Modeling the Distribution of H 2 O and HDO in the upper atmosphere of Venus Mao-Chang Liang Research...

Modeling the Distribution of H2O and HDO in the upper

atmosphere of Venus

Mao-Chang LiangResearch Center for Environmental Changes, Academia Sinica

C. D. Parkinson (U. of Michigan), S. Bougher (U. of Michigan), A. Brecht (U. of Michigan), S. Rafkin (SwRI Boulder), B. Foster

(NCAR HAO), Y. L. Yung (Caltech)

2008 EGU Conference at Vienna, Austria (April 14-18)

Photochemical modeling• 1-D and 2-D Catech/JPL KINETICS• Cloud top at 58 km to 112 km• 24 species including chlorine, hydrogen, oxygen,

and carbon compounds• 210 chemical reactions• Transport by eddy mixing (1-D) and eddy mixing

+ advection (2-D)– Homopause >> 112 km

• Advection– Downwelling at -0.3 cm s-1: 75-85 km– Upwelling at 0.5 cm s-1: 85-95 km– Downwelling at -0.5 cm s-1: >95 km

Chemical fractionation

• Water photolysis– H2O + h H + OH

– HDO + h H + OD or D + OH

• Hydrogen chloride photolysis– HCl + h H + Cl– DCl + h D + Cl

Photoabsorption cross sections

Dynamical fractionation

• Hydrogen escape (Gurwell and Yung 1993)– H escape: 3.5106 atoms cm-2 s-1

– D escape: 3.1104 atoms cm-2 s-1

• H production rate– HCl photolysis: 8.11010 atoms cm-2 s-1

– H2O photolysis: 4.8108 atoms cm-2 s-1

1-D model results

0

-0.3

0.5

-0.5

advection

Photochemistry at 75-85 km

• d[H2O]/dt = -JH2O * [H2O]

• d[HDO]/dt = -JHDO * [HDO]

• R = { HDO(t)/H2O(t) } / { HDO(0)/H2O(0) }

= exp[(JH2O - JHDO)t]

• r = H2O(t) / H2O(0) = exp(-JH2Ot)

• R = r-f, f = (JH2O - JHDO)/JH2O = 0.46

2-D model results

Estimate of downwelling• Diabatic heating by air subsidence

T = -w(T/z + T/H) t = (-1) = 1/4 (=Cp/Cv)

t = trad ~ 3 day– T = 175 K, T/z = -4 K/km, H = 4 km T ~ -18w– w ~ 1 cm s-1

– Needed to explain the increase of H2O above 95 km

Temperature field

Vertical wind

Summary

• If the decrease of H2O mixing ratio at ~75 km is caused by photolysis, the isotopic compostion can be explained by photolytic fractionation

• Inferred downwelling is explained by NO influx in the polar region

• Inferred upwelling remains unknown

Thank you

UPDATE Re-vitalized National Center for Atmospheric Research (NCAR)

thermospheric general circulation model for Venus (VTGCM); Lowered the bottom boundary (from ~95km to ~80km) to insure

all dynamical influences contributing to the NO and O2 nightglow layers can be captured;

Applied new Near-IR heating and CO2 15-micron cooling rates (Roldan et al, 2000);

Analyzed solar cycle variations (F10.7 = 70 to 200) Used observations from Pioneer Venus Orbiter (PVO) and

Venus Express (VEX), including measurements of NO, O2, and H airglows plus temperatures. Interpreted these global tracers of the thermospheric circulation with the VTGCM;

Goal: To determine the dynamical processes that link the Venus middle and upper atmospheres through general circulation modeling of the upper mesosphere and thermosphere.

Venus Thermospheric General Circulation Model (VTGCM):

Current Formulation and Structure

Altitude range: ~80-200 km (day); ~80-150 km (night) Horizontal resolution: 5x5º latitude-longitude grid (pole-to-pole) Pressure vertical coordinate (1/2-H intervals): 46-levels Major Fields: T, U, V, W, O, CO, N2, CO2, Z PCE ions Fields: CO2+, O2+, N2+, NO+, O+, Ne Minor Fields: O2 (and O2 IR nightglow at 1.27 m) Future Minor Fields: N(4S), N(2D) (and NO nightglow) Full 2-hemispheric capability. Timestep = 30.0 secs. Venus obliquity ~ 177.4º (“seasonal” cases possible) Rayleigh friction used to slow SS-AS winds. Upgraded airglow capability: O2-IR, NO-UV(future)