Land – sea contrast of diurnal cycle characteristics and rain event propagations over Sumatra according to different rain duration and seasons

Abstract

This study investigated fundamental issues on diurnal cycle variations over Sumatra, especially concerning land - sea contrasts and rain event propagations according to changing rain duration and seasons. Based on the Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement (IMERG) final-run product version 6 covering 2015–2019 and observed precipitation data from 97 rain gauge stations, precipitation amount (PA), frequency (PF) and intensity (PI) were computed and analyzed. The land-sea contrasts in PA, PF and PI were apparent but the contributions to diurnal cycle of precipitation came mainly from PA and PF. Early morning peaks of PA and PF were observed in the coastal sea in contrast to afternoon and evening peaks in the land regions. Furthermore, the variations of peak times of PA and PF were also dependent on rain duration, particularly over land regions. The peak time of short-duration rain events (〈3h) over land regions manifested much earlier than long-duration rains (> 6 h). The precipitation system's peak time dependence on rain duration tended to affect westward and eastward propagations. Long-duration rain can migrate hundreds of kilometers from the coastline toward inland and offshore regions, whereas short-duration rain travels only tens of kilometers. For rain events over mainland, the initiation point of rain propagation often coincided with the highest point of the topography, indicating the prominent role of the island's mountainous areas in the development and propagation of the precipitation system. The propagation of rain events was also dependent on the width of Sumatra island where it was more prominent in equatorial and southern regions compared to the northern region where the width is narrower. Hence, the peak time of PA and PF showed little variation across northern Sumatra. In addition, the spatial patterns of PA, PF, and the number of rain events for different durations also showed seasonal variations due to the influence of the Inter-tropical Convergence Zone (ITCZ). However, the propagation features of rain events including peak time showed little evidence of variations with season.