Ultrahigh-Resolution Numerical Weather Prediction with a Large Domain and Size Dependency of Simulated Convective Cores on Model resolutions

On 19-20 August 2014, Hiroshima City, western Japan, was struck by several debris flows caused by a line-shaped rainband. The rainband was formed by “back-building” without a strong synoptic forcing, and its horizontal scale was small. Using the K computer, we conducted ultra-high resolution (500 and 250 m) numerical weather prediction (NWP) experiments with a large domain for this heavy rain event, and compared the findings with the Izu Oshima heavy rainfall event in October 2013 (Oizumi et al. 2018).

We investigated important factors in a NWP model that influence on the forecast of the event. In the Izu Oshima case, the PBL schemes strongly influenced the position of the rainband, while in the Hiroshima case, the grid spacings had a larger impact on the position of rainband. The higher resolution (500 and 250-m) models showed a better reproducibility of the rainband in position and intensity than the coarser resolution (5 and 2-km) models.

In sensitivity experiments, finer terrain data improved the position of the rainband and its intensity. The 250-m experiments showed the best score in a quantitative verification. The impacts of the model domain size and lateral boundary conditions were less than those of the Izu Oshima case, because the horizontal scale of the line-shaped rainband was small.

We also investigated the size dependency of simulated convective cores (CCs) on model resolutions. The horizontal size of CCs tended to converge at grid spacing less than 500-m, though the number of total CCs still increase at 250 m grid spacing. These results showed that the resolution higher than 500 m is needed to properly simulate the heavy rain event.

Keywords high resolution; heavy rainfall; JMA-NHM; K computer

Reference:

Oizumi, T., K. Saito, J. Ito, T. Kuroda, and L. Duc, 2018: Ultra-high-resolution numerical weather prediction with a large domain using the K Computer: A case study of the Izu Oshima heavy rainfall event on October 15-16, 2013. J. Meteor. Soc. Japan, 96, 25-54. Doi: https://doi.org/10.2151/jmsj.2018-006