Intensive Radiosonde Observations of Environmental Conditions on the Development of a Mesoscale Convective System in the Baiu Frontal Zone

• Published in: Earth and space science (Hoboken, N.J.) Vol. 11; no. 7
• Main Authors: Manda, A., Tachibana, Y., Nakamura, H., Takikawa, T., Nishina, A., Moteki, Q., Zhao, N., Iizuka, S.
• Format: Journal Article
• Language: English
• Published: Hoboken John Wiley & Sons, Inc 01.07.2024; American Geophysical Union (AGU)
• Subjects: Altitude; Boundary layers; convection; entrainment; Environmental conditions; Flash floods; heavy rain; moisture transport; Precipitation; Troposphere; Wind shear; Japan
• DOI: 10.1029/2023EA003486
• ISSN: 2333-5084

Mesoscale convective systems (MCSs) that occur in the Baiu frontal zone (BFZ) can cause devastating flash floods during early summer in Japan; however, the environmental conditions necessary for their development require further investigation. High‐frequency atmospheric soundings, conducted using multiple marine vessels in the East China Sea on 19 June 2022, captured the detailed environmental conditions pertaining to the development of an MCS within the BFZ. The MCS, which developed rapidly without any remarkable preceding synoptic or mesoscale disturbance in the mid‐ or upper troposphere, caused intense precipitation exceeding 80 mm/hr. The MCS persisted for approximately 6 hr, and it intensified when the influx of nearly saturated air near the sea surface toward a weak surface front overlapped with the influx of free‐tropospheric moist air. The influx of nearly saturated air near the sea surface ensured conditional instability within the lower troposphere. The influx of moist air in the free troposphere contributed to the near‐saturation conditions above the boundary layer, a feature inherent to the BFZ, and played an important role in minimizing the reduction in the buoyancy of air parcels. The results of this study indicate that a better forecast of the horizontal distribution of free tropospheric moist air is beneficial for limiting the potential area of genesis of MCS in the BFZ, and a more comprehensive understanding of the vertical variations in moisture transport contributes to an improved forecast skill for MCS in the BFZ. Plain Language Summary A very long, stagnant atmospheric front that forms over East Asia in early summer is often associated with a mesoscale convective system (a cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area) that can cause flash floods. The recent increase in the number of heavy rainfall disasters has led to increased interest in the conditions under which such systems develop around the front. In this study, the conditions favorable for the development of such a system with no remarkable upper‐tropospheric low pressure system were investigated using an unprecedented high‐frequency and dense observation network. The system developed when the influxes of two very moist air masses, one near the sea surface and the other well above the sea surface, overlapped vertically. The moist air near the sea surface ensured that the atmosphere was unstable, and the moist air above contributed to the development of the system by suppressing convection weakening. The findings of this study indicate that a more comprehensive understanding of moist air transport may facilitate more accurate forecasting of convective systems. Key Points Unprecedented high‐frequency atmospheric soundings of a mesoscale convective system were conducted within the Baiu frontal zone The system began to rapidly develop when the influxes of nearly saturated near‐surface and free‐tropospheric air overlapped The areas where systems potentially develop are sensitive to small variations in the humidity distribution in the free troposphere