Validation of WRF Rainfall Model during the November 4th, 2021 Flash Flood Event in Batu City – Indonesia

Abstract

Flash floods in Batu City, Indonesia on November 4th, 2021, caused damage to property, agricultural land, settlements, death of livestock, and loss of human life. One of the important factors triggering this flash flood is heavy rainfall. This study aims to analyse the spatial and temporal dynamics of rainfall as a trigger for the event. Rainfall modelling was conducted using the WRF model with 2 microphysical schemes and 3 cumulus schemes. The data used is GFS data on October 30th, 2021, with a resolution of 0.25 x0.25 degrees which is used to predict rain events until November 4, 2021. Contingency table verification, RMSE value verification, and verification using tolerance limits are used to verify the results of modelling data based on BMKG observation data. The results of this study show that this model can predict rain and non-rain events very well. However, this model is not good enough to predict the thickness of rain until November 4, 2021. The best scheme in modelling rainfall is scheme 5 (Lin and Betts Miller Janjic). Scheme 5 has the smallest RMSE value of 77, the data fit is appropriate and the medium error rate in scheme 5 is 84%. Scheme 5 with GFS data input started from 02 November 2021 can record rain events on 04 November 2021 with extreme rain categories. This extreme rain is influenced by convective clouds. The temperature of the cloud tops changes significantly from 13.00 WIB to 14.00 local time, namely from −4 degrees, and decreases to −76 degrees Celsius.