Fig. 5: Evaluation of squall-line dynamics based on environmental factors and rainband propagation characteristics.
a Scatterplot of the low-level band-normal vertical shear vs. the band-relative band-normal wind component for all outer tropical cyclone rainband (TCR) cases identified in this study. The low-level vertical shear and band-relative flow are calculated and averaged below 1.5–2 km MSL (see details in the text) from ERA5 reanalysis data nearest the formative location of each outer TCR case. The number of outer TCR cases and the corresponding percentage observed within each quadrant are also indicated. b The box and whisker plot of the observed cross-band propagation velocities (m s−1) relative to the tropical cyclone (TC) motion for the squall-line and non-squall-line TCR groups. Boxes denote 25th–75th percentiles (the interquartile range), with a horizontal line at the mean value. Vertical thick lines (whiskers) extend from the 5th to 95th percentile values. c Observed cross-band propagation velocities relative to the TC motion (small blue circles) and theoretically predicted velocity ranges of cold pools (red shading) for each of the cases of the squall-line TCR group. Positive propagation velocities represent motions away from the TC center, and vice versa. d Same as in (c) but showing the results for the non-squall-line TCR group. Source data are provided as a Source Data file.
