Characteristics of mesoscale convective systems over the east part of continental China during the Meiyu from 2001 to 2003

摘要

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-beta-scale convective systems (M beta CSs) around the Meiyu front. Convective and stratiform. portions were separated from M beta CSs 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 M beta CSs 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 M beta CSs, 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 M beta CSs around the Meiyu front, observed M beta CSs were categorized into 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 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. The 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 x 10(-5) s(-1)) 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.