CloudRoots: integration of advanced instrumental techniques and process modelling of sub-hourly and sub-kilometre land–atmosphere interactions

Abstract

Abstract. The CloudRoots field experiment was designed to obtain a comprehensive observational dataset that includes soil, plant, and atmospheric variables to investigate the interaction between a heterogeneous land surface and its overlying atmospheric boundary layer at the sub-hourly and sub-kilometre scale. Our findings demonstrate the need to include measurements at leaf level to better understand the relations between stomatal aperture and evapotranspiration (ET) during the growing season at the diurnal scale. Based on these observations, we obtain accurate parameters for the mechanistic representation of photosynthesis and stomatal aperture. Once the new parameters are implemented, the model reproduces the stomatal leaf conductance and the leaf-level photosynthesis satisfactorily. At the canopy scale, we find a consistent diurnal pattern on the contributions of plant transpiration and soil evaporation using different measurement techniques. From highly resolved vertical profile measurements of carbon dioxide (CO2) and other state variables, we infer a profile of the CO2 assimilation in the canopy with non-linear variations with height. Observations taken with a laser scintillometer allow us to quantify the non-steadiness of the surface turbulent fluxes during the rapid changes driven by perturbation of photosynthetically active radiation by cloud flecks. More specifically, we find 2 min delays between the cloud radiation perturbation and ET. To study the relevance of advection and surface heterogeneity for the land–atmosphere interaction, we employ a coupled surface–atmospheric conceptual model that integrates the surface and upper-air observations made at different scales from leaf to the landscape. At the landscape scale, we calculate a composite sensible heat flux by weighting measured fluxes with two different land use categories, which is consistent with the diurnal evolution of the boundary layer depth. Using sun-induced fluorescence measurements, we also quantify the spatial variability of ET and find large variations at the sub-kilometre scale around the CloudRoots site. Our study shows that throughout the entire growing season, the wide variations in stomatal opening and photosynthesis lead to large diurnal variations of plant transpiration at the leaf, plant, canopy, and landscape scales. Integrating different advanced instrumental techniques with modelling also enables us to determine variations of ET that depend on the scale where the measurement were taken and on the plant growing stage.

Surface representation impacts on turbulent heat fluxes in the Weather Research and Forecasting (WRF) model (v.4.1.3)

“…3. RUC (Rapid Update Cycle) (Smirnova et al, 2016). It uses nine soil layers with higher density close to the surface.…”

“…Indeed, water transfers from the soil to the air (and vice versa) are significantly linked to the vegetation type and soil properties: infiltration, runoff, soil moisture, and evapotranspiration (ET) (Zhang and Schilling, 2006). All these changes in the surface energy balance have direct or indirect feedbacks on the planetary boundary layer (PBL) development (Combe et al, 2015), cloud formation (Vilà-Guerau De Arellano et al, 2012), atmospheric temperatures (Koster et al, 2006;Christidis et al, 2013), and rainfall (Koster et al, 2003). This may impact surface characteristics and vegetation activity again, and in the long term it will restart the whole cycle by changing species (vegetation included), which need to adapt to the modified environmental conditions (Pielke et al, 1998), with the direct or indirect associated impacts on the first triggers: the humans (Meyer and Turner, 1994;Rulli et al, 2020).…”

“…the Project for Intercomparison of Land-surface Parameterization Schemes (PILPS; Henderson-Sellers et al, 1996) and the Global Land-Atmosphere Coupling Experiment (GLACE; Koster et al, 2006). Recently, the Cloud-Roots field experiment (Vilà-Guerau de Arellano et al, 2020) offered an integrated multi-scale approach from leaf to landscape measurements complemented with models.…”

“…This influence of the surface heterogeneous patches on the lower troposphere is known as static heterogeneity (e.g. Patton et al, 2005;van Heerwaarden and Guerau de Arellano, 2008) and also has impacts on the PBL processes (e.g. Margairaz et al, 2020a, b), creating new (dynamical) inhomogeneities such as clouds or modified turbulence.…”

“…(1) Noah (Chen and Dudhia, 2001), (2) Noah-MP (Multi-Physics) (Niu et al, 2011), and (3) RUC (Rapid Update Cycle) (Smirnova et al, 2016). The different experiments were designed as follows.…”