Some Important Mesoscale Features in HUAMEX

George Tai-Jen Chen
Department of Atmospheric Sciences
National Taiwan University
Taipei, Taiwan (ROC)

1. Introduction

Mei-Yu front established its quasi-stationary position over South China and Taiwan in the period of mid-May to mid-June. The seasonal rainfall distribution reaches a maximum during this period primarily due to the repeated occurrence of the Mei-Yu front. The mean position of Mei-Yu front migrates northward to the Yangtze River Valley in the period of mid-June to mid-July and leads to the ``traditional'' Chinese Mei-Yu season over central China. Heavy rain / flash flood in the Mei-Yu season is one of the most serious meteorological disasters over Taiwan. To better understand the mesoscale processes responsible for heavy rain, the National Science Council of the ROC on Taiwan established a ten-year program ``Taiwan Area Mesoscale Experiment (TAMEX)'' (Chen 1985; Chen and Shieh 1993). The field phase of TAMEX was carried in May-June 1987 (Wu and Chen 1987; Kuo and Chen 1990). Chen (1992) discussed the mesoscale features observed in the Taiwan Mei-Yu season. These include Mei-Yu front, low level jet (LLJ), mesoscale convective systems (MCSs), mesolow, land-sea breezes, and island circulations. Recently, Chen (1994; 1995) reviewed the heavy rain research in the Taiwan Mei-Yu season and suggested that a multi-scale field program covering South China and Taiwan is needed to better understand the atmospheric processes responsible for heavy rain. The main purpose of this short article is to discuss some important mesoscale features over South China and Taiwan in the Mei-Yu season based on author's personal viewpoint.

2. Mei-Yu front

The geographical distribution of frontogenesis frequency showed that the Mei-Yu fronts affecting Taiwan area primarily formed to the south of 35N and gradually moved southward. (Fig. 1; Chen and Chi 1980). In contrast, the polar front usually formed to the north of this latitude and mainly moved eastward. A diagnostic case study of the Mei-Yu front by Chen and Chang (1980) indicated that the structure and the dynamics of the eastern (near Japan) and central (the East China Sea) sections are quite different. An observational and theoretical study by Cho and Chen (1995) suggested that a Mei-Yu front is characterized by a line of high values of low-level potential vorticity and large wind shear across the front (Fig. 2). Frontogenetic process is maintained through the interaction between the PV anomaly and the cumulus convection induced by Ekman-layer pumping. The scale contraction produced by the convergence flow of the convection provides the basic frontogenetic forcing. The convective frontogenesis mechanism proposed for the Mei-Yu front is in contrast to the deformation mechanism for the polar front. Also the properties of the front produced by the convective frontogenesis mechanism are different from those of the classic cold front. Thus, the intensive observing data as proposed in HUAMEX (Huanan and Taiwan Area Mesoscale Meteorological Experiment; Zhou 1995) would provide a great opportunity to better understand the nature and the dynamics of the Mei-Yu front. Another intriguing feature is the formation of wave-like MCSs located at several hundred km apart along the Mei-Yu front. A case study by Chin (1994) suggested that the barotropic instability associated with strong horizontal wind shear within the Mei-Yu frontal zone is a possible mechanism for producing these MCSs. However, the width of shear zone resolved by the current observation network may not be the actual width. Thus, the wavelength derived from the barotropic instability is too large as compared to the observed one. The observational data as proposed in HUAMEX will be very helpful to understand this wave-like feature along the front. Finally, the structure and dynamics of intermedium scale disturbances along the Mei-Yu front as well as the interaction between these disturbances and the MCSs are also operationally and scientifically important topics.



Fig. 1. Frequency of frontogenesis at 1° lat x 1° long grid interval during the Twiwan Mei-Yu period of 15 May - 15 June 1968-77 (1970 and 1975 missing). The heavy solid line remarks the boundary between the formation of the polar front and the Mei-Yu front (Chen and Chi 1980).



