Abstracts Geography

A comparison of two heavy rainfall events and impacts of data assimilation on a coastal heavy rainfall event during TiMREX (2008)

by Chuan-Chi Tu

Ph.D. University of Hawaii at Manoa 2013. Two contrasting localized heavy rainfall events during Taiwan's early summer rainy season with the daily rainfall maximum along the windward mountain range and coast were studied and compared using a combination of observations and numerical simulations. Both events occurred under favorable large-scale settings including the existence of a moisture tongue from the tropics. For the 31 May case, heavy rainfall occurred in the afternoon hours over the southwestern windward slopes after a shallow surface front passed central Taiwan. The orographic lifting of the prevailing warm, moist, west-southwesterly flow aloft, combined with a sea breeze-upslope flow at the surface provided the localized lifting needed for the development of heavy precipitation. On 16 June before sunrise, pronounced orographic blocking of the warm, moist, south/southwesterly flow occurred because of the presence of relatively cold air at low levels with an offshore wind component as a result of nocturnal and rain evaporative cooling. This also caused convective systems to intensify as they moved toward the southwestern coast. During the daytime, the cold pool remained over southwestern Taiwan without the development of onshore/upslope flow. Furthermore, with a southsouthwesterly flow aloft parallel to terrain contours, orographic lifting was almost absent, and pre-existing rain cells offshore diminished after they moved inland. Over northern Taiwan on the leeside, a sea-breeze/onshore flow developed in the afternoon hours, resulting in heavy thundershowers. Our results demonstrate the importance of diurnal and local effects on determining the location and timing for the occurrences of localized heavy precipitation during the early summer rainy season over Taiwan. Constellation Observing System for Meteorology, Ionosphere, and Climate global positioning system (GPS) radio-occultation (RO) soundings were used to depict the prefrontal moist tongue for a heavy rainfall case (16 June 2008) during the early summer season over Taiwan. Cycling model runs assimilate Terrain-influenced Monsoon Rainfall Experiment (TiMREX) data, global telecommunications system (GTS) data and GPS RO sounding data in order to improve the initial conditions for the outermost domain as well as all nested domains. This leads to better representations of the prefrontal moist tongue over the open ocean, a weak 500-hPa prefrontal trough, and the modifications of the planetary boundary layer structure by the antecedent rains of 14-15 June. The GPS RO data have positive impacts on subsynoptic features including the upper-level low/trough location and orientation resulting in a better initial condition for sequence forecast for upward motion and rainfall patterns. The initial air temperature in the boundary layer over southwestern Taiwan in the early morning of 16 June is relatively cold with significant orographic blocking as compared with the control run initialized with the National Centers for Environmental Prediction (NCEP) Global…