UNDERSTANDING THE PROCESSES RESPONSIBLE FORHEAVY RAINFALL IN TWO MEI-YU FRONT CASESJennifer DeHart1, Alexandra Cole1, Michael Bell1, Yunji Zhang2, Yi-Leng Chen3
1
Colorado State University, Fort Collins, CO, 2The Pennsylvania State University, University
Park, PA, 3University of Hawai’i at Mānoa, Honolulu, Hawaii
ABSTRACTThis presentation analyzes numerical simulations to examine the mesoscale conditions associatedwith heavy rainfall in two mei-yu frontal events near Taiwan. For the first case, from early June
2017, the simulation reasonably reproduces the frontal progression and heavy rainfall
surrounding northern Taiwan from the event. The front is identified through time using aclustering algorithm, which enables analysis of the characteristics of the rainfall, surrounding
environment, and underlying mechanisms through space and time. Heavy rain is produced by aseries of eastward propagating MCSs and maximizes on the eastern side of the Taiwan Strait.
Convective available potential energy (CAPE), counter-rotating vortices spanning the front, andtwo-dimensional frontogenesis are not well correlated with the rainfall intensity. Meridional
moisture flux, influenced by the low-level barrier jet, and potential vorticity (PV) are better
correlated with the rainfall intensity. The results suggest that low-level moist air ascending
adiabatically along isentropes towards the front provides an important contribution to the rainfallintensity.
In the second case, a peak measurement of 600 mm of rainfall and widespread accumulation over
100 mm were observed near Yonaguni, Japan on June 09, 2020. Global model forecast skill waspoor in both the rainfall location and intensity. A 40-member ensemble of numerical simulations
showed liminal conditions between heavy rainfall or little to no rainfall. The two most accurate
and two least accurate ensemble members are selected for analysis via validation against
estimated rainfall totals from the Quantitative Precipitation Estimation and Segregation Using
Multiple Sensors (QPESUMS) system. Synoptic conditions of both the most and least accurate
ensemble members are quite similar, featuring moisture-rich environments, moist neutral
soundings with low levels of free convection (LFC), and sufficient instability for deepconvection. Our analysis reveals that the location and intensity of the shallow near-surface
frontal boundary are key to ensemble member accuracy. In the two best members, southsouthwesterly flow ascends over the sloped isentropes, rises above the LFC, and produces backbuilding deep convection. In the two worst members, the near-surface gradients are weaker andmore confined along Taiwan’s coast, which leads to less rainfall located west of the observed
rainfall. In addition, as the least accurate simulations progress, stronger southerly winds advectdrier mid-level air into the region of interest and shift the near-surface boundary further northand west. Winds in the best performing members are weaker and more southwesterly with lessdry air advection and frontal displacement. The analyses suggest that subtle details in thesimulation of frontal boundaries and meso-scale flow structures can lead to bifurcations in
producing extreme or almost no rainfall.
Keywords: mei-yu front; heavy rainfall; modeling