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Climatological feature and heating mechanism of the Foehn phenomena
over the north of the central mountain range in Japan by using nonhydrostatic
RCM
Noriko Ishizaki and Izuru Takayabu
Meteorological Research Institute, 1-1 Nagamine, Tsukuba 305-0052, Japan. E-mail: nishizak@mri-jma.go.jp
1. Introduction
Foehn phenomena can be observed in many parts of the
world, and studies have been made by scientists for many
decades. In these days, 3-dimensional numerical simulations
have become popular to investigate detailed structure of
foehn. The contributions of precipitations for foehn warming
often appear to be a controversial point (e.g., Seibert 1990,
Zangle 2003).
In Japan, Arakawa (1982) studied condition under which
foehn event occurred using observation data-sets. Inaba et al.
(2002) analyze prolonged foehn along the coast of Japan Sea
using observation and AOGCM. They concluded that the PJ
pattern and associated synoptic-scale disturbances are
important for the persistence of foehn. However, there are
few studies of which aim is to reveal local structure of foehn
or role of precipitation. The purpose of this study is to
capture the climatological feature and heating mechanism of
foehn in Japan using RCM.
2. Data and Model
The Automated Meteorological Data Acquisition System
(AMeDAS) data, which widely spread in Japan, is utilized to
capture climatological aspect of foehn phenomena. To
determine the foehn duration, we propose the definition as
follows. At first, in each station, the deviation from average
value is calculated. The average values are derived using
data at the same hour each day from 10 days before to 10
days later. If both the surface temperature and wind speed
deviation values exceed 5K and 3m/s respectively, we
regard this period as a foehn event. While the conditions are
satisfied continuously, it is assumed to be one event.
We used the non-hydrostatic model NHRCM which is based
on the operational model of JMA. Details are described in
Sasaki et al. (2008). We used two sets of RCM
configurations. One has 145x155 grids with 20km resolution
to examine long-term simulation. Mechanism study is
conducted with finer grids model. The horizontal resolution
is 10km and domain has 145 by 145 grids and 40 vertical
layers as shown in Figure 1. Both configuration sets use
Japanese reanalysis (JRA-25) to make initial and boundary
condition data.
3. Climatological Aspect
Through the observation analysis, we found that Japan Sea
side of the Central Mountainous region often undergoes
foehn events. Most of these events occur under strong
southerly wind. According to the composite analysis,
migratory cyclonic system over Japan Sea is prominent
while foehn period observed around north part of Central
Mountain region. It has a key role to bring a favorable
situation for foehn with strong and persistent winds.
The foehn frequency distribution derived from long-term
simulation using 20km-NHRCM, which is covering the
whole Japan, is basically similar to those of observation
(Fig.2). Furthermore, the typical pressure patterns at the
foehn event for each area appear to be similar to each other
(Fig.3). Thus, 20km-NHRCM can reproduce basal
characteristics features of the foehn.
Figure 1. Calculation domain and topography. Gray
colored zone indicates buffer zone.
Figure 2. Frequency of occurrence of foehn events
from 2002-2004. (a) is for 20km-NHRCM and (b) is
derived from observational station.
Figure 3. Composited sea level pressure for near
Toyama station (see Fig. 1) derived from (a) 20km-
NHRCM for 2002-2004 and (b) JRA-25 for 1979-
2004.