Three-Dimensional Variational Data Assimilation with Rapid Update Cycling for Short-Range Precipitation Forecasting: A Case Study of Heavy Rainfall in Bali, Indonesia

Abstract

This study evaluates the effectiveness of three-dimensional variational (3D-Var) data assimilation coupled with a Rapid Update Cycle (RUC) framework for improving short-range precipitation forecasts over the Indonesian Maritime Continent (IMC). We employ the Weather Research and Forecasting (WRF) model and its data assimilation component (WRFDA) to assimilate surface observations from Automatic Weather Stations (AWS) at cycling intervals of 1, 3, 6, and 12 hours. Our test case is a heavy rainfall event on 7 July 2023 in Bali Province, during which accumulated precipitation exceeded 193 mm.day$^{-1}$. The 1-hour cycling interval yields the lowest root-mean-square error (RMSE) for both 2-meter temperature (0.0-0.3$\,^\circ$C) and hourly precipitation (1.295 mm.h$^{-1}$), corresponding to reductions of roughly 75% and 57%, respectively, relative to non-assimilated forecasts. Frequent cycling constrains initial-condition errors and captures mesoscale convective evolution, as confirmed by improved spatial agreement with radar reflectivity observations. These results demonstrate that high-frequency assimilation cycling offers clear advantages for nowcasting in tropical maritime environments.

Figures (7)

• Figure 1: Computational domain configuration. Domain 1 (outer) has 9 km horizontal resolution; Domain 2 (inner, rectangle) has 3 km resolution. Shading shows topographic elevation (m). Bali Province lies within Domain 2.
• Figure 2: Locations of the five AWS sites used for DA and verification. Topographic elevation (m) is shown by shading.
• Figure 3: Schematic of the RUC experimental design. Colored blocks mark analysis times; horizontal lines denote forecast integrations. The four cycling configurations use intervals of 12, 6, 3, and 1 hours. The verification target time is 08:00 UTC (16:00 WITA) on 7 July 2023.
• Figure 4: RMSE of 2-meter temperature ($^\circ$C) at the Tabanan AWS for all experiments, 6 July 08:00 to 8 July 08:00 WITA. Lower values indicate closer agreement with observations.
• Figure 5: Accumulated precipitation (mm day$^{-1}$) for 7 July 2023 (00:00--23:00 WITA). Top: GPM satellite estimate. Remaining panels: Cycle 1, Cycle 3, Cycle 6, Cycle 12, DA, and Non-DA.