McRALI: a Monte Carlo high spectral resolution lidar and Doppler radar simulator for three-dimensional cloudy atmosphere remote sensing

Abstract. The aim of this paper is to present the Monte-Carlo code McRALI that provides simulations, under multiple scattering regimes of polarized high spectral resolution (HRS) lidar as well as Doppler radar observations for three-dimensional (3D) cloudy atmosphere. The effects of non-uniform beam filling (NUBF) on HSR lidar and Doppler radar signals related to the EarthCARE mission are investigated with the help of an academic 3D box-cloud, characterized by a single isolated jump in cloud optical depth, assuming vertically constant wind velocity. Regarding Doppler radar signals, it is confirmed that NUBF induces a severe bias in velocity estimates. The correlation of the NUBF bias of Doppler velocity with the horizontal gradient of reflectivity shows a correlation coefficient value around 0.15 m s−1(dBZ km−1)−1 close to that given in scientific literature. Regarding HSR lidar signals, we confirm that multiple scattering processes are not negligible. We show that NUBF effects on molecular, particulate and total attenuated backscatter are mainly due to unresolved variability of cloud inside the receiver field of view, and to a lesser extent, to the horizontal photon transport. This finding gives some insight into the reliability of lidar signal modelling using independent column approximation (ICA).