Guided by a holistic approach, the combined effects of direct and diffuse radiation on the atmospheric boundary layer dynamics over vegetated land are investigated on a daily scale. Three numerical experiments are designed that are aimed at disentangling the role of diffuse and direct radiation below shallow cumulus at the surface and on boundary layer dynamics. A large-eddy simulation (LES) model coupled to a land surface model is used, including a mechanistically immediate response of plants to radiation, temperature, and water vapor deficit changes. The partitioning in direct and diffuse radiation created by clouds and farther inside the canopy is explicitly accounted for. LES results are conditionally averaged as a function of the cloud optical depth. The findings show larger photosynthesis under thin clouds than under clear sky, due to an increase in diffuse radiation and a slight decrease in direct radiation. The reduced canopy resistance is the main driver for the enhanced carbon uptake by vegetation, while the carbon gradient and aerodynamic effects at the surface are secondary. Because of the coupling of CO2 and water vapor exchange through plant stomata, evapotranspiration is also enhanced under thin clouds, albeit to a lesser extent. This effect of diffuse radiation increases the water use efficiency and evaporative fraction under clouds. The dynamic perturbations of the surface fluxes by clouds do not affect general boundary layer or cloud characteristics because of the limited time and space where these perturbations occur. It is concluded that an accurate radiation partitioning calculation is necessary to obtain reliable estimations on local surface processes

We studied the effects of shading by shallow cumulus (shallow Cu) and the subsequent effect of inducing heterogeneous conditions at the surface on boundary-layer characteristics. We placed special emphasis on quantifying the changes in the characteristic length and time scales associated with thermals, shallow Cu and induced thermal circulation structures. A series of systematic numerical experiments, inspired by Amazonian thermodynamic conditions, was performed using a large-eddy simulation model coupled to a land-surface model. We used four different experiments to disentangle the effects of shallow Cu on the surface and the response of clouds to these surface changes. The experiments include a 'clear case', 'transparent clouds', 'shading clouds' and a case with a prescribed uniform domain and reduced surface heat flux. We also performed a sensitivity study on the effect of introducing a weak background flow. Length and time scales were calculated using autocorrelation and two-dimensional spectral analysis, and we found that shading controlled by shallow Cu locally lowers surface temperatures and consequently reduces the sensible and latent heat fluxes, thus inducing spatial and temporal variability in these fluxes. The length scale of this surface heterogeneity is not sufficiently large to generate circulations that are superimposed on the boundary-layer scale, but the heterogeneity does disturb boundary-layer dynamics and generates a flow opposite to the normal thermal circulation. Besides this effect, shallow Cu shading reduces turbulent kinetic energy and lowers the convective velocity scale, thus reducing the mass flux. This hampers the thermal lifetime, resulting in a decrease in the shallow Cu residence time (from 11 to 7 min). This reduction in lifetime, combined with a decrease in mass flux, leads to smaller clouds. This is partially compensated for by a decrease in thermal cell size due to a reduction in turbulent kinetic energy. As a result, inter-cloud distance is reduced, leading to a larger population of smaller clouds, while maintaining cloud cover similar to the non-shading clouds experiment. Introducing a 1 m s −1 background wind speed increases the thermal size in the sub-cloud layer, but the diagnosed surface-cloud coupling, quantified by characteristic time and length scales, remains.

Achieving higher canopy photosynthesis rates is one of the keys to increasing future crop production; however, this typically requires additional water inputs because of increased water loss through the stomata. Lowland rice canopies presently consume a large amount of water, and any further increase in water usage may significantly impact local water resources. This situation is further complicated by changing the environmental conditions such as rising atmospheric CO concentration ([CO ]). Here, we modeled and compared evapotranspiration of fully developed rice canopies of a high-yielding rice cultivar (Oryza sativa L. cv. Takanari) with a common cultivar (cv. Koshihikari) under ambient and elevated [CO ] (A-CO and E-CO , respectively) via leaf ecophysiological parameters derived from a free-air CO enrichment (FACE) experiment. Takanari had 4%-5% higher evapotranspiration than Koshihikari under both A-CO and E-CO , and E-CO decreased evapotranspiration of both varieties by 4%-6%. Therefore, if Takanari was cultivated under future [CO ] conditions, the cost for water could be maintained at the same level as for cultivating Koshihikari at current [CO ] with an increase in canopy photosynthesis by 36%. Sensitivity analyses determined that stomatal conductance was a significant physiological factor responsible for the greater canopy photosynthesis in Takanari over Koshihikari. Takanari had 30%-40% higher stomatal conductance than Koshihikari; however, the presence of high aerodynamic resistance in the natural field and lower canopy temperature of Takanari than Koshihikari resulted in the small difference in evapotranspiration. Despite the small difference in evapotranspiration between varieties, the model simulations showed that Takanari clearly decreased canopy and air temperatures within the planetary boundary layer compared to Koshihikari. Our results indicate that lowland rice varieties characterized by high-stomatal conductance can play a key role in enhancing productivity and moderating heat-induced damage to grain quality in the coming decades, without significantly increasing crop water use.

