Gravity waves near convection: causes and consequences

Transcription

1 Gravity waves near convection: causes and consequences Robert Fovell UCLA Atmospheric & Oceanic Sciences

2 Outline Basics Mechanically generated stratospheric gravity waves Thermally induced low frequency tropospheric gravity waves Thermally/mechanically excited high frequency tropospheric gravity waves Mechanically forced obstacle effect gravity waves

3 Terms Frequency ω and period P Wavelengths L x, L z ; wavenumbers k, m Intrinsic frequency (relative to mean flow)

4 Terms (continued) Dispersion equation (N = B-V frequency) Phase line tilt from vertical Phase speed (flow relative)

5 Multicellular squall line storm

6 Vertical cross-section

7 Vertical cross-section

8 Vertical cross-section Storm-relative airflow

9 Animation Storm is heat and momentum source; high frequency and low frequency components Fovell and Tan (1998)

10 Theme Convective storms are an excellent source of gravity waves Some of these waves significantly impact the surrounding environment Some of those impacts feed back onto the convection GCM import: subgrid phenomena that do not remain subgrid

11 Gravity waves above convection

12 Fovell, Durran and Holton (1992)

13 Fovell, Durran and Holton (1992)

14 S(0) case Fovell, Durran and Holton (1992)

15 S(-8) and S(8) cases Fovell, Durran and Holton (1992)

16 An S(0) case

17 An S(0) case

18 An S(0) case

19 Aspect ratio not 1:1

20 (principal cell period)

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23 FDH s S(0) gravity wave fan Fovell, Durran and Holton (1992) Wave period given in hours

24 Impact of stratospheric GWs Alexander and Holton (1997)

25 Evolution of stratospheric waves

26 Simple oscillator model Tropospheric momentum source mimicking a convective cell updraft Oscillate at set period P Source may be tilted Source may be moved horizontally

27 Fovell, Durran and Holton (1992)

28 Fovell, Durran and Holton (1992)

29 Fovell, Durran and Holton (1992)

30 Fovell, Durran and Holton (1992)

31 S(+8) case Storm s convective region acts as equivalent obstacle

32 Fovell, Durran and Holton (1992) S(+8) case

33 Fovell, Durran and Holton (1992) S(+8) case

34 S(+8) case What happened to the high frequency waves? Fovell, Durran and Holton (1992)

35 Low frequency tropospheric gravity waves

36 Vertical half-sine heating profile For z H

37 Nicholls et al. (1991)

38 Dry heat source model Compressible, nonlinear with stratosphere No mean flow Maintained, vertically oriented heat source

39 Dry heat source model Temperature perurbation (colored); potential temperature (contoured)

40 Dry heat source model

41 Animation Note displaced air does not return to original elevation

42 Phase speed

43 Dry heat source model Heat source deactivated halfway through animation

44 Two vertical modes Nicholls et al. (1991)

45 No applied cooling needed top heavy profile Mapes (1993)

46 Dry heat source model Top heavy heating profile in vertically sheared atmosphere

47 Animation Result of the two modes: Net ascent in lower troposphere in vicinity of source

48 Rear side of storm

49 Rear inflow current Pandya and Durran (1996)

50 Rear inflow current Pandya and Durran (1996)

51 Microphysical impact

52 Squall line forward environment Gentle, sustained lower tropospheric lifting generates cool and moist tongue ahead of storm

53 Water vapor perturbations 12 km 135 km

54 Mature phase moist tongue cold pool Import of this low frequency GW response: lower troposphere is more moist, less stable (especially in near-field)

55 In theory, there is no difference between theory and practice. But, in practice, there is. -- Jan LA van de Snepscheut (Caltech professor)

56 BAMEX IOP6

57 BAMEX IOP6 radar and dropsonde locations

58 BAMEX IOP6 dropsondes Mullendore and Fovell (in progress)

59 21 June 2003 (midnight-5am), Oklahoma

60 High frequency tropospheric gravity waves

61 Dry heat source model Steady heat source Unsteady heat source

62 Dry heat source model Steady heat source Unsteady heat source

63 Idealized nocturnal simulation OU ARPS cloud model - 2D and 3D Idealized setup Integrated 6 PM till past sunrise Model physics: surface physics atmospheric radiation (clear sky & cloud) ice microphysics

64 Idealized nocturnal simulation Fovell, Mullendore and Kim (2006)

65 Idealized nocturnal simulation Fovell, Mullendore and Kim (2006)

66 Idealized nocturnal simulation Fovell, Mullendore and Kim (2006)

67 Trapping of gravity waves Associated with sharp decrease of Scorer parameter l 2 with height Forward anvil serves as wave duct Decreased stability Jet-like wind profile

68 upstream sounding Fovell, Mullendore and Kim (2006)

69 U zz min upstream sounding Fovell, Mullendore and Kim (2006)

70 U zz min upstream sounding Fovell, Mullendore and Kim (2006)

71 Gravity wave-induced clouds

72 Fovell, Mullendore and Kim (2006)

73 Fovell, Mullendore and Kim (2006)

74 Obstacle effect gravity waves above convective rolls

75 Fovell (2004)

76 Fovell (2004)

77 Fovell (2004)

78 Fovell (2004)

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84 Summary Convective phenomena (deep convection, rolls) superb source of gravity waves High frequency gravity waves Vertically propagating above deep convection Trapped waves ahead of convection Low frequency gravity waves Responding to maintained heating/cooling Owing to flow over obstacles How gravity waves impact convective environment Feedback of impact on convective source Subgrid in GCMs for some time to come