Ensemble rainfall-runoff and inundation simulations using 100 and 1000 member rainfalls by 4D LETKF on the Kumagawa River flooding 2020

This paper presents the 1000 ensemble flood simulations using ensemble rainfalls simulated by 4D LETKF. The number of ensemble rainfall members is large as 1000 compared to the operational rainfall products of two-digit numbers to avoid sampling errors in the three-dimensional meteorological simulation based on chaotic theory. Using the large data set, 1000 ensemble rainfall-runoff for dam catchments and high-resolution inundation simulations of large area are carried out focusing on the Kumagawa river catchment. Herewith, the comparisons were carried out with 21-member ensemble rainfalls of an operational forecast by Japan Meteorological Agency and 100-member 4D-LETKF ensemble rainfalls simulated independent of 1000-member 4D-LETKF. At the same time, the accuracy of selective 100-member ensembles out of 1000 members is investigated. As a result, although many previous research works show a large number of ensemble simulations are necessary for three-dimensional meteorological field, the number could be reduced in the catchment-average rainfall-runoff and 2.5-dimensional inundation simulations given that the rainfall prediction has a certain level of accuracy since improving the discharge prediction accuracy with lower dimension is sometimes possible by adjusting the horizontally/vertically integrated model parameters determined by topography and soil characteristics in advance against the observed rainfall. Also, the 1000 ensembles could be classified into several patterns in horizontally accumulated 2D rainfall field. Likewise, the flood flow moves toward the low elevation area and river; thus, the resultant 2.5-dimensional flood field does not show much variety as three-dimensional meteorological simulation. The paper summarizes these studies.

Automated summary

This paper presents 1000 ensemble flood simulations based on ensemble rainfalls simulated by 4D LETKF. The large ensemble size of 1000 members is used to avoid sampling errors in the three-dimensional meteorological simulation. The study focuses on the Kumagawa river catchment and compares the results with operational forecast rainfalls and independent 4D-LETKF ensemble rainfalls. The findings suggest that a large number of ensemble simulations may not be necessary for catchment-average rainfall-runoff and 2.5-dimensional inundation simulations, and the flood flow patterns can be classified in horizontally accumulated 2D rainfall field.