Greenhouse Effect & Venusian Atmospheric Balance. Evan Anders

Transcription

1 Greenhouse Effect & Venusian Atmospheric Balance Evan Anders

2 Greenhouse Effect Strong absorption bands of gases and aerosols trap the heat in the lower atmosphere...raising the surface temperature High IR opacity leads to higher Tsurface necessary to balance solar flux

3 Greenhouse Effect & Contributors Greenhouse effect: ~500 K on Venus Different atmospheric components contribute uniquely to the greenhouse. 30 ppm! The effects in this table are NOT additive effects, and are calculated using simple models [Titov et al. 2007, EVTP; Bullock et al 2007]

4 Runaway greenhouse H2O dominated atmospheres have limited ability to radiate to space There is a critical flux above which atmospheric balance is impossible. (What goes in can t go out) Runaway greenhouse, H2O and CO2 (from outgassing) build up in atmosphere

5 Critical Greenhouse Values Four critical solar forcing values. F < F1 One equilibrium F1 < F < F2 Two equilibria F2 < F < F3 Two equilibria, one unstable branch F3 < F < F4 One equilibrium, one unstable branch F > F4 Unstable [Renno, Tellus 49A, 1997]

6 Radiation Limits for Earthlike planets The Stratospheric Limit (the KomabayashiIngersoll limit) The Tropospheric Limit (convection, F2) [Renno 1997]

7 The Spectrum of Venus Where does Venus receive and get rid of its energy?

8 Venus Spectrum Reflected solar light Cloud deck thermal emission Night side surface/lower atmosphere emission [Titov et al. 2007, EVTP]

9 Albedo (Optical Spectrum) Bond albedo estimates: 0.80 ± ± ) Wavelength < 0.32 microns: SO2 in upper cloud decks 2.) Wavelength microns: Unknown absorber [Titov et al. 2007, EVTP; Tomasko et al, 1980b; Moroz et al, 1985]

10 Cloud-top IR spectrum 20 m 5 m [Titov et al. 2007, EVTP]

11 Lower Atmosphere IR spectrum Surface emission Emission at ~35 km [Titov et al. 2007, EVTP; Bezard et al, 1990; Allen and Crawford, 1984]

12 Atmospheric Balance

13 Emission/Absorption v. Latitude Earth & Venus receive more radiation at the equator than at the poles Earth emits more thermal radiation at the equator than the poles Venus radiates at an almost constant value from equator to pole absorption emission [Schofield & Taylor, Icarus 52, 1982]

14 Energy Budget Venus only absorbs 157 W/m2 (Earth: 240 W/m2) Venusian clouds and unknown UV absorber absorb most energy. [Titov et al. 2007, EVTP]

15 Entropy Budget Entropy flux Radiative Flux / Temperature Planets typically have NEGATIVE entropy balance [Titov et al. 2007, EVTP]

16 Solar Fluxes (Up & Down) Flux profiles tell us about atmospheric structure & heating/cooling Net flux: upward flux - downward flux Shows where energy is deposited (via its derivative) bottom of cloud deck [Titov et al. 2007, EVTP]

17 Heating and cooling ~Half of atmospheric energy is absorbed by the unknown UV absorber above 57 km Radiative cooling at high latitudes can t compensate Requires convective cell (rise at low latitudes, fall at high latitudes) Venus is a special case in our solar system: atmosphere is heated from the top. Heating Cooling [Titov et al. 2007, EVTP; Crisp 1986]

18 Solar radiation vs. depth Intensity of downward solar radiation decreases with altitude. H2O mixing ratio is ~30 ppm between cloud base and 16 km Mixing ratio increases to ppm in the lower atmosphere. [Titov et al. 2007, EVTP]

19 Convergence at high altitude (60 km) Thermal Flux v. Altitude Implies a latitudinal trend in the water mixing ratio. Dashed lines are different water mixing ratio models Convergence at low altitude H2O Mixing ratio varies with latitude at mid altitudes [Titov et al. 2007, EVTP; Revercomb et al 1985]

20 Thermal Flux v. Altitude A little easier to see [Revercomb et al, Icarus 61, 1985]

21 Venus Mesosphere Strong radiative disequilibrium Cold collar at N with temperature inversion Adiabatic cooling in rising branch and compressional heating in descending polar branch Downflow (subsidence) velocities of ~1 cm/s necessary to produce required adiabatic heating [Titov et al. 2007, EVTP]

22 The Takeaway Venus atmosphere likely had traumatic things happen to it long ago (runaway greenhouse) Flux imbalance was necessary to cause such trauma Venus atmosphere is now balanced in flux, but looks nothing like the other terrestrial planets in our solar system. Heating from the upper atmosphere creates interesting dynamics in the atmosphere (more in a couple weeks)

23 Solar Fluxes (Down) UV + blue absorption Easier to appreciate downward flux in non-log form. Cloud base [Moroz et al., 1995]