Tracking 3D Cloud-Induced Radiative Effects of individual cumulus cells within ICON-MYSTIC Case studies

Clouds are the largest source of uncertainty in the Earth's energy budget and in operational weather and climate models. However, because radiative transfer is among the most computationally expensive physical parameterisations it typically relies on the independent column approximation (ICA). This neglects horizontal photon transport and therefore introduces systematic biases in surface shortwave CRE up to 120% depending on solar zenith angle (Hogan et al. 2016), particularly beneath and around spatially heterogeneous broken cumulus clouds. The radiative impact of these conditions is moreover highly transient and evolves over the lifetime of a single cumulus cell. As the cloud grows into a thicker, more reflective cloud, ICA cannot correctly capture shadow position and extent on hectometer scales and entirely misses the surface irradiance enhancements above clear-sky levels caused by side-scattering at the edges or neighboring thin clouds.

 

The DFG funded research unit C3SAR (Cloud 3d Structure And Radiation) combines ground-based, in situ, and satellite observations with ICON-NWP simulations and the Monte Carlo radiative transfer model MYSTIC to investigate how 3D cloud variability alters radiative fluxes and the Earth's energy budget. We use ICON cloud microphysics and atmospheric profiles as offline input to MYSTIC to compare ICA and fully 3D radiative calculations against clear-sky simulations. The tobac (Tracking and Object-Based Analysis of Clouds) package is used to track single cumulus cells and their (3D)CRE throughout the simulation. This framework enables physically consistent 3D radiative studies on regional scales larger than most large-eddy simulations and with more realistic cloud scenes than previous idealised studies.

 

In the presented case study we analyse the temporal evolution of individual cumulus cells to quantify cloud-induced and three-dimensional radiative effects on surface solar irradiance. The preliminary results demonstrate the value of ICON-MYSTIC synergy for closure studies combing pyranometer network, cloud radar and allsky imager observations from the C3SAR 2026 campaign.