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

Abstract

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 km 2. 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.

Initial CEOP-based Review of the Prediction Skill of Operational General Circulation Models and Land Surface Models

“…The NSC site (full names are listed in Table 2) is located on a subtropical island and is subjected to heavy rainfall. EOP3 observations reveal that the maximum rainfall hour at this site is 1700-1900 LST, which is similar to the findings (1600-1700 LST) of Chen et al (1999) based on more than 300 stations and the findings (1700 LST) of Kishtawal and Krishnamurti (2001) based on Tropical Rainfall Measuring Mission (TRMM) data. At the HIM site, EOP3 data reveal that the maximum precipitation hour is 2300 LST; the same result was also reported by Tartari et al (1998) and Ueno et al (2001) for nearby stations and Hirose and Nakamura (2005) for a station located south of Mt.…”

Characteristics of Heavy Summer Rainfall in Southwestern Taiwan in Relation to Orographic Effects

“…For the first event (9 August 1999), up to 393 mm fell over the slopes in southwestern Taiwan, while in the second event (13 August 1994), up to 450 mm fell over the coastal and lowland areas of southwestern Taiwan. The European Center for Medium-Range Weather Forecasts/ Tropical Ocean-Global Atmosphere (EC/TOGA) data, rainfall data from the Automatic Rainfall and Meteorological Telemetry System (ARMTS, Chen et al 1999), conventional surface stations over Taiwan, and MM5 were employed to investigate the causes of heavy rainfall observed over the slopes and/or lowlands. The synoptic situation in both events showed that a low pressure belt extended from the east coast of China into the west Pacific south of Japan, and a monsoon trough stretched from southern China into the South China Sea.…”

Maintenance mechanisms for the early‐morning maximum summer rainfall over southeast China

“…Gray and Jacobson, 1977;Chen et al, 2010), and convergence of surface wind and water vapor flux (e.g. Wai et al, 1996;Chen et al, 1999). Here, we propose that all these mechanisms have impacts on the formation of the early-morning maximum P SEC as well.…”

“…Such a feature also predominates in some areas east of 110 • E, including the middle and lower reaches of the Yangtze River, the North China Plain, and the coastal regions of China (Peacock, 1952;Chan and Ng, 1993;Yin et al, 2009;Chen et al, 2010). In contrast, the northern and southern parts of areas east of 110 • E are dominated by daytime precipitation (Chen et al, 1999Yu et al, 2007a). Because the diurnal rainfall over EA varies with a high geographical dependence, many previous studies (e.g.…”

“…Such an afternoon rainfall maximum was generally attributed to moist convection connected with solar heating (e.g. Chen et al, 1999;Dai, 2001). In contrast, causes of the earlymorning maximum P SEC have rarely been discussed and are not well documented.…”