Mesoscale Airflow Over Mountains: Orographic Drag and Upstream Convective Initiation

山脉对天气和气候有着深远的影响。本项目将调查山脉与大气之间相互作用的两个重要方面。第一个将是研究山脉对气流施加的阻力如何在大气中不同的垂直高度上分布，以及这种阻力如何反馈到更大尺度的气流上。第二项工作将研究在何种情况下，低层阻塞可以触发深对流雷暴上游的山，以及由此产生的对流如何演变的山和随时间变化的大规模流动的存在。 智力优势：当气流遇到山脉屏障时，屏障上方和下游可能会产生山波，低空气流可能会被阻挡并转向山脉两侧。 这些扰动的尺度太小，无法在全球天气和气候模型中解决，但它们对必须参数化的大尺度流动产生了重要的阻力。 本项目将试图将这种参数化置于更坚实的理论基础上，并通过高分辨率数值模拟分析确定对地形阻力的大规模反应。 将进行类似的高分辨率模拟，以检查被山脉阻挡的潮湿气流能够经历足够的低层不稳定和提升以触发山脉本身上游的深对流的条件。 由此产生的雷暴本质上是不稳定的，我们将在随时间变化的大尺度流中研究它们的开始和衰减。 这种上游对流触发被认为是调节南亚季风降水分布的一个重要因素。 首席研究员还将研究热驱动环流对这些对流事件产生的影响。 更广泛的影响：两名研究生的研究和专业发展将得到支持。 研究结果将有助于改善全球天气预报和许多气候研究所必需的大尺度模式中低层重力波阻力的参数化。 全球气候模型表明，随着全球变暖，风和水分通量将发生变化，但这些模型的分辨率太粗糙，无法直接模拟对流的变化。 因此，了解山脉在南亚季风和其他区域触发深对流方面所起的作用，将有助于随着气候变暖对水资源进行管理。

Mountains exert a profound influence on the weather and climate. This project will investigate two important aspects of the interaction between mountains and the atmosphere. The first will be to examine how the drag exerted by mountains on an air stream is distributed among different vertical levels in the atmosphere and how that drag feeds back on the larger scale flow. The second effort will examine the circumstances in which low-level blocking can trigger deep convective thunderstorms upstream of a mountain and how the resulting convection evolves in the presence of the mountain and a time-varying large-scale flow. Intellectual Merit: When an air stream encounters a mountain barrier, mountain waves may be set up above and downstream of the barrier and the low-level flow may be blocked and diverted around the flanks of the mountain. These disturbances are too small in scale to be resolved in global weather and climate models, but they produce an important drag on the large-scale flow that must be parameterized. This project will attempt to place such parameterizations on a firmer theoretical footing and also determine the large-scale responses to orographic drag through the analysis of high-resolution numerical simulations. Similar high-resolution simulations will be conducted to examine the conditions under which a moist air stream blocked by a mountain can undergo sufficient low-level destabilization and lifting to trigger deep convection upstream of the mountain itself. The resulting thunderstorms are inherently non-steady, and we will examine their onset and decay in time varying large-scale flows. Such upstream convective triggering is thought to be a significant factor in regulating the distribution of precipitation in the south Asian monsoon. The Principal Investigator also will examine the influence of thermally driven circulations on the generation of these convective events. Broader Impacts: The research and professional development of two graduate students will be supported. Results should help improve the parameterization of low-level gravity wave drag in large-scale models essential for global weather forecasting and much climate research. Global climate models suggest there will be shifts in winds and moisture fluxes in response to global warming, but the resolution of those models is too coarse to allow them to directly simulate changes in the convection. Therefore, understanding the role played by mountains in triggering deep convection, both in the south Asian monsoon and in other regions, will be helpful in the management of water resources as the climate warms.