Processes controlling temperatures near the tropopause Steve Sherwood (with thanks to: Alex Costa Takeshi Horinouchi Frank Robinson Heidi Zeleznik)
Jan 02, 2016
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