Radiative energy balance of Venus based on improved models of the middle and lower atmosphere
Introduction
Physical processes in a planetary atmosphere are based on three fundamental influence quantities, the input of solar energy including spatial and temporal variations due to both the planet's movement around its sun and its axial rotation, the release of energy by heated bodies, and the forces that result from planetary gravitation and rotation. Radiative energy conversion stimulates dynamical processes at all atmospheric levels and determines climate and weather in the lower atmosphere via a number of coupling effects. One of the main mysteries in our Solar System is the atmospheric superrotation that is present on Venus and on Saturn's Moon Titan. The origin of this phenomenon and the driving forces for the winds that blow faster than the bodies rotate are not well understood yet. To improve insights into atmospheric dynamics especially on Venus, detailed radiative energy balance studies are necessary. They will provide input parameters for General Circulation Models (GCMs) that are used to simulate and explain observed dynamical properties (e.g. Lebonnois et al., 2010). Calculation of the radiative budget at each level of the atmosphere that is determined by absorption and scattering of solar radiation as well as thermal emissions requires precise knowledge of the thermal state, gaseous and particulate constituent distributions, and specific interaction processes between radiation and matter considering spatial and temporal variations of atmospheric parameters.
Early orbital and lander missions to Venus (Mariner 2, 5, 10; Venera 4–16; Pioneer Venus 1 + 2; Vega 1 + 2; Magellan) and space experiments during the Venus flybys of Galileo/NIMS and Cassini/VIMS revealed basic information about pressure and temperature conditions on the surface, chemical composition, thermal structure and cloud composition of the atmosphere, large-scale atmospheric circulation features as well as surface topography (Arnold et al., 2012). But in spite of the many successful measurements, some fundamental problems in the physics of the planet remained unsolved. In particular, a systematic and long-term survey of the atmosphere was missing (Titov et al., 2008a).
Former analyses of radiative processes in the atmosphere of Venus were mainly based on Pioneer Venus and Venera-15 data (e.g. Pollack et al., 1980, Schofield and Taylor, 1982, Schofield and Taylor, 1983 Tomasko et al., 1985, Crisp, 1986, Crisp, 1989 Haus and Goering, 1990, Titov, 1995). Summaries of knowledge about the planetary energy balance characteristics and of open issues were given by Crisp and Titov, 1997, Taylor, 2006, and Titov et al., 2006, Titov et al., 2007, Titov et al., 2013). These investigations already revealed a strong sensitivity of radiative energy balance to atmospheric parameter variations like changes of temperature structure, cloud microphysical properties, and vertical distributions of individual cloud mode abundances. However, all these parameters were not strongly constrained by measurements before Venus Express with its great planetary mapping potential. Moreover, early analyses were hampered by computational constraints during that time, and the use of band models for gaseous absorbers was mostly unavoidable.
The latest mission to Venus (ESA's Venus Express, VEX) has carried the most powerful remote sensing suite of instruments ever flown to Earth's sister planet. Together with the other instruments (VMC, SPICAV/SOIR, VeRa, ASPERA, MAG), the Visible and Infrared Thermal Imaging Spectrometer VIRTIS has enabled for the first time a long-time study of the structure, composition, chemistry, and dynamics of the atmosphere and the cloud system, as well as investigations of the thermal and compositional characteristics of the planetary surface. VIRTIS (Piccioni et al., 2007a, Drossart et al., 2007, Arnold et al., 2012) provided an enormous amount of new data and a four-dimensional picture of the planet Venus (2D imaging, spectral dimension, temporal variations) on global scales. The spectral dimension permits a sounding of atmospheric properties at different altitude levels. VIRTIS-M-IR measurements during eight Venus solar days between April 2006 and October 2008 have been used by Haus et al., 2013, Haus et al., 2014, Haus et al., 2015b) to retrieve information on mesospheric nightside thermal structure and cloud features and on trace gas distributions in the lower atmosphere using new methodical approaches. Resulting maps for the southern hemisphere have covered parameter variations with altitude, latitude, local time, and mission time.
Temperature profile and cloud top altitude retrieval results using VIRTIS-M-IR spectra were also reported by Grassi et al., 2010, Grassi et al., 2014). Tellmann et al. (2009) used VeRa data to determine temperature profiles at altitudes between 45 and 90 km, while SPICAV/SOIR occultation measurements provided profiles at 90–140/80–170 km (Piccialli et al., 2014, Mahieux et al., 2012). Altimetry of the Venus cloud top based on VIRTIS, VMC, and VeRa data was also investigated by Titov et al., 2008b, Ignatiev et al., 2009, and Lee et al. (2012). Trace gas distributions below the cloud bottom based on VIRTIS-M-IR and -H measurements were studied by Tsang et al., 2008, Tsang et al., 2009, Tsang et al., 2010), Marcq et al., 2008, Bézard et al., 2009. The only recent two-dimensional analysis (altitude-latitude) of both radiative cooling and radiative heating in the atmosphere of Venus that considers atmospheric parameters retrieved from VEX instrument data (VeRa temperatures, VIRTIS-M-IR cloud parameters) was performed by Lee et al. (2015).
