Dust and sand brought by distant sandstorms from the north affected Hong Kong starting from late Sunday (21 March). With its passage and as 'the dust settled', it is time to answer some of the questions people have on the science of sandstorms and what we at the Observatory can do.
What is a sandstorm ?
Sandstorms are characterized by a mass of dust and sand particles lifted to great heights by a strong and turbulent wind. The visibility could sometimes be reduced to below 1000 metres.
What was the weather in Hong Kong the day dust and sand arrived ?
Because of the long distance, sandstorms from the north would have lost most if not all their character by the time dust and sand arrived in Hong Kong on late Sunday, 21 March. The dust and sand took the visibility down to between 2000 and 4000 metres over most of the territory.
Figure 1. Visibility reports over Hong Kong at 5 p.m. on 21 March 2010
How does the Observatory monitor the dust and sand ?
There are several tools:-
a) Reports every 3 hours from weather centres in our neighbouring areas, including mainland China, Korea, Japan and Taiwan, China. During the dust and sand episode in late March 2010, no stations in Guangdong or mid/southern Fujian (south of 27 degrees N) reported any sandstorm, duststorm or widespread dust.
b) Satellite pictures from low-flying satellites, available once or twice a day. Such satellites fly close to the Earth (from a few hundred kilometres to thousands of kilometres above Earth) and enable dust and sand to be discerned by the experienced eye. However, any overlying clouds would obscure the dust and sand. This was what happened on 22 March (Monday).
Figure 3. (a) True colour image from Earth Observing Satellite at 1:06 p.m., 21 March.
Dust and sand had spread to central and eastern China.
Figure 3. (b) The image at 10:38 a.m., 22 March showed the dust and sand and dust further
spreading to Taiwan and its adjacent waters. South China was generally covered by
clouds, obscuring any dust or sand.
c) Backward trajectories of air mass. The service on the Observatory website is available since 2008. A backward trajectory enables one to appreciate the possible origin, 3 days ago, of the air mass arriving at Hong Kong. From these trajectories, one can relate the local visibility to the possible origin of the air mass 3 days ago. If past experience is any guide, relatively low visibility (excluding fog, mist and rain) is normally associated with air masses coming from the north and some coming from the east. Those coming from the south, being of maritime nature, normally bring good visibility and blue skies. However, there are limitations to the application of this tool to monitor dust and sand. This is because the tool offers at best a qualitative (i.e. not quantitative) assessment, for dust and sand are different from air in terms of their physical characteristics and may not necessarily follow the same trajectory as that for air.
Figure 4. Past 72-hour backward trajectory of air mass reaching Hong Kong at 8 a.m., 25 Mar (Thursday)
How can dust and sand be forecast ? Can we forecast dust and sand for Hong Kong, say, everytime dust, sand or sandstorm is reported in Taiwan, or in mainland stations close to Hong Kong ?
Prediction of dust and sand for Hong Kong is difficult at the moment because real-time (or near real-time) measurements of dust and sand concentration are needed, not only those taken in Hong Kong but also those taken in neighbouring areas including south China and Taiwan. It would not be possible to accurately predict the arrival of dust and sand without such 'ground truth'. Computer modelling of sand and dust a couple of days ahead may not be helpful because it generally suffers from the lack of ground truth. If advance alert or forewarning is to be given, real-time dust concentration data from neighbouring areas (in addition to local data) are needed. Without this, people have to accept quite a bit of false alarms.
What the Observatory can/will do when another dust and sand incident occurs ?
The Observatory was able to forecast low visibilities for 21 and 22 March, the days worst affected by dust and sand from distant sandstorms in the north. But the public has difficulties relating that forecast to the dust and sand. If there is a repeat of the 21 March episode, the Observatory will enhance its information service for the public. Upon confirmation of dust and sand either by the Observatory observer or through real-time measurements of dust and sand available to us, we can bring people's attention to it by mentioning it in the weather bulletin. In other words, the public will be informed 'after the fact'. If real-time dust concentration data from neighbouring areas are available, some forewarning may be possible through coordination with the Environmental Protection Department (EPD).
What is the way forward ?
Hitherto, the Observatory has been providing EPD with weather data from local surface and upper-air stations, and twice-daily numerical weather prediction (NWP) products obtained with the Observatory's regional weather model. We also make meteorological assessment and engage in consultation with EPD during low-visibility or high-pollution days. The Observatory stands ready to assist EPD with more weather information and NWP products as and when required.