Preliminary Results on the seasonal evolution of atmospheric conditions
surrounding the subtropical frontal zone in East Asia (with special attention tothe subtropical high area around the Southwest Islands region)
１

Kuranoshin Kato, ２Shouichiro Ikeda and ３Yuuki Nakayama
１
Faculty of Education, Okayama University
２
Graduate School of Natural Science and Technology, Okayama University
３
Graduate School of Education, Okayama University
Tsushima-naka 3-1-1, Okayama-city, 700-8530 Japan. 1E-mail: kuranos@cc.okayama-u.ac.jp

1. Introduction
It is well known that the air mass isextremely humid in the subtropical region justto the south of the Meiyu/Baiu front (asignificant subtropical front) in China and thewestern part of Japan. On the other hand,
although the extremely dry and hot region islocated just to the north or northwest of theMeiyu front in China, the moisture content inNorth China increased at the onset of theMeiyu in Central China, according to a casestudy for 1979 (Kato et al., 1995).
Ikeda and Kato (2004 and 2005: this and thelast LAPS International Workshop) examine
the maintenance processes of moisture content
in both regions to the north and the south of theMeiyu front in China in the 1998 Meiyu.
However, behaviors of the subtropical front inEast Asia present considerable seasonal andintraseasonal variations, as well as the regionalvariety. Thus it would be also necessary to
examine seasonal evolution of the atmosphericconditions associated with the maintenance ofthe huge moisture content to the south of thesubtropical front, as well as the moistening
process to the north of the front.
Ikeda and Kato (this workshop) point out
that once the Meiyu rainfall belt stagnatedaround the Huaihe River Basin at the beginningof July 1998, the moisture content there wassustained rather high value, even when theMeiyu rainfall belt moved southward to theChanjiang River Basin in late July. Theysuggested the important role of the relatively
large sensible and latent heat fluxes there,
which might be due to the moistening of thesoil by the duration of the Meiyu frontal
rainfall belt there. However, we are interested
in the atmospheric and land surface conditions
also befere the Meiyu season. Thus thepresent study will report the evolution during
spring to Meiyu season based on the GAMEre-analysis data for 1998.
On the other hand, although the considerable
rainfall climatology is realized throughout thewarm season around the Japan Islands (aroundthe frontal zone such as the Baiu/Akisamefront), moisture export processes of thesubtropical high area to the frontal zone during
the warm season are not are not fully described
yet. Thus the present study will examine
these features with special attention to thecomparison among the Baiu, midsummer andthe Akisame (the autumn rainfall season in theJapan Islands) seasons around the Southwest
Islands area, the LAPS special observationregion, based on the NCEP/NCAR re-analysis
data for 1970 to 2004.
2. Large-scale thermodynamic and land
surface conditions around the Huaihe River
Basin in 1998
Figure 1 shows the time series of daily meanlatent heat flux at the ground (LH) and daily
precipitation (PR) based on the 6-hourly
GAME re-analysis data. The series of daily

data and their 5-day running mean values arepresented for 31.25-35N/110-120E (roughly
corresoponding to the Huaihe River Basen) and27.5-31.25N/110-120E (the Changjiang RiverBasin ) are shown in (a) and (b), respectively.
LH(W/m2) & PR (mm/day) Huaihe(31.25-35N/110-120E)

160
140
120
100806040204/10(a)LH(W/m2) & PR (mm/day) Changjiang(27.5-31.25N/110-120E)

160
140

process of the moisture throughout someperiod can be realized effectively only aftersome persistent precipitation events, such as
the stagnation of the Meiyu front. The similarfeatures are found for the Huanghe River Basin(not shown here) (the latent heat flux increasedaround the middle of July after the Meiyu
frontal precipitation zone once stagnated there).
As for the Changjiang River Basin, thepersistent precipitation event by the Meiyu
front (the two heavy rainfall periods; in lateJune and late July) brought much more total
precipitation amount there than in the HuaiheRiver Basin. Maybe, reflecting that, the latent
heat flux increased greatly in the middle of July
(Period 4 as discussed by Ikeda and Kato (thisand the last workshop). However, it isinteresting that the latent heat flux decreases
again rather rapidly, after the relatively dry
period persists for 2 or 3weeks.

1201008060
40
204/10(b)Fig. 1. Time series of LH and PR based on
the 6-hourly GAME re-analysis data. Details
are referred to the text.
As pointed out by Ikeda and Kato (thisworkshop), the latent heat supplied from theground to the atmosphere in the Huaihe River
Basin has increased in early July (just after thepersistent rainfall event associated with theMeiyu in this basin). Interestingly, although
the rainfall events sometimes appeared thereintermittently also in April and May, the latent
heat flux once decreased gradually in June
when the precipitation was especially small.
This might imply that the recycling-like

3. Seasonal evolution of low-levelsoutherly wind component around theSouthwest Islands area
Figure 2 illustrates the time-longitudesection of 5-day running mean southerly windcomponent at 850 hPa level (V850) at 25N
averaged for 35years.
That for theyear-to-year variation of 31-day running mean
V850 in each year (standard deviation at eachdate for the 35 years) at 25N is shown in Fig. 3.
Furthermore, series of deviation of the 5-day
running mean V850 from that 31-day runningmean were calculated and the series of the"31-day running standard deviation" of themfor the 35 years is presented in Fig. 4.
As for the seasonal cycle of the 35-year
averaged field (Fig. 2), strong southerly wind
toward the Meiyu/Baiu front (contributing tothe huge moisture transport northward) prevails
in the subtropical high area from late June toearly July (mature stage of the Meiyu/Baiu
there). Around September (Akisame season
in the Japan Islands), the time mean V850 isnot so strong as in the Baiu season.
However, the year-to-year variation of V850
in the subtropical high area in about monthly
mean scale shows relatively large variationfrom 120-150E (just to the south of the JapanIslands) from August to early October (Fig. 3).
Furthermore, intraseasonal variation of V850
with period from about 5 days to 30 days islarge as shown in Fig. 4, although this figure
also includes its year-to year difference.
These facts imply that the strong low-levelsoutherly wind frequently appears there as theshort-term
intraseasonalvariationor
year-to-year variation of about monthly mean
field, and so an. In other words, the variationsof moisture transport structure of thesubtropical high to the south of the frontal zone,such as in the Southwest Islands area, would
play an important role in the climatologcal
values of rainfall amount in the frontal zone
around the Japan Islands after the mature stageof the Baiu season there.

Fig. 2. Time-longitude section of 5-day
running mean southerly wind component at
850 hPa level (V850) at 25N averaged for
35years, m s-1. Areas with V850 more than 4
ms-1 are shaded.

Fig. 3.
Time-longitude section ofyear-to-year variation of 31-day running mean
V850 in each year (standard deviation at eachdate for the 35 years) at 25N. Areas with
V850 more than 1.6 ms-1 are shaded.

Fig. 4. Time-longitude section of "31-day
running standard deviation" of difference of the5-day running mean V850 from that 31-day
running mean for the 35 year.