Numerical simulation of a severe squall event in Hong Kong

The possibility of forecasting severe squalls using a numerical weather prediction model is studied for a squall line case in Hong Kong on 9 May 2005. The squall line swept across the Hong Kong International Airport around noon on that day, with the wind gusting to about 21 m/s on the airfield. Gusts reaching 30 m/s also brought about damages at many other places.

The Regional Atmospheric Modelling System (RAMS) version 4.4 is used in this study. It is nested with the 4D-VAR Regional Spectral Model of the Observatory. Simulation is performed down to a horizontal resolution of 1.3 km. The method developed by Brasseur (2001) is employed to estimate the gusts associated with the squall line. The estimated gusts are found to agree better with actual observations compared to the other methods (such as GUSTEX). The study demonstrated that it was feasible to forecast the wind strength associated with squally lines using RAMS and the Brasseur's gust estimation technique.

The use of cloud-top turbulence in RAMS is found to have a great impact on several aspects of the simulation results, such as the development of convection (as shown in the updraft strength) and the speed of movement of the squall line (which is related to the strength of the cold pool associated with the squall line). These will also be discussed in the paper.

Reference: Brasseur, O., 2001: Development and application of a physical approach to estimating wind gusts, Mon. Weather Rev. 129, 5 – 25.