The diversity of mechanisms responsible for interactions between the (vegetated) land and the atmosphere range from the size of the stomata (10 – 100μm) to the size of the atmospheric boundary layer (~1 km). This includes temporal dynamics on time scales of minutes (passing clouds and plant responses), days (diurnal solar cycle) and seasons (seasonal solar cycle and vegetation dynamics) (TABLE).
Clouds as well as atmospheric-boundary layer dynamics disturb radiation and turbulence conditions in and above the vegetation canopy and, subsequently, the energy and moisture fluxes that affect clouds. Although these effects occur at short spatiotemporal scales, they have profound impacts on the regional and global CO2 budget. CloudRoots aims to advance current understanding of land-atmosphere dynamics. Therefore, it is essential to investigate, using first-principles, the cross-scale interactions between the relevant processes in an integrated observation – simulation system.
At the leaf level, CloudRoots will observe and attempt to mechanistically represent the exchange of carbon dioxide and water vapor through the stomatal aperture. One crucial aspect of this exchange is to understand to what degree plant stomatal control depends on the partitioning of direct and diffuse radiation perturbed by clouds (cloud flecks). We also need to understand to what extent the penetration of radiation into the canopy as well as temperature and vapor pressure deficit control photosynthesis. Photosynthesis and stomatal aperture are described by the leaf mechanistic model A-gs (A for photosynthesis and gs for the conductivity at leaf level, see figure below). This representation also includes the exchange of the light and heavy stable isotopologues of CO2 and H2O (CHEMISTRY). A-gs is used in the atmospheric models DALES and CLASS (see MODELLING). Soil evaporation and carbon dioxide respiration fluxes are represented by models that link soil properties to the in-canopy atmosphere. All these representations are supported by eco-physiological measurements (see ECOPHYS).
The CloudRoots representation of leaf, soil and cloud droplet CO2 and H2O exchanges
CloudRoots focuses on how the available energy is partitioned on surface fluxes at the soil and by plants. Measuring the concentration and isofluxes of stable isotopologues of CO2, H2O enable us to quantify how much of the evaporation is driven by soil or by plants. Particular attention is paid on how to scale up the leaf processes to the canopy processes. To this end, the A-gs model (see LEAF) is upscaled to the canopy level. It is at the canopy level where the turbulent fluxes need to be accurately represented to adequately reproduce the dynamics of the clear and cloudy boundary-layers. In all the planned field campaigns, we attempt to measure these fluxes using eddy-covariance and scintillometer techniques (see INSTRUMENTS). With the later technique, we will measure 1-minute turbulent measurements of the carbon dioxide and water vapor stable isotopologues (isofluxes). Here, we expect to collect evidence on how the turbulent fluxes evolve under transition periods (clouds, diurnal and weather variability) and their partitioning. This research is connected to the explicit simulation of these turbulent fluxes. Here, we can make use of two sorts of canopy representations: multi-layer and bulk. Using the first approach, we can represent the transfer of radiation and the state meteorological and atmospheric composition variables within the canopy at the sub-meter scale. The bulk canopy approach enables us to determine the performance of more crude representations, like the ones used in weather and climate models, on the canopy turbulent fluxes.
Moving up in the spatial scales, CloudRoots focuses on the landscape to study the effect of surface heterogeneity on the surface exchange fluxes and their impact on the boundary-layer dynamics. The landscape scale (the sub-kilometers scales) are still not well-resolved in weather and climate models. By measuring and simulating turbulent flux measurements of the carbon dioxide and water vapor stable isotopologues (isofluxes), we can determine how the turbulent fluxes change due to the contribution and aggregation of distinct fluxes controlled by different landscapes, and therefore, different partitioning properties. This will be further supported by simulating the isofluxes connected to the boundary-layer development using DALES and CLASS. Special attention will be paid to how grasslands and forests at the ecosystem level (see ECOSYSTEMS) react under different cloud, stress (drought) and heatwave conditions.
The CloudRoots representation of processes and their interactions at canopy, landscape and cloud scales
It is at the regional and global scales where we need to quantify how an improved description of the processes will yield better surface fluxes and improved descriptions of the interactions with clouds. Our hypothesis is that better representations of surface fluxes will lead a more accurate description of boundary-layer dynamics and clouds. To this end, our plan is to embed the large-eddy simulation to a high-resolution global model, ECMWF-IFS, and perform validations of surface and upper atmospheric conditions for multiple ecosystems over timescales ranging from one week to a full season.