Fig 2. Time-latitude cross section on the 850 hPa surface of a) east-west velocity u (ms-1) and b) potential vorticity (10-2 PVU) along the W section of the case analyzed in Chen and Chang (1980) (Cho and Chen 1995)


3. Low level jet

The close relationship between the LLJ and heavy rain is generally observed over South China and Taiwan (Chen and Yu 1988; Ding 1992). Composite study of LLJ observed over South China in TAMEX period by Chen and Hsu (1995) indicated the highly ageostrophic (supergeostrophic) nature of the LLJ at its formation and mature stages. The observed vertical shear was stronger than the geostrophic shear such that a thermally indirect circulation across the jet core and upstream region was indicated. This is in contrast to that associated with an upper level jet streak. Upward motion was observed over the downstream region primarily due to speed convergence by ageostrophic wind along the jet axis. It was suggested that the ageostrophic wind induced by MCSs perhaps contributed to strengthen a LLJ through Coriolis acceleration (Chou et al. 1990). In addition, different length scales of LLJs were observed in the Mei-Yu season. The observational data from HUAMEX are, therefore, very important to study the relevant issues related to LLJ such as the ageostrophic nature, structure and dynamics, and the formation mechanisms of LLJ at different scales as well as the interaction between the LLJ and MCSs.

4. Mesoscale convective systems

Regional heavy rains are primarily produced by the mesoscale convective systems (MCSs). The MCSs affecting Taiwan are mainly originated from South China. It is clear that a better understanding of the initiation and evolution processes of MCSs over South China is necessary to improved the QPF and heavy rain forecast over Taiwan. The Doppler radar observations over Taiwan and southeastern China coast area designed in HUAMEX (HSP 1996) will provide an excellent opportunity for studying the structure and dynamics of those MCSs. Another interesting mesoscale feature is the line convection (squall line) over northern Taiwan and its vicinities. The lines mainly formed over the northern Taiwan Strait and northwestern coast in May and over northern Taiwan and northeastern coast in June. They then moved southeastward and dissipated over northern Taiwan, northeastern coast, and the sea to the northwest of Taiwan in May and over northern Taiwan in June. Observational and numerical studies are needed to better understand the mesoscale triggering mechanisms and the favorable regional- and local-scale environmental conditions.

5. Concluding remarks

``International Workshop on Mesoscale Research and TAMEX Program Review'' (TAMEX 1993) was held in Taipei in 1993 to conclude a ten-year TAMEX program. Scientists from various countries get together to discuss the future research beyond TAMEX. To promote the collaborated mesoscale and heavy rainfall studies, several conferences were then held at various countries. These include "Workshop on Weather Research in Taiwan Area" in Taipei (WWRTA 1994), ``International Workshop on Heavy Rainfall in East Asia'' in Seoul, Korea (IWHREA 1995), ``Bilateral Workshop on Numerical Modelling and Precipitation Processes between Taiwan and Canada'' in Toronto, Canada (BWTC 1995), and ``The Workshop on Mesoscale Meteorology and Heavy Rain in East Asia'' in Fuzhou, China (WMMHR 1995). HUAMEX was proposed and discussed at Fuzhou Workshop. It was recommended to carry out the field phase of HUAMEX in May-June of 1998 and/or 1999 in collaborating with the SCSMEX. Mesoscale features over South China and Taiwan discussed in this paper are the major concerned in the experimental design of HUAMEX (HSP 1996).

Acknowledgments

The author wishes to express his sincere appreciation to Mr. T. S. Wang for preparing the manuscript. This research is partially supported by the National Science Council of ROC under Grant NSC 86-2111-M-002-006-AP6.

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George Tai-Jen Chen
Department of Atmospheric Sciences
National Taiwan University
Taipei, Taiwan (ROC)
E-mail: george@asalpha2.as.ntu.edu.tw