Numerical simulation of a squall line over Hong Kong

Abstract

The objective of this study is to investigate the squall line that swept through Hong Kong on 9th May 2005 through simulation using the Regional Atmospheric Model System (RAMS). The investigation includes the genesis, lifetime and propagation of the squall line system. Two numerical models, the PSU/NCAR mesoscale model (MM5) and RAMS were used for trial propose and the latter was selected in this study. Three types of data are used for trial as well including MM5-exported, Operational Regional Spectral Model (ORSM)-exported and National Centers for Environmental Prediction (NCEP) data. NCEP data are introduced to the model as initial and boundary conditions. The squall line system is simulated by 3 nested grids with horizontal resolutions of 10, 4 and 1 km. Automatics Weather Station and Meteorological Information Comprehensive Analysis and Processing System (MICAPS) data are also used for data assimilation in the finest grid. The lifetime of the squall system is about 50 min from 0410UTC to 0500UTC. By visualizing the locations of the well-defined gust front during its mature stage just before the dissipating stage, it is estimated that its propagation speed is about 11.0 m s-1 from 308˚ (southeastward). The speed is similar to previous studies. However, the horizontal dimensions of the squall system in this study are much smaller than those that often exist in meso-alpha scale. The simulated kinematic and thermodynamic structures also agree well with those found in previous studies. In the kinematics aspects, a confluent line in the wind flow pattern, which corresponds to the gust front, can be seen just behind a linear-oriented convergence zone. An inflow entering the system ahead of the gust front contributes to the convergence zone while the rear outflow behind the front is associated with the divergence zone that results in the cold pool developing underneath the leading edge. A characteristic convective tower can also be visualized by plotting cloud mixing ratio through a selected vertical cross-section normal to the front. In the thermodynamic aspects, the pattern of equivalent potential temperature (θe) across the front resembles that described in previous conceptual models. The low θe region is located ahead of the gust front aloft while lying on the surface underneath the convective cumulus, coinciding with the cool pool. The low θe value of the cold pool can be explained by evaporation of precipitation at low levels, which leads to downdraft of air and brings the cold air down, spreading outward on the ground eventually. The simulated results are also validated using available observations. It found that the model is able to simulate the evolution of the squall system, with only about a 10-minute delay with respect to the actual system. However, the model tends to underestimate the wind speed in the vicinity of the gust front. The wind pattern in the pre-squall environment is similar to the observed one but the post-squall environment is not well simulated. It is believed that this disagreement leads to an angle between the deduced and the simulated gust front, which become more significant with at the later time. Overall, the evolution of the squall line system from mature to dissipating stage can be described in this simulation.