Investigating the Relationships Between Rotation and Heavy Rainfall Along the Mei-yu Front During PRECIP 2022

Extreme rainfall is an important weather phenomenon due to the high potential for loss of life and property; however, our forecast skill remains limited, especially on smaller spatial scales. Recent research has shown a positive relationship between the strength of low-level mesoscale rotation and rainfall rates through nonlinear dynamic accelerations driven by negative pressure perturbations. This relationship has been explored in continental convective storms and a landfalling tropical storm, but has not yet been explored in the context of mei-yu frontal precipitation. Recent results from a numerical simulation show a positive relationship between potential vorticity and rainfall rates in association with meridional moisture flux and isentropic ascent along the mei-yu front. Other nonlinear interactions between vorticity and the boundary layer can also produce localized ascent, including boundary layer convergence underneath rotating flow and isentropic ascent ahead of PV anomalies in sheared flow. The relationships between rotation and rainfall, and which of these mechanisms plays a role in the context of mei-yu frontal precipitation, remain to be explored.

This study analyzes Doppler radar data from operational and research radars collected during the NSF-funded Prediction of Rainfall Extremes Campaign in the Pacific (PRECIP) experiment held in Taiwan and southern Japan in 2022. Multi-Doppler analyses generated by the SAMURAI software are used to compare the kinematic and precipitation structures during the 2022 mei-yu period. Over 680 analyses across 8 days were generated from 4 radars, including NCAR’s S-Pol radar. The three-dimensional analyses provide best estimates of vertical motion, vorticity, and precipitation characteristics to evaluate the relationship between rotation, ascent, and rainfall intensity.

This study examines mei-yu frontal convection over two intensive operating periods during the 2022 mei-yu season to examine the statistical relationships between these quantities on different spatial scales. A positive relationship between rotation and rainfall is identified, as the vorticity distributions shift to higher values with increasing rain rates. The strongest relationship occurs at 5-km altitude, though a weaker signal is present at 1.5- and 8-km altitudes. This study will also examine the relative character of the vorticity (i.e., curvature versus shear vorticity) and evaluate the role of the aforementioned mechanisms linking rotation with rain rate intensity.