Radiative energy balance of Venus based on improved models of the middle and lower atmosphere

Abstract

The distribution of sources and sinks of radiative energy forces the atmospheric dynamics. The radiative transfer simulation model described by Haus et al. (2015b) is applied to calculate fluxes and temperature change rates in the middle and lower atmosphere of Venus (0-100 km) covering the energetic significant spectral range 0.125-1000 μm. The calculations rely on improved models of atmospheric parameters (temperature profiles, cloud parameters, trace gas abundances) retrieved from Venus Express (VEX) data (mainly VIRTIS-M-IR, but also VeRa and SPICAV/SOIR with respect to temperature results). The earlier observed pronounced sensitivity of the radiative energy balance of Venus to atmospheric parameter variations is confirmed, but present detailed comparative analyses of possible influence quantities ensure unprecedented insights into radiative forcing on Venus by contrast with former studies. Thermal radiation induced atmospheric cooling rates strongly depend on temperature structure and cloud composition, while heating rates are mainly sensitive to insolation conditions and UV absorber distribution. Cooling and heating rate responses to trace gas variations and cloud mode 1 abundance changes are small, but observed variations of cloud mode 2 abundances and altitude profiles reduce cooling at altitudes 65-80 km poleward of 50°S by up to 30% compared to the neglect of cloud parameter changes. Cooling rate variations with local time below 80 km are in the same order of magnitude. Radiative effects of the unknown UV absorber are modeled considering a proxy that is based on a suitable parameterization of optical properties, not on a specific chemical composition, and that is independent of the used cloud model. The UV absorber doubles equatorial heating near 68 km. Global average radiative equilibrium at the top of atmosphere (TOA) is characterized by the net flux balance of 156 W/m2, the Bond albedo of 0.76, and the effective planetary emission temperature of 228.5 K in accordance with earlier results. TOA radiative equilibrium can be achieved by slight adjustments of either UV absorber or cloud mode abundances. Ratios of synthetic spectral albedo values at 0.36 μm calculated for different abundance factors of the UV absorber are suggested to provide a possible tool to interpret observed VMC/VEX brightness variations with respect to actual absorber abundances. 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). This radiative forcing field has to be balanced by dynamical processes to maintain the observed thermal structure. A similar but much smaller meridional gradient is also observed at altitudes between 62 and 72 km where the unknown UV absorber provides additional heating. At these altitudes, equatorial net heating dominates net cooling from about 07:30 h until 16:30 h local time. Intermediate altitudes (72-82 km) are characterized by net cooling at all latitudes in case of VIRTIS temperature data. This planet-wide net cooling region is not observed when calculations are based on VeRa temperatures, and low latitudes are then characterized by small net heating. When a warm atmospheric layer as detected by SPICAV/SOIR around 100 km is considered, strong global average net cooling occurs above 90 km that is far away from radiative equilibrium. A weak net cooling layer (1-2 K/day) exists at altitudes between 55 and 60 km, while very weak net heating (0.1-0.5 K/day) takes place near the cloud base (48 km). Almost zero net heating prevails in the deep atmosphere below 44 km. On global average, the entire atmosphere of Venus at altitudes between 0 and 90 km is not far away from radiative equilibrium (usually within± 2 K/day). Maximum temperature change rate deviations from mean values at each altitude and latitude are defined based on retrieved atmospheric parameter single standard deviations using VIRTIS data. This is an important prerequisite to investigate parameterization approaches for the calculation of atmospheric temperature change rates that can be used in Global Circulation Models. This will be a major topic of future studies on radiative energy balance of Venus' atmosphere.