Surface roughness – a key control on land-atmosphere exchanges of heat and momentum – differs between dormant and growing seasons. However, how surface roughness shifts seasonally at fine time scales (e.g., days) in response to changing canopy conditions is not well understood. This study: (1) explores how aerodynamic resistance changes seasonally; (2) investigates what drives these seasonal shifts, including the role of vegetation phenology; and (3) quantifies the importance of including seasonal changes of aerodynamic resistance in “big leaf” models of sensible heat flux (H). We evaluated aerodynamic resistance and surface roughness lengths for momentum (z_0m) and heat (z_0h) using the kB⁻¹ parameter (ln(z_0m/z_0h)). We used AmeriFlux data to obtain surface-roughness estimates, and PhenoCam greenness data for phenology. This analysis included 23 sites and ∼190 site years from deciduous broadleaf, evergreen needleleaf, woody savanna, cropland, grassland, and shrubland plant-functional types (PFTs). Results indicated clear seasonal patterns in aerodynamic resistance to sensible heat transfer (R_ah). This seasonality tracked PhenoCam-derived start-of-season green-up transitions in PFTs displaying the most significant seasonal changes in canopy structure, with R_ah decreasing near green-up transitions. Conversely, in woody savanna sites and evergreen needleleaf forests, patterns in R_ah were not linked to green-up. Our findings highlight that decreases in kB⁻¹ are an important control over R_ah, explaining > 50% of seasonal variation in R_ah across most sites. Decreases in kB⁻¹ during green-up are likely caused by increasing z_0h in response to higher leaf area index. Accounting for seasonal variation in kB⁻¹ is key for predicting H as well; assuming kB⁻¹ to be constant resulted in significant biases that also exhibited strong seasonal patterns. Overall, we found that aerodynamic resistance can be sensitive to phenology in ecosystems having strong seasonality in leaf area, and this linkage is critical for understanding land-atmosphere interactions at seasonal time scales.

地表粗糙度——陆地-大气热量和动量交换的关键控制因素——在休眠季节和生长季节之间有所不同。然而，表面粗糙度如何在精细的时间尺度（例如，天）响应不断变化的冠层条件而发生季节性变化尚不清楚。本研究：（1）探索空气动力阻力如何随季节变化；(2) 调查是什么驱动了这些季节性变化，包括植被物候的作用；(3) 量化了在感热通量 ( H ) 的“大叶”模型中包含空气动力阻力季节性变化的重要性。我们使用kB ^-1评估了动量 ( z _0m ) 和热量 ( z _0h ) 的空气动力学阻力和表面粗糙度长度参数 (ln( z _0m / z _0h ))。我们使用 AmeriFlux 数据来获得表面粗糙度估计值，以及用于物候学的 PhenoCam 绿度数据。该分析包括落叶阔叶、常绿针叶、木本稀树草原、农田、草地和灌木植物功能类型 (PFT) 的 23 个地点和约 190 个地点年。结果表明空气动力学对显热传递的阻力 ( R _ah ) 有明显的季节性模式。这种季节性跟踪了 PFT 中 PhenoCam 派生的季初绿化过渡，显示了冠层结构中最显着的季节性变化，R _ah在绿色过渡附近减少。相反，在木本稀树草原和常绿针叶林中，R _ah中的模式与绿化无关。我们的研究结果强调kB ^{-1 的}减少是对R _ah的重要控制，解释了大多数站点中R _{ah 的}> 50％的季节性变化。绿化期间kB ^{-1 的}减少可能是由于对较高的叶面积指数做出响应而增加z _0h引起的。考虑kB ^{-1 的}季节性变化也是预测H 的关键；假设kB ^-1保持不变会导致显着的偏差，这些偏差也表现出强烈的季节性模式。总体而言，我们发现空气动力学阻力对叶面积季节性强的生态系统中的物候敏感，这种联系对于理解季节性时间尺度上的陆地 - 大气相互作用至关重要。