Possibility of Rain Shower Forecasting in Tropical Isolated Stations

Abstract

In order to find a reliable and effective means for forecasting the tropical rain showers with respect to the time of occurrence and their characteristics by using a single-station data, brief survey was made of the hourly surface and three-hourly pilot balloon observation data obtained at Soelavaya, Java, from May 1, 1942 to June 30, 1942.
This inspection revealed the existence of several well established four to five-day weather sequences that recurred regularly with high frequency showing a significant characteristics of rain shower type.
In this report, the following six shower types are classified as balow;
1. Whole-day shower type: Date, May 22-25, June 18-22.
2. Night shower type: Date, May 1-5, 18-22, June 7-10.
3. Morning shower type: Date, May 15-17, June 27-29.
4. Afternoon shower type; Date, May 10-14, 26-30.
5. Fair weather type: Date, May 31-June 6, June 23-26.
6. Clear weather type: Date; May 6-9, June 11-17.
As an aid in forecasting the comming weather type in a transitional season from rainy to dry, the following means are proposed hereunder as a conclusion.
A. Six types alternate with about the same frequency.
Their persistency for each type varies from 3 to 7-day period with an average of 4.6-day.
B. In majority of the cases, there is a good parallelism between a phase of each type and daily mean pressure waves having a period of 4 to 5-day with an average of 4.6-day.
C. Strong east winds are accompanied by night and morning shower types, while strong southeast winds are accompanied by fair and clear weather types.
D. When a weak westerly wind is observed aloft, afternoon shower type follows in most cases.
E. Common in all cases, whenever northeast winds begin to blow, showers will follow one or half a day later.
F. Same types are mostly recurred about 20 days apart except for morning shower and clear weather type.
G. During this period, the following three long waves were pointed out of the smoothed pressure march.
May 3-17 (descending half) Period 28-day, Amp. 2.5mm
May 26-June 19 (perfectly) Period 24-day, Amp. 2.5mm
June 19-July 5 (ascending half) Period 32-day, Amp. 2.7mm
H. It must be the most helpful for the practice of long range forecasting to make a map with consequent 5 days average of pressure and lower cloudiness as an abscissa and ordinate, respectively, Map represents visual designations of the shower type and prevailing winds on each day point plotted. (See Fig. 3 in Japanese report attached.)
In such a composite map, it will be noted that there is an outstanding transitional rule from one type to another