The characteristics of squall lines in the Southeast Asia region

The Maritime Continent experiences active convective activity all year round, with strong diurnal rainfall variation. Squall lines are frequent in this region, which is surrounded by the warm sea, and coastlines mostly run perpendicular or parallel to the monsoon winds. This study investigates the characteristics of squall lines using Integrated Multi-satellite Retrievals for the Global Precipitation Measurement (IMERG GPM) half-hourly precipitation data from 2001 to 2020 during the extended boreal summer monsoon. The squall lines are identified using the area-overlapping technique and based on the size and rainfall thresholds. A total of 173,831 squall lines are identified and classified into four regional clusters using K-means clustering. The clusters are Peninsular Malaysia (PM), northwestern Borneo (NwB), northern Borneo (NB) and South China Sea (SCS). The analysis reveals notable regional differences in propagation speed and direction, size, intensity and lifespan. Squall lines in the PM cluster are smaller and intense but have shorter lifespans, while those in the Borneo clusters are larger, of moderate intensity, and have longer lifespans. The squall-line propagation speeds are only weakly linked to the background large-scale flow, indicating that the internal storm dynamics and local circulations play an important role. Composite case studies further reveal that nighttime squall lines are driven by the convergence of offshore winds and monsoonal winds. Along coastal regions, land-breeze fronts, which act as density currents, may also interact with storm-generated outflows. Overall, these findings provide important statistical insights into the structure and evolution of squall lines. The findings can support operational forecasters in anticipating squall lines' behaviour and serve as a foundation for developing regionally tailored nowcasting tools.