It is the main goal of the present paper to investigate atmospheric radiation fluxes (F) and temperature change rates (Q) in the middle and lower atmosphere of Venus (0–100 km) that are mainly based on improved three-dimensional atmospheric models (altitude-latitude-local time) retrieved from VIRTIS-M-IR data. An additional focus is the response of Q to the replacement of VIRTIS temperature profiles by VeRa data. The used approach is premised on the recently published precursor work of Haus et al. (2015b) where the mathematical and computational tools have been comprehensively outlined. Moreover, detailed studies of F and Q variability for possible atmospheric parameter variations based on initial atmospheric standard models have been performed there. F and Q responses to spectroscopic model changes have been also analyzed. These results enabled many new and improved insights into radiative cooling and heating characteristics. They now serve as reference for present investigations.
A summary of methodical approaches and recent modeling results to investigate the radiative energy balance of Venus is provided in retrospect in Section 2. Section 3 gives an overview of atmospheric parameter retrieval results obtained from VIRTIS-M-IR measurements by some of the present authors. Section 4 describes the used model of the unknown UV absorber in Venus’ mesosphere. Section 5 presents detailed results on radiative temperature change rates that are based on these improved models of the middle and lower atmosphere and compares the recalculated quantities with previous results obtained for an atmospheric standard model. Section 6 contains additional discussions on the role of the UV absorber, the Q variability based on VIRTIS data, comparisons of calculated Q profiles for VIRTIS temperature data with those determined from VeRa and SPICAV/SOIR temperature data, and comparisons of Q with recent results from the literature. The main results are summarized in Section 7.
Section snippets
Retrospect
This section recites the main methodical approaches and modeling results on Venus’ radiative energy balance from the recently published paper by Haus et al. (2015b). Calculated responses of radiative fluxes and temperature change rates to atmospheric and spectroscopic parameter variations were based on distinct variations of initial (standard) model data sets rather than using actual retrieval results especially with respect to cloud feature changes with latitude.
A radiative transfer simulation
Atmospheric parameter retrieval results based on VIRTIS measurements
New methodical approaches for self-consistent temperature profile and cloud parameter retrievals from VIRTIS-M-IR nightside radiation measurements were applied to investigate both the thermal structure and cloud features of Venus’ atmosphere on the northern and southern hemisphere (Haus et al., 2013, Haus et al., 2014). VIRTIS-M-IR was the mapper optical subsystem of the VIRTIS instrument at moderate spectral resolution (FWHM ∼17 nm) in the infrared spectral range from 1.0 to 5.1 μm. Temperature
Unknown UV absorber model
There is a broad depression in the observed spectral Bond albedo of Venus at wavelengths between 0.32 and about 0.8 µm that cannot be explained by known absorption features of gases or clouds. Shortward of 0.32 µm, SO2 UV absorption provides sufficient opacity to match the observed albedo features. A new model for this additional opacity source, which may be either composed of aerosol particles or of gaseous molecules or solid atom conglomerates or even mixtures of all these agents, was recently
Radiative cooling
Radiative temperature change rates are calculated according to Haus et al. (2015b), QΔλ(z, φ)is the atmospheric temperature change rate with time (t*) at level z and latitude φ considering integrated contributions from a defined spectral interval ∆λ per time unit [1 s]. Fn∆λ= F−∆λ − F+∆λ is the atmospheric net flux of radiative energy where F−∆λ and F+∆λ refer to the downward and upward directed flux components of either solar (sol) or thermal (the)
Influence of the UV absorber on planetary albedo and energy balance and a possible way of its quantification using VMC images
For a planet in global radiative balance, the total outgoing radiation flux must compensate the incoming flux. According to Haus et al. (2015b), this equilibrium condition is described by Eq. (5), E* = 2618.4 W/m2 is the solar constant at mean Venus–Sun distance (0.723 AU) that is based on the synthetic solar irradiance model of Kurucz (2011) here. σ is the Stefan–Boltzmann constant (σ=5.6704 × 10−8 W/(m2K4), TP is the effective planetary emission temperature. The quantity A is the Bond
Summary and conclusions
The radiative transfer simulation model described by Haus et al. (2015b) is applied to calculate fluxes and temperature change rates in the atmosphere of Venus at altitudes between 0 and 100 km. The calculations are performed separately for thermal (1.67–1000 µm) and solar (0.125–1000 µm) flux components. Improved models of atmospheric parameters are utilized that have been retrieved mainly from VIRTIS-M-IR (VEX) measurements. This concerns nightside temperature altitude profiles as well as cloud
Acknowledgments
R.H. is funded by the German Research Foundation under grant number HA 2887/2-2. We acknowledge the work of the VIRTIS/VEX and VeRa/VEX teams and also the entire Venus Express team of ESA and Astrium, who made the measurement data available that were used in this study.
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