Orographic Enhancement of Precipitation in Mid-latitude Baroclinic Waves

这项研究的目的是增进对降雨系统在经过主要山脉时的地形变化的了解。这项研究将综合最近的两个实地方案的数据：1999年在欧洲阿尔卑斯山实施的中尺度阿尔卑斯方案(MAP)，以及2001年在俄勒冈州级联上开展的通过观测验证实验改进微物理参数化的第二阶段。这两个项目的数据都是从各种机载和地面先进天气雷达收集的。这一雷达的收集有效地显示了两个项目中迎风坡上的主要降水机制。在地图上，移动多普勒仪在固定地点雷达看不到的深谷中提供了进一步的信息。在Improve II中，华盛顿大学Convair飞机在高空收集冰粒样本，而地面观测者在瀑布顶端收集冰粒样本。从MAP收集的数据推断的地形降水增加的初步概念模型表明，低于0摄氏度的水滴的合并和产生重力的边缘(略高于0摄氏度)是产生粒子快速落在迎风斜坡上的关键过程。这一过程似乎在地形上畅通无阻的水流中效果最好，也就是说，当水流强烈且弱稳定时，它拥有直接上升到屏障上方所需的能量。当地形上的气流稍有不稳定时，这一过程甚至效果更好，因此嵌入的对流单体可以在地形的第一个迎风大峰上形成并加速合并和海风形成过程。对新的改进II数据集的初步分析表明，类似的过程也在级联上起作用。经验概念模式将指导高分辨率数值模式运行的关键评估地图和改进II风暴。数值模式输出提供了一个数据库，将评估该数据库与地图和改进的地形增强经验模式II的一致性。数值模式还将用于特定的情况，以获得更好的空间分辨率和模式中发生的微物理过程的更多细节。雷达数据将对模型结果提供动态约束，并将检查数据与模型模拟的微物理场的一致性。对数值模式的改进将有助于推进山区降水和径流的预报。

The goal of this research is to advance understanding of the orographic modification of precipitation systems as they pass over major mountain ranges. The study will integrate data from two recent field programs: the Mesoscale Alpine Programme (MAP), conducted over the European Alps in 1999, and the second phase of the Improvement of Microphysical PaRameterization through Observational Verification Experiment (IMPROVE II), conducted over the Oregon Cascades in 2001. Data in both projects was collected from a variety of airborne and ground-based advanced weather radars. This collection of radars was effective at indicating the predominant precipitation mechanisms over the windward slopes in both projects. In MAP, mobile Dopplers provided further information in deep valleys hidden from the view of fixed site radars. In IMPROVE II the University of Washington Convair aircraft collected ice particle samples aloft, while ground observers collected ice particle samples at the crest of the Cascades.The tentative conceptual model of orographic precipitation enhancement inferred from the data collected in MAP suggests that coalescence of drops below the 0 degree C level and riming that generates graupel, just above the 0 degree C level, are key processes in producing particles that fall out quickly on the windward slopes. This process seems to work best in unblocked flow over the terrain, i.e., when the flow is strong and weakly stable, so it possesses the energy required to rise directly over the barrier. The process works even better when the flow up the terrain is slightly unstable, so that embedded convective cells can form and accelerate the coalescence and graupel-forming processes over the first large windward peak of terrain. Preliminary analysis of the new IMPROVE II data set suggests that similar processes are at work over the Cascades.The empirical conceptual models will guide the critical evaluation of high resolution numerical model runs for the MAP and IMPROVE II storms. Numerical model output provides a database which will be evaluated for consistency with the MAP and IMPROVE II empirical models of orographic enhancement. The numerical models also will be run for select cases to obtain finer spatial resolution and more details on the microphysical processes taking place in the model. The radar data will provide dynamical constraints on the model results and the data will be examined for consistency with the model-simulated microphysical fields. Improvements to the numerical models will help to advance the forecasting of precipitation and runoff in mountainous regions.