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首页> 外文期刊>Journal of hydrometeorology >Direct and Diffuse Radiation in the Shallow Cumulus-Vegetation System: Enhanced and Decreased Evapotranspiration Regimes
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Direct and Diffuse Radiation in the Shallow Cumulus-Vegetation System: Enhanced and Decreased Evapotranspiration Regimes

机译：浅层植被系统中的直接和漫射辐射：蒸发制度增强和减少

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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.
机译：通过整体方法引导，每日规模研究了直接和漫射辐射对植被土地的大气边界层动力学的综合影响。设计了三个数值实验，其旨在解开漫反射和直接辐射在表面和边界层动态的浅层下方的作用。使用耦合到陆地模型的大型涡流模拟（LES）模型，包括植物的机械方式直接响应辐射，温度和水蒸气缺损变化。明确地解释了由云创造的直接和漫射辐射的分区和漫射辐射。随着云光学深度的函数，LES结果被条件平均。由于漫射辐射的增加和直接辐射的略微降低，该研究结果显示薄云下的光合作用薄弱的光合作用。降低的冠层电阻是植被增强碳吸收的主要驱动器，而表面的碳梯度和空气动力学效果是次要的。由于CO2和水蒸气交换通过植物气孔的偶联，在薄云下也增强了蒸散蒸腾，尽管程度较小。漫射辐射的这种效果增加了云下的用水效率和蒸发级分。由于这些扰动发生的时间和空间有限，云的表面通量的动态扰动不会影响一般边界层或云特征。结论是，需要准确的辐射分配计算，以获得对局部表面过程的可靠估计。

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