No abstract

Cumulus clouds make a significant contribution to the Earth's energy balance and hydrological cycle and are a major source of uncertainty in climate projections. Reducing uncertainty by expanding our understanding of the processes that drive cumulus convection is vital to the accurate identification of future global and regional climate impacts. Here we adopt an interdisciplinary approach that integrates interrelated scales from plant physiology to atmospheric turbulence. Our explicit simulations mimic the land‐atmosphere approach implemented in current numerical weather prediction, and global climate models enable us to conclude that neglecting local plant dynamic responses leads to misrepresentations in the cloud cover and midtropospheric moisture convection of up to 21% and 56%, respectively. Our approach offers insights into the key role played by the active vegetation on atmospheric convective mixing that has recently been identified as the source of half of the variance in global warming projections (i.e., equilibrium climate sensitivity).

The explicit coupling at meter and second scales of vegetation's responses to the atmospheric-boundary layer dynamics drives a dynamic heterogeneity that influences canopy-top fluxes and cloud formation. Focusing on a representative day during the Amazonian dry season, we investigate the diurnal cycle of energy, moisture and carbon dioxide at the canopy top, and the transition from clear to cloudy conditions. To this end, we compare results from a large-eddy simulation technique, a high-resolution global weather model, and a complete observational data set collected during the GoAmazon14/15 campaign. The overall model-observation comparisons of radiation and canopy-top fluxes, turbulence, and cloud dynamics are very satisfactory, with all the modeled variables lying within the standard deviation of the monthly aggregated observations. Our analysis indicates that the timing of the change in the daylight carbon exchange, from a sink to a source, remains uncertain and is probably related to the stomata closure caused by the increase in vapor pressure deficit during the afternoon. We demonstrate quantitatively that heat and moisture transport from the subcloud layer into the cloud layer are misrepresented by the global model, yielding low values of specific humidity and thermal instability above the cloud base. Finally, the numerical simulations and observational data are adequate settings for benchmarking more comprehensive studies of plant responses, microphysics, and radiation. Plain Language Summary Clouds and forest in the Amazonian rainforest region are closely related. We investigated the final month of the Amazonian dry season in order to study interactions between the rainforest and the overlying atmosphere, placing particular emphasis on studying small spatiotemporal effects, such as that of cloud shading on photosynthesis. We employed three different methods: a cloud-turbulence resolving model, a global weather model, and a complete set of canopy-top and atmospheric observations. We holistically studied these relationships by systematically analyzing the characteristics of incoming solar radiation, evapotranspiration, and cloud cover and thickness. This comparison enabled us to make two relevant findings related to these diurnal carbon and cloud cycles. First, we observed that photosynthesis is offset by the soil carbon dioxide efflux earlier than the two models calculations. With respect to cloud formation and intensification, we showed quantitatively that the inefficiently modeled moisture transport leads to less active shallow convection, which may be insufficient to trigger deep convection. This systematic study paves the way for more comprehensive studies that would include more complex descriptions of microphysics processes and radiation, as well as chemistry and aerosol formation.

The effects of increases in carbon dioxide and temperature on the vegetation‐atmosphere‐cloud interaction are studied with a bottom‐up approach. Using the 3‐D large‐eddy simulation technique coupled with a CO2‐sensitive dynamic plant physiological submodel, we aimed to spatially and temporally understand the surface and vegetation forcing on the coupled land‐atmosphere interactions in future scenarios. Four simulations were designed: a control simulation for current conditions, an enhanced carbon dioxide simulation (current +200 ppm), an elevated temperature simulation (current +2 K), and a simulation covering the combination of both elevations in temperature and CO2. With elevations in carbon dioxide, plant transpiration is reduced due to stomatal closure, resulting in reduced latent‐ and increased sensible heat fluxes. Although no effects on cloud cover were found in this simulation, the in‐cloud moisture flux was enhanced. Elevations in temperature yielded opposite results with reduced sensible and increased latent heat fluxes, which reduced the turbulent kinetic energy and buoyancy rates, thereby negatively impacting cloud formation. Our future climate mimicking simulation shows minimal changes in the regional energy balance due to offsetting effects between increased temperature and [CO2], while plant photosynthesis increased and transpiration decreased. The atmospheric boundary layer was drier, even though surface fluxes were very similar current conditions, thereby hampering cloud formation and development. Our results highlight the necessity of small scales and interactions, which require a bottom‐up approach to be able to accurately capture the nonlinear plant‐atmosphere interactions.

Vegetation and atmosphere processes are coupled through a myriad of interactions linking plant transpiration, carbon dioxide assimilation, turbulent transport of moisture, heat and atmospheric constituents, aerosol formation, moist convection, and precipitation. Advances in our understanding are hampered by discipline barriers and challenges in understanding the role of small spatiotemporal scales. In this perspective, we propose to study the atmosphere-ecosystem interaction as a continuum by integrating leaf to regional scales (multiscale) and integrating biochemical and physical processes (multiprocesses). The challenges ahead are (1) How do clouds and canopies affect the transferring and in-canopy penetration of radiation, thereby impacting photosynthesis and biogenic chemical transformations? (2) How is the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.