Abstract
Abstract. This paper is a continuation of the authors' previous paper (Part 1) on the feasibility of ensemble flood forecasting for a small dam catchment (Kasahori dam; approx. 70 km2) in Niigata, Japan, using a distributed rainfall–runoff model and rainfall ensemble forecasts. The ensemble forecasts were given by an advanced four-dimensional, variational-ensemble assimilation system using the Japan Meteorological Agency nonhydrostatic model (4D-EnVar-NHM). A noteworthy feature of this system was the use of a very large number of ensemble members (1600), which yielded a significant improvement in the rainfall forecast compared to Part 1. The ensemble flood forecasting using the 1600 rainfalls succeeded in indicating the necessity of emergency flood operation with the occurrence probability and enough lead time (e.g., 12 h) with regard to an extreme event. A new method for dynamical selection of the best ensemble member based on the Bayesian reasoning with different evaluation periods is proposed. As the result, it is recognized that the selection based on Nash–Sutcliffe efficiency (NSE) does not provide an exact discharge forecast with several hours lead time, but it can provide some trend in the near future.
Highlights
Flood simulation driven by ensemble rainfalls has attracted more attention in recent years, resulting in a lot of useful information that ensemble flood forecasts can provide in flood control, such as forecast uncertainty, probabilities of rare events and potential flooding scenarios
In much of their review, the resolution of numerical weather prediction (NWP) model is relatively coarse, the number of ensembles is moderate (10–50) and the target catchment size is often large (e.g., Danube River Basin). They basically reviewed global and European ensemble prediction systems (EPSs) and introduced some research on regional EPSs nested into global EPSs (e.g., Marsigli et al, 2001)
The study used 1600-member ensemble rainfalls produced by 4D-ensemble variational method (EnVar) which contain various rainfall fields with different rainfall intensities
Summary
Flood simulation driven by ensemble rainfalls has attracted more attention in recent years, resulting in a lot of useful information that ensemble flood forecasts can provide in flood control, such as forecast uncertainty, probabilities of rare events and potential flooding scenarios. In much of their review, the resolution of NWP model is relatively coarse (over 10 km), the number of ensembles is moderate (10–50) and the target catchment size is often large (e.g., Danube River Basin). They basically reviewed global and European ensemble prediction systems (EPSs) and introduced some research on regional EPSs nested into global EPSs (e.g., Marsigli et al, 2001). They stated that “One of the biggest challenges in improving weather forecasts remain to increase the resolution and iden-
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