Atmospheric conditions analysis of the heavy rain phenomenon in Biak (case study 4-5 December 2014)

Abstract

Heavy rain in Biak on 4–5 December 2014 caused a flood. The accumulation of rainfall until 5 December was 102 mm, it exceeded BMKG's threshold about the intensity of extreme condition. This research aimed to find out the atmospheric conditions related to super heavy rain phenomenon. The atmospheric dynamical analysis used ERA-Interim Reanalysis data 0.125x0.125 degrees resolution. Divergence, relative vorticity, and streamline are the parameters investigated. Their dynamic fluctuations in significant pressure levels proved there were strong convergence and negative vorticity. Streamline analysis showed that there was a convergence wind pattern. While, the physical analysis used NOAA/NESDIS SST data to find sea surface temperature anomaly, RMM index data to find MJO phase and ONI index data to find ENSO phase. At the time, SST around Biak was in warmer condition, and there was a weak El Nino influenced by moderate MJO phase 5. Upper air sounding analysis is used to study the predictability of extreme weather. The indices indicated moderately convective activity and very unstable atmospheric layer before the event, while marginally convective activity and unstable boundary layer with possible inversion aloft during the event. All the conditions supported to produce convective cloud (Cumulonimbus) so that cloud had a long lifetime and produced thunderstorm and heavy rainfall. Biak meteorological station observation and MTSAT imagery used to verify the atmospheric conditions.