Abstract A surface energy budget underclosure, equivalent to an undersampling of sensible and latent heat fluxes at the surface, is commonly observed in the daytime convective boundary layer over land. The two mechanisms currently held responsible for such underclosure are, on one hand, entrainment-induced reduction of heat fluxes due to low-frequency circulations spanning the depth of the convective boundary layer, and on the other hand, horizontal advection effects induced by surface horizontal heterogeneity. Given the involvement of both heat fluxes in the budget equation, it is expected that the turbulent perturbations of variables involved in these fluxes (potential temperature θ ′ and specific humidity q ′ ) contain the signature of both mechanisms. Here, we assess to what extent non-local entrainment effects contribute to the budget underclosure in complex terrain using a quadrant analysis and multiresolution flux decomposition approach. We use data collected in the semi-arid Owens Valley, CA, USA, during the Terrain-Induced Rotor Experiment (T-REX) campaign. The budget components are sampled near the surface of the valley floor and the adjacent slope, with turbulence flux measurements, pyranometers, pyrgeometers, as well as ground heat flux measurements. We found that the budget underclosure was within the range of underclosures reported in past studies, with a systematically poorer closure over the valley slope. Although quadrant analysis revealed a dominance of scalar-similar events, associated with convective boundary layer overturning, a high degree of disagreement was found between the scalar-dissimilar quadrants which represent the counter-gradient motions. Specifically, those turbulent motions associated with entrainment of free tropospheric air to the surface, were found to be of minor importance in degrading scalar similarity. Instead, we postulate that local horizontal advection and hectometre-scale land-use heterogeneity in the vicinity of both the valley floor and slope sites, are the major factors in controlling the relationship between the budget underclosure and scalar similarity.

中文翻译：

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研究半干旱山谷中夹带驱动的标量差异与表面能平衡欠封闭之间的关系

摘要 地面能量收支欠封闭，相当于地表感热和潜热通量的欠采样，通常在陆地上空的白天对流边界层中观察到。目前认为造成这种欠封闭的两种机制是，一方面，由于跨越对流边界层深度的低频环流，夹带引起的热通量减少，另一方面，由表面水平面引起的水平平流效应异质性。鉴于预算方程中涉及两种热通量，预计这些通量中涉及的变量的湍流扰动（潜在温度 θ ′ 和比湿度 q ′ ）包含两种机制的特征。这里，我们使用象限分析和多分辨率通量分解方法评估非局部夹带效应对复杂地形中的预算欠封闭的影响程度。我们使用在地形诱导转子实验 (T-REX) 活动期间在美国加利福尼亚州半干旱的欧文斯谷收集的数据。预算分量在谷底表面和相邻斜坡附近采样，使用湍流通量测量、总辐射表、高温计以及地热通量测量。我们发现预算不足在过去研究中报告的不足范围内，在山谷斜坡上有系统性较差的关闭。尽管象限分析揭示了与对流边界层翻转相关的标量相似事件占主导地位，在代表反梯度运动的标量不同象限之间发现了高度的分歧。具体来说，那些与自由对流层空气夹带到地表相关的湍流运动被发现在降低标量相似性方面的重要性不大。相反，我们假设谷底和坡地附近的局部水平平流和百米尺度的土地利用异质性是控制预算欠封闭和标量相似性之间关系的主要因素。