This study analyzes data obtained by intensive observation during a pilot field campaign of the Years of the Maritime Continent Project (Pre-YMC) to investigate the diurnal cycle of precipitation in the western coastal area of Sumatra Island. The diurnal cycle during the campaign period (November–December 2015) is found to have a number of similarities with statistical behavior of the diurnal cycle as revealed by previous studies, such as afternoon precipitation over land, nighttime offshore migration of the precipitation zone, and dependency on Madden–Julian oscillation (MJO) phase. Composite analyses of radiosonde soundings from the Research Vessel (R/V) Mirai, deployed about 50 km off the coast, demonstrate that the lower free troposphere starts cooling in late afternoon (a couple of hours earlier than the cooling in the boundary layer), making the lower troposphere more unstable just before precipitation starts to increase. As the nighttime offshore precipitation tends to be more vigorous on days when the cooling in the lower free troposphere is larger, it is possible that the destabilization due to the cooling contributes to the offshore migration of the precipitation zone via enhancement of convective activity. Comparison of potential temperature and water vapor mixing ratio tendencies suggests that this cooling is substantially due to vertical advection by an ascent motion, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves emanating from convective systems over land play a significant role in the offshore migration of the precipitation zone.

The diurnal cycle over tropical coastal waters is characterized by offshore migration of precipitation area during nighttime. This study analyzes in situ observational data collected during the YMC-Sumatra 2017 field campaign around the western coast of Sumatra Island, Indonesia, to examine the offshore migration phenomenon during 5–31 December 2017, when the Research Vessel Mirai was deployed about 90 km off the coast to perform observation. The offshore migration is observed in only less than a half of the 27 days. A comparison of radiosonde data at the vessel between days with and without the offshore migration reveals that vertical wind shear in the lower troposphere is a key environmental condition. In late afternoon of the days with the offshore migration, offshore (northeasterly) wind shear with height with considerable magnitude is observed, which is due to weaker daily mean southwesterly wind in the lower free troposphere, stronger southwesterly wind in the boundary layer, and sea breeze. As this condition is considered favorable for regeneration of convective cells to the offshore side of old ones, these results support an idea that the regeneration process is critical for the offshore migration. The Madden–Julian oscillation and cold surges play some roles in the weakening of the free-tropospheric wind. The migration speed is estimated at 2–3 m s−1, which is lower than that observed in another field campaign conducted in 2015 (Pre-YMC 2015). This difference is partly due to the difference in the environmental wind in the lower to midtroposphere.

The present study has used reflectivity data from 8-year observations of the TRMM precipitation radar to investigate diurnal variations of the Meiyu/Baiu rain belt associated with the Meiyu/Baiu frontal zone and their relations to diurnal variations on both sides of the frontal zone. A front-relative coordinate system has been designed for analysis. It is found that reflectivity in the frontal zone has a remarkable diurnal cycle that peaks in the early morning. Meanwhile, reflectivity both south and north of the frontal zone possesses a distinct diurnal cycle with a different phase from that in the frontal zone and has a peak in the late morning and afternoon. Based on the analysis, a possible regulation of the diurnal cycle of the Meiyu/Baiu rain belt through the thermally forced diurnal evolution of rainfall systems on both sides of the frontal zone is suggested.

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.

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A Baiu-frontal meso-a-scale depression was generated on 18 June 2001 in the mesoscale observational region, which was located in the downstream region of the Yangtze River. Based mainly on the data collected in this observation, we analyze the three-dimensional structure of a meso-a-scale convective system. We describe this structure and discuss the evolution of Baiu-frontal convective systems near the east coast of the continent.This convective system was composed of several meso-b-scale convective groups, which were bandshaped echoes located near the depression center, cellular echoes formed one after another ahead of the center, and a band-shaped echo extending more than 500 km in the rearward of the depression. There were three flows toward the convective system in the lower troposphere: moist southwesterly jet, dry west-to-northwesterly wind, and cold shallow easterly wind, which appeared near the coast. These convective groups were formed where the southwesterly wind, with convectively-unstable air, was lifted up to the free-convection level. This lifting resulted from low-level convergence, which was formed both by easterly and west-to-northwesterly wind in the central portion of the depression, by easterly wind at the forward portion, and by west-to-northwesterly wind at the rear portion.The system-relative speed of the southwesterly wind in the forward portion was larger than that in the rear. It is suggested that this strong inflow caused the formation of a deep convective structure in the forward portion, which reached the tropopause. The easterly wind played an important role in lifting such a strong inflow, and forming the deep convective groups in the meso-a-scale convective system, and then contributed to evolving the depression through the extension of the convective-updraft area. The Corresponding author: Hiroyuki Yamada, Frontier Observational Research System for Global Change, 3173-25, Shouwa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan. E-mail: yamada@jamstec.go.jp ( 2003, Meteorological Society of Japan mechanism that the easterly wind was formed locally near the coastline is discussed. The enhancement of low-level convergence, due to the formation of such a local wind system, is suggested to be one of the causes for the development of a mesoscale convective system, near the east coast of the continent during Baiu seasons.

Numerical simulations of a cloud cluster over a Meiyu-Baiu front were conducted using a cloudresolving non-hydrostatic model. The purpose was to demonstrate thermodynamic impact of the diurnal heating of the mainland China upon the nocturnal evolution of the cluster. Simulations were conducted within a domain covering eastern and southern China to reproduce land-surface heating on the southern side of the front. The case simulated was a rainband associated with the cluster, which formed on the late afternoon of 22 June 2003. The environment is characterized by a synoptic-scale southerly inflow of warm and moist air in the lower troposphere. A control simulation reproduced the rainband successfully, and showed that the evolution resulted from the strong latent instability over the frontal convergence zone, due to the southerly inflow of the warm and moist air. The origin of this air was the surface heat flux over a cloud-free area to the south of the front. The importance of the heating was proved by a sensitivity simulation without insolation, which failed to reproduce deep convective updrafts. The water budget analysis demonstrated that the evaporation from the ground, rather than the moisture convergence, contributed to the increase in the precipitable water before the rainband evolution. This study, thus, indicates that the continental surface, heated by insolation, has a significant impact upon the nocturnal evolution of a cloud cluster over a Meiyu-Baiu front.