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Migrating Solar Tides and other Waves in the polar region of Venus

Migrating Solar Tides and other Waves in the polar region of Venus. J.Peralta, D.Luz, D.L.Berry CAAUL/Observatorio Astronómico de Lisboa (Universidade de Lisboa, PORTUGAL). ESTEC , 2011-11-09. Nature of Atmospheric Tides.

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Migrating Solar Tides and other Waves in the polar region of Venus

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  1. Migrating Solar Tides and other Waves in the polar region of Venus J.Peralta, D.Luz, D.L.Berry CAAUL/Observatorio Astronómico de Lisboa (Universidade de Lisboa, PORTUGAL) ESTEC, 2011-11-09

  2. Nature of Atmospheric Tides • Persistent global oscillations found in wind, temperature, pressure, etc… • Global-scale atmospheric gravity waves excited by different sources. • Focus on the ones caused by the periodic heating of the Sun due to planet’s rotation (Solar Tides).

  3. Solar Tides in Venus About this study: • Extend coverage (lack of studies for polar regions) • Characterization using wavelengths sensing day/night at the same time with VIRTIS images (3.8 µm and 5.0 µm) • Which is their role in the atmospheric circulation? • More than 14,000 wind vectors • Spatial Coverage: 70ºS — 85ºS • Time Coverage: 289 days (but only 16 VEX orbits) Solar Tides detected in Venus: • Thermal IR emissions (Ingersoll, 1974; Apt, 1980) • Temperature (Taylor, 1980; Zasova, 2002; Tellmann, 2009) • Aerosol concentrations (Zasova, 2002) • UV Cloud brightness(Del Genio, 1990) • Winds (Rossow, 1990; Toigo, 1994; Limaye, 2007; Moissl, 2009; Sánchez-Lavega, 2008)

  4. Procedure to detect Solar Tides • Solar Tides appear as Stationary Wave in Solar-fixed Coordinates (local time). • Low-quality winds are filtered using the Weighted Mean in Lat. Intervals. • Zonally Averaged Profiles inferred. • Averaged Wind is substracted to get the Wind Disturbances:ΔU = U – <U>ΔV = V – <V> • Disturbed Winds are then: • Sorted in Local Time. • Mean and SDs for each hour. • LS Periodograms applied. • Sine Fit to characterize Solar Tide.

  5. So, to detect Tides in Venus: • Samples of 0.5 local hours. • Latitude intervals 1º. • As time variation not expected, all orbits studied at same time. Sensitivity Studies • Noise • Local Time Resolution • Latitude/Period in terms of Confidence

  6. Solar Tides in the Zonal Wind • K=2 and K=4. • Mean Amplitudes:~ 2 m/s [K=2]~ 5 m/s [K=4](≤ 20% zonal comp.) • K=4 dominant at lower latitudes. • Retrograde acceleration at midday.

  7. Solar Tides in the Meridional Wind • K=1. • Mean Amplitudes:~ 6 m/s [K=1] (>>100% merid. wind) • Variations with the Latitude. • Poleward acceleration at midday.

  8. Luz et al. 2011 Diurnal Tides: Main implications Zonal Disturbances: • Effect weaker than meridional. • Absence for Diurnal Tide. Meridional Disturbances: • Strong effect of Diurnal Tide. • DAY: Poleward accelerationNIGHT: Equatorward acceleration V V SOLAR-TO-ANTISOLARinstead ofHADLEY-CELL ??

  9. Sánchez-Lavega, 2008 -2.5 m·s-1/hour Solar Tides: Comparison with other works • K=1 dominance for winds also found in previous missions (Rossow et al. 1990; Limaye, 2007). • K=1 also dominant in high lats for aerosols and temperature (Schofieldet al. 1983; Zasova et al. 2002)Exception: Tellmann et al. 2009 • Solar harmonics UNCOUPLED:Isothermal Conditions (Volland, 1998) • Semidiurnal effect on zonal wind is consistent with dayside acceleration found in other VEX results(Sánchez-Lavega, 2008; Moissl, 2009).

  10. Diurnal Tides: Spatial Structure • Dispersion relation for global-scale Gravity Waves with N2~constant:(Pechmann & Ingersoll, 1984)λz< 1 km • Meridional structure in terms of Legendre Polynomials:(Lindzen, 1966)Ky~ 6 ?? • Wave fronts matching lines of constant SZA explains absence of K=1 on Zonal Wind.

  11. Diurnal Tides: Thermal Structure Assumptions (Xu et al. 2009): • <v>≈0 , <w>≈0 • Coriolis terms are neglected • Perturbations: F=A(Ф,z)·ei(Ωt+λ+δ) • Ideal gases • Images display “isosteres” (Houghton, 2002)

  12. Non Solar-fixed Waves in the Zonal Wind • K=1 and K=2. • Mean Amplitudes:~ 4.5 m/s [K=1]~ 3.6 m/s [K=2](≤ 30% zonal comp.) • Only small changes for the Amplitude.

  13. Non Solar-fixed Waves in the Meridional Wind • K=1, K=2 and K=3. • Mean Amplitudes:~ 8.0 m/s [K=1]~ 3.9 m/s [K=2] • Important variations in the Amplitude.

  14. Non Solar-fixed Waves: Phase Variation • Opposite sense for K=1 and K=2 • Phase Variation ~ 1.4 hours/day Cx ~ - 5.7 m/s

  15. Conclussions for the Solar Tides: • The Southern region of Venus is studied in search of the effect of Solar Tides on the windfield at the cloud tops. • For the first time, day and night are studied simultaneously using VIRTIS images at 3.8 and 5.0 µm • Semidiurnal (K=2) and Quarter-diurnal Tide (K=4) found in the Zonal Wind. • Diurnal Tide (K=1) is the strongest and determines sense of Meridional Wind: • Ky~6 and λz < 1 km. • Wave fronts match SZA=const • Forces: Poleward Winds on Dayside Equatorward on Nightside. • Thermal effects can be derived from wind disturbances with discrepancies < 2 K

  16. Conclussions for the non Solar-fixed Waves: • The Southern region of Venus was also studied to look for non solar-fixed waves. • VIRTIS images at 3.8 and 5.0 µm were used studying the orbits separately. • Wavenumbers K=1, K=2 and K=3 found. • Amplitudes of 4-8 m/s for Zonal and Meridional winds. • Amplitude changes with time. Relation with vortex morphology? (Luz et al 2011) • Further work required !!

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