Processes controlling temperatures near the tropopause

Processes controlling temperatures near the

tropopause

Steve Sherwood (with thanks to:Alex Costa

Takeshi HorinouchiFrank RobinsonHeidi Zeleznik)

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Theories of cold point / tropopause

• Basic Theory: stratosphere = radiative equilibrium; intersection of dynamic and radiative profile (e.g. Held, 1982).

• Stratospheric theory: Cold point formation independent of convection (e.g. Kirk-Davidoff et al. 1999; Thuburn and Craig 2002). (1-D RCE models)

Observations

• In tropics, T deviates from adiabat starting ~11 km; Tcold (cold point) is ~16-17 km.

• cold point matches sea surface es-- mean and seasonal variations (Reid and Gage 1981)

• Budget near CP not closed without convection (Sherwood 2000)

• 100 hPa trends match those in troposphere

11 km limit to the moist adiabat

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Folkins, 2002

Radiativesubsidence

Parcel modelsubsidence

---- 11 km

Interannual T variations at 200 hPa

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(SSTc SST where OLR < 250 Wm-2. T includes 1-month lag.)

Minschwaner and Dessler 2004

Cold-point tracking

• Assume cold point stays on same material surface during convection/lifting event* (for null hypothesis = no convection reaches it)

• Track cold point through life cycle

* tests confirm this should hold.

Sherwood et al. 2003

At location of Tcold

p T

Sherwood et al. 2003

WRF simulations of near-tropopause entrainment and diabatic effects in deep convection

x = 250mKessler warm microphysics onlyNo radiationCAPE ~ 2700 K/kgInitial tropical DJF sounding

Substantial effects also likely in midlatude summer for several km above tropopause (e.g., Dessler and Sherwood, In press)

See also Kuang and Bretherton, In press.

CRM-simulated climate equilibria

Change in temperature profile with doubling of CCN concenration, fixed SST. (see also Grabowski, 2003).

GCM parameterizations cannot represent this effect.

Observed trends

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Lanzante et al.2003b (green=Tropics)

WP region, ‘79-’97

Can ozone explain TTL trends?

p

Ozone concentration QRAD

Indonesia

“Tropical”

20-day radiative --> peak T of 5K!

Conclusions

• Convection appears to cool significantly through the cold point in the tropics, and probably also in midlatitudes several km above tropopause.

• This cooling/mixing will blur radiatively induced temperature change across the tropopause, as observed.

• However, ozone trends may also produce cooling at the tropical tropopause.

• GCM convective schemes may be overzealous in clamping T(z) to an adiabat, but may also cut off convective effects too abruptly near the tropopause.

Courtesy of T. Lane, NCAR

Entrainment zone?

• If deeply convecting troposphere behaves like simpler convective layers, an entrainment zone will exist….

• Objective: Q1 near cold point…=0? <0?

Sherwood and Wahrlich, 1999

…cont’d

Courtesy ofG. Kiladis

Drive wave model (Horinouchi & Yoden) w/idealized Q

From Sherwood, Horinouchi and Zeleznik, in Press

Composite obs.Model

bimodal Uni, shallow Uni, deep

Tropical T trend 1985-99

From Salby et al., In press

Cb

Tcold

DJF