tokens: shown to scale. The temperature and abundance maps of the ∇ − T case are almost the same, except the scale height of the atmosphere is smaller. Fig. 5 Abundance profiles for H 2 O and H 2 plotted for a range of temperature between 2000 and 3000 K. At a given pressure, H 2 O and H 2 dissociation depends strongly on the temperature. The higher the temperature, the lower the abundance in the atmosphere. At a fixed temperature the variation of the dissociation rate is a function of P and P 2 , respectively, for H 2 and H 2 O. 2.3 Retrieval model: TauREx The spectral retrievals has been computed with the TauREx retrieval code ( Waldmann et al. 2015b , a ). TauREx consists in aBayesian retrieval code which uses the ExoMol line lists ( Yurchenko et al. 2011 , 2014 ; Tennyson Yurchenko 2012 ; Barton et al. 2013 , 2014 ), HITRAN ( Rothman et al. 2009 ; Gordon et al. 2013 ) and HITEMP ( Gordon et al. 2010 ). TauREx assumes a plane-parallel exoplanetary atmosphere with 1D distribution elements and temperature-pressure profile. We assume the radius at the base of the model of Wasp-121b as the value for which the atmospheric pressure is 10 bar. We simulate the exoplanetary atmosphere assuming a pressure between 10 6 and 10 −4 Pascals. We consider an atmosphere dominated by hydrogen and helium, with a mean molecular weight of 2.3 amu. We also suppose the presence of CO and H 2 O as trace gases. An important assumption is that TauREx does not take into account thermal dissociation of species and assumes that molecular abundances are constant with the atmospheric pressure (see Changeat et al. 2019 for a detailed discussion on the effect of this assumption). We set up the prior range for the molecular volume mixing ratios of CO and H 2 O between 10 −12 and 10 −1 , which allows us to explore a wide range of solutions. Even if we never add clouds in our input simulation, we set up the prior range for clouds between 10 −12 and 10 −1 to allow theretrieval code to have more freedom. We also feel that this better simulates a real situation where clouds would be put in the retrieval without prior knowledge about their occurrence in the real planet. The allowed range for the planetary radius and the atmospheric temperature (assumed constant in the vertical) are, respectively, 1.3 R Jup − 4.0 R Jup and 400−3600 K, with R Jup the radius of Jupiter. The range for the temperature is linked to the minimum and maximum temperature in the GCM simulation of Wasp-121b, as shown in Table 1 . As we simulate JWST observations, we set up the range of wavelengths between 0.6 to 10 μ m. For the non-retrieved parameters such as masses and stellar radius, we use those describe in Table 2 . Table 1 Parameters used in the three simulation configurations of WASP-121b. Table 2 Mass, radius, and temperature parameters of the WASP-121 star (top) and its planet Wasp-121 b (bottom). Notes. They are used for the observable generations and for the retrievals. We note that the TauREx retrieval
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