Cloud-edge and in-cloud parallax effects in the polarimetric cloud property retrievals

Abstract:

In practice, the realistic 3D nature of the radiative transfer has to be replaced by the 1D radiative transfer assumptions in many remote sensing applications mainly for the sake of computational efficiency. When the realistic atmosphere deviates from these assumptions, the retrieval technique is susceptible to the biases that are often referred to as "3D Radiative Transfer (RT) effects”. A large volume of studies has been performed to investigate the 3D RT effects in the well-known bi-spectral (Nakajima-King) retrievals. However, the 3D RT effects in the polarimetric retrieval technique which exploits the angular pattern of the polarized reflectance to retrieve the cloud effective radius (CER) and cloud effective variance (CEV) have not been extensively investigated. In this work, we used large-eddy simulation (LES) cloud fields coupled with MSCART (Multiple-Scaling-based Cloudy Atmospheric Radiative Transfer) radiative transfer simulations to investigate the 3D RT effects in the polarimetric cloud retrieval technique. We identified two mechanisms, namely (1) The cloud-edge parallax effects; (2) The in-cloud parallax effects that could potentially induce biases in the polarimetric cloud retrievals. The biases due to both effects are more prominent in the scattered cumulus cloud fields, especially when the accurate cloud top determination is challenging while the biases are less significant in the stratocumulus cloud fields with minimal cloud top variability.