1. 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

2. Objectives 1)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 2)Start to address the question of why most of these wave disturbances move westward

3. Outline 1)Brief historical review of tropical squall lines - how did we come to know about them; current state of knowledge 2)Analysis of observational data - provide evidence to support the idea 3)Explicit simulations of convection on an equatorial beta-plane - test hypothesis about what causes westward bias 4)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” V n > 7 m/sV n < 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 Haertel and Johnson (1998) ~ 1500 km

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

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

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

19. TOGA-COARE (1992; Eq. west Pac.) Takayabu et al. (1996) 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?

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 Wheeler and Kiladis (1999) Power Spectrum of OLR (symmetric component) Westward Eastward 96 days 3 days -1515 1.25 days

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

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

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

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

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

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

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

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

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

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

35. 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)

46. 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

51. Composite WIG rain evolution (2E,10N) 18 m/s Note: data averaged between 7.5-12.5 N

52. Composite WIG rain evolution (2E,10N) 18 m/s ~2 day period Note: data averaged between 7.5-12.5 N

53. Composite WIG rain evolution Plan views at lags: -12,0,12 hr +12 hr 0 hr -12 hr

54. Comparison to the west Pac.

55. Composite WIG wave evolution (155E, 5N) Note: data averaged between 2.5-7.5 N

56. Composite WIG wave evolution (155E, 5N) 18 m/s Note: data averaged between 2.5-7.5 N

57. Composite WIG wave evolution (155E, 5N) 18 m/s ~2 day period Note: data averaged between 2.5-7.5 N

58. Side by side comparison West Pacific West Africa

59. Side by side comparison West Pacific West Africa

60. Side by side comparison West Pacific West Africa

61. Side by side comparison (Plan view at lag 0) West Pacific West Africa

62. Side by side comparison (Plan view at lag 0) West Pacific West Africa

63. Side by side comparison (Plan view at lag 0) West Pacific West Africa

64. Oceanic WIG waves as traveling “V”s or “U”s West Pacific Takayabu (1994)

65. Oceanic WIG waves as traveling “V”s or “U”s West Pacific Takayabu (1994)

66. And squall lines too! West Pacific Zipser (1969)

67. Conclusions thus far Tropical squall line systems and linear MCSs appear to be associated (if not synonymous) with convectively coupled gravity wave disturbances Westward-moving waves dominate, especially over Africa

68. Idealized numerical experiment Explicit, nested simulations of convection on an equatorial beta-plane Two types of runs: 1)Zonal-mean u-wind relaxed to zero 2)Zonal-mean u-wind relaxed to shear profile

69. Idealized numerical experiment Explicit, nested simulations of convection on an equatorial beta-plane Two types of runs: 1)Zonal-mean u-wind relaxed to zero 2)Zonal-mean u-wind relaxed to shear profile

70. Idealized numerical experiment Explicit, nested simulations of convection on an equatorial beta-plane Two types of runs: 1)Zonal-mean u-wind relaxed to zero 2)Zonal-mean u-wind relaxed to shear profile

71. Further details Model: WRF (most recent version) Forcing: Spatially uniform radiative-like cooling to drive deep convection SST: Zonally uniform; peaked at eq.

72. Further details Model: WRF (most recent version) Forcing: Spatially uniform radiative-like cooling to drive deep convection SST: Zonally uniform; peaked at eq.

73. Nesting strategy: 3 grids dx, dy = 27 km 8000 km 9900 km Equator 45 N 45 S Grid 1

74. Nesting strategy: 3 grids dx, dy = 27 km 8000 km 9900 km Equator 45 N 45 S Periodic Grid 1

75. Nesting strategy: 3 grids dx, dy = 27 km 8000 km 9900 km Equator 45 N 45 S Periodic Rigid wall Grid 1

76. Nesting strategy: 3 grids dx, dy = 9 km 8000 km 15 N 15 S 45 N 45 S 3300 km Grid 2

77. Nesting strategy: 3 grids 8000 km 15 N 15 S Periodic 45 N 45 S Grid 2 dx, dy = 9 km

78. Nesting strategy: 3 grids 8000 km 15 N 15 S Periodic 45 N 45 S dx, dy = 9 km Grid 2

79. Nesting strategy: 3 grids 8000 km 15 N 15 S 45 N 45 S 5 N 5 S Grid 3 dx, dy = 3 km

80. Coriolis force acts only on perturbation winds (about the zonal mean) Prevents the formation of unwanted zonal jets and tradewinds One last detail

81. Results

82. Rain hovmoller: No shear

84. Rain spectrum: No shear

85. Rain hovmoller: Shear

87. Rain spectrum: Shear

89. Other shear profiles

90. Hovmoller for shear reversal

91. Conclusions Vertical shear of background zonal wind is essential for producing westward bias in convective wave propagation Simulated “V”-pattern in cloudiness consistent with observations of oceanic squall lines and 2-day waves

92. Implications of “V” pattern Radar 1 Radar 2

93. Implications of “V” pattern Radar 1 Radar 2

94. Implications of “V” pattern Radar 1 Radar 2

95. Implications of “V” pattern Radar 1 Radar 2

96. Implications of “V” pattern Radar 1 Radar 2 Fast-mover; Shear perpendicular Slow-mover; Shear parallel

97. Open Questions Why are two-day periodicities absent from the model? Why is low-level shear important? Role of topography/diurnal forcing? What determines the “V” vs. N-S line structure? Implications of westward bias towards the QBO?

98. What about the squall lines observed during GATE? Going back to the first geostationary satellite IR dataset (SMS-1; Smith & Vonderhaar 1976, CSU Tech note.) hourly at ~ 0.1 deg

99. What about the squall lines observed during GATE? Going back to the first geostationary satellite IR dataset (SMS-1; Smith & Vonderhaar 1976, CSU Tech note.)

100. What about the squall lines observed during GATE? Going back to the first geostationary satellite IR dataset (SMS-1; Smith & Vonderhaar 1976, CSU Tech note.)

101. Typical (18-day window) power spectrum of SMS Tb observed during GATE

102. Hovmoller of SMS Tb (<250 K) during GATE Squall line dates reported by Houze and Rappaport (1984)

103. Hovmoller of CLAUS Tb during TOGA COARE (Cruises 1 and 2)

105. 1