Mesoscale Structure and Along-Front Variation of a Meiyu Front and AssociatedPrecipitation Observed in the Downstream Region of the Yangtze River
Biao Geng, Hiroyuki Yamada, Krishnareddigari Krishna Reddy,
IORGC, JAMSTEC, 3173-25, Showa-machi, Kanazawa-ku, Yokohama, 236-001, Japan(Email: bgeng@jamstec.go.jp)
Hiroshi Uyeda
IORGC, JAMSTEC/HyARC, Nagoya University
Yasushi Fujiyoshi
FRCGC, JAMSTEC/ILT, Hokkaido University
Abstract

A Meiyu front evolved on 21 June, 2002 in the downstream region of the Yangtze River has been observed byfive Doppler radars. The front was much deeper and precipitation systems were much stronger in the western partsof the observational area than those in the eastern parts. It seems that the different frontal structure would be
important in the formation of different types of precipitation along the Meiyu front.
Keywords: Meiyu front, Precipitation systems, Doppler radar

1. Introduction
Since 2001, the Institute of Observational Researchfor Global Change (IORGC) of the Japan Agency for
Marine-Earth Science and Technology (JAMSTEC)
has conducted a project study on Meiyu fronts andassociated precipitation systems in the downstream
region of the Yangtze River. Intensive observations
have been taken during the Meiyu season of each year.
The main purpose of this project is to elucidate
processes of mesoscale precipitation systems
associated with Meiyu fronts. The results of theobservations and analyses will contribute to thedevelopment and improvement of numerical modelsof global atmosphere.
On 21 June, 2002, a Meiyu front and associatedprecipitation system were observed by five Doppler
radars in the downstream region of the Yangtze River.
This study will describe the structure of the Meiyu
front and precipitation system. We will emphasize thealong-front variation of the front and precipitation
system.

2. Structure of Meiyu front and precipitation
systemThe Meiyu front evolved in the observational areain the morning on 21 June 2002 (Fig. 1). The surface
front was well defined by the intense horizontal
gradient of the equivalent potential temperature andthere was nearly a 180-degree shift in wind direction
across the front. A linear mesoscale convective system

Fig. 1. Horizontal distribution of surface winds(arrows) and surface equivalent potential temperature(contoured every 2 K) for 0800 LST 21 June 2002. The
reflectivity of low elevation PPI (shaded) and theposition of the surface Meiyu front obtained fromweather map are superimposed. Two boxes indicate twoareas suitable for multiple-Doppler radar analysis.

had developed along the Meiyu front. Along the front,
the contour of equivalent potential temperature wasmore crowded in the western portion than that in theeastern portion, suggesting that the front was stronger
in the western portion. The variation of the intensityfor the convective system along the front also has thesame tendency.
There are two areas suitable for multiple-DopplerDBCAABFig. 2. Top panel: horizontal section of radarreflectivity (shaded) and ground-relative winds (arrows)
at the 3-km level for 0642 LST 21 June 2002 in the area
shown by the western box in Fig. 1. Bottom panel:vertical section of radar reflectivity (shaded) andground-relative winds (arrows) along the line shown in
the top panel. The bold solid line marks the interface ofwinds from both sides.

analysis, as shown by two boxes in Fig. 1. The
western box covered region where the front andprecipitation were stronger, while the eastern box
covered the region where the front and precipitation
were weaker.
The results of Doppler radar analysis in two areas
are shown in Figs. 2 and 3, respectively. The westernsegment of the front was characterized by a deep layerof east-northeasterly flow behind the front (Fig. 2).
The frontal surface reached as high as 6 km. No
prefrontal low-level jet was found in this region.
Strong convection evolved in a narrow belt and close
to the leading edge of the front. Little stratiform
precipitation was found behind the front.
In contrast, the eastern segment of the front wasshallow, less than 1 km in depth (Fig. 3). A
southwesterly low-level jet was identified ahead of thefront. Precipitation system in this region wasorganized with new convection successively
developing south of the front, moving across the frontCDFig. 3. Top panel: same as that of Fig. 2, only in thearea shown by the eastern box in Fig. 1. Bottom panel:same as that of Fig. 2, only along the line shown in the
top panel of Fig. 3. The bold dashed line is the isotach ofhorizontal winds with the speed of 12 m s-1.

toward the north, and becoming stratiform precipitation behind the front. Convective updrafts and cells
were much weaker than those in the western region

3. Conclusions
Data from intensified observation taken in thedownstream region of the Yangtze River were used toanalyze the detailed mesoscale structure of a Meiyu
front and associated precipitation system. The front
was observed on 21 June 2002. The east-west oriented
front could be divided into two parts which have been
observed by Doppler radars very well. The westernsegment of the front was characterized by a deeper
frontal surface and strong narrow convective line. In
contrast, the eastern segment of the front was shallow.
Although a southwesterly low-level jet was identified
ahead of the front in this region, only weaker
convective cells and stratiform precipitation were
found in a broader area across the front.
It seems that the different frontal structure would be
important in the formation of different types ofprecipitation along the Meiyu front.