Influence of sub-mesoscale topography on daytime precipitation associated with thermally driven local circulations over a mountainous region
Influence of sub-mesoscale topography on daytime precipitation associated with thermally driven local circulations over a mountainous region
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次中尺度地形对山区热驱动局部环流相关白天降水的影响
DOI：
10.1175/jas-d-20-0332.1
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发表时间：
2021
影响因子：
3.1
通讯作者：
Kajikawa Yoshiyuki
中科院分区：
文献类型：
--
作者：
Nishizawa Seiya;Yamaura Tsuyoshi;Kajikawa Yoshiyuki
在这项研究中，亚中尺度地形（即，地形特征（小于几公里）对山区热力驱动局地环流降水的影响，通过100 m网格大涡模拟试验，在没有天气尺度降水系统的情况下进行了研究。观测到的地形对降水的影响不同于以往的研究;在本研究中，观测到亚中尺度地形对降水的减弱作用，而以往的研究表明，亚中尺度地形增强降水。研究之间的这种差异是由于所考虑的地形和降水诱导系统的规模不同。以往的研究主要集中在与天气尺度系统，其中机械地形强迫占主导地位的降水。本文阐明了以热地形强迫为主的地形效应的机制。在热力驱动的局地环流作用下，大尺度山脊附近的上坡气流辐合是造成降水的主要原因之一。次中尺度地形特征促进了上坡气流与山体表面的分离和边界层的垂直混合。这种分离和混合导致辐合和上升气流减弱，脊周围的等效位温降低，解释了观测到的降水减弱效应。降雨蒸发形成的冷池与上坡气流共同作用，增强了减弱效果。这些结果证实了次中尺度地形在地形降水中的重要性。
In this study, the effect of submesoscale topography (i.e., topographical features smaller than a few kilometers in size) on precipitation associated with thermally driven local circulations over a mountainous region is examined in the absence of synoptic-scale precipitation systems through a 100-m-mesh large-eddy simulation experiment. The observed effect of topography on precipitation is different than that identified in previous studies; submesoscale topography is observed to induce a weakening effect on precipitation in this study, while previous studies have suggested that submesoscale topography enhances precipitation. This discrepancy between studies is owing to differences in the scale of the topography and the precipitation-inducing system under consideration. Previous studies have focused on precipitation associated with synoptic-scale systems, where mechanical orographic forcing is dominant. The mechanism of the topographic effect where thermal orographic forcing is dominant was clarified in this study. Under thermally driven local circulation, the convergence of upslope flow near large-scale mountain ridges is one of the main causes of precipitation. Submesoscale topographic features promote the detachment of upslope flow from the mountain surface and vertical mixing in the boundary layer. This detachment and mixing result in a weakening of convergence and updraft and reduction of equivalent potential temperature around the ridge that explains the observed weakening effect on precipitation. Cold pools formed by evaporation of rainfall associated with upslope flow enhance the weakening effect. These results confirm the importance of submesoscale topography in orographic precipitation.