| dc.description.abstract | After the cessation of Mei-Yu season (mid-May to mid June), which in one of the major rainfall periods in Taiwan, rainfall declines quite significantly in late June (16 to 30). Rain showers are the main sources of rainfall over southwestern Taiwan. The averaged annual heavy rainfall occurrence is about 8.8 rainfall stations. However, in 2008, there are 55 heavy rainfall occurrences during late June (Shu 2010). On 27 and 28 2008, the heavy rainfall occurrences are 10 and 4, respectively. During these two day period, the occurrences of heavy rainfall are significantly larger then annual average. On 27, continuous rainfall appeared over southwestern Taiwan with peak rainfall rate over sloped areas during the period from 0900 to 1600 LST and produced 204 mm maximum accumulated rainfall over sloped areas. On 28 June, most of rainfall appeared over lowland areas near coasts from 0900 to 1600 LST and the maximum accumulated rainfall was 160 mm over lowland areas. The occurrences of localized heavy rainfall over either sloped areas or lowland areas present a difficult, yet important, challenge to skilful weather forecasting.
The hypothesis for causing localized heavy rainfall over sloped areas on 27 June is that the inland movement of the existing convection from nearby ocean embedded in the moist southwesterly winds is enhanced by orographic lifting over sloped areas in southern Taiwan. On 28 June, the lifting on the existing convection from nearby ocean embedded in the moist southwesterly winds over low level convergence areas in lowland due to prevailing southwesterly flow deflection resulting from orographic effects may facilitate heavy rainfall over lowland areas. In addition, the cool air from sloped areas may enhance the interaction between off shore flow and the prevailing wind to strengthen low level convergence.
The objective of this study is to perform observational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) data, satellite imagery, radar reflectivity, and rainfall data for the synoptic and mesoscale processes to support the importance of the above two mechanisms. The mesoscale processes associated with the development of the heavy rainfall event and the orographic effects on the enhancement of accumulated rainfall are examined by the Weather Research and Forecast (WRF) model.
The WRF model was employed to help understand the details of the dynamics and physical processes responsible for the localized high rainfall on 27 and 28 June 2008. In order to better resolve the orographic effect on the production of rainfall over southwestern Taiwan, nested grids were used at horizontal resolutions of 27km, 9km, and 3 km. All domain were comprised of 31 vertical levels from the surface to 50 hPa. The moisture processes included the subgrid-scale convective parementerization of Grell in Domains 1 and 2 and the grid-resolvable WSM 6-class microphysics scheme. Planetary boundary layer processes were represented by the YSU PBL paramenterization (Hong et al. 2006). The model was initialized and lateral boundary conditions derived from the ECMWF/TOGA analyses at 0800LST (0000 UTC) 26 June 2008 and integrated for 66h. In order to examine the Taiwan orographic effects on the occurrence of the heavy rainfall over southwestern Taiwan, we performed two sensitivity test without Taiwan’s topography (here after abbreviated to NT) and with the decrease of height of topography to half of those in the control run (here after abbreviated to HC) on 9 and 3-km grid spacing simulations, while keeping everything else identical to the control run. The procedure of the setup of topography in the NT is similar to that in Chiao et al. (2004). In order to investigate the effect of the cool air on the enhancement of rainfall over lowlands over southwestern Taiwan, a sensitivity experiment without the evaporation of raindrops (here after abbreviated to NEV) is performed.
Simulation results confirm our hypothesis that orographic effects offer an important role on generating localized heavy rainfall episode in southwestern Taiwan in late June during post-Mei-Yu period.
| en_US |