Abstract
Global Water Cycle Experiment (GEWEX)-Asian Monsoon Experiment (GAME)/Huaihe River Basin Energy Water Cycle Experiments (GAME/HUBEX) were conducted during the Meiyu period in 1998 and 1999. Using infrared brightness temperature (TBB) data of the Geostationary Meteorological Satellite (GMS)-5, we investigated the diurnal variation of 61 long-lasting cloud clusters that developed during GAME/HUBEX Intensive Observation Periods (IOPs). More than two-thirds of the cloud clusters, named nocturnal-type clusters, attained a convective peak between midnight and early morning, with most peaking between 00 and 02 LST. Almost all of these nocturnal-type cloud clusters developed in, or south of the Meiyu frontal zone. The other clusters, named evening-type cloud clusters, peaked from late afternoon to evening, and were less intense than the nocturnal clusters.
GAME-Reanalysis (Version 1.5) data showed that the ageostrophic wind component over a large domain to the south of the Meiyu front shifted from easterly to southerly, and increased the velocity of the southwesterly airflow largely at low levels at nighttime (02 LST). This southwesterly ageostrophic synoptic-scale low-level jet (S-LLJ) transported large amounts of water vapor to the Meiyu front, forcing large moisture convergence within, and immediately south of the Meiyu frontal zone. The low-level moisture convergence, and the S-LLJ itself are proposed to support the nocturnal evolution or redevelopment of cloud clusters.
