Seasonal variability of gravity waves and tidal/gravity-wave interactions in the Arctic and Antarctic MLT Charlie Beldon &...
Outline <ul><li>Introduction </li></ul><ul><li>Measuring gravity waves with meteor radar </li></ul><ul><li>Height and seas...
Rothera, Antarctica (68ºS, 68ºW) Esrange, Sweden (68ºN, 21ºE) Location of Radars
Introduction – meteor radar winds <ul><li>Assume: </li></ul><ul><li>Flow is homogeneous within a height gate (a “slab” mod...
Introduction – meteor radar winds Final Data product: Winds at heights of  ~ 80-100 km Time resolution  ~ 1-2 hours Height...
Gravity waves - method <ul><li>‘ Homogeneous wind field’  –  </li></ul><ul><li>mean wind, planetary waves,  </li></ul><ul>...
Gravity waves - method <ul><li>Remove the homogeneous wind field from  each individual  </li></ul><ul><li>meteor  -  leave...
Gravity waves - method Final Data Product: Variance from waves with period ~ 20 mins – 2 hours, λ h ~< 400 km <ul><li>Remo...
Example of the data
Seasonal behaviour
 
Summer Profiles:
 
Summer Profiles:
What controls the summer wave activity? Rothera Esrange
<ul><li>Zonal mean wind reversal?  </li></ul>What controls the summer wave activity? 100 80 60 -50  0  50 Zonal Wind
<ul><li>Zonal mean wind reversal?  </li></ul><ul><li>Peak in meridional mean wind? </li></ul>What controls the summer wave...
<ul><li>Zonal mean wind reversal?  </li></ul><ul><li>Peak in meridional mean wind? </li></ul><ul><li>Mesopause? </li></ul>...
Exclusion circles 20 km Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed URAP zonal winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 30 km URAP zonal winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 40 km URAP zonal winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 50 km URAP zonal winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 60 km Meridional Phase Speed URAP zonal winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 70 km URAP zonal winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 80 km URAP zonal winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 81.1 km Radar zonal and meridional winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 84.6 km Radar zonal and meridional winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 87.5 km Radar zonal and meridional winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 90.4 km Radar zonal and meridional winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 93.3 km Radar zonal and meridional winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed 96.8 km Radar zonal and meridional winds
Exclusion circles Zonal Phase Speed -50  0  50  50 0 -50 Meridional Phase Speed <ul><li>Result: </li></ul><ul><li>Filterin...
What controls the wave activity? (Temperature data from L ü bken  et al,  2004, falling sphere climatology)
(Temperature data from L ü bken falling sphere climatology) What controls the wave activity? <ul><li>Change in dT/dz can e...
Equinoctial minima – filtering of low phase-speed waves Seasonal cycle
COMPOSITE YEAR VARIANCE, ESRANGE Note : Esrange data shifted by 6 months Arctic/Antarctic comparisons COMPOSITE YEAR VARIA...
<ul><li>Higher activity in spring over Rothera  </li></ul><ul><li>Lower activity in summer over Rothera at greater heights...
<ul><li>An origin in the differing underlying stratospheres? </li></ul>Arctic/Antarctic comparisons
<ul><li>An origin in the differing underlying stratospheres? </li></ul>Arctic/Antarctic comparisons
Tidal/gravity-wave interactions
Modulations of gravity-wave activity AMPLITUDE (m 2 s -2 ) PERIOD (DAYS)
Modulation of gravity-wave activity - tides <ul><li>Summer </li></ul><ul><li>Composite day -  January 2007  </li></ul><ul>...
Modulation of gravity-wave activity - tides VARIANCE ZONAL WIND VARIANCE ZONAL WIND <ul><li>Winter </li></ul><ul><li>Compo...
<ul><li>Meteor radar cannot resolve individual gravity waves </li></ul><ul><li>But,  variance of horizontal velocities loo...
 
 
 
 
 
 
i.e.,  more gravity-wave activity in spring over Rothera than Esrange less gravity-wave activity in summer over Rothera at...
 
Meridional Winds
Modulation of Gravity-wave Activity
Gravity wave scales sizes Meteor radars cannot easily resolve individual waves of short horizontal wavelength (< ~ 1,000 k...
Equinoctial minima – filtering of low phase-speed waves Seasonal Cycle - Filtering
NB, six routine height gates of 5, 3, 3, 3, 3, 5 km  Meteor radar winds January 6, 2007 over Rothera (68 ˚S) Can not easil...
January 6, 2007 over Rothera (68 ˚S) Meteor radar winds NB, six routine height gates of 5, 3, 3, 3, 3, 5 km
Seasonal cycle <ul><li>Equinocial minima </li></ul><ul><li>Summer and winter maxima </li></ul>
<ul><li>Uncertainties are very small compared to the scatter  </li></ul><ul><li>- i.e.  The scatter is predominantly geoph...
<ul><li>An origin in the differing underlying stratospheres? </li></ul>Inter-hemispheric comparisons
Seasonal cycle and short-term variability Rothera (68˚S) Variance (m 2 s -2 ) Variance (m 2 s -2 )
12-hours 24-hours Modulation of gravity-wave activity
Modulation of gravity-wave activity
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Seasonal variability of gravity waves and tidal/gravity-wave interactions in the Arctic and Antarctic MLT

