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首页> 外文期刊>Journal of hydrometeorology >Understanding the Daily Cycle of Evapotranspiration: A Method to Quantify the Influence of Forcings and Feedbacks
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Understanding the Daily Cycle of Evapotranspiration: A Method to Quantify the Influence of Forcings and Feedbacks

机译：了解蒸发蒸腾的每日周期：一种量化强迫和反馈影响的方法

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摘要

A method to analyze the daily cycle of evapotranspiration over land is presented. It quantifies the influence of external forcings, such as radiation and advection, and of internal feedbacks induced by boundary layer, surface layer, and land surface processes on evapotranspiration. It consists of a budget equation for evapotranspiration that is derived by combining a time derivative of the Penman-Monteith equation with a mixed-layer model for the convective boundary layer. Measurements and model results for days at two contrasting locations are analyzed using the method: midlatitudes (Cabauw, Netherlands) and semiarid (Niamey, Niger). The analysis shows that the time evolution of evapotranspiration is a complex interplay of forcings and feedbacks. Although evapotranspiration is initiated by radiation, it is significantly regulated by the atmospheric boundary layer and the land surface throughout the day. In both cases boundary layer feedbacks enhance the evapotranspiration up to 20 W m(-2) h(-1). However, in the case of Niamey this is offset by the land surface feedbacks since the soil drying reaches -30 W m (2) h (1). Remarkably, surface layer feedbacks are of negligible importance in a fully coupled system. Analysis of the boundary layer feedbacks hints at the existence of two regimes in this feedback depending on atmospheric temperature, with a gradual transition region in between the two. In the low-temperature regime specific humidity variations induced by evapotranspiration and dry-air entrainment have a strong impact on the evapotranspiration. In the high-temperature regime the impact of humidity variations is less pronounced and the effects of boundary layer feedbacks are mostly determined by temperature variations.
机译：提出了一种分析陆地蒸散量每日周期的方法。它量化了辐射和对流等外部强迫以及边界层，表层和陆面过程引起的内部反馈对蒸散的影响。它由一个蒸发蒸腾的预算方程组成，该方程通过将Penman-Monteith方程的时间导数与对流边界层的混合层模型相结合而得出。使用以下方法分析了两个相对位置的天数的测量结果和模型结果：中纬度（荷兰卡鲍）和半干旱地区（尼亚美，尼日尔）。分析表明，蒸散量的时间演变是强迫和反馈的复杂相互作用。尽管蒸发蒸腾是由辐射引起的，但全天受大气边界层和陆地表面的调节。在这两种情况下，边界层反馈都可以将蒸散量提高到20 W m（-2）h（-1）。但是，在尼亚美的情况下，由于土壤干燥达到-30 W m（2）h（1），因此被地面反馈所抵消。值得注意的是，在完全耦合的系统中，表面层反馈的重要性微不足道。对边界层反馈的分析表明，取决于大气温度，该反馈中存在两种状态，两者之间有一个逐渐过渡的区域。在低温条件下，由蒸散作用和干空气夹带引起的特定湿度变化对蒸散量有很大影响。在高温状态下，湿度变化的影响不太明显，边界层反馈的影响主要由温度变化确定。

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