Just like on the Earth, there are winds and clouds on Venus... but contrary to our planet, the surface of Venus is very warm, more than 700K, and the pressure is huge, leading to a lot of exotic meteorological events.
Venus' atmosphere is a complete and complex system, governed by specific physical, dynamical and chemical processes, which can be observed and understood.
The diurnal cycle, combined to the EUV solar flux and cloud albedo variabilities, make the planet's climate quite variable in space and time.
These variations can be modelled and predicted, and this is what the Venus Climate Database is about.
What is the Venus Climate Database?
The Venus Climate Database (VCD) aims at providing scientists, engineers and enthusiasts with a realistic and reliable modelisation of the Venusian climatological system.
The VCD is a database of meteorological fields derived from General Circulation Model (GCM) numerical simulations of the Venusian atmosphere and validated using available observational data.
The GCM is developed at LMD (Paris, France), in collaboration with LATMOS (both laboratories belonging to the Insitute Pierre Simon Laplace, IPSL) with support from CNES and from the European Space Agency (ESA).
The VCD is freely distributed and intended to be useful and used in the framework of engineering applications as well as in the context of scientific studies which require accurate knowledge of the state of the Venusian atmosphere.
The sign convention for wind components in the Venus Climate Database
Note that in the Venus Climate Database the convention for zonal winds is reversed, compared to Earth: Venus zonal wind is taken to be positive when westward (i.e. when going from East to West).
As for the meridional component, the convention is the same as for Earth, a northward wind is positive.
Combinations of solar and cloud albedo scenarios have been used because these are
the two forcings that are highly variable from year to year.
On the one hand, the solar conditions describe variations in the Extreme UV input which control the
heating of the atmosphere above around 100 km, which typically varies on an 11-year cycle. Depending
on the scenarios, fixed (i.e. constant over time) solar minimum, average or maximum conditions
are provided. The minimum, average and maximum EUV scenarios correspond, respectively,
to a value of E10.7 factor of 70, 140 and 300 sfu (solar flux unit : 1 sfu = 10e-22 W/m²/Hz).
The user also has the possibility to automatically deduce the EUV value from the input Earth date
(computed from the "Solar2000 empirical solar irradiance model" of
Tobiska et al. (2000)),
or to directly set the solar EUV at a desired value. A linear interpolation is then used between
minimum and average or average and maximum solar EUV scenarios.
On the other hand, there are still uncertainties about the Venus solar heating rate. As shown in the
work of Lee et al. (2019),
this Venus solar heating rate is directly related to the cloud albedo through a radiative transfer model.
Therefore the heating rate is calculated for each scenario in order to simulate the albedo measured by
Hubble and MESSENGER mission during its flight over Venus. Three kinds of cloud albedo scenarios are
proposed:
The Standard cloud albedo scenario is calculated using our standard heating rate profile, based on
Haus et al. (2014).
The Low cloud albedo scenario is calculated using the maximum ratio profile
from Fig. 13 (right panel) in Lee et al. (2019), with a maximum ratio of 50%
(instead of 40% in Lee et al. (2019)).
The High cloud albedo scenario is calculated by decreasing the standard heating rate profile by 30%.
Every cloud albedo scenarios are provided with each of the three previously mentionned solar EUV conditions (average, minimum and maximum).
The Venus Climate Database has been compiled from the output of a General Circulation Model
in which the topography is very smoothed because of its low resolution. The VCD includes (and uses)
topography at the GCM resolution (96x96 in longitudexlatitude). The access software includes
a high resolution mode with a high resolution (23 pixels/degree) topography measured with
Magellan
radar altimeter (combined with a few topography measurements from
Pioneer Venus to fill in
the gaps) in order to compute surface pressure as accurately as possible. In order to correct the
atmospheric mass, we also add a correction with a surface pressure measurement from
Vega 2.
The surface pressure is then used to reconstruct vertical pressure levels and hence, within the
restrictions of the procedure, to yield high resolution values of atmospheric variables.
A General Circulation Model (GCM) is a numerical model that computes the temporal evolution of the atmosphere of a planet. It solves well known equations of motion and thermodynamics on a 3D grid that covers the entire atmosphere.
The Venus Planetary Climate Model, mainly developed at LMD and LATMOS inside the IPSL, computes in 3D the atmospheric circulation and climate taking into account radiative transfer through the gaseous atmosphere, includes a representation of non-LTE processes, EUV heating, conduction and molecular diffusion in the thermosphere. Sub-grid processes such as convection in the boundary layer, non orographic and orographic gravity waves are also simulated. Photochemistry that controls the atmospheric composition is also implemented in the model.