no cloud below: causes heating

Clouds Lower LZH

Convection Tb < 200K

Gettelman, Salby & Sassi, JGR 2002

HALOE H2OConvection Frequency (0.5, 1, 5, 10%)TropopauseECMWF Temperatures (Shading)

Large Scale TTL Transport

Randel et al 2001, fig 6

TTL Trajectory Transport(also see Haynes talk)

Analytical microphysical model on trajectories Reproduces seasonal, interannual UTH variations(Gettelman et al 2002; Dessler & Sherwood, 2000; Fueglistaler et al 2004)

Observations Model

Microphysics in the TTL

Cloud formation important:• Clouds are important for Climate• Chemical impact of clouds

• Clouds control condensation of H2O

1. Clouds do not form at RH=100%2. Microphysics affect cloud

formation

TTL SupersaturationPDF from Aircraft & Satellites: E. Pac, Jan 2004

Gettelman et al, J. Climate, in press, 2006

H2O Sensitivity to Sice

Sice can change 100hPa H2O by 30% (Gettelman et al 2002, GRL)

SSiceice

Analytic Model: Change in 100hPa H2O

Waves Tropical waves affect T & H2O

• Waves affect minimum T (& H2O)

• Many scales of variability• Examples:

– Equatorial Kelvin waves– Gravity Waves from Convection

Tropical Waves, Clouds & T

Boehm & Verlinde, 2000 GRL2000, GRL

Cloud Lidar

Temp variations (- +)

Sensitivity of H2O to Waves

Ratchet Effect: Increasing Temp Variance (t)

Decreases H2O in TTL (Gettelman et al 2002, GRL)

Microphysical model: waves change 100hPa H2O by 25%

Chemistry in the TTL(also see Lawrence Talk)

• Chemistry affects aerosols– Aerosols affect cloud microphysics and H2O vapor

• TTL clouds and H2O affect chemistry– UTLS H2O affects Ozone (through HOx)

• TTL Ozone affected by short lived species– Bromine may affect Ozone in TTL– NOx & short lived compounds from surface affect O3

• Cirrus may contain Nitric Acid – Is this like polar ozone loss? Not really, but might affect cirrus cloud formation

• TTL Ozone is important for heating rates

TTL Summary (1)• TTL transition between Strat-Trop

– Many definitions, Thermodynamic one convenient– Trends in the TTL exist!

• Convection present up to cold point– Some into stratosphere, key is convection above Qclear=0

• Radiative heating above Qclear=0 (15km)

• Transport after convective lofting, mixing

• 4-D circulation plays a big role – Monsoon circulations important (bypasses deep tropics)

TTL Summary (2)• Microphysics can be important

– Supersaturation strongly affects water vapor

– Also impacts radiation, short lived species

• Tropical waves– Wave fluctuations dehydrate in ‘Ratchet effect’ (GW)

– Coherent wave structures (Kelvin waves, MJO)

• Chemistry– May affect microphysics and H2O (HNO3)– Short lived species processed in TTL affect L Strat O3

Sorting out TTL Processes

• Use coupling between processes – Transport, Condensation/Microphysics

• Use natural modes of variability and observed changes to sort out processes

• Annual Cycle, [ENSO, QBO], interannual

• Do global models resolve the TTL?

HALOE H2OConvection Frequency (0.5, 1, 5, 10%)TropopauseECMWF Temperatures (Shading)

Interactions: TTL H2O & Clouds

Randel et al 2001, fig 6

TTL Interannual Variations

82 hPaWater vaporanomalies

Tropopausetemperatureanomalies(radiosondesand ERA40)

Randel et al, 2004

Trajectory Models

[H2O]e Correlated with T along trajectories

Models Can Simulate TTL

WACCM3 coupled model1km vertical resolution~6 levels in TTLGets basic relations right

See poster for details!

CP & LR Tropopause

Qclear=0

Min O3

Min

Simulated Tape RecorderMOZART3 H2O (ppmv)UARS / HALOE

Randel, et al., JGR, 106, 14313, 2001 MATCH CCM3.6 column physics

Park et al., 2003, JGR

How Does Air Enter Strat?

• Infrequent convection up to the cold point

• Radiative heating above Qclear=0 (15km)• Large scale transport after convective lofting, mixing– Key is convection above Qclear=0

• 4-D circulation plays a big role – Monsoon circulations important– Some air may bypass tropical tropopause

• Tropical wave mixing/forcing (GW, Kelvin)

Schematic: Air Motion in TTL

Schematic

Gettelman & Forster, JMSJ 2002

How Does H2O Enter Strat?

• H2O tied to TTL & cold point temperaturesAnnual -> ENSO -> Interannual Changes in transport play a roleMethane partial cause of long term trend

• Cloud microphysics/waves change H2O– Changes to IN (aerosol), nucleation, RH

– Changes in chemistry– Changes to tropical wave spectra

What Controls TTL Temps?• Convection

– Direct input of low e air

• Wave driven (Brewer-Dobson) circulation• Radiative effects of convection, clouds

– Long radiative damping time (low absorption)

– Increases importance of forcing (bigger response)

• Dynamical responses to Waves & Clouds– Non-local equatorial wave response to convection

– Gravity wave ratchet effect

The Last Word• TTL definable region

– Lapse rate minimum to cold point tropopause

• TTL exists due to balance of processes– Convection (lower bound)– Strat Circ, Waves (upper bound)– Radiation important in the region

• H2O in LS governed (0 order) by temp– Transport, Waves, microphysics important– Convection helps set temps (response to heating)

• Expect modest changes in TTL over time– Due to radiation changes. Convective changes uncertain

• Global models can simulate this region pretty well

Conceptual Picture(also see Sherwood talk)

Convective Dehydration

“Cold Trap” DehydrationSherwood & Dessler,

GRL, 2000; JAS, 2001Holton & Gettelman, GRL, 2001; GRL, 2002

Cold Pool

18km, 420K

14km, 355K

Tropopause

Stratosphere

Troposphere