So far, the vast majority of close-in exoplanets that have been discovered are gaseous giants or rocky planets roughly of the size, respectively, of Jupiter and the Earth. One of the most common explanation to account for the lack of intermediate size planets, also called the Neptune's desert, is a high atmospheric escape in their early stage of life leading to the incapacity for the planet to retain a Hydrogen/Helium (H/He) envelope. In recent years, the high-resolution transmission spectroscopy of transiting exoplanets in the metastable Helium triplet line (He $2^3 \mathrm{S}$) has been extensively used to infer basic properties of exoplanets' thermosphere. However, most of the studies rely on 1D Parker wind models to generate the upper atmosphere which does not account for asymmetries between the day- and night-sides. This limitation is particularly important when advanced 3D codes are used to simulate the frame by frame transit. In this context, we plan to develop a 2D model for a H/He upper atmosphere including the chemistry of the Helium triplet. This model will incorporate a self-consistent dynamics of the flow and winds as well as a temperature profile of the planet assuming symmetry with respect to the star-planet axis. Interactions with the star will be encapsulated through a radiative transfer assuming a simplified geometry. This 2D thermospheric model will be implemented in the 3D framework of the \texttt{EVE} code to simulate the time-dependent transit of the planet, accounting for the inhomogeneities of the stellar surface and the 3D geometry of the transit. We hope with this model to characterise the structure of exoplanets' upper atmosphere and quantify day/night-side asymmetries through high-resolution transmission spectroscopy of the Helium triplet line.