Processes controlling temperatures near the tropopause

1. Processes controlling temperatures near the tropopause Steve Sherwood (with thanks to: Alex Costa Takeshi Horinouchi Frank Robinson Heidi Zeleznik)

3. 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)

4. 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

5. 11 km limit to the moist adiabat Parcel model subsidence ---- 11 km Radiative subsidence Folkins, 2002

6. Interannual T variations at 200 hPa Minschwaner and Dessler 2004 (SSTc SST where OLR < 250 Wm-2. T includes 1-month lag.)

7. 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

8. At location of Tcold T p 

9. Sherwood et al. 2003

10. WRF simulations of near-tropopause entrainment and diabatic effects in deep convection x = 250m Kessler warm microphysics only No radiation CAPE ~ 2700 K/kg Initial tropical DJF sounding See also Kuang and Bretherton, In press. Substantial effects also likely in midlatude summer for several km above tropopause (e.g., Dessler and Sherwood, In press)

11. 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.

12. Observed trends WP region, ‘79-’97 Lanzante et al. 2003b (green=Tropics)

13. Can ozone explain TTL trends? Ozone concentration QRAD p Indonesia “Tropical” 20-day radiative  --> peak T of 5K!

14. 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.

15. Courtesy of T. Lane, NCAR

16. Entrainment zone? • If deeply convecting troposphere behaves like simpler convective layers, an entrainment zone will exist…. • Objective: Q1 near cold point…=0? <0?

17. Sherwood and Wahrlich, 1999

18. …cont’d

20. Courtesy of G. Kiladis

21. Drive wave model (Horinouchi & Yoden) w/idealized Q From Sherwood, Horinouchi and Zeleznik, in Press

22. Model Composite obs. bimodal Uni, shallow Uni, deep

23. Tropical T trend 1985-99

24. DJF Tcold Cb From Salby et al., In press