East Asia and Western Pacific METEOROLOGY AND CLIMATE

323
Numerical Simulations of topographical effects on airflow and precipitation
Su-Tzai Soong, Mukut Mathur 1 and Wei-Kuo Tao 2
University of California, Davis, CA 95616
* National Meteorological Center, Washington, DC 20233
2 Goddard Space Flight Center, Greenbelt, MD 20771
ABSTRACT
A primitive equation model was used to simulate the topographic and diurnal
heating/cooling effects on the airflow pattern and precipitation during
undisturbed conditions over Taiwan. The model uses the s coordinate and
includes the soil and boundary layer physics. The simulation without
heating/cooling reached a quasi-steady state in 12 hours. The mountain blocking
effect is clearly evident and there is almost no cross mountain flow except in the
southern most part of Taiwan, where the mountain range is lower. The
simulation with diurnal heating/cooling generated a distinct cyclonic circulation
over southeast Taiwan where the heating in the afternoon was a maximum. This
airflow pattern created an area of low level convergence and precipitation
extending from the west side of the mountain in southern Taiwan, across the
central mountain range, and to the northeast coast of Taiwan
1. INTRODUCTION
One of the major objectives of Taiwan Area Mesoscale Experiment (TAMEX) is to
study the effect of topography on airflow and precipitation during undisturbed conditions.
During the period of June 19-21, 1987, no frontal passage was anticipated and the
Intensive Observing Period (IOP) No. 11 of TAMEX was carried out specifically for this
purpose. Isolated thunderstorms were predicted for all three days, especially in the late
afternoons due to sea breeze effect. The NOAA P-3 aircraft also flew a topographic
mission around the Taiwan island on June 20, 1987. No deep clouds were observed by
satellite at 0800 LST on that morning. By 2000 LST, deep clouds from the southwest to
the northeast of the island were observed by satellite. Precipitation was reported over the
northeast part of Taiwan, where Taipei recorded 27 mm and I-Lan 19 mm (Wu and Chen,
1987). Precipitation was also reported over the Central Mountain Ranges.