Separating the impact of individual land surface properties on the terrestrial surface energy budget in both the coupled and uncoupled land–atmosphere system

Changes in the land surface can drive large responses in the atmosphere on local, regional, and global scales. Surface properties control the partitioning of energy within the surface energy budget to fluxes of shortwave and longwave radiation, sensible and latent heat, and ground heat storage. Changes in surface energy fluxes can impact the atmosphere across scales through changes in temperature, cloud cover, and large-scale atmospheric circulation. We test the sensitivity of the atmosphere to global changes in three land surface properties: albedo, evaporative resistance, and surface roughness. We show the impact of changing these surface properties differs drastically between simulations run with an offline land model, compared to coupled land–atmosphere simulations that allow for atmospheric feedbacks associated with land–atmosphere coupling. Atmospheric feedbacks play a critical role in defining the temperature response to changes in albedo and evaporative resistance, particularly in the extratropics. More than 50% of the surface temperature response to changing albedo comes from atmospheric feedbacks in over 80% of land areas. In some regions, cloud feedbacks in response to increased evaporative resistance result in nearly 1 K of additional surface warming. In contrast, the magnitude of surface temperature responses to changes in vegetation height are comparable between offline and coupled simulations. We improve our fundamental understanding of how and why changes in vegetation cover drive responses in the atmosphere, and develop understanding of the role of individual land surface properties in controlling climate across spatial scales—critical to understanding the effects of land-use change on Earth’s climate.

Methods
Simulations with the CESM model coupled to a simple land model as described in Laguë et al. 2019 linked here.

地表变化可在局地、区域乃至全球尺度上对大气产生显著影响。地表属性控制着地表能量收支中能量向短波辐射、长波辐射、感热、潜热以及土壤热存储的分配。地表能量通量的变化可通过改变气温、云量以及大尺度大气环流，在不同尺度上对大气产生影响。

我们针对反照率（albedo）、蒸发阻力（evaporative resistance）与地表粗糙度（surface roughness）这三种地表属性的全球变化对大气的敏感性开展了测试。研究表明，相较于考虑陆气耦合相关大气反馈的耦合陆气模拟（coupled land–atmosphere simulations），采用离线陆面模型（offline land model）运行的模拟中，改变上述地表属性所带来的影响差异悬殊。

大气反馈在定义反照率与蒸发阻力变化引发的气温响应方面发挥着关键作用，尤其在中高纬区（extratropics）。在超过80%的陆地区域，反照率变化引发的地表气温响应中有50%以上源自大气反馈。在部分区域，针对蒸发阻力增大的云反馈（cloud feedbacks）可带来近1开尔文的额外地表增温。与之相反，植被高度（vegetation height）变化引发的地表气温响应幅度在离线模拟与耦合模拟中基本相当。

本研究深化了对植被覆盖变化如何以及为何会引发大气响应的基础认知，并厘清了各类地表属性在不同空间尺度上调控气候的作用——这对于理解土地利用变化（land-use change）对地球气候的影响至关重要。

方法
本研究采用与Laguë等人2019年研究中描述一致的、耦合了简单陆面模型的CESM开展模拟，相关链接见此处。