Mesoscale convective systems along the Meiyu frontin a numerical model |
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Authors: | S-J Chen W Wang K-H Lau Q-H Zhang Y-S Chung |
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Institution: | (1) State Key Laboratory for Severe Storm Research, Department of Geophysics, Peking University, Beijing, China, CN;(2) National Center for Atmospheric Research, Boulder, USA, US;(3) Center for Coastal and Atmospheric Research, Hongk Kong University of Science and Technology, Hong Kong, HK;(4) Korea-China Center for Atmospheric Research, Korea National University of Education, Chongwon, Korea, CN |
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Abstract: | Summary Two organized mesoscale convective systems (MCSs) developed sequentially along the Meiyu front over the Yangzi-Huai River
basin and caused severe flooding over eastern China during 12–13 June 1991. In this paper, the structure and evolution of
these MCSs are studied with a high-resolution (18 km) numerical simulation using the Fifth Generation Penn-State/NCAR Mesocale
Model (MM5).
The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded
clearly on satellite-derived cloud-top black body temperature (T
bb
) maps. A mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respectively, to the south
and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection
associated with the MCS. The mLLJs transported warm, moist air with equivalent potential temperature greater than 352 K into
the MCSs, and strong low-level convergence can be identified on the left-front end of the mLLJ. This strong convergence was
associated with intense upward motion in the MCS with speed up to 80 cm s−1. Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ.
The development of the MCSs was also associated with substantial increase in potential vorticity (PV). The build up of PV
in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential
temperature gradient, suggesting a relationship between the MCSs and the local enhancement and cyclogenesis of the front.
In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about
300 km were simulated. This result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence
of moist-diabatic process. Since the horizontal velocity gradient across the Meiyu front near the synoptic-scale low-level
jet (LLJ) was quite large while the corresponding temperature gradient across the frontal zone was rather weak, we speculate
that barotropic process may be responsible for triggering these MCSs along the Meiyu front.
Received December 28, 1999 Revised May 11, 2000 |
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