2025 年 21C 巻 Special_Edition 号 p. 21-29
The shear line is a narrow zone of maximum horizontal wind shear, typically identified as a confluence zone of low-level wind streams. Previous studies showed that this system can trigger heavy to extreme rainfall events in the Philippines during the northeast monsoon season. However, a research gap remains in objectively identifying and locating this system. Thus, this study develops a detection method that may be used for monitoring and forecasting the location of shear lines. Results show that the meridional gradient of the 925 hPa meridional winds (∂V925hPa/∂y), the boundary layer moisture flux convergence, and the 925 hPa relative vorticity may be used to estimate the location of the shear line. These parameters are then utilized in the detection algorithm. Applying the detection algorithm to reanalysis data from 1979 to 2022 for the northeast monsoon season revealed that shear lines contribute up to 20% of the total extreme rainfall days, with the greatest influence observed in November and December. This system is also best depicted in the 925 hPa and 850 hPa levels. The detection method developed in this study may be useful for improving the forecast of heavy to extreme rainfall events in the Philippines, especially during this season.
