Radiative transfer modeling in the Earth–Atmosphere system with DART model

Abstract

The atmosphere strongly affects satellite measurements of Earth surfaces in the optical domain. Modeling this influence is complex. This is typically the case of the “Earth–Atmosphere” radiative coupling in the presence of Earth surfaces with spatially variable optical properties. In that case, it may be very difficult to couple Earth and cloud-free atmosphere radiative transfer models. This explains why an atmosphere module was input into the Earth radiative transfer (R.T.) model DART (Discrete Anisotropic Radiative Transfer) in order to simulate accurately satellite images of natural and urban Earth surfaces. This paper presents how DART simulates the atmosphere R.T. in the short wave and thermal infrared domains. The atmosphere is divided into 3 zones: bottom atmosphere (BA), mid atmosphere (MA) and high atmosphere (HA). The 3D distribution is arbitrary in BA and horizontally constant with any vertical distribution in MA and HA. The “Earth–Atmosphere” R.T. is modeled in 5 stages. 1) Atmosphere R.T. (i.e., atmosphere thermal emission and/or sun radiation scattering). 2) Earth surface R.T. (i.e., Earth thermal emission and/or atmosphere and direct sun radiation scattering). 3) Atmosphere R.T. (i.e., Earth radiation scattering). 4) Earth surface R.T. (i.e., scattering of downward atmosphere radiation). 5) Simulation of satellite reflectance and/or brightness temperature images. The approach takes into account the earth curvature and the atmosphere non-Beer law behavior in the presence of strongly varying spectral properties. It uses optimally located scattering points for improving atmosphere R.T. accuracy, and it reduces computer time through the use of pre-computed transfer functions that transfer radiation between the different atmosphere levels (BA, MA, HA). Moreover, it can simulate automatically an atmosphere geometry that optimizes the trade-off “Computer time–Accuracy” of simulations. The robustness and accuracy of the DART atmosphere modeling were successfully validated with theoretical cases and with the MODTRAN atmosphere R.T. model.