A comparison of two Mei-Yu front cases during TAHOPE: exploring the physical mechanisms for the differences in precipitation features

Abstract During the Taiwan-Area Heavy rain Observation and Prediction Experiment (TAHOPE) in 2022, two Mei-Yu front cases were selected for investigation, corresponding to the Special Observation Period 1 (SOP1) and Intensive Observation Period 3 (IOP3), respectively. Although the two Mei-Yu fronts at 975 hPa were similarly positioned over the central to northern Taiwan Strait between 0700 and 1900 UTC on 25 May (SOP1 case) and 6 June (IOP3 case), the rainfall totals observed over northern Taiwan in the SOP1 case (area-averaged 12-h rainfall of 188 mm) were nearly three times as much as those in the IOP3 case (62 mm). This study aimed to uncover the physical mechanisms responsible for this contrast. Our analysis revealed that although the IOP3 case exhibited a steeper frontal slope, greater frontal intensity, and stronger synoptic-scale kinematic features, the SOP1 case possessed more favorable thermodynamic conditions and mesoscale-to-convective-scale kinematic structures (e.g., persistent low-level convergence zones repeatedly initiating new convection), along with higher low-level liquid water content and greater cloud-microphysics precipitation efficiency. These factors collectively led to higher accumulated rainfall over northern Taiwan in the SOP1 case than in the IOP3 case, suggesting that the rainfall amount during the frontal passages may not be solely determined by the structure and intensity of the front, but rather by the combined effects of environmental moisture and both mesoscale-to-convective-scale and microphysical processes.