Synoptic and Mesoscale Analyses of Heavy Precipitation Events During TAMEX

本项目将基于数值模式的结果和台湾地区中尺度试验（TAMEX）收集的资料，对TAMEX期间有利于强降水发展的天气尺度、中尺度和局地过程进行研究。当梅雨锋面接近台湾岛时，西南季风锋面（或低空急流）从南方带来了温暖潮湿的不稳定空气。除了低层抬升（地形和锋面）外，高层强迫的存在（与分流气流有关的高层辐散或与喷气锋系统有关的锋生强迫）是副热带大范围暴雨发展的必要条件。在本研究中，我们将基于模型生成的高分辨率数据来诊断LLJ。特别是，将进一步研究气旋形成与LLJ发展之间的联系。此外，我们将研究导致大范围降水发展的高层和低层环流之间的耦合。天气尺度的环流调节是否会发生大范围降水，而中尺度环流、局地环流和地形效应则精确地控制对流发生的时间和地点的细节。本文将利用雷达、飞机和常规资料，以及嵌套网格模式，研究台湾地区的锋前中尺度气流。我们将特别关注高空低空槽/低空急流接近时迎风面的气流。在锋面西南气流中，沿海岸偏转的南风急流形成有利于深部对流发展的局部辐合带。当锋面在台湾北部移动时，偏转偏南气流可能与西北沿海台湾海峡的非偏南气流合并，形成有利于深对流发展的局部辐合带。当锋面在台湾北部移动时，偏转的偏南气流可能与锋面后东北风汇合，导致锋面辐合区沿海岸增强。为了评估由日加热循环引起的热驱动环流对降水的影响，我们将研究岛上不同地区在日循环期间的降雨特征。此外，我们将基于IOP #11期间收集的数据，对西南季风锋面流下的岛内局地环流进行个案研究。

9421060 Chen In this project, the synoptic-scale, mesoscale and local processes favorable for the development of heavy precipitation during TAMEX will be studied based on the results form numerical models and the data collected during the Taiwan Area Mesoscale Experiment (TAMEX). As the Mei-Yu front approached the island of Taiwan, the prefrontal southwest monsoon flow (or the low-level jet) brought in the warm, moist unstable air from the south. In addition to the low-level lifting (orographic and frontal), the presence of upper-level forcings (the upper-level divergence related to diffluent airflow or the frontogenetical forcing associated with a jet-front system) is required for the development of widespread rainstorms in the subtropics. In this study, we will diagnose a LLJ based on the model generated high resolution data. In particular, the link between the cyclongenesis and the development of LLJ will be further investigated. In addition, we will study the coupling between the upper-level and low-level circulations leading to the development of widespread precipitation. The synoptic-scale circulations regulate whether or not widespread precipitation will occur, whereas mesoscale circulations, local circulations and orographic effects govern the details of precisely when and where the convection will occur. We will study the prefrontal mesoscale airflow over the Taiwan area based on radar, aircraft and conventional data as well as a nested grid model. In particular, we will focus on the airflow on the windward side as the upper-level trough/low-level jet approached. In the prefrontal southwesterly flow, the deflected southerly jet along the coast and results in a localized convergence zone favorable for the development of deep convection. As the front moved over the northern Taiwan, the deflected southerly flow may merge with the undeflected southwesterly flow in the Taiwan Strait off the northwestern coast and results in a localized convergence zone favorable for the development of deep convection. As the front moved over the northern Taiwan, the deflected southerly flow may converge with the postfrontal northeasterlies and results in an enhanced frontal convergence zone along the coast. To assess the effects of thermally driven circulations caused by the diurnal heating cycle on precipitation, we will study the rainfall characteristics during the diurnal cycle at different parts of the island. In addition, we will conduct a case study of local circulations over the island under the prefrontal southwest monsoon flow based on the data collected during IOP #11.