Characteristics of Mesoscale Convective Systems over the East Part of Continental China during the Meiyu from 2001 to 2003

Abstract

Hefei Doppler radar observation data over the downstream region of the Yangtze River during the Meiyu period from 2001 to 2003, were analyzed in order to reveal the predominant structural characteristics of meso-b-scale convective systems (MbCSs) around the Meiyu front. Convective and stratiform portions were separated from MbCSs using the bright-band fraction (BBF) method. The daily and yearly mean vertical profiles of radar reflectivity for the convective portion were calculated.Results showed that the vertical profile of the convective portion of MbCSs for 3 years was characterized by low altitude of radar reflectivity peaks (around 3 km), and large decrease of reflectivity with height above the melting level. To understand these characteristics of MbCSs, the convection of medium depth (CMD) is defined as a group of convective cells whose echo top height, with the reflectivity of 15 dBZ, is equal to or less than 8 km, and in which the reflectivity peak is below 4 km throughout their lifetime.To investigate the structural characteristics of MbCSs around the Meiyu front, observed MbCSs were categorized into slow-moving (a 3 m s À1 ) and south-of-front (SSF) type, slow-moving and along-the-front (SAF) type, fast-moving (b 7 m s À1 ) and along-the-front (FAF) type, and slow-moving and north-of-front (SNF) type; according to their movement speed, and their locations relative to the surface front. The predominant convection in the SSF type was the CMD, and it covered 51% of the convective area. The CMD and deep convections (DC) coexisted in the SAF, with the CMD covering 34% of convective area. TheCorresponding author: Cheng-Zhong Zhang, Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya 464-8601, Japan. E-mail: zhang@rain.hyarc.nagoya-u.ac.jp ( 2006, Meteorological Society of Japan FAF type was organized from the DC, and the SNF type primarily consisted of the CMD. The environmental conditions under which the SSF type formed were characterized by a weak wind convergence (<2 Â 10 À5 s À1 ) near the surface, a low level of neutral buoyancy and humid atmosphere below the middle level. The large contribution of the CMD to convective rainfall amount in the SSF type, and its unnegligible contribution in the SAF type, indicate that the CMD has one of the main structures of the Meiyu frontal convective precipitation systems.

Characteristics of Convections of Medium Depth to South of the Meiyu Front Analyzed by Using Numerical Simulation

“…Convection dominated by warm rain process was found to exist to south of the front (Maesaka 2001). Such convection, whose reflectivity structure was characterized by low altitude of reflectivity peak and low echo top height (15 dBZ ), corresponds to the CMD introduced by Zhang et al (2006). They analyzed 3 years of radar data over the east part of continental China, and concluded that the CMD predominated to the south of the Meiyu front.…”

“…Both LNB and LCL are low (Fig. 8), which are favorable condition for formation of the CMD south of the front (Zhang et al 2006). In the region between the front and 30 km south of the front, the CMD could form in the place where both LNB and LCL are low.…”

“…To further understand the characteristics of the convection of medium depth (CMD) that was introduced by Zhang et al (2006), the dynamic and thermodynamic aspects of the CMD were examined with the use of a cloud-resolving numerical model. A typical case of the CMD south of the Meiyu front over the east part of continental China, which occurred between 00 UTC and 12 UTC 22 July 2002, was simulated and analyzed.…”

“…In an attempt to understand the characteristics of relatively low echo top convection, Zhang et al (2006) proposed the concept of convection of medium depth (CMD). The CMD was defined as a group of convective cells whose echo top height with the reflectivity of 15 dBZ was equal to or less than 8 km, and in which the reflectivity peak was below 4 km throughout their lifetime, by examining 3 years of radar and other observational data during the Meiyu period over the east part of continental China.…”

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AbstractTo further understand the characteristics of the convection of medium depth (CMD) that was introduced by Zhang et al (2006), the dynamic and thermodynamic aspects of the CMD were examined with the use of a cloud-resolving numerical model. A typical case of the CMD south of the Meiyu front over the east part of continental China, which occurred between 00 UTC and 12 UTC 22 July 2002, was simulated and analyzed.

Based on realistic simulation, results of a case study indicated that the CMD possessed the following features: 1) dominated by warm rain process, 2) rapid formation of raindrops and their rapid falling down to the surface, by comparison with those of deep convection.

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