The Shedding Light On Cloud Shadows project: measuring and simulating surface solar irradiance under broken clouds

Boundary-layer clouds trigger large fluctuations in solar surface irradiance and in surface heat fluxes. The incoming radiation in shadows is almost an order of magnitude less than under clear sky, while peaks near clouds shadows can sometimes reach a 50% increase with respect to clear sky, due to scattering of sunlight in cloud edges. Performing large-eddy simulation (LES) with realistic surface solar irradiance patterns under broken clouds remains a challenge. First, this is due to the absence of spatial radiation observations that capture individual cloud shadows at a typical LES resolution (~50 m), second, because cloud fields need to be accurate up to a very high detail level and, third, the 3D aspects of radiative transfer needs to taken into account.

The Shedding Light On Cloud Shadows (SLOCS) project aims to overcome these challenges by i) gathering spatial observations in a spatial grid fine enough to capture individual clouds with a novel instrument, and ii) further developing 3D radiative transfer models for LES with optimal balance between detail level and performance. The FESSTVaL campaign in Lindenberg, Germany in early summer 2021 provided a unique opportunity for the SLOCS team to address those challenges. In FESSTVaL, we performed grid measurements of radiation, while benefiting from complementary boundary-layer and cloud observations of other research groups. In addition, FESSTVaL brought a boundary-layer modelling community together ranging from people working on NWP models to people working on fine-scale LES. This permitted comparison of the ability of different modelling techniques to capture surface irradiance variability driven by clouds.

Here, we will present the design and the results of LES of four selected case studies based on FESSTVaL data: one clear sky case, two shallow cumulus cases with different cloud depths, and one deep convection case with cold pools. First, we will show how we have extended and accelerated the RTE+RRTMGP radiation model to take into account 3D interactions between clouds and radiation, to enable comparison against our grid observations near the Falkenberg measurement tower. Second, we will present the outcome of the LES of the four cases and evaluate simulated solar and turbulence surface fluxes, boundary layer structures and cloud properties against FESSTVaL observations. Third, we will show a comparison between our own 25 m resolution large-eddy simulations with GPU-accelerated MicroHH against 100 m resolution simulations of the ICON large-eddy model and 2 km resolution simulations of the ICON NWP model. In our analyses, we compare the different modelling techniques in their ability to reproduce the FESSTVaL cases. This comparison will address the balance between grid resolution and domain size as well as the necessity for lateral inflow and outflow boundary conditions in modelling accurate surface solar irradiance fluxes.