Formation and Maintenance Processes of the Fukui
Heavy Rainfall Simulated by the Cloud Resolving Model
In northern Fukui prefecture, a heavy rainfall was observed during the period of 00-12 JST July 18, 2004.
The heavy rainfall is called 'Fukui Heavy Rainfall (Fukui HR)'. The Fukui HR event has two stages. The
period of the first stage is from 01 to 03 JST July 18. That of the second stage is from 05 to 12 JST
July 18. Rainbands were observed during all period of the first stage and 2 hours from 05 JST in the
second stage. The rainband in the second stage changed from line-shaped precipitation to oval-shaped
around 07 JST. The maximum precipitation intensity of Fukui HR was recorded when the oval-shaped
precipitation was formed.
The target of the present study is the second stage of the FukuiHR. A numerical simulation of the Fukui HR
is performed using Cloud Resolving Model (CReSS) to make clear the formation and maintenance processes of
the Fukui HR. The result of the simulation shows that precipitation distribution and amount of precipitation
are similar to those of the observation. Using this result, we study the formation and maintenance processes
of the Fukui HR. We also made sensitive experiments. The sensitive experiments are no evaporation and modified
topography. These results are compared with the simulation experiment to make clear maintenance process of the
We examine the region where the precipitation area occur using JMA-RANAL (Japan Meteorological Agency Regional
objective ANALysis) and simulation results. In this region, The environmental field is high-convectively unstable
and weak convergence region is present in lower layer. After the precipitation area occurred, the precipitation
area move to northern Fukui prefecture.
The precipitation do not show buck-building formation when it move to Fukui prefecture. Because precipitation
cell occur in front of the pre-existing precipitation cells. However, when the low layer convergence zone with
this precipitation area reach Fukui prefecture, the convergence zone stay for about one hour over the mountain
to the west of Fukui plain. This stagnation of the convergence zone is caused by orographic effect and outflow
by water evapolation.
When the convergence zone is stagnated, new cells are developed in convergence zone. Some of the precipitation
cells are drifted to the rear of pre-existing cells by upper-level wind. Consequently, large amount of rain is
brought to northern Fukui prefecture.
In the present study, we find that Fukui HR is occurred by convergence zone which is maintained by a orographic
effect and outflow by water evapolation.