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Presentation given at COSPAR 2008. Looking at using meteor radar to study atmospheric gravity waves. Covers introduction to the method and then Arctic / Antarctic comparisons and seasonal behaviour

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  • Seasonal variability of gravity waves and tidal/gravity-wave interactions in the Arctic and Antarctic MLT

    1. 1. Seasonal variability of gravity waves and tidal/gravity-wave interactions in the Arctic and Antarctic MLT Charlie Beldon & Nick Mitchell Centre for Space, Atmospheric and Oceanic Science Department of Electronic and Electrical Engineering University of Bath [email_address] COSPAR 2008
    2. 2. Outline <ul><li>Introduction </li></ul><ul><li>Measuring gravity waves with meteor radar </li></ul><ul><li>Height and seasonal behaviour </li></ul><ul><li>Polar inter-hemispheric differences </li></ul><ul><li>Tidal modulation of gravity-wave activity </li></ul><ul><li>Conclusions </li></ul>
    3. 3. Rothera, Antarctica (68ºS, 68ºW) Esrange, Sweden (68ºN, 21ºE) Location of Radars
    4. 4. Introduction – meteor radar winds <ul><li>Assume: </li></ul><ul><li>Flow is homogeneous within a height gate (a “slab” model) </li></ul><ul><li>Horizontal scale of features of interest are large compared to the collecting volume </li></ul><ul><li>Then: </li></ul>Calculate zonal and meridional winds by a least-squares fit of horizontal velocities
    5. 5. Introduction – meteor radar winds Final Data product: Winds at heights of ~ 80-100 km Time resolution ~ 1-2 hours Height resolution ~ 2-3 km <ul><li>Assume: </li></ul><ul><li>Flow is homogeneous within a height gate (a “slab” model) </li></ul><ul><li>Horizontal scale of features of interest are large compared to the collecting volume </li></ul><ul><li>Then: </li></ul>Calculate zonal and meridional winds by a least-squares fit of horizontal velocities
    6. 6. Gravity waves - method <ul><li>‘ Homogeneous wind field’ – </li></ul><ul><li>mean wind, planetary waves, </li></ul><ul><li>tides, long period gravity waves </li></ul><ul><li>‘ Inhomogeneous wind’ – </li></ul><ul><li>assume to be short-period, </li></ul><ul><li>small horizontal-scale gravity </li></ul><ul><li>waves </li></ul>
    7. 7. Gravity waves - method <ul><li>Remove the homogeneous wind field from each individual </li></ul><ul><li>meteor - leaves horizontal velocities of small-scale motions </li></ul><ul><li>The variance of remaining horizontal velocities = a proxy for </li></ul><ul><li>high-frequency gravity-wave activity </li></ul>
    8. 8. Gravity waves - method Final Data Product: Variance from waves with period ~ 20 mins – 2 hours, λ h ~< 400 km <ul><li>Remove the homogeneous wind field from each individual </li></ul><ul><li>meteor - leaves horizontal velocities of small scale motions </li></ul><ul><li>The variance of remaining horizontal velocities = a proxy for </li></ul><ul><li>high-frequency gravity-wave activity </li></ul>
    9. 9. Example of the data
    10. 10. Seasonal behaviour
    11. 12. Summer Profiles:
    12. 14. Summer Profiles:
    13. 15. What controls the summer wave activity? Rothera Esrange
    14. 16. <ul><li>Zonal mean wind reversal? </li></ul>What controls the summer wave activity? 100 80 60 -50 0 50 Zonal Wind
    15. 17. <ul><li>Zonal mean wind reversal? </li></ul><ul><li>Peak in meridional mean wind? </li></ul>What controls the summer wave activity? 100 80 60 -50 0 50 Zonal Wind 100 90 80 -20 -10 0 Meridional Wind
    16. 18. <ul><li>Zonal mean wind reversal? </li></ul><ul><li>Peak in meridional mean wind? </li></ul><ul><li>Mesopause? </li></ul>What controls the summer wave activity? 100 80 60 -50 0 50 Zonal Wind 100 90 80 -20 -10 0 Meridional Wind 120 140 160 180 200 100 90 80 Temperature
    17. 19. Exclusion circles 20 km Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed URAP zonal winds
    18. 20. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 30 km URAP zonal winds
    19. 21. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 40 km URAP zonal winds
    20. 22. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 50 km URAP zonal winds
    21. 23. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 60 km Meridional Phase Speed URAP zonal winds
    22. 24. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 70 km URAP zonal winds
    23. 25. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 80 km URAP zonal winds
    24. 26. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 81.1 km Radar zonal and meridional winds
    25. 27. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 84.6 km Radar zonal and meridional winds
