Large‐eddy simulations of the atmospheric boundary layer over an Alpine glacier: Impact of synoptic flow direction and governing processes
Large‐eddy simulations of the atmospheric boundary layer over an Alpine glacier: Impact of synoptic flow direction and governing processes
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DOI：
10.1002/qj.4263
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发表时间：
2021-08
影响因子：
8.9
通讯作者：
Brigitta Goger;I. Stiperski;L. Nicholson;T. Sauter
中科院分区：
文献类型：
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作者：
Brigitta Goger;I. Stiperski;L. Nicholson;T. Sauter
山地冰川的物质平衡对一些应用（例如，当地水文或气候预测）很有意义，在强烈融化事件期间，湍流通量可能是冰川表面物质平衡的一个重要因素。潜在的复杂地形导致了湍流通量的空间异质性和非平稳性。由于热诱导流和重力波的作用，交换机制完全是三维的，而不仅仅是垂直的。此外，冰川有自己独特的小气候，由冰川下的倾斜风控制，它保护冰川冰并在多个尺度上与周围的流动相互作用。在这项研究中，我们使用天气研究与预报模型在48米的水平网格间距上进行大涡模拟，以深入了解阿尔卑斯山谷冰川腹地的边界层过程。我们选择了两个不同天气风向（西南和西北）的Hintereisferner实验测量活动的案例。在冰舌和冰川周围的涡动相关站对模型进行了评价，结果表明该模型能够模拟冰川边界层的一般结构。在西南气流下，平行于冰川轴的天气风支持冰川下风，冰面上存在稳定的边界层，局地过程控制湍流动能的产生。在西北气流作用下，跨冰川山谷流和破碎重力波导致强烈的湍流混合和随后的冰川边界层侵蚀。对感热通量的平稳性分析表明，在两个案例研究日都存在非平稳性，而在重力波事件期间，西北日的非平稳性最高。这些结果表明，天气风向除了上游地形和大气稳定性外，还对局部冰川边界层能否形成产生强烈影响，从而影响冰川能否维持自身的小气候。
The mass balance of mountain glaciers is of interest for several applications (e.g., local hydrology or climate projections), and turbulent fluxes can be an important contributor to glacier surface mass balance during strong melting events. The underlying complex terrain leads to spatial heterogeneity and non‐stationarity of turbulent fluxes. Owing to the contribution of thermally induced flows and gravity waves, exchange mechanisms are fully three‐dimensional, instead of only vertical. Additionally, glaciers have their own distinct microclimate, governed by a down‐glacier katabatic wind, which protects the glacier ice and interacts with the surrounding flows on multiple scales. In this study, we perform large‐eddy simulations with the Weather Research and Forecasting model at a horizontal grid spacing of 48 m to gain insight into the boundary‐layer processes over an Alpine valley glacier, the Hintereisferner. We choose two case studies from the Hintereisferner experiment measurement campaign with different synoptic wind directions (southwest and northwest). Model evaluation with an array of eddy‐covariance stations on the glacier tongue and surroundings reveals that the Weather Research and Forecasting model is able to simulate the general glacier boundary‐layer structure. Under a southwesterly airflow, the down‐glacier wind is supported by the synoptic wind parallel to the glacier axis, a stable boundary layer is present over the ice surface, and local processes govern the turbulence kinetic energy production. Under northwesterly airflow, a cross‐glacier valley flow and a breaking gravity wave lead to strong turbulent mixing and to the subsequent erosion of the glacier boundary layer. Stationarity analysis of the sensible heat flux suggests non‐stationary behaviour for both case study days, whereas non‐stationarity is highest on the northwesterly day during the gravity‐wave event. These results suggest that the synoptic wind direction has, in addition to upstream topography and the atmospheric stability, a strong impact on whether a local glacier boundary layer can form or not, influencing whether a glacier is able to maintain its own microclimate.