總計畫暨子計畫:台灣準滯留性豪大雨降水系統之形成機制與可預報度研究(I)

Heavy-rainfall events are major weather hazards in Taiwan, and the most severe hazards are often produced by quasi-stationary rainfall systems (QSRSs), for they can lead to prolonged rainfall at high intensity with extreme total accumulation values. Therefore, it is necessary to specifically target QSRSs and perform more in-depth studies in order to improve the quantitative precipitation forecasts (QPFs) for them and eventually to prevent and reduce the hazards caused by them. This is the purpose of this three-year project. In the first year, we have performed idealized cloud-model simulations of mei-yu front and its associated rainfall distribution in Taiwan, specifically in northern Taiwan, to better understand the role played by the two important factors: orientation and moving speed of the mei-yu front. Since real events are affected by many different factors such as the southwesterly flow strength, the moisture content, the front, topographic effect, and meso- and small scale disturbances. To clarify the role of certain factors, idealized simulations that rule out the influence of some of the other factors are needed and benefical. Thus, a series of idealized simulations are designed and performed, using realistic Taiwan topography. A total of eight different orientation angle and three moving speeds are designed, while the front is assumed to be a straight line at the initial time. The eight orientations are every 10° from -20° to +50°, and the three speeds are fast, medium and slow. The vertical structure ahead of and behind the front are obtained by averaging observed conditions from the gridded analyses from the super heavy-rainfall event of 11-12 June 2012, and are each horizontally uniform. The northerly wind behind the front is assumed to be at 45° from the front, whose slope is also prescribed based on observation. Using the geospheric wind relationship, the three-dimensional southwesterly pre-frontal flow and the northerly post-frontal flow fields are constructed and combined according to the specified mei-yu front. Then, the combined field is fed into the Cloud-Resolving Storm Simulator to simulation the evoluation of the front and the rainfall in Taiwan for each of the 24 scenarios. For each experiment, the rainfall in northern Taiwan is examined, and its accumulated rainfall, rainfall intensity, and rainfall duration are analyzed. For the same moving speed of the front, the closer the orientation is to 20°-30° (ENE-WSW), the higher the rainfall intensity; the closer to 20° (WNW-ESE), the longer the duration. For the same orientation, the faster the front moves, the higher the rainfall intensity but the shorter the duration. The simulations that produce more rainfall in northern Taiwan are associated with one of the two following scenarios: long duration or high intensity. Also, faster-moving fronts are more capable to produce frontal uplifting and are associated with stronger upward motion, so slow fronts do not always produce more rain in such an idealized setting. When the orientation is close to 20°-30° (ENE-WSW), the front can produce stronger upward moving as well, presumable due to stronger confluence between the front and Taiwan’s topography.