Diurnal and Seasonal Variations of the Rainfall Measured by the Automatic Rainfall and Meteorological Telemetry System in Taiwan

Under a weather hazard prevention program in Taiwan, the Vaisala Meteorological Instrument, Inc., of Finland and the Environmental Science and Engineering Corporation of Taiwan jointly developed and installed more than 300 stations of the Automatic Rainfall and Meteorological Telemetry System (ARMTS) over this subtropical island with an area of approximately 30 000 km². The Taiwan ARMTS network can provide the hourly rainfall data of not only a densely instrumented, but also very uniquely observed, geographically complex island embedded in the east Asian monsoon environment. The hourly ARMTS rainfall for the period 1993-96 supplemented with the hourly surface winds and pressure observed by 25 conventional surface stations around this island are analyzed to understand three aspects of the diurnal and seasonal variations of Taiwan rainfall. The preliminary results of the analysis are presented in this paper. 1) The maximum daily rainfall occurs at about 1600 - 1700 LST, which is about 2-3 h behind the maximum horizontal convergence of surface airmass flux. This time lag suggests that the interaction between mountains and the land-sea breeze circulation may have a timing regulation effect on the maximum daily rainfall. 2) The daily rainfall variations in Taiwan exhibit a bimodality (an early morning minor maximum and an early evening maximum). Although the timing of these two rainfall maxima coincides with the surface pressure minima of semidiurnal variation, the surface pressure tendency analysis does not support this possible cause of the bimodal rainfall variation. The contrast between daily rainfall variation and divergence of surface airmass flux suggests that the early morning rainfall maximum may be induced by the nocturnal drainage flow along the western slope of the Taiwan central mountain range. 3) The seasonal variation of Taiwan rainfall consists basically of a seasonal seesaw between the summer rainfall over west Taiwan and the fall-winter rainfall over northeast Taiwan. Four rainfall regimes over a year are identified around the island. The seasonal transitions of Taiwan rainfall are essentially formed by a counterclockwise rotation of these four rainfall regimes.