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

Abstract

About a quarter to a half of all rainfall from mid-July through August from 1994 to 2000, on the windward side of southwestern Taiwan, came from convective systems embedded in the monsoon flow associated with the southwesterly monsoon. In this study, the causes of two heavy rainfall events (daily rainfall exceeding 100 mm day À1 over at least three rainfall stations), observed over the slopes and/or lowlands of southwestern Taiwan were examined. Data from the European Center for Medium-Range Weather Forecasts/Tropical Ocean-Global Atmosphere (EC/TOGA) analyses, the rainfall stations of the Automatic Rainfall and Meteorological Telemetry System (ARMTS) and the conventional surface stations over Taiwan, and the simulation results from a regional-scale numerical model were used to accomplish the objectives. In one event, on 9 August 1999, heavy rainfall of up to 393 mm was observed over the windward slopes of southern Taiwan in a potentially unstable environment, with very humid air around 850 hPa. The extreme accumulation was simulated and attributed to orographic lifting effects as preexisting convection did not drift into western Taiwan from the Taiwan Strait.In another event, on 13 August 1994, with a maximum rainfall of 450 mm, westerly flow with potentially unstable and very moist air in the lowest layers, converged with northwesterly flow from the central Taiwan Strait over the southwestern coast. The westerly flow decelerated while approaching Taiwan, resulting in enhanced convergence and upward motion over the coastal area and a high amount of rainfall. In addition, a mesoscale convergence area was produced over the southern Taiwan Strait near the southwestern coast of Taiwan due to synoptic circulations. Some convection formed in the ocean near the southwest coast and drifted inland. This situation also enhanced precipitation over the coastal areas. Northwesterly flow formed over the southwestern slopes as weak westerly flow approached the mountains. As rainwater from the Strait and coastal areas moved eastward toward the slopes, the northwesterly flow transported the rainwater southeastward away from the southwestern slopes. Subsequently, less rainfall occurred over the southwestern slopes than over the coastal areas.

Orographic effects on precipitating clouds

“…In Taiwan, extreme convective rainfall and flooding events occur in association with mesoscale convective systems that are affected by the presence of the Central Mountain range [e.g., G. T.‐J. Chen et al , 2003, 2005; C.‐S. Chen et al , 2004, 2005, 2010, 2011; Chang et al , 2008; Kerns et al , 2010].…”

Diurnal Circulations and Rainfall in Taiwan during SoWMEX/TiMREX (2008)

“…In the mountainous subtropical island of Taiwan, rainfall is controlled throughout the year by such circulations, which cause a pronounced diurnal oscillation in island-averaged low-level divergence (Johnson and Bresch 1991;Chen et al 1999;Kishtawal and Krishnamurti 2001;Kerns et al 2010;Lin et al 2011). Great complexity is added to this system in the presence of monsoon flow, which interacts with Taiwan's main topographic barrier, the Central Mountain Range (CMR), to generate enhanced upstream convergence, barrier jets, offshore rainbands, and lee vortices (Sun and Chern 1993;Li and Chen 1998;Yeh and Chen 2002;Chen et al 2004;Alpers et al 2007;Yu and Lin 2008;Yu and Hsieh 2009). The manner in which such features interact with the diurnal cycle needs further investigation, as these two major aspects of Taiwan's meteorology have been studied quite separately to date.…”

Projections of Future Changes in Precipitation and the Vertical Structure of the Frontal Zone during the Baiu Season in the Vicinity of Japan Using a 5-km-mesh Regional Climate Model

“…When compared with observational data, however, even the super-fine hydrostatic atmospheric general circulation model with a horizontal resolution of 20 km (AGCM-20km) underestimate intense precipitation and overestimate the number of wet days (Kanada et al 2010). Heavy rainfall events in East Asia are characterized by spatially and temporally concentrated events of intense precipitation (e.g., rain rate greater than 100 mm day À1 ) associated with the Baiu frontal zone and typhoons (Biao et al 2004;Kato 2006;Chen et al 2004;Shin and Lee 2005).…”