Validation of the IPSL Venus general circulation model with Venus Express data

Several numerical models devoted to the simulation of Venus atmosphere have been developed. These models are useful instruments in the understanding of the mechanisms behind the observational features. However, before using their outputs to drive any conclusion about the dynamics and the structure of the atmosphere of Venus, we need to validate them. The process of validation passes by a comparison of the modelled and observational features. Among these numerical models, the Institut Pierre Simon Laplace (IPSL) Venus GCM is the one with a more physical approach, being capable to solve the radiative transfer for each layer of the simulated atmosphere. Our study makes use of Venus Express data – in particular VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) and VeRa (Venus Express Radio Science Experiment) observations – in order to validate this model. This work will analyze the temperature and wind fields in the atmosphere of Venus between 50 and 90 km, that is the range covered by observations. In this range – covering from the upper troposphere to the upper mesosphere – two different regimes are found in the observational thermal field, above and below ~ 76 km, with temperatures increasing towards the pole and towards the equator, respectively. At cloud top level (~ 68 km), permanent cold features, the cold collars, encircle the warmer poles. Winds velocities reach their maximum values (~ 120 m/s) at cloud top, but are faster than the solid body through the entire range of altitudes, determining a condition called superrotation. Seasonal thermal tides are negligible in data, but those related to the diurnal cycle, are present and have a large impact, especially in the upper atmosphere. Venus modelling has always suffered from the strong dependence of the simulation by the initial conditions and the different dynamical cores. Winds far weaker than observed, as well as the inability to reproduce the complex polar vortexes and the subpolar cold regions, have been the major issues for all the numerical simulations for Venus. However, to simulate the fast rotation of the atmosphere and to properly model the thermal structure associated to the polar and subpolar regions, means to understand the physical conditions under which these characteristics develop. Thus, the first objective of our validation of the IPSL Venus GCM is to estimate the general characteristics of the modelled atmosphere and their resemblance of observations. A first, qualitative comparison, is fundamental in recognizing the main dynamical regions in Venus atmosphere. Being the main goal of this work, the validation of a model through its comparison with the thermal and winds field in observational data, we need to clarify the adopted ingredients and the state of the art of our knowledge of Venus. Thus, in chapter 1 we present the overall characteristics of Venus atmosphere, in terms of its composition, thermal structure and dynamics. In chapter 2 we discuss the Venus Express mission, with a focus on the VIRTIS and VeRa experiments and the datasets that we used in this analysis. In chapter 3 we describe the evolution and the present state of the major numerical models trying to simulate the atmosphere of Venus, with a particular emphasis on the IPSL Venus GCM. Chapter 4 and chapter 5 present our validation: the former concerns the analysis of the average temperature and wind fields, the latter is about the thermal tides affecting the temperature and wind fields. As a result, we recognize the capability of the model to reproduce the main observational feature and we propose future steps in order to overcome the major discrepancies that we found in our validation.