Thesis

Gravity waves: a key process in Venus and Mars middle/upper atmosphere

• Call:

  IDPASC Portugal - PHD Programme 2019

• Academic Year:

  2019 / 2020

• Domain:

  Astrophysics

• Supervisor:

  Pedro Machado

• Co-Supervisor:

  Gabriella Gilli

• Institution:

  Faculdade de Ciências - Universidade de Lisboa

• Host Institution:

  Faculdade de Ciências - Universidade de Lisboa

• Abstract:

  Recent observations by the ESA satellite Venus Express (VEx) (2006-2014) [Drossart2007, Titov2008, Limaye2017] and on-going ground-based campaigns, also led by IA Team Members, allowed to carry out an unprecedented characterization of winds [Machado2014, Machado2017, Widemann2008, Peralta2017] and atmospheric temperatures of Venus [Gilli2015, Piccialli2015, Mahieux2015]. Those new measurements significantly improved our comprehension of Venus atmospheric circulation, and achieved new valuable constrains in atmospheric dynamics of planets with superrotation. At the same time, they put in evidence the high variability of the Venus atmosphere and they opened new scientific questions such as: what processes control the transition region (70-120 km) between the retrograde superrotating zonal flow and day-to-night circulation? How does the interplay of planetary and small-scale waves control the circulation features? Recent advancements in observational techniques are expected to bring new constrains on Venus atmospheric models at cloud top level including the capability to track short-term variations. In particular, temporal and spatial variability of winds need to be better quantified and the role of waves and the mechanisms that allow the topography to influence the upper cloud motions need to be better addressed with the help of 3D models. An important new result led by IA team using the Doppler velocimetry technique is the evidence of a symmetrical, poleward meridional Hadley flow in both hemispheres of Venus [Machado2017]. A complete interpretation of those GW induced temporal and spatial variations in the atmospheres of planets is possible with the application of 3D models. Sophisticated modeling tools have been developed by Team Members, such as Global Circulation Models (GCM) for Mars [Forget1999, Bougher2000] and Venus [Lebonnois2010, Brecht2012]. They are unique tools to support the exploration by remote sounding. Current improved versions of Venus and Mars GCM developed at the Laboratoire de Meterorologie Dynamique (LMD) described in Gilli2017a and GonzalezGalindo2005, respectively, are nowadays the only ground-to-thermosphere available GCMs for those terrestrial planets. In order to address key measurements in the middle/upper atmosphere of Venus and Mars and to interpret the large observed variability, a non-orographic GW parameterization, following the formalism developed for the Earth LMD-GCM [LottGuez2012], was recently implemented in those models [Gilli2017a,Gilli2017b]. The preliminary results are very promising: the inclusion of this key physical mechanism could partially explain data-model biases at mesospheric layers. However, given the uncertainty in the wave basic characteristics, excitation mechanisms and sources of GW, it is difficult to quantify the GW effects using a unique set of parameters, and detailed sensitivity studies are required. Scientific Goal: For this proposal of PhD Fellowship, we propose a systematic characterization and classification of the waves apparent on Venus using remote sensing images from Venus Express (VMC and VIRTIS-M) (Svedhem et al. 2007) and Akatsuki cameras (UVI, IR1, IR2 and LIR) (Nakamura et al. 2007). In the case of remote sensing images, the wave amplitudes can be derived by means of devoted Radiative Transfer models. Of critical importance for the CGMs is the systematic characterization and classification of atmospheric waves on Venus, what will allow to provide accurate estimations of the energy transport in these atmospheres. Proposed Strategy: This PhD project is intended to combine synergistically space and ground-based observations with model simulations to improve the understanding of physical and dynamical processes in the atmosphere of terrestrial planets. After a systematic search and characterization of waves present in the imagery data sets of Venus (VEx/VMC and VIRTIS-M, Akatsuki/UVI, IR1, IR2 and LIR), the PhD student will build dispersion graphs to enable a classification of the real nature of these waves and the restoration forces responsible for them (Peralta et al. 2014a; 2014b). Their wave amplitude will be first-time estimated thanks to the use of different Radiative Transfer Models (RTM): (a) the amplitude of waves in 280-nm images from VIRTIS-M and UVI will be obtained with retrievals of SO2 abundance using the RTM NEMESIS (Irwin et al. 2008); (b) 1.74, 2.26, and 2.3-μm images from VIRTIS-M and IR2 can be used to estimate normalized temperature amplitudes of waves from clouds’ opacity using the RTM by McGouldrick & Toon (2008); (c) to estimate the amplitude of the mesoscale stationary waves apparent on the nightside upper clouds of Venus, we will estimate the brightness temperature using the 4.3-µm CO2 band (ranges 4.24–4.54 µm and 4.77–5.01 µm) observable in the spectra of the VIRTIS-M cubes (Peralta et al. 2017b) and the RTM by García-Muñoz et al. (2013); (d) the 10-μm images from Akatsuki/LIR will enable to constrain the amplitudes of the main giant stationary waves apparent on the brightness temperature of Venus’s upper clouds (Kouyama et al. 2017). On the modeling side, the candidate will make use of sophisticated theoretical tools such as Global Circulation Models (GCM). In particular, we plan to use a current improved version of the Venus GCM developed at the Laboratoire de Meteorologie Dynamique (LMD) (hereinafter LMD-VGCM) [Lebonnois2016, Gilli2017a] and the last improved version of the Mars GCM, also developed at LMD (hereinafter LMD-MGCM) [Forget2017]. Models validation and inter-comparison will be performed thanks to the collaboration with partners at Michigan University and NASA Ames Institute, world references for Venus and Mars thermosphere GCMs (hereinafter VTGCM and MTGCM) [Bougher2000, Brecht2012].