## 4 Evolution of the Squall Line from 3D Retrieved Wind  
The 1 km Constant Altitude Plan Position Indicator (CAPPI) radar reflectivity and the radar-retrieved horizontal wind at a height of 3 km above sea level are shown in Figure 2a. The corresponding vertical wind at 3 km is shown in Figure 2b. A vertical cross section, with location given in Figure 2a, is shown in the inset of that figure, with the wind arrows  
Figure 1: Wind and gust observations at 07:00 UTC ((**a**) and (**b**), respectively), 07:30 UTC ((**c**) and (**d**), respectively) and 08:00 UTC ((**e**) and (**f**), respectively) on 15 March 2025. “M” in (**b**,**d**,**f**) indicates gust observations not available from weather buoys or automatic weather stations under maintenance. “V” in (**a**,**c**,**e**) indicates the wind direction was variable. showing the wind component projected on the cross-sectional plane with the colour shaded as the speed of that wind component. It is shown that, at the time of Figure 2, the squall line was severe and mature. Significant wind convergence occurred at the fore-front of the squall line with strong radar echo as well as high reflectivity gradient. Strong upward vertical motion with vertical velocity exceeding 10 m/s could be found in that region. At the northern part of the squall line, there was clear signature of a cyclonic vortex which can be perceived as a bookend vortex and typically appears at midlevels (3-7 km) behind the leading edge of active convection [2; 29; 30]. A bookend vortex is possibly generated through upward tilting of vortex lines that are originated from low-level baroclinically forced or ambient shear-forced horizontal vorticity [2]. Radar observations showed that the squall line exhibited features of an elongated bow echo evolving to become a comma echo as described by [PERSON] [31] with apparent curvature stretching a distance of over 80 km from north to south as it moved across Hong Kong from west to east. In the vertical cross section, complete vertical circulation could be found in the fore-front of the squall line. Behind it was subsiding flow and strong horizontal winds as high as 50 knots or around 25.7 m/s at a height of 3 km, which is consistent with radar wind profiler observations. This seemed to resemble the feature of Rear-Inflow Jet (RJI) which suggested strong acceleration of midlevel flow, causing the amplification of concavity and giving rise to the bow-shaped echo [32]. The presence of rear inflow was signified by an area of weak radar echo in the stratiform region (the black ellipse in Figure 2a) which was likely caused by evaporation and melting [33; 34; 35]. This weak-echo channel is termed as \"rear-inflow notch\" (RIN) by [PERSON] [36]. About half an hour later, while the radar echo remained strong as in Figure 3a, the upward vertical motion area became less extensive as in Figure 3b. The complete circulation at the fore-front of the squall line was not so apparent in the inset of Figure 3a. This trend continued as in Figure 4a,b. In particular, there was just an extensive area of upward motion at the front of the squall line, without signature of vertical circulation (inset of Figure 4a). The subsiding motion was also not apparent in Figure 4b, consistent with the weaker surface gusts in Figure 1f. The whole radar picture sequence shows that, in the progress of time following the movement of the squall line across Hong Kong, the squall line shows signs of weakening with less pronounced vertical structure (vertical circulation), subsiding motion behind the fore-front part and the weakening of the surface gusts. Figure 2: _Cont._  
Figure 3: Horizontal winds at 3 km above sea level overlaying on 1 km CAPPI radar reflectivity (**a**) and vertical wind at 3 km height (**b**) at 07:06 UTC on 15 March 2025 (inset in **(a,b)**) shows the vertical cross section of winds along the A–B line).