A study of cloud variation on the commencement of South-West monsoon around the Sri Lankan region

Due to the high uncertainty involved in weather forecasting, more accurate methods need to be explored. In this paper a novel approach based on the cloud shape stability to predict the arrival of South-West monsoon is proposed. The cloud shape stability which is determined by the pixel orientation based on the neighbourhood of cloud images begins to drop in April-May and remains in that low level in subsequent months of peak monsoon period (May-September) in all three years 2012, 2013 & 2014 considered in this research. The cloud cover which is determined by the brightness of the pixel starts to go up in the same period and remains in that high level in the same monsoon peak period. When the seven day moving average cloud shape life time is plotted for the months April, May & June for all three years, a bell shaped gradual increase and decrease phenomenon can be observed for late May. Moreover, after decrease of the life time, no momentous life time increase or fluctuation is observed until the end of June. The above decrease happens due to the fact that, the monsoon onset week is determined right after the highest point of that bell shape. Therefore, probable days or a week of the SW monsoon onset can accurately be confirmed as May 18 - May 21, May 25 - May 28 & May 17 - May 20 in the years 2012, 2013 & 2014 respectively according to the above phenomenon. The proposed method can be adopted to predict the arrival of S-W monsoon more accurately in the future.