The 8-hour Tide in the Atmosphere

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A longer version of the department seminar. More introductory material about the radar and tidal source mechanisms.

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The 8-hour Tide in the Atmosphere

  1. 1. The 8-hour Tide in the Atmosphere Charlotte Beldon Centre for Space, Atmospheric and Oceanic Science, Department of Electronic and Electrical Engineering 5th December 2005
  2. 2. Outline •The Mesosphere and Lower Thermosphere •What causes a meteor •The meteor radar technique •Distribution of meteor echoes •Tides in the atmosphere •The 8-hour tide
  3. 3. Atmospheric Temperature Profile 0 20 40 60 80 100 120 TEMPERATURE (DEGREES) HEIGHT(km) -120 -80 -40 0 40 Meteors Everest (8848m) Planes Weather Balloons Satellites
  4. 4. Meteors Meteor seen by eye •‘Shooting Stars’ • Particle enters the atmosphere •Heating and ablation •Column of ionisation •De-excitation emits light
  5. 5. Tx Antenna Five-Antenna Rx Array ~ 90 km Radio Pulses Mesosphere / lower- thermosphere region Meteor Radar
  6. 6. Tx Antenna Five-Antenna Rx Array ~ 90 km Radio Pulses Mesosphere / lower- thermosphere region Meteor Radar
  7. 7. Radar Meteor trail Reflection point Meteor Radar
  8. 8. TIME (ms) AMPLITUDE Meteors seen by radar TIME (ms) AMPLITUDE Meteors •Overdense (>1014 e- m-1 ) •Underdense (<1014 e- m-1 )
  9. 9. Parameters from a Meteor Radar Tx Antenna Five-Antenna Rx Array Radio Pulses • Date and Time • Range • Height • Radial Velocity • Elevation • Azimuth • Meteor Brightness • Meteor Decay Time • Meteor Entry Speed • Signal to Noise Ratio
  10. 10. Calculating Horizontal Winds •Meteor trails act as tracers •Assume : -Atmosphere moves as a slab above the radar -Vertical winds are small compared with horizontal winds •Plot velocity of trails against angle around the radar •Sine curve amplitude = speed phase = direction
  11. 11. Meteor Radar in the UK •January 1988 – 2005 •Two Beam System (~30o ) •No height determination •3000 – 4000 meteors per day Castle Eaton, UK (52.6 N, 2.2 W) Frequency: 25 MHz Peak Transmission Power: 20 kW Pulse Repetition Frequency: 300 Hz Pulse Length: 30 μsec
  12. 12. Meteor Radar in the Arctic •October 1999 – 2005 •SKiYMET – All sky •5 Receiving Antennas, 1 Transmission Antenna •~4000 meteors per day Esrange, Sweden (67.9 N, 21.1 E) Frequency: 32.5 MHz Peak Transmission Power: 6 kW Pulse Repetition Frequency: 2144 Hz Pulse Length: 13 μsec
  13. 13. Distribution of Meteor Echoes 2nd November 2005 5402 Meteors found 100 200 300 400 NORTH SOUTH WEST EAST Norway Sweden Finland Russian Federation
  14. 14. Distribution of Meteor Echoes Distribution in Range METEORCOUNT(x105 ) 100 200 300 400 500 RANGE (km) 0 1 2 3 4 6 5 •Number of echoes decreases with range •Power decays as a function of range
  15. 15. Distribution of Meteor Echoes METEORCOUNT HOUR 0 5 10 15 200 5 10 15 20 0 50 100 150 200 250 300 350 METEORCOUNT HOUR (UT) 0 50 100 150 200 250 300 350 Distribution in Time •Change in number of echoes over one day •Peaks in early morning •More meteors on the leading hemisphere of the earth
  16. 16. Distribution of Meteor Echoes 0 50 100 150 200 250 300 350 400 450 500 70 75 80 85 90 95 100 105 110 METEOR COUNT HEIGHT(km) Distribution in Height HEIGHT(km) METEOR COUNT 0 100 200 300 400 500 70 75 80 85 90 10 0 95 10 5 11 0 •Detect echoes ~70 to ~110 km •3 km height resolution •Strongly peaked at ~90 km
  17. 17. Horizontal Winds Observed by the radar
  18. 18. East-West Horizontal Winds •Wind blows alternately east and west •Regular 12-hour oscillation
  19. 19. Solar Atmospheric Tides 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 1 2 3 4 5 6 7 FREQUENCY(CYCLES PER DAY) AMPLITUDE(m/s) FREQUENCY (CYCLES PER DAY) AMPLITUDE(m/s) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.50 1 2 3 4 5 6 7 12-hour tide 24-hour tide 8-hour tide 6-hour tide •Winds dominated by solar tides •Analogous to ocean tides •12-hour tide is largest over the UK •Source of the 8-hour tide is uncertain
  20. 20. The 8-hour Tide Band passed zonal winds, October 2000 DAY OF YEAR WINDSPEED(m/s) 24-hour tide 12-hour tide 8-hour tide •High day-to-day variability •8-hour tide can match 12- hour tide on occasion •Variability from changes in excitation and environment
  21. 21. Sources of the 8-hour Tide – Solar Heating Solar Heating HEIGHT(km) TEMPERATURE (DEGREES) Troposphere Thermosphere Mesosphere Stratosphere O2 H2O O3 0 20 40 60 80 100 120 -120 -80 -40 0 40 •Solar heating - periods are integer sub- harmonics of a solar day •24-, 12-, 8- and 6-hour tides are generated
  22. 22. Sources of the 8-hour Tide – Nonlinear coupling 24-hour tide 12-hour tide Nonlinear interaction Family of secondary waves : SumDifference •Sum and difference waves •Related frequencies and wave numbers •Vertical wavelength : •Testable predictions λ8 = (λ24 * λ12) (λ24 + λ12)
  23. 23. Observations of the 8-hour Tide - UK Amplitudes of the 8-hour tide •UK radar has no height finding •Clear seasonal behaviour •Maximum amplitudes in autumn •Elevated values in winter
  24. 24. Observations of the 8-hour Tide - Esrange 8 5 9 0 9 5 HEIGHT(km) 0 1 2 3 4 5 6 7 8 9 1 0 J F M A M J J A S O N D AMPLITUDE(ms-1 ) •Height resolution •Tide grows with height •Large tide in autumn •Small tide in winter – different to UK Amplitudes of the 8-hour tide
  25. 25. Conclusions •Meteor radar used to measure horizontal winds between 80-100 km in the atmosphere •Atmospheric tides dominate motion in the mesosphere and lower thermosphere •The 8-hour tide has two possible sources •There is a clear seasonal cycle that changes with latitude
  26. 26. Thank you Charlotte Beldon Centre for Space, Atmosphere and Oceanic Science, Dept. Electronic and Electrical Engineering

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