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Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward?

Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward?. Stefan Tulich 1 and George Kiladis 2 1 CIRES, University of Colorado, Boulder CO, USA 2 NOAA ESRL, Boulder CO, USA Funding: NSF ATM-0806553. Objectives.

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Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward?

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  1. Tropical squall lines as convectively coupled gravity waves: Why do most systems travel westward? Stefan Tulich1 and George Kiladis2 1CIRES, University of Colorado, Boulder CO, USA 2NOAA ESRL, Boulder CO, USA Funding: NSF ATM-0806553

  2. Objectives Provide evidence that many tropical “squall line systems” are part of a broad family of disturbances that arise through coupling between convection and tropospheric gravity waves Start to address the question of why most of these wave disturbances move westward

  3. Outline Brief historical review of tropical squall lines - how did we come to know about them; current state of knowledge Analysis of observational data - provide evidence to support the idea Explicit simulations of convection on an equatorial beta-plane - test hypothesis about what causes westward bias Conclusions and future work

  4. Historical Review of Tropical Squall Lines If one goes back to the earliest papers by leading authors, they’ll be pointed to two even earlier papers on west African squall lines

  5. West African “Disturbance Lines” Hamilton and Archibald (1945; QJRMS; No previous articles referenced!) Eldridge (1957; QJRMS; 2 articles referenced)

  6. West African “Disturbance Lines” Hamilton and Archibald (1945; QJRMS; No previous articles referenced!) Eldridge (1957; QJRMS; 2 articles referenced) 25 deg / 45 hr = 17 m/s

  7. The Thunderstorm Project (1947; USA) Newton (1950; J. Meteor.) “Structure and mechanisms of the prefrontal squall line”

  8. The Thunderstorm Project (1947; USA) Newton (1950; J. Meteor.) “Structure and mechanisms of the prefrontal squall line”

  9. The Line Islands Exp. (1967 Cntrl. Pac.) Zipser (1969; J. Appl. Meteor.) “The role of organized unsaturated downdrafts in the structure and decay of an equatorial disturbance” 15 m/s

  10. The Line Islands Exp. (1967 Cntrl. Pac.) Zipser (1969; J. Appl. Meteor.) “The role of organized unsaturated downdrafts in the structure and decay of an equatorial disturbance”

  11. GATE (1974; Eastern Atlantic) Several squall lines sampled as they passed across the IFA Barnes and Sieckman (1984; MWR) “The environment of fast- and slow-moving tropical mesoscale convective cloud lines”

  12. GATE (1974; Eastern Atlantic) A number of squall lines sampled as they passed across the IFA Barnes and Sieckman (1984; MWR) “The environment of fast- and slow-moving tropical mesoscale convective cloud lines” Vn > 7 m/s Vn < 3 m/s

  13. TOGA-COARE (1992; Eq. west Pac.) Similar to GATE but satellite data more accessible Linear MCS-scale bands dominate total rainfall Numerous fast-moving “2-day waves” were sampled

  14. TOGA-COARE (1992; Eq. west Pac.) 2-day wave composite evolution Haertel and Johnson (1998)

  15. TOGA-COARE (1992; Eq. west Pac.) 2-day wave composite evolution ~ 1500 km Haertel and Johnson (1998)

  16. TOGA-COARE (1992; Eq. west Pac.) 2-day wave composite evolution 16 m/s Haertel and Johnson (1998)

  17. TOGA-COARE (1992; Eq. west Pac.) 2-day wave vertical cloud evolution Takayabu et al. (1996)

  18. TOGA-COARE (1992; Eq. west Pac.) 2-day wave vertical cloud evolution Are 2-day waves just large-scale squall lines? Takayabu et al. (1996)

  19. TOGA-COARE (1992; Eq. west Pac.) 2-day wave vertical cloud evolution Are 2-day waves just large-scale squall lines? Or are squall-lines mini- versions of 2-day waves? Takayabu et al. (1996)

  20. Observational Analysis Goal: Advance the idea that many tropical squall line systems are part of a broader family of convectively coupled gravity wave disturbances Strategy: Space-time spectral (Fourier) analysis of high-resolution satellite data

  21. Space-time spectral analysis: Previous work Power Spectrum of OLR (symmetric component) 1.25 days 3 days 96 days -15 15 WestwardEastward Wheeler and Kiladis (1999)

  22. Space-time spectral analysis: Previous work Power Spectrum of OLR (symmetric component) Wheeler and Kiladis (1999)

  23. Space-time spectral analysis: Previous work Power Spectrum of OLR (symmetric component) Westward inertia-gravity waves (1.3-2.5 day) Kelvin waves (3-10 day) Eq. Rossby waves (6-50 day) Wheeler and Kiladis (1999)

  24. Spectral Analysis of TRMM TRMM 3B42 Rainfall Product 1) Global from 50N-50S 2) 0.25 deg. resolution in space 3) 3-hourly in time (1999-present) TRMM TMI CPC Global Merged IR

  25. Spectral Analysis of TRMM TRMM 3B42 Rainfall Product 1) Global from 50N-50S 2) 0.25 deg. resolution in space 3) 3-hourly in time (1999-present)

  26. TRMM rainfall spectrum 96 days 3 days 1.7 days

  27. Looking at smaller scales 96 days 12 hrs 1 day -80 80

  28. Looking at smaller scales 96 days 12 hrs 1 day Sharp diurnal peak -80 80

  29. Looking at smaller scales hn ~ 20-40 m 96 days 12 hrs 1 day Sharp diurnal peak -80 80

  30. Looking at smaller scales cn ~ 14-20 m/s 96 days 12 hrs 1 day Sharp diurnal peak -80 80

  31. Looking at even smaller scales 6 hrs 12 hrs 96 days

  32. Looking at even smaller scales 6 hrs 12 hrs 96 days ~ 6-hr periods & ~ 400-km wavelengths

  33. Where are these signals most active? 6 hrs 12 hrs 96 days “WIG” filter window

  34. Map of WIG-filtered variance (Boreal Summer JJA)

  35. Focus on N. Africa (JJA)

  36. Focus on N. Africa (JJA)

  37. Hovmollers of rainfall over N. Africa (7.5-12.5N) 2005 2006 2007

  38. Hovmollers of rain over N. Africa (7.5-12.5N) 2005 2006 2007

  39. How do these systems relate to objectively identified squall lines? AMMA 2006 Field Experiment (ROP: July 5 – Sept 27)

  40. Analysis of Niamey Radar Data Rickenbach et al. (2009; JGR) “Radar-observed squall line propagation…”

  41. Rain Hovmoller + Radar Identified Squall Lines

  42. Linear convective bands during TOGA COARE? Rickenbach and Rutledge (1998)

  43. Linear convective bands during TOGA COARE? Rickenbach and Rutledge (1998)

  44. Hovmoller of CLAUS Tb during TOGA COARE (Cruises 2 and 3)

  45. Hovmoller of CLAUS Tb during TOGA COARE (Cruises 2 and 3)

  46. Inclusion of EIG-filtered rainfall

  47. Inclusion of EIG-filtered rainfall

  48. What is the typical evolution of these disturbances? Strategy: Lagged linear regression of WIG-filtered rainfall to construct statistical composites

  49. Location of base point Base point (2E, 10N)

  50. Composite WIG rain evolution (2E,10N) Note: data averaged between 7.5-12.5 N

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