Convective Influence on the Heat Balance of the Tropical Tropopause Layer

Convective Influence on the Heat Balance of the Tropical Tropopause Layer

Chris Bretherton and Peter BlosseyDepartment of Atmospheric Sciences

University of Washington

Zhiming KuangHarvard University

The TTL is the transitional layer between the troposphere and the stratosphere

Mean soundings over Samoa

TTLcold point tropopause

mixing barrier or asecondary tropopause

Many processes may combine to affect TTL T, q, trace species

Holton

Is cold point T, q significantly affected by convection?

Maybe not…

Argument 1

Convective clouds

are rare in the TTL (Gettelman et al., JGR, 2002).

… but rare might be enough, for a slow TTL radiative response time.

Argument 2

A cold point tropopause solely in radiative equilibrium can exist well above the convectively adjusted region in a 1D model (Thuburn and Craig,

JGR, 2002)

…but is a 1D model good enough for this problem?

Convection top

Our approach: Use a cloud resolving model to examine TTL heat balance in radiative-convective equilibrium. Kuang and Bretherton 2004 JAS Similar approach, different result: Kuepper et al. 2004 JGR

• Colorado State University CRM (Khairoutdinov and Randall, JAS, 2003), anelastic, bulk microphysics, interactive radiation, 100 day run.

• Doubly periodic, 64km×64km domain, 1km horizontal grid.

• Uniform SST, zero wind shear, no diurnal cycle.

• Fixed ozone.• Init lower-strat water vapor = 4 ppmv.• Control simulation has <w> = 0.• Results robust for larger domain and

finer grid sizes.

Sponge layer

Heat Balance in the TTL

convective

Cold point tropopause level

radiative

Convectivecooling

Level of zero rad. heating

Cold point level

EntrainmentLayer

LNB

0

LNB

Δθ Convective Mass flux

ConvectionRadiation

Entrainment of warm TTL air by occasional overshooting tops

The radiative spring is weak in the TTL: rad = T/Qrad ~ 30 d.

How much effect does convective cooling have on the temperature of the TTL?

Convective cooling removed

With convective cooling

A convective event

BL tracer (12 h decay)

cloud ice

Domain averaged values 500m above the cold point

Long term behavior at the cold point tropopause

In perturbed simulations, convective cooling maximum is also collocated with the cold point in this model.

Filled: convective heating rate Open: radiative heating rate

SST+2K W+0.3mm/s

radiativedynamical

Cold point

Cold point

Convective impactnegligible (like Thuburn+Craig)

Why different?- Radiation?- Humidity?- Transport?…an open problem.

Mock-Walker 2D CRM simulation (Blossey et al. poster)

• Radiatively-forced subsidence above convection.• Occasional convective penetration to cold point but little conv. cooling

100-200 d means

Summary• In some, but not other, CRM simulations of radiative-

convective equilibrium, eddy cooling due to overshooting convection affects cold point height and temperature.

• High CRM vertical resolution (100 m) is necessary to avoid spurious gravity-wave damping in the TTL.

• Walker-circulation results suggest that radiatively-driven subsidence above convective anvils may be at least as important.

• We still need to develop a TTL cloud-resolving modeling framework that includes appropriate feedbacks between convection and large-scale circulations.