A broad understanding of orographic precipitation processes is vital to the forecasting of heavy rain, floods, and severe weather. Most precipitation on earth occurs in connection with frontal systems, convective storms, or tropical cyclones. The precipitation of these storms is enhanced, redistributed and concentrated when the storms occur near or over mountains. This study tries to advance understanding of these effects, for all three types of storms. The research effort will be based on existing but not fully exploited datasets from several field campaigns. High-resolution model simulations with several of the latest ice-phase microphysical schemes will supplement the field observation studies. The study leverages collaborations to facilitate examination of a range of storm types near different mountain ranges (Alps, Cascades, Canadian Coast Mts., Sierra Nevada, Andes, Himalayas, and Taiwan Central Mts.).
For frontal systems passing over mountain ranges, this research will test hypotheses of precipitation enhancement suggested by the MAP and IMPROVE II field programs and determine if these ideas have applicability to storms in the vicinities of diverse mountain ranges. In particular, data from SNOW-V10 will be used to determine the existence and characteristics of decoupled flows, accompanying shear, and down-valley flows in the mountain setting of the Canadian western coast. Comparison of data from Alps, Coast Mountains, and Sierra Nevada range will be used to determine whether the turbulence in these layers has the specific form of Kelvin-Helmholtz waves. The development of decoupled layers by barrier jet formation will be explored by model simulation of fronts approaching the Andes.
For convective systems, we will examine the triggering and downstream development of intense convective systems as a result of the impingement of low-level flow on mountains and the enhancement of mesoscale convective systems passing over large mountains. The triggering mechanism will be examined with model simulations and data analysis in the Andean region. The modeling will test the hypothesis that orographic subsidence holds back the deep convection by capping of the moist low-level moist jet until the jet breaks the cap by striking a foothill. Downstream evolution of the orogenic convection into a mesoscale system will be examined with the model. The enhancement of a mesoscale convective system passing over steep terrain will be examined by simulating the 2010 Leh flood case over the Himalayas. The precipitation mechanisms in the orographically triggered and modified convective systems will be examined using radar data from operational and research radars in Taiwan.
For tropical cyclones, Taiwan radar data will be analyzed for Typhoon Morakot (2009), which caused devastating flooding when its rain was orographically enhanced. We will explore the hypotheses that warm-rain growth was crucial to the enhancement, and model simulations will test the results.
Intellectual merit: The results of this study will improve basic understanding of the role of mountains in affecting all major precipitation systems across the globe. The research will use existing but unexploited field campaign datasets and the latest versions of high-resolution numerical forecast models to examine the precipitation processes occurring in all major storm types (fronts, convective storms, and tropical cyclones) when they occur near or over mountains. Inclusion of all major storm types and examination over geographically separated mountain ranges will achieve generality.
Broader impacts: Improved observations and understanding of how mountains affect precipitation will lead to improved prediction of severe flooding, hail, tornadoes, and winds. The storms investigated in this study have caused hundreds of casualties and great property damage, and the research will incorporate investigation of their human impacts.
