Journal directory listing - Volume 11-20 (1966-1975) - Volume 13 (1968)

A Study of Thunderstorm Forecasting In Taiwan
Author: Yen-huai Liu


In the year before last year the author made a detailed study of the thunderstorms in Taiwan and their forecasting. The first part of this Study was published in Geographical Studies No.l June 1966, entitled Geographical Distribution, Seasonal Frequency And Precipitation of The Thunderstorms In Taiwan. This paper is other part of that study. It treats upon the types of Taiwan thunderstorms and the time of their occurrence, general rules of forecasting, methods of objective forecasting and results of thunderstorm forecasting made in different parts of Taiwan.
According to weather map conditions and the process which causes their generation, thunderstorms in Taiwan may be classified into two principle types, the air mass type and the frontal type. Air mass thunder storms are those which occur within relatively homogeneous air masses in the warm seasons of the year and in the warm part of the day. They are local storms with less intensity and few precipitation. Frontal thunderstorms are commenly more severe and they are not confined to any particular time of the day or any particular season. Figure 1 and Figure 2 show the synoptic situations of the migration of the severe thunderstorms in Taiwan in April 1954.
The movement of the thunderstorms in Taiwan is closely correlated with the average wind through the depth of the cloud bearing layer. The small storm moves to the left of the mean wind, and the large storm moves to the light of the mean wind. Small-and medium-sized storms generally travel nearly at the speed of the mean wind, and large storms move much more slowly.
Thunderstorms in Taiwan are prevalent in the afternoon hours of the day. In Taipei, the time at which thunderstorms, are most frequent is 15 :00-16 :00L. T. About 44% of the summer thunderstorms occurred in this tune, and 90% of these storms occurred between 14:00 and 18:00 L. T.
Thunderstorm forecasting may be done a few hours ahead with a little practice by observations of the clouds. Radio static increasing is frequently a good sign of approaching thunderstorms. From pseudoadiatic diagram it is possible to determine the stability of the atmosphere, the steeper the lapse rate, the more unstable is the atmosphere. Inversions at higher levels may limit the development to strato-cumulus clouds.
When strong differences of wind exist at different levels, air mass thunderstorms fail to appear. Low pressure and troughs favor thunderst-orms. Convergence effects may produce thunderstorms.
Objective methods have been developed for the forecasting of the thunderstorms. Stability indices are computed for this purpose, such as Gardner index, Showalter index, tornado index, lifted index, Fawbush-IVIiller index, etc. Methods using combinations of some other parameters as predictor are also introduced, such as combination of atmospheric pre-ssure and characteristic latitude, combination of thickness difference and saturation deficit, and combination of the two combinations just mentioned.
The development and use of weather radar for locating, indentifying and traceing the thunderstorms in the last decade were amazing. There is great promise that doppler radar in particular will prove to be most important as a quantitative indicator of turbulence as well as other internal thunderstorm particle motions. Simultaneous measurement with a high degree of accuracy at two or more wavelengths would be helpful in res-olving some of the ambiguities in determination of precipitation particle sizes, types and concentrations.
Weather forecasters of the Chinese Air Force in Taiwan have develo-ped several objective methods to forecast the summer thunderstorms in Taipei, Taichung, Hualien, Chiayi, Tainan, Kangshan and Pingtung. The results of forecasting are satisfactory in the place where data of radioso-nde and wind aloft observations are available. The forecasts are based on considerations of the stability indices and other meteorological variable such as atmospheric pressure, maximum temperature, dew point, relative humidity, wind direction and wind force and synoptic patterns. Figure 3 and figure 4 are diagrams used for thunderstorm prediction of Taichung area, and figure 5 is another one of Pingtung area. If the radar storm det-ection information in Taiwan can be used properly in the forecasting, the efficiency of thunderstorm forecasts should be better.

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