Study on Characteristics of Convection of Medium Depth
around the Meiyu Front over the East Part of Continental China

Cheng-Zhong ZHANG

In order to reveal the predominant structural characteristics of meso-β-scale convective systems (MβCSs) around the Meiyu front, Hefei Doppler radar observation data over the downstream region of the Yangtze River during the Meiyu period from 2001 to 2003 were analyzed. Convective and stratiform portions were separated from MβCSs using the bright-band fraction (BBF) method. Convective and stratiform portions were separated from MβCSs using the bright-band fraction (BBF) method. The daily and yearly mean vertical profiles of radar reflectivity for the convective portion were calculated.

Both the daily and yearly mean vertical profiles of radar reflectivity for the convective portion identified by the BBF method exhibited remarkable features of a low altitude of reflectivity peak (around 3 km) and a large decrease of reflectivity with height above the melting level.

To understand the characteristics of the convection around the Meiyu front, MβCSs observed there was analyzed. The results confirmed that a number of convective cells with a low altitude of reflectivity peak (≦ 3 km) and a relatively low echo top height (≦ 8 km) existed around the Meiyu front. A concept of the convection of medium depth (CMD) is proposed. The 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 MβCSs around the Meiyu front, MβCSs were categorized into four types; slow-moving (≦ 3 m s-1) and south-of-front (SSF) type, slow-moving and along-the-front (SAF) type, fast-moving (≧ 7 m s-1) 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 Meiyu front. The contribution of the CMD to convective rainfall amount for four types of MβCSs was examined. This contribution in the SSF type (to south of the front region) is large, and those in SAF types (along the front) are un-negligible, whereas the contribution in the FAF and SNF type is negligible. The CMD has one of the main structures of the Meiyu frontal convective precipitation systems.

In order to extend understanding of characteristics of the CMD to south of the Meiyu front, a case of the CMD south of the front was investigated. The case occurring between 00 UTC and 12 UTC 22 July was selected, since the ratio of the CMD to convective area was large from analyzing daily ratios of the CMD during the Meiyu period of 2001, 2002 and 2003. To investigate the characteristics of the CMD from viewpoint of dynamics and thermodynamics, a could-resolving numerical model (CReSS) was used to simulate this case.

Based on the simulation, five cells south of the front were traced. Results showed that echo top height, hydrometeors peak and lifetimes of these cells coincided well with observational CMD, verified that they were CMD in nature. Analysis results of output of simulation indicated that the CMD to south of the front exhibited two conspicuous features: 1) maximum updraft was moderate(< 6 m s-1); 2) rapid formation and rapid falling out of raindrops, 3) little or no snow and graupel forms from the CMD. To better understand these features, we compared the CMD with convections along the front. It was found that deep convection (DC) and the CMD coexisted along the front. This agreed well with that in observation. The features of the CMD along the front were similar to those to south of the front. Formation process of the DC was similar to the CMD before its mature stage.

To understand the environment in which the CMD occurs, relationship between the CMD around the Meiyu front and its environment was discussed by combination of observational sounding data, regional objective reanalysis data and results of the simulation. Environment where the CMD frequently occurs was characterized by low LCL (due to humid atmosphere), weak wind convergence near the surface, and low LNB. On the other hand, the environment where the DC frequently forms was characterized by strong wind convergence near the surface and high LNB. It is considered that one of favorable conditions for formation of the CMD around the Meiyu front is low LCL, low LNB and weak wind convergence near surface.

In combination with the features of the CMD and its environment obtained from both simulation and observation, the formation process of the CMD to south of the front could be explained as follow; under humid atmospheric condition and weak convective instability at lower level, convective cell could be formed by a weak wind convergence at lower troposphere in the place where the atmosphere is the most unstable. Updrafts within the cell are moderate, the growing hydrometeor particles easily become heavy enough in a short time to overcome the updraft, falling out to the ground before the cell reaches mature stage. Because the LNB, which represents potential cloud top height, is low, and updrafts are not sufficiently strong to uplift large amount of hydrometeor particles penetrating the melting level, resulting in formation of the CMD. The CMD is characterized by predominance of warm rain process around the Meiyu front.

It is concluded that the CMD is one of significant and common phenomena around the Meiyu front over continental China. This study contributes to understand the characteristics of convection formed in the region where environment is characterized by humid atmospheric condition below middle level.