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Using GPS data to study the tropical tropopause

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). Overview. GPS radio occultation temperature measurements GPS observations of the tropical tropopause :

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Using GPS data to study the tropical tropopause

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  1. Using GPS data to studythe 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. Occulting GPS 20 msec data (LINK 1) Ionosphere Neutral atmosphere Occulting LEO Earth GPS Radio Occultation 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) each LEO satellite ~ 100-200 occultations/day number of tropical profiles per month (20 N – S)

  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 NH winter climatology deep convection

  9. Variability of tropopause temperature

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

  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 instratospheric 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 convectionfrom OLRmeasurements 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 maximum in tropics (see Alexander et al.,JAS,2002) Tsuda et al., JGR, 2000

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

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

  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 cold lower stratosphere high, cold tropopause - - - cross section monsoon circulation near 15 km winds tropopause deep convection warm troposphere

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

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

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

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

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

  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 mean bias std. dev. ~ uncertainty of single measurement Hajj et al., JGR, 2004

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

  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) cold lower stratosphere tropopause warm troposphere deep convection

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

  49. Seasonal variation from GPS/MET data

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