Inland freshwater bodies form the largest natural source of carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. The eddy-covariance method is derived for solely local processes from the surface (lake). Non-local processes, such as entrainment or advection, would add erroneous contributions to the eddy-covariance flux estimations. Here, we use four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden. When the lake is covered with ice, unexpected lake fluxes were still observed. A statistical approach using only surface-layer data reveals that non-local processes produce these erroneous fluxes. The occurrence and strength of non-local processes depend on a combination of wind speed and distance between the instrumented tower and upwind shore (fetch), which we here define as the time over water. The greater the wind speed and the shorter the fetch, the higher the contribution of non-local processes to the eddy-covariance fluxes. A correction approach for the measured scalar fluxes due to the non-local processes is proposed and also applied to open-water time periods. The gas transfer velocity determined from the corrected fluxes is close to commonly used wind-speed based parametrizations.

中文翻译：

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对涡旋协方差空气湖 $$\hbox {CO}_2$$ 通量的非局部影响

内陆淡水水体是大气中最大的天然碳源。为了研究这种对大气碳循环的贡献，湖泊站点的涡流协方差通量测量变得越来越流行。涡流协方差方法仅针对来自表面（湖泊）的局部过程推导出来。非局部过程，例如夹带或平流，会给涡流协方差通量估计增加错误的贡献。在这里，我们使用了四年来对瑞典中部淡水湖 Erken 中二氧化碳的涡旋协方差测量。当湖泊被冰覆盖时，仍然观察到意想不到的湖泊通量。仅使用表层数据的统计方法表明，非局部过程会产生这些错误的通量。非局部过程的发生和强度取决于风速和仪表塔与上风岸（取力）之间的距离的组合，我们在这里将其定义为水上时间。风速越大，获取时间越短，非局部过程对涡流协方差通量的贡献就越大。提出了一种对由于非局部过程而导致的测量标量通量的校正方法，该方法也适用于开放水域时间段。从修正的通量确定的气体转移速度接近于常用的基于风速的参数化。提出了一种对由于非局部过程而导致的测量标量通量的校正方法，该方法也适用于开放水域时间段。从修正的通量确定的气体转移速度接近于常用的基于风速的参数化。提出了一种对由于非局部过程而导致的测量标量通量的校正方法，该方法也适用于开放水域时间段。从修正的通量确定的气体转移速度接近于常用的基于风速的参数化。