Water Vapour Abundance and Distribution in the Lower Atmosphere of Venus

Sarah Chamberlain – CAAUL / Lisbon Observatory, Portugal.Jeremy Bailey – University of New South Wales, Australia.

Vikki Meadows – University of Washington, U.S.A.David Crisp – Jet Propulsion Laboratory, U.S.A.

IRIS2,Anglo-

AustralianTelescope

4th PHC/Sakura France-Japan Workshop / 3rd Europlanet Strategic Workshop / 5th – 8th March

VenusNear Infrared Wavelengths

Venus Crescent

λ Alt.2.3µm – 35km1.74µm – 24km1.18µm – 15km

H2O absorption CO2 absorption

Chemical models suggest the lower atmosphere should have a constant water vapour abundance and even distribution.

(Meadows and Crisp, 1996)AAT/IRIS

(This Study)AAT/IRIS2

(De Berg et al, 1995)CFHT/FTS

VEX: VIRTIS M

VEX: VIRTIS H

VEX: SPICAV

2500

TheObservations

Venus : Near infrared wavelengths

Venus : Optical wavelengths

–Anglo-Australian Telescope /IRIS2 instrument

Thermal image

O2 Airglow

Full-disk spatially resolved spectra - R ~ 2500Full-disk spatially resolved spectra - R ~ 2500

Removing the correct terrestrial water vapour contributionIs complex:

Standard star observations are usually obtained At a different time, airmass and through a different path Length.

Solar spectrum

Venus

1: Model the transmittance spectra for the earth2: Multiply the modelled transmittance spectra to modelled Venus spectra

for various Venus water vapour abundances.3: Find the best fit modelled spectra for each extracted observed spectra

from various locations.

THEModel

Model sensitivity with altitude for the 1.18 micron window

Note for later: that this region is insensitive to water vapour close to the Venus surface

Model Parameters that DO NOT affect the 1.18 µm window shape and therefore the contrast of the water vapour absorption peaks.

- Emission angle Zenith angle- Emissivity/Albedo- Lapse Rate

Emission Angle

Emissivity Lapse Rate

- Abundance gradient (at sensitive altitudes)- Line list completeness- CO2 Line shape (at high temperatures

and pressures)

Model Parameters that DO affectthe 1.18 µm window shape and

therefore the contrast of the water vapour absorption peaks

Line shape

Figure from Meadows and Crisp, 1996

Where

and is a modification of that of Perrin and Hartmann (1989) as determined by Meadows and Crisp (1996). The values of

,

and are given in the table below.

Coefficients for the CO2 χ factor

CO2 Line shape

Venus spectra were thenextracted from across thedisk of Venus and RMSfitted against various water vapour abundancesand for different spectralregions of the 1.18 µmwindow.

The ResultsHave been checked for spatial variations and for the best fit water vapour abundance.

Matched to

F3

Matched to F1 ~ 1.174 µm Matched to F2 ~ 1.178µm

Matched to F3 ~ 1.182 µm Matched to 1.175 – 1.185 µm

Best fit water vapour abundance against x position

Best fit water vapour abundance against y positionMatched to F1 ~ 1.174 µm Matched to F2 ~ 1.178 µm

Matched to F3~ 1.182 µm Matched to 1.175 – 1.185 µm

Matched to F1 ~ 1.174 µm

Matched to F1 ~ 1.174 µm

Matched to F1 ~ 1.174 µm Matched to F2 ~ 1.178 µm

Matched to F3 ~ 1.182 µm Matched to 1.175 - 1.185 µm

Vatriations in Water Vapour with altitude

Matched to F3~ 1.182 µm

Matched to F1~ 1.174 µm Matched to F2~ 1.178 µm

Matched to 1.175 – 1.185 µm

f1

f2

f3

Observed Spectra

Higher spectral resolutions will obtain a smaller spread of results.

Observed Spectramultiple unresolved

absorption peaks

f1

D

B

A

CE

F Position F (0 km) / Position A (4km)

4kmaltitude

0kmaltitude

Water Vapour absorption from the lowest 4 km of the Venus atmosphere is observed in the gradient and individual features.

Water vapour in the Lowest 4 km

Conclusions- 32ppmv water vapour at around 16km altitudes This result agrees with previous studies.

- Uncertainties connected to :The far wing absorption / continuumThe completeness of the CO2 line list

- Higher spectral resolution observations would aid this study by better defining the absorption peak shape and also resolving the multiple bands that contribute to some of the absorption peaks.

-There is a possibility that near surface (0 – 4 km) water vapour abundances can be determined from remote observations.

Installation of the Venus GCM at the Lisbon Observatory

David Luz CAAUL/Obs. Astronomico de LisboaSarah Chamberlain CAAUL/Obs. Astronomico de LisboaSebastien Lebonnois Lab. de Meteorologie Dynamique

/Lawrence Livermore National Lab.

Venus General Circulation Model (0 – 100km altitude)

The dynamical core of the GCM is based on the LMDZ Earth modeldeveloped at the Laboratoire de meteorologie Dynamique. (Hourdin et al., 2006)

Key Features:- Topography- Diurnal cycle

- Dependence of the specific heat on temperature - Consistent radiative transfer module based on net exchange rate matrices

(consistent computation of the temperature field as opposed to simple temperature forcing).

Consistent with observations:- Superrotation above roughly 40km with comparatively small winds beneath- Meridional circulation consists of equator to pole cells- Temperature structure is globally consistent (with discrepancies in the stability of the lowest layers and equator to pole temperature contrasts within the clouds)- Convective layers at the base of the clouds and the middle of the clouds

Figure from lebonnois et al., 2010

Intended use of the Model:

Angular Momentum Budget with respect to circulation components. Mean meridional circulation

Transient waves

Polar Regions:The dynamical behaviour at polar regionsThe solar tides in the polar regionsThe rotational and thermal properties near the poles (the presence of the dipole).

S-shaped pattern of the southern polar vortex (polardipole). The centroid is shown to be displaced by 3 degrees from the geographic south pole.(VIRTIS - 5 µm radiance map from orbit 38.)Luz et al., 2011

Current Status:

Sebastien Lebonnois has provided us with two models:

- A reference model that has been run for 250 Venus days where the atmosphere was started with the superrotation fully developed.- A second model that has been run for 1050 Venus days from an atmosphere

initially at rest.

We are currently working on stabilising a zoomed versionof the reference GCM that isfocussed on the polar regions.

Image shows the Zonal flow as Produced by the Reference model After 254 Venus days.