Searching for Relationships Between Dynamics and Microphysics to Understand Extreme Rainfall in a Mei-Yu Front During PRECIP 2022

Extreme rainfall can quickly lead to dangerous flash flooding, however, our understanding of its formation is incomplete and forecasting skill is limited. Previous work has found that the tallest convection is not always associated with the heaviest rain rates and found mixed results on the relative importance of warm rain and ice phase microphysical processes in extreme rainfall. The Prediction of Rainfall Extremes Campaign in the Pacific (PRECIP) conducted in 2022 sought to understand how heavy rain differs from light rain by examining the kinematic, thermodynamic, and microphysical properties of precipitation near Taiwan.

This study leverages Doppler and polarimetric radar data from the PRECIP 2022 field campaign, using the National Center for Atmospheric Research’s (NCAR) S-Pol radar along with other research and operational radars to investigate heavy rainfall in a mei-yu front. We hypothesize that collision-coalescence and accretion are the most important microphysical processes in high intensity rainfall, and ice aggregation is the most important in low intensity rainfall. Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI), Radx2Grid in Lidar Radar Open Software Environment (LROSE), and Fast Reorder and CEDRIC Technique in LROSE (FRACTL) are used to produce gridded 3-dimensional kinematic and polarimetric fields. The investigation focuses on the period from 0600 to 1200 UTC 6 June 2022, during which a zonally-oriented band of heavy rainfall moved southward over the ocean west of Taiwan. Case studies of representative heavy and light rain regions are subjectively identified based on radar reflectivity intensity and available S-Pol Range Height Indicator scans (RHIs). The S-Pol RHIs and vertical cross-sections taken from the gridded kinematic fields are used to understand the horizontal and vertical structure of the regions. Grid points that meet heavy and light rain rate criteria across the 6-hour period at 1.5-km altitude are identified and the statistics of vertical profiles for polarimetric and kinematic variables are calculated.

This study finds that there is a relationship between the microphysical processes and dynamics in precipitation from this mei-yu front. The results from this 6-hour event support the hypothesis that in heavy rain regions, collision-coalescence and accretion are the most important microphysical processes due to polarimetric variable characteristics and an updraft that supports cloud and supercooled liquid water formation throughout the column. In contrast, the light rain regions show a lack of dynamical support, so less cloud water is produced, and the vertical profiles of the polarimetric variables indicate that aggregation is the primary microphysical process, occurring above the melting layer. Further research using longer duration events is required to better understand the complexity of these microphysics-dynamics relationships with rainfall intensity, and therefore create better forecasts.