5:15 PM - 6:45 PM
[AAS08-P14] INFLUENCES OF DIFFERENT MEI-YU FRONTAL ORIENTATION AND MOVING SPEED ON RAINFALL OVER NORTHERN TAIWAN: IDEALIZED SIMULATIONS
Keywords:Mei-yu, Frontal orientation, Movement of front, Idealized simulations
The May–June rainy season is an important water source in Taiwan. During this period, continuous rainfall occurs due to the influence of fronts, and there are often cases of heavy rain. The amount and distribution of rainfall produced by different Meiyu fronts are affected by many complex factors, including water vapor content, southwest airflow, frontal and topographic effects, and mesoscale disturbances. It is difficult to compare and clarify the role of a single factor in different rainfall cases. Therefore, this study uses idealized simulation to simplify the complex frontal system and explore the influence of two major factors—orientations and moving speeds of the mei-yu front—over realistic Taiwan terrain.
We examine eight different orientation angles and three moving speeds, with the front assuming a straight line at the initial time. The eight orientations are every 10° from -20° to +50° (measured counterclockwise from E-W), and the three speeds are fast (20 km h-1), medium (15 km h-1), and slow (10 km h-1). The vertical structure ahead of and behind the front is each horizontally uniform and is obtained by averaging observed conditions from the gridded analyses during the super heavy-rainfall event of 11–12 June 2012. 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 geostrophic 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 (CReSS) to simulate the evolution of the front and the rainfall in Taiwan for each of the 24 scenarios (8 orientations × 3 speeds = 24 runs). For each scenario, 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 (more consistent with the reference case of 11–12 June 2012) or high intensity. Also, faster-moving fronts are more capable of producing frontal uplifting and are associated with a stronger upward motion, so slow fronts do not always produce much rain in such an idealized setting. When the orientation is close to 20°–30° (ENE–WSW), the front produces stronger upward movement as well as low-level convergence, presumably due to stronger confluence between the front and Taiwan’s topography.
We examine eight different orientation angles and three moving speeds, with the front assuming a straight line at the initial time. The eight orientations are every 10° from -20° to +50° (measured counterclockwise from E-W), and the three speeds are fast (20 km h-1), medium (15 km h-1), and slow (10 km h-1). The vertical structure ahead of and behind the front is each horizontally uniform and is obtained by averaging observed conditions from the gridded analyses during the super heavy-rainfall event of 11–12 June 2012. 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 geostrophic 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 (CReSS) to simulate the evolution of the front and the rainfall in Taiwan for each of the 24 scenarios (8 orientations × 3 speeds = 24 runs). For each scenario, 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 (more consistent with the reference case of 11–12 June 2012) or high intensity. Also, faster-moving fronts are more capable of producing frontal uplifting and are associated with a stronger upward motion, so slow fronts do not always produce much rain in such an idealized setting. When the orientation is close to 20°–30° (ENE–WSW), the front produces stronger upward movement as well as low-level convergence, presumably due to stronger confluence between the front and Taiwan’s topography.