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Total solar eclipse

Surface layer turbulence measurements Guy Schayes. Institut d’Astronomie et de Géophysique Georges Lemaître, Louvain-la-Neuve, Belgium. Total solar eclipse. 11 August 1999, Anelles, France. center line. Anelles. Location of experiment. Totality Zone 130 km wide. Pictures of event.

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Total solar eclipse

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  1. Surface layer turbulence measurements Guy Schayes Institut d’Astronomie et de Géophysique Georges Lemaître, Louvain-la-Neuve, Belgium Total solar eclipse 11 August 1999, Anelles, France

  2. center line Anelles Location of experiment Totality Zone 130 km wide

  3. Pictures of event Gill propeller 3D anemometer

  4. Photosphere: depth 350 km i.e. 1/2000 of Ro T = 5780 K, coldest zone of solar atmosphere

  5. Interest of atmospheric measurements during an eclipse • Effect on radiation balance at first look similar to a passing big Cu • This effect is much larger : the totality shadow is more than 100 km wide; the duration of the partial occultation is of the order of two hours • → we are in presence of a mesoscale atmospheric feature with a predominant 1D structure • As for the diurnal cycle, the SL is still in equilibrium with the phenomenon; the hole BL is not • Possibility of finding an upper boundary for the time constant of the of atmospheric turbulence decay

  6. Diurnal variation of the radiation balance 1000 W/m2 (summer) Eclipse effect global solar radiation (R) Net radiation Balance (Bn) 0 12 24 h infra-red (Qt-Qa)

  7. Rg Radiation data Rn Totality duration : 2 min 01s Tg

  8. u Raw turbulent data v t = 1 s 17 000 points w T

  9. Statistical results Slices of 15 min each (900 data pts)

  10. Conditions at eclipse begin (6): H0 = 40 W m-2 hi = 400 m (est.) w* = 0.77 m s-1 u* = 0.5 m s-1 t* = 520 s Longueur de M-O :

  11. Decay phase TKE Ratios :  u w T B T u v u* w

  12. Decay characteristics • Previous LES studies : • Nieuwstadt F., Brost R. (1986). The decay of convective turbulence • J. Atm. Sc. 43, 532-546 • Sorbjan R. (1997). Decay of convective turbulence revisited. • BLM, 82, 501-515. • This experiment agrees on : • Rapid and immediate decay of var T • var u stays constant for about 2 t*then decays slowly • var w decays more rapidly than var u • TKEdecays with a -2 power law • Decay time (e-1) : less than 15 min for T, and 30 min for TKE.

  13. TKE decay Comparison of data points with turbulence model Goulard et al. (2003), BLM 107, 143-155  Anfossi D., Schayes G. et Goulard, BLM, 2004

  14. Wavelet Transform of T Zoom on central part

  15. WT of w

  16. Conclusions • The measurements succeeded in measuring the variations in the characteristics of the SL turbulence • One of the (very) few measurements of this kind • Model study to be improved … • Sky conditions were not optimal but : •  Next occurrences : • Spain, on 3 October 2005 (annular) • Libya, Turkey, on 29 March 2006 • Siberia, on 1 August 2008 • …

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