## 1 Introduction  
Squall lines are common in southern China in spring and summer time. They could be associated with cold fronts which mark the interface between the cooler continental air from the north and the warmer maritime airstream to the south over the south China coast. Squall lines are known to produce high surface winds and small tornadoes [1] and they can be damaging because of the occurrence of high surface gusts, sometimes in excess of 100 km/h. Timely alerting of the high winds is an essential service of the weather authorities in the region in these seasons.  
A severe squall line moved across Hong Kong, China, from west to east in the early afternoon of 15 March 2025. Early alerting of the high gusts associated with the squall line had been provided by the Hong Kong Observatory (HKO) to the public a couple of hours before the arrival of the squall line. However, there was still an event of workers trapped in a swinging gondola in strong wind at a place at the northwestern part of Hong Kong (a news report can be found in [[https://www.scmp.com/news/hong-kong/health-env](https://www.scmp.com/news/hong-kong/health-env)]([https://www.scmp.com/news/hong-kong/health-env](https://www.scmp.com/news/hong-kong/health-env))  
ironment/article/3302509/hong-kong-braces-squally-thunderstorms-violent-gusts-sa  
turday-afternoon (accessed on 30 July 2025)). Fortunately, the workers managed to find a way to stabilize the gondola and there was no injury or death during the event, but the incident did raise general concern in Hong Kong about the safety of workers in high-wind situations during thunderstorms. It also shows the possible damaging powers of strong winds associated with squall lines. In fact, many parts of Hong Kong had registered wind gusts in excess of 25 m/s, with some places reaching 28 m/s. The paper serves to document this case of severe squall line for reference of the weather forecasters.  
There have been quite a number of studies about the physical mechanisms of squall lines over southern China [2,3]. These studies are mostly based on radar observations and simulation of the events using numerical models. However, there is a lack of in-depth discussion about the change in vertical velocity and turbulence severity associated with the evolution of squall lines, not to mention the availability of valuable aircraft data to verify radar-based observations. In the present study, the squall line structure is studied using three-dimensional (3D) winds retrieved from multiple weather radars and the HKO has the unique advantage of having a dense network of weather radars in the vicinity of the Pearl River Estuary to enable derivation of 3D wind fields for evaluating the dynamic change of severe weather systems. The study is thus novel with the inclusion of aircraft data in the observational study, such as vertical wind velocity and eddy dissipation rate (EDR). The vertical winds and EDR are also compared with the values retrieved from the weather radars. The flight data provide first-hand information to verify the radar-based retrieved winds and the EDRs. The results of the study would be useful for the study of the quality of the latter data and would have useful applications such as alerting of turbulence to be encountered by aircraft in squall lines. A previous study with similar nature can be found in [PERSON] et al. [4] based on one single flight only, but the present study involves multiple flights covering different locations and heights with respect to the main convective areas of the squall line. The vertical wind and EDR are derived from the quick access recorder (QAR) data of commercial aircraft using the algorithms of [PERSON] and [PERSON] [5] (\"NLR\") and EDR2W of [PERSON] et al. [6] (\"Spectrum\").