## 5 Comparison with Flight Data  
Despite the approach and the passage of a squall line across Hong Kong, the Hong Kong International Airport (HKIA) continued to operate in the period. There were a number of flights passing through the squall line and the associated rain areas. The flights provide valuable first-hand information about the weather within the squall line. Data from a number of flights are obtained, and they are briefly analysed here. The data for the first flight are shown in Figure 5. On the left hand side, they are the EDR, vertical wind, horizontal wind and altitude from the flight. On the right hand side, there is the radar-based EDR map and the radar-retrieved vertical wind. At the time of this flight, the squall line had just reached HKIA. The aircraft departed to the west and flew around the western edge of Lantau Island. The maximum EDR has reached around 0.6 m\({}^{2}\)/3s\({}^{-1}\), i.e., severe turbulence [37]. The maximum vertical velocity is in the region of \(-\)10 m/s. The EDR value is consistent with the radar-derived value, and strong downdraft is indeed retrieved from the radar-based, three-dimensional winds. Figure 4: 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 08:00 UTC on 15 March 2025 (inset in (**a,b**) shows the vertical cross section of winds along the A–B line). Figure 5: Cont. The second, third and fourth flights are shown in Figures 6-8, respectively. In that period of time, the squall line was passing the central and the eastern parts of Hong Kong. All these three flights had to conduct missed approach for the first time, and successfully landed at the second time. During the two landings, all the aircrafts encountered severe convection and highly turbulent flows were measured for both times. The maximum EDR could reach 0.8 to 1.0 m\({}^{2/3}\)s\({}^{-1}\), i.e., highly turbulent flows. Both updrafts and downdrafts were encountered during the landings, generally reaching a magnitude of 10 m/s. For the third flight (Figure 8), the downdraft (positive value of vertical wind) could reach around 14 m/s during the first approach, with the corresponding EDR reaching the exceptionally high value of around 1.0 m\({}^{2/3}\)s\({}^{-1}\). Interesting instances of the radar observations are shown in the bottom two panels of the figures (refer to Figure 5b,c for the case of Figure 6). The radar-derived EDRs are generally consistent with the actual observations, particularly the maximum value recorded onboard the aircraft. The vertical velocity, especially the maximum value, appears to be weaker in the radar-retrieved wind particularly for 1 km height or below. Since radar data are only available every 6 min and are pulse volume averages, they may not capture the maximum vertical winds from the point (spatially) and instantaneous (temporally) measurements of the aircraft. Another reason may be that the radar-based wind retrieval has not handled well the vertical motion in the lower atmosphere as a result of complex topographic effects [19; 27]. There is also the possibility of artifacts involved in deriving the vertical velocity in the interpolation and smoothing of the polar radar data to a Cartesian coordinate system [19; 38]. The last two flights are shown in Figures 9 and 10. In that period, the squall line had reached the eastern coast of Hong Kong. The aircraft just flew through the squall line once, and successfully landed at HKIA. Even though in the previous discussion, the squall line may have appeared weaker with the loss of vertical circulation and strong subsiding flow, the maximum EDR and the maximum vertical velocity associated with the squall line are not particularly low.