    26. 28. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 87.5 km Radar zonal and meridional winds
    27. 29. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 90.4 km Radar zonal and meridional winds
    28. 30. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 93.3 km Radar zonal and meridional winds
    29. 31. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed 96.8 km Radar zonal and meridional winds
    30. 32. Exclusion circles Zonal Phase Speed -50 0 50 50 0 -50 Meridional Phase Speed <ul><li>Result: </li></ul><ul><li>Filtering will remove waves above ~ 87 km </li></ul><ul><li>Doesn’t explain summer behaviour </li></ul>
    31. 33. What controls the wave activity? (Temperature data from L ü bken et al, 2004, falling sphere climatology)
    32. 34. (Temperature data from L ü bken falling sphere climatology) What controls the wave activity? <ul><li>Change in dT/dz can explain the summer behaviour </li></ul>
    33. 35. Equinoctial minima – filtering of low phase-speed waves Seasonal cycle
    34. 36. COMPOSITE YEAR VARIANCE, ESRANGE Note : Esrange data shifted by 6 months Arctic/Antarctic comparisons COMPOSITE YEAR VARIANCE, ROTHERA
    35. 37. <ul><li>Higher activity in spring over Rothera </li></ul><ul><li>Lower activity in summer over Rothera at greater heights </li></ul>Arctic/Antarctic comparisons RATIO OF COMPOSITE YEAR VARIANCES, ROTHERA/ESRANGE
    36. 38. <ul><li>An origin in the differing underlying stratospheres? </li></ul>Arctic/Antarctic comparisons
    37. 39. <ul><li>An origin in the differing underlying stratospheres? </li></ul>Arctic/Antarctic comparisons
    38. 40. Tidal/gravity-wave interactions
    39. 41. Modulations of gravity-wave activity AMPLITUDE (m 2 s -2 ) PERIOD (DAYS)
    40. 42. Modulation of gravity-wave activity - tides <ul><li>Summer </li></ul><ul><li>Composite day - January 2007 </li></ul><ul><li>Tide-like structure in variance, a 12-hour modulation </li></ul><ul><li>More wave activity when the tidal wind is westward </li></ul><ul><li>Filtering? </li></ul>VARIANCE ZONAL WIND
    41. 43. Modulation of gravity-wave activity - tides VARIANCE ZONAL WIND VARIANCE ZONAL WIND <ul><li>Winter </li></ul><ul><li>Composite day - May 2006 </li></ul><ul><li>Tide-like structure in variance, a 12-hour modulation </li></ul><ul><li>More wave activity when the tidal wind is eastward </li></ul><ul><li>Filtering? </li></ul>
    42. 44. <ul><li>Meteor radar cannot resolve individual gravity waves </li></ul><ul><li>But, variance of horizontal velocities looks to be a useful proxy for gravity-wave activity (periods ~ 20 mins – 2 hours and λ h <~ 400 km) </li></ul><ul><li>Polar wave field has abrupt transition near the summer mesopause – effect of temperature? </li></ul><ul><li>Clear seasonal variation (filtering) </li></ul><ul><li>Filtering below the mesosphere can account for some inter-hemispheric differences </li></ul><ul><li>Clear evidence of wave-field modulation by tides (and PW) </li></ul>Conclusions
    43. 51. i.e., more gravity-wave activity in spring over Rothera than Esrange less gravity-wave activity in summer over Rothera at greater heights Inter-hemispheric Comparisons
    44. 53. Meridional Winds
    45. 54. Modulation of Gravity-wave Activity
    46. 55. Gravity wave scales sizes Meteor radars cannot easily resolve individual waves of short horizontal wavelength (< ~ 1,000 km) or high frequency (periods < ~ 2 hours)
    47. 56. Equinoctial minima – filtering of low phase-speed waves Seasonal Cycle - Filtering
    48. 57. NB, six routine height gates of 5, 3, 3, 3, 3, 5 km Meteor radar winds January 6, 2007 over Rothera (68 ˚S) Can not easily resolve individual waves of short horizontal wavelength (< ~ 1,000 km) or high frequency (periods < ~ 2 hours)
    49. 58. January 6, 2007 over Rothera (68 ˚S) Meteor radar winds NB, six routine height gates of 5, 3, 3, 3, 3, 5 km
    50. 59. Seasonal cycle <ul><li>Equinocial minima </li></ul><ul><li>Summer and winter maxima </li></ul>
    51. 60. <ul><li>Uncertainties are very small compared to the scatter </li></ul><ul><li>- i.e. The scatter is predominantly geophysical </li></ul>How much variance from measurement uncertainty?
    52. 61. <ul><li>An origin in the differing underlying stratospheres? </li></ul>Inter-hemispheric comparisons
    53. 62. Seasonal cycle and short-term variability Rothera (68˚S) Variance (m 2 s -2 ) Variance (m 2 s -2 )
    54. 63. 12-hours 24-hours Modulation of gravity-wave activity
    55. 64. Modulation of gravity-wave activity

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