曲克恭

On the Effect of the Central Mountain on Typhoons in Taiwan by Field Data Study

Chu Ko-Kung

Abstract

1. Data of 45 typhoons which have affected Taiwan from 1956 to 1976 are studied by using the methods of case analysis and statistics in order to know the effect of the Central Mountain in Taiwan on the vertical distributions of wind field, moisture field and hourly rianfalls.

2. The vertical wind field below 50,000 ft indicated by rawin data is approximately barotropic as the characteristics of tropical air before it is affected by typhoon circulation. Under the typhoon influence, it would be changed considerably. Individual typhoon has its own wind field over Taiwan islands and the level of the strongest wind (typhoon jet stream) may forms at different height above the mountain or below it. There are close relations between it and the rainfalls.

3. There is considerable influence of the Central Mountain on the typhoon moisture field over Taiwan. The typhoon air is much moist near the mountain than the air 50 kilometers west and even much drier 200 kilometers far west over the Taiwan Strait. Using the data of the three rawinsonde stations in Taiwan to compute the averaged mixing ratio, the result is about 9.1 g/kg which is comparable to Bell and Tsui’s (1973) result, 8.9 g/kg, but the maximum mixing ratio which has been found in Taiwan was 12.0 g/kg (Trix) and the minimum, 6.8 g/kg (Mary). In general, the most frequently occurred typhoon mixing ratio over Taiwan is 7.0-10.9 about 88 per cent of the total.

4. Using the surface observed dew point to estimate the total precipitable water in typhoon air column which has been affected by the mountain in Taiwan is not adequate, but using the reported dew points by the mountain weather stations to integrate the precipitable water from the surface to the pressure height of the highest mountain weather station is much better than preceding method.

5. It is found that there are specific types of curve of the hourly rainfall change relative to certain typhoon tracks； and the configurations of it are similar if the reporting stations are under the same topographic influence, except that its amplitutes are much different and its phase differences are not much significantly.

Therefore, we could normalize the hourly rainfall curves as unit rainfall graphs