Impact of clouds on the forest albedo measured at the Leipzig Canopy Crane - A pilot study

Albedo, defined as the ratio between reflected radiation and total incoming radiation, is a key variable in the Earth radiative budget. In a fast changing climate with more frequent extreme events, such as droughts and excessive heat, vegetation is under constant stress. Such stress factors might modify the tree physiology, the reflectivity of individual leaves, and, eventually, the forest albedo as an entity. This might alter the local radiative budget and contribute to changes in the local climate, e.g., intensifying drought - a potential feedback loop. The understating of those effects might be further complicated by the occurrence of clouds. Therefore, this study presents spectral solar measurements of upward and downward irradiance that are used to determine the spectral albedo over a forest canopy. Since June 2021, ongoing measurements are performed on top of the Leipzig Canopy Crane located in the Leipzig floodplain forest. The measurements are separated for illumination geometries, i.e., the solar zenith angle, as well as for different cloud conditions. The interpretation of the measurements is aided and validated by coupled radiative transfer simulations using the library for radiative transfer model (libRadtran) and the Soil Canopy Observation of Photosynthesis and Energy fluxes (SCOPE2.0) model. Both models allow for simulations in the visible, near- and far-infrared wavelength range. By that, the impact of clouds on the spectral and broad band albedo, as well as the net radiative budget can be investigated. First simulations revealed that the presence of clouds enhance the spectral forest albedo. The magnitude of the effect is controlled by the cloud optical thickness, i.e., the ratio of direct and diffuse radiation. The enhancement is more pronounced for small solar zenith angles. However, the effect from clouds appears to be smaller than influences of variations in the surface properties. The presentation aims to outline the measurement set-up and strategy, and to discuss preliminary results. Furthermore, the new, iterative coupling of the atmosphere and soil-vegetation model is presented, which aims to improve the understating of cloud-vegetation radiation interactions.