Investigation of the radiative energy balance of Venus based on improved models of the middle and lower atmosphere
Final Report Abstract
Scientific task-oriented new archives of high-quality VIRTIS and PMV Venus spectra were generated that are optimized for use in atmospheric state parameter retrieval procedures. A new multi-spectrum retrieval algorithm (MSR) was verified for adaptation to short-wavelength VIRTIS measurements. A multiple-scattering radiative transfer model and a multi-window retrieval technique (MWR) for selfconsistent temperature profile and cloud parameter retrievals were applied to separate temperature and cloud influences on measured nightside spectra. Latitude and local time dependent mesospheric temperature profiles were retrieved from VIRTIS data. Zonally averaged temperature structure was additionally determined from both VIRTIS and PMV spectra. Temperature deviations are usually below 3 K between 58 and 80 km. The zonally averaged temperature structure of Venus is N-S axial-symmetric and nearly stable with mission time. The dawn side below 75 km is colder than the dusk side and vice versa above 75 km. A wavelength-dependent universal CO2 opacity correction (continuum) was retrieved applying the new multi-spectrum retrieval (MSR) algorithm. The continuum is common to all spectra and independent from local atmospheric structure and composition. This correction is essential for reliable lower cloud parameter retrievals and an important step forward in the investigation of gaseous opacity features under extreme pressure and temperature conditions. A comparative analysis of VIRTIS and PMV spectra was used for the first time to determine an optimal initial model of chemical composition and individual cloud mode altitude distributions. Cloud top altitude at 1 µm decreases from 71 km at low/mid-latitudes to 62 km at the poles. Cloud particle size increases from mid-latitudes towards the equator and the poles. Zonally averaged cloud opacities at 1 µm in the southern hemisphere vary between 32.5 at mid latitudes and 42 at polar latitudes. Zonal averages of CO abundances at 35 km increase by 35% from equatorial latitudes to 65° and S then slightly decrease towards the poles. This latitudinal variation is consistent with a Hadley-cell-like circulation. The latitudinal distribution of OCS at 35 km is anticorrelated with that of CO. H2O abundances slightly decrease with increasing absolute latitude by about 10%. The new retrieval results on atmospheric thermal structure, cloud parameters, and trace gas abundances provided a profound data base for improved radiative energy balance studies. Radiative energy balance calculations require radiative transfer (RT) simulations extending over the huge spectral range 0.1-200 µm in contrast to comparatively narrow ranges utilized for atmospheric state parameter retrievals. Modifications and optimizations of RT routines were performed. Retrieved atmospheric parameter fields have very different dimensions. An optimum data flow for radiative flux studies was implemented considering day and night conditions. Both spectral resolution and spectral range of gaseous absorption coefficients бa used in the retrieval procedures were not sufficient for radiative balance calculations. A tremendous computational effort was required to calculate new high-resolution бa look-up tables for temperature and pressure conditions in the upper mesosphere. A new model for the unknown UV absorber was developed. In contrast to earlier models, it is not directly linked to cloud particle modes and permits an investigation of radiative effects regardless of the absorber’s chemical composition. Detailed and very computer time consuming studies have encompassed all possible atmospheric state parameter variations and their influence on radiative fluxes and temperature change rates. The calculated cooling (heating) rates are very reliable at altitudes below 95 (85) km with maximum uncertainties of about 0.25 K/day. Heating uncertainties may reach 3-5 K/day at 100 km. Cooling rates strongly respond to variations of atmospheric thermal structure, while heating rates are less sensitive. The latter are mainly sensitive to insolation conditions and UV absorber distribution. The influence of mesospheric trace gas variations on radiative temperature change rates is small, but may become more important near the cloud base and in case of episodic SO2 boosts. Variations of cloud mode parameters (except of mode 1) (abundances, cloud top and base altitudes) may significantly alter radiative temperature change rates by more than 50% in Venus’ lower mesosphere and upper troposphere. Different kinds of 2D maps of atmospheric fluxes and temperature change rates were created. Atmospheric net heating dominates the low and mid latitudes above 82 km, while net cooling prevails at high latitudes at all mesospheric altitudes (60-100 km). On global average, the entire atmosphere of Venus at altitudes between 0 and 90 km is not far away from radiative equilibrium. General Circulation Models (GCM’s) require parameterized descriptions of radiative processes. A parameterization approach was developed that permits a fast and reliable calculation of thermal and solar temperature change rates at altitudes between 0 and 100 km. Mesospheric radiative equilibrium temperatures above the cloud top at the poles are up to 70 K lower and equatorial temperatures up to 10 K higher than observed values (e.g. VIRA) resulting in an average equator to pole gradient of about 35 K. The observed thermal structure can only be maintained by dynamical processes.
Publications
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2013. Self-consistent retrieval of temperature profiles and cloud structure in the northern hemisphere of Venus using VIRTIS/VEX and PMV/VENERA-15 radiation measurements. Planet. Space Sci. 89, 77-101
Haus, R., Kappel, D., Arnold, G.
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2014. Atmospheric thermal structure and cloud features in the southern hemisphere of Venus as retrieved from VIRTIS/VEX radiation measurements. Icarus 232C, 232-248
Haus, R., Kappel, D., Arnold, G.
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2015. Lower atmosphere minor gas abundances as retrieved from Venus Express VIRTIS-M-IR data at 2.3 µm. Planet. Space Sci. 105, 159-174
Haus, R., Kappel, D., Arnold, G.
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2015. Radiative heating and cooling in the middle and lower atmosphere of Venus and responses to atmospheric and spectroscopic parameter variations. Planet. Space Sci. 117, 262-294
Haus, R., Kappel, D., Arnold, G.
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2016. Multi-spectrum retrieval of Venus IR surface emissivity maps from VIRTIS/VEX nightside measurements of Themis Regio. Icarus 265, 42-62
Kappel, D., Arnold, G., Haus, R.
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2016. Radiative energy balance of Venus based on improved models of the middle and lower atmosphere. Icarus 272, 178-205
Haus, R., Kappel, D., Tellmann, S., Arnold, G., Piccioni. G., Drossart, P., Häusler, B.
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2016. Sensitivity of net thermal flux to the abundance of trace gases in the lower atmosphere of Venus. J. Geophys. Res. (Planets) 121(9), 1737-1752
Lee, Y.J., Sagawa, H., Haus, R., Stefani, S., Imamura, T., Titov, D.V., Piccioni, G.
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2017. Radiative energy balance of Venus: An approach to parameterize thermal cooling and solar heating rates. Icarus 284, 216-232
Haus, R., Kappel, D., Arnold, G.
