In-Situ Observations of Contrasting Cloud Microphysics and Governing Mechanisms within the Meiyu Front: Frontal Zone vs. Warm Sector

The Meiyu front is the primary summer precipitation system over the Jianghuai region, China, yet in situ measurements of its cloud microphysical properties remain limited. Using observations from the Guangmingding Meteorological Station on Mount Huangshan (1840 m a.s.l.) during the 2008 Meiyu period, this study categorizes heavy precipitation processes into frontal zone and warm sector types based on their locations relative to the Meiyu front to investigate the differences in cloud microphysical properties and dominant mechanisms. Results indicate that, compared to the warm sector case, the frontal zone case shows higher droplet number concentrations, smaller droplet sizes, and narrower droplet spectra. As rainfall intensity increases, the droplet number concentration decreases in both the frontal zone and warm sector cases; conversely, the volume mean diameter decreases in the frontal zone case and increases in the warm sector case. These differences can be attributed to contrasting dominant microphysical and dynamical processes. In the frontal zone case, the strong dynamical lifting and abundant moisture promote the sustained activation and condensation of numerous small droplets. As a result, the droplet spectrum becomes narrower with decreasing volume mean diameter. In the warm sector case, strong collision-coalescence between cloud droplets accelerates the depletion of small droplets and promotes the formation of larger droplets, broadening the droplet spectrum. These observations reflect differences in cloud microphysical properties and underlying mechanisms between the Meiyu frontal zone and warm sector, enhancing the understanding of the cloud microphysical processes in the Meiyu frontal precipitating cloud system.