1. Using GPS data to study the tropical tropopause Bill Randel National Center for Atmospheric Research Boulder, Colorado “You can observe a lot by just watching” (Yogi Berra)

2. Overview GPS radio occultation temperature measurements GPS observations of the tropical tropopause : –low frequency variability (seasonal cycle, QBO) –large and small-scale waves

3. GPS Radio Occultation Occulting LEO Occulting GPS 20 msec data Ionosphere Neutral atmosphere Earth (LINK 1) Basic measurement principle: Deduce atmospheric properties based on precise measurement of phase delay and amplitude. * high vertical resolution! ~100 m

4. Availability of GPS data: GPS/MET (1995-1997) CHAMP (2001-present) SAC-C (2001-2002) COSMIC (launched April 2006) (6 satellites) number of tropical profiles per month (20 N – S) each LEO satellite ~ 100-200 occultations/day

5. Sample of GPS tropical temperature profiles Temperature profiles are characterized by high variability (planetary waves, gravity waves), closely linked to convection. GPS data offer a new tool to understand this variability.

6. Comparison of GPS with radiosondes very good agreement for wave structures

7. Tropical temp variability studied with GPS data Seasonal climatology and annual cycle Quasi-biennial oscillation Planetary-scale Kelvin waves Small-scale waves (inertia-gravity waves) references: Randel et al., JGR, 2004 Randel and Wu, JGR, 2005

8. Cold point tropopause temperatures deep convection NH winter climatology

9. Variability of tropopause temperature

10. Vertical structure at equator (NH winter) Indonesia Africa South America ‘top’ of convection note eastward tilt with height, characteristic of Kelvin wave TTL

11. high, cold tropopause over South Asian Monsoon NH summer climatology deep convection

12. Seasonal variation from GPS data Equator 18 km

13. Amplitude of annual cycle in temperature strong maximum just above the tropopause (~8 K ) Why? cold point

14. Amplitude of annual cycle in temperature strong maximum just above the tropopause (~8 K ) Why? thermodynamic balance small long radiative time scale in lower stratosphere hence, amplified T response

15. Quasi-biennial oscillation (QBO) in temperature contours: +/- 0.5, 1.5,... cold point result: QBO influence of ~ 0.5 K on tropical tropopause

16. Recent cooling of tropical tropopause echoed in stratospheric water vapor decreases stratospheric water vapor from HALOE satellite tropical tropopause temperatures r=.72 Randel et al, JGR, 2006

17. Space-time variability on daily time scales using CHAMP + SAC-C data Kelvin waves –identification –forcing by tropical deep convection Small scales (gravity waves) –coupling with background winds

18. Equatorial sampling of CHAMP and SAC-C

19. Simple gridding procedure

20. Kelvin waves near the tropopause eastward traveling Kelvin waves

21. Vertical structure tropopause eastward phase tilt with height characteristic of Kelvin waves

22. How are Kelvin waves linked to deep convection?

23. Variations in tropical convection from OLR measurements Nov Dec Jan Feb Mar

24. Correlation of waves with convection (OLR) wave variance at 16.5 km OLR near Indonesia

25. Global-scale Kelvin wave forced by convection note cold anomalies above convection, as part of large-scale wave structure modulation of cold point

26. Residual temperature variance (small scales)

27. Sample of GPS tropical temperature profiles note enhanced variability above ~15 km

28. Gravity waves observed by GPS/MET Tsuda et al., JGR, 2000 maximum in tropics (see Alexander et al.,JAS,2002)

29. Residual (small-scale) wave variance maximum near tropopause

30. Residual (small-scale) wave variance maximum just below u=0 line QBO winds

31. Gravity waves interacting with a critical level

32. Key points: GPS data allow high resolution view of ubiquitous wave variability near tropical tropopause. Kelvin waves (and smaller scales) strongly linked to tropical deep convection. Global-scale dynamical response in TTL, with cooling near tropopause over convection. Maximum wave variance near tropopause (why?). Waves are coupled to background winds (QBO) Future: COSMIC (6 more satellites) EQUARS (equatorial orbit)

33. Future: COSMIC + EQUARS Soundings in a Day COSMIC EQUARS Radiosondes

34. high, cold tropopause over South Asian Monsoon NH summer climatology deep convection

35. Circulation of the South Asian summer monsoon deep convection monsoon circulation near 15 km - - - cross section cold lower stratosphere high, cold tropopause warm troposphere winds tropopause

36. 16 km clouds from HIRDLS 100 hPa relative humidity from MLS Persistent high clouds over monsoon region

37. observed 100 hPa circulation (zonal mean removed) response to low frequency tropical heating (Gill, 1980)

38. Persistent cirrus clouds over monsoon region (HIRDLS measurements) cold tropopause

39. Correlation of GPS temps and OLR near Indonesia convection varies over this region easterly winds in lower stratosphere tropopause TTL

40. convection varies over this region westerly winds in lower stratosphere (waves do not propagate vertically) Correlation of GPS temps and OLR near Indonesia tropopause TTL

41. Model simulation of gravity waves forced by deep convection Alexander and Holton, 2000

42. Gravity waves interacting with a critical level critical level

43. Comparison of near-coincident CHAMP and SAC-C retrievals Hajj et al., JGR, 2004 mean bias std. dev. ~ uncertainty of single measurement

44. from Pan et al., JGR, 2004 zonal wind (from analysis) tropopause from aircraft profiler measurements potential vorticity (from analysis) double tropopause associated with break near subtropical jet poleequator

45. Understanding the Tropical Tropopause Layer (TTL) Gettelman and Forster, 2002

46. Using GPS data to study the tropical tropopause Bill Randel, NCAR

47. Vertical section through anticyclone (60-120 E) warm troposphere cold lower stratosphere tropopause deep convection

48. zonal winds Background: stratospheric QBO temperatures CHAMP + SACC

49. Seasonal variation from GPS/MET data