Orbiting around the non-closure problem in eddy covariance, a new generation of high-resolution thermal imagery has revealed that advection may be more common than previously expected. To investigate this, we conducted an extensive study over an irrigated alfalfa field that experienced heat and moisture advection. Over the course of five analysis periods (37 days total), multiple tower arrays and profile measurements were deployed to measure the horizontal advection and vertical heat flux divergence. Latent heat flux (λE) measured at the anchor tower showed an enhancement (i.e., increase) due to both local and non-local processes. Locally, as a result of the upwind λE, advection humidified the atmosphere and increased stomatal opening, enhancing the downwind λE. Simultaneously, with lowered atmospheric demand, λE was suppressed downwind. Our results suggest that stomatal regulation played a dominant role in the enhancement, but not by itself. Spectral analysis revealed that low frequency (i.e., large) eddies contributed high heat and moisture via advection. In combination with thermal remote sensing observations from ECOSTRESS and Landsat 8/9, we found that these large eddies were generated over the upwind surface, and they were independent of the local boundary layer conditions. Consequently, spatiotemporal heterogeneity in land-surface conditions induced large eddies, further enhancing λE through non-local transport of heat and moisture. Lastly, by conditionally including the advective fluxes, the energy balance closure improved from 89 % to 97 % (r = 0.97, < 0.001) over the five analysis periods. Results from this improved energy balance closure suggest an alternative approach for developing validation datasets for remote sensing evapotranspiration (ET) models rather than forcing closure with Bowen-ratio. Furthermore, our findings provide insights for algorithms that may improve remote sensing ET products that treat pixels as isolated columns rather than also considering the lateral effects of heat and moisture transport.

中文翻译：

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平流如何影响苜蓿灌溉田的表面能量平衡及其闭合


围绕涡流协方差的非闭合问题，新一代高分辨率热图像显示平流可能比之前预期的更为常见。为了调查这一点，我们对经历过热量和水分平流的灌溉苜蓿田进行了广泛的研究。在五个分析期（总共 37 天）的过程中，部署了多个塔阵列和剖面测量来测量水平平流和垂直热通量散度。由于局部和非局部过程，在锚塔处测量的潜热通量 (λE) 显示出增强（即增加）。局地方面，由于上风λE的作用，平流使大气湿润，气孔开口增大，从而增强了下风λE。同时，随着大气需求的降低，顺风处的λE受到抑制。我们的结果表明，气孔调节在增强过程中发挥了主导作用，但其本身并不是。光谱分析表明，低频（即大）涡流通过平流产生高热量和水分。结合ECOSTRESS和Landsat 8/9的热遥感观测，我们发现这些大涡是在迎风表面产生的，并且与局部边界层条件无关。因此，地表条件的时空异质性引发了大涡流，通过热量和水分的非局域传输进一步增强了 λE。最后，通过有条件地包括平流通量，五个分析周期内的能量平衡闭合率从 89% 提高到 97%（r = 0.97，< 0.001）。 这种改进的能量平衡闭合的结果提出了一种为遥感蒸散 (ET) 模型开发验证数据集的替代方法，而不是强制使用鲍文比闭合。此外，我们的研究结果为算法提供了见解，这些算法可以改进遥感 ET 产品，将像素视为孤立的列，而不是考虑热量和水分传输的横向影响。