The precursors of high rainfall intensity during June in Southern Central Java: A case study of flash floods 18 June, 2016 in Purworejo

Abstract

High rainfall intensity in Indonesia, even if it happens during the dry season can cause floods and landslides that cause a lot of loss of property and life, as happened in Purworejo. Generally, the dry season such as June has a little rainfall, and even more, heavy rain is very rare. However, the disaster in Purworejo became an important experience in anticipating disasters by identifying the precursors of extreme rainfall. This research aims to obtain high-intensity rain precursors in the south of Central Java, especially Purworejo. Identification is done using rainfall intensity classification and is related to atmospheric phenomena that affect Indonesian rainfall variability. The results show that if there is very heavy rainfall and evenly distributed, precursors, can be identified 3 days before the incident, such as a low outgoing long radiation (OLR) value of <150 Wm-2. Moreover, in an analysis of the average 3 days before the incident, the value of the OLR anomaly in the Central Java region is between –60 to –20 Wm-2.Meanwhile, the vertical humidity profile has a value of more than 80% up to a level of 500 Mb, as the temperature value of more than 30.2 °C in the Indian Ocean bordering the southern part of Java Island occurs three to four days before the extreme rain event in Purworejo Regency. This change occurs when the value of the Indian Dipole Mode is negative and the ENSO is neutral. Existing low air pressure in the northern part of Kalimantan and in the southern part of Java, which persists for a long time, can support very heavy rains in June in Purworejo. Especially if the wind speed decreases to less than 4 m/s and occurs in phases 3 and 4 of the MJO. The 20 mm/30-minute rainfall that occurs for 7 hours without stopping needs to be watched out for because it has the potential to cause flash floods in the sandy loam area of Purworejo.