Sensitivity and Predictability of an Extreme Rainfall Event in Indonesia

The Makassar Peninsula in southwestern Sulawesi, Indonesia, experienced its largest flood in its recorded history in January 2019. Four-day accumulated rainfall exceeded 350 mm with devastating impacts on the community, including 53 perished and over 14000 evacuated. Previous studies find a convectively coupled Kelvin wave and convectively coupled equatorial Rossby wave associated with the Madden-Julian Oscillation to be likely contributors to the onset of the mesoscale convective system responsible for the flooding. We employ an adjoint model to identify and dynamically link specific components of the mesoscale and environmental flow affecting the flooding event.The adjoint simulations indicate that enhancing the moisture and low-level convergence associated with the mesoscale convective system can substantially increase rainfall. The sensitivity patterns are complex, with low-level convergence and vorticity sensitivity in quadrature and projecting onto the larger-scale Kelvin and Rossby waves. The vorticity sensitivity enhances waves along the dynamic equator. Small adjoint-based perturbations made to the initial state can increase the 36-h rainfall maximum by greater than 30%. The sensitivity analysis supports the importance of a mesoscale convective system, orographic ascent, and equatorial wave components in contributing to the flood. The rapid growth of small initial perturbations underscores the need for probabilistic forecasts.