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    Atmospheric and hydrospheric sciences


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

    Kobayashi K, Le Duc, Kawabata T, Tamura A, Oizumi T, Saito K, Nohara D, Sumi T

    1000 ensemble, 4D LETKF, rainfall-runoff, storage function model, inundation, shallow water equation, Kumagawa river

    Upper left: Overview of the Kumagawa River catchment.In the figure, Ichifusa and assumed Kawabegawa dam catchments are also shown. Likewise, the computational area of the detailed inundation simulation with 5m resolution is shown.

    Lower left: The reproduced flooding with 5m resolution by the radar-raingauge rainfall at the July 2020 Kumagawa severe rainfall event. The inundation depth is shown in the figure which is in general in good agreement from the range with the record taken by Geospatial Information Authority.

    Upper right:1000 discharge simulations at Ichifusa and assumed Kawabegawa dams driven by 1000 (LETKF)

    Lower right: Given that the inundation depth simulated by radar-raingauge rainfall without dam peak discharge cuts is an observation, the probabilities [%] of 1000 ensemble inundation depths beyond the observation are shown.

    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.