Lau Fau Shan becomes part of the reference anemometer network
As the typhoon season is round the corner, let me introduce the adjustment made by the Observatory this year (2013) to the network of reference anemometers in the tropical cyclone warning signal system.
When the Observatory considers issuing a tropical cyclone warning signal, it makes reference to the winds recorded or expected to be recorded from a network of reference anemometers covering the whole of Hong Kong. Located at the northwestern part of the New Territories, the Wetland Park station was originally one of the reference anemometers. The station is fairly open with natural wetlands to its east and some high-rise buildings are located to its far west. As compared with the Lau Fau Shan station, which is also located at the northwestern part of the New Territories, the Wetland Park station is closer to residential areas and the sea level. Therefore, the fairly well exposed Wetland Park station was chosen as a reference anemometer.
In recent years, we are seeing increasing density of buildings near the Wetland Park station. Statistics over the years indicated that the monthly mean wind speed of the station was on a gradual decreasing trend, at a rate of about 0.15 kilometres per hour every year. In addition, there are planned construction projects about 300 metres to the west of the station. Buildings of 3 to 11 storeys, with the tallest one of about 50 metres high, will be constructed. This is expected to further degrade the exposure to the west of the station.
Figure 1 The Lau Fau Shan station replaces the Wetland Park Station and becomes the
reference anemometer in the tropical cyclone warning signal system in 2013.
To ensure that the reference anemometer network can continue to reflect the general wind condition over Hong Kong, the Observatory has made an adjustment of the network layout by replacing the Wetland Park station with the Lau Fau Shan station this year. Reference anemometers are selected on account of their geographical distribution and how well they represent the prevailing wind condition of their respective regions. The Lau Fau Shan station, located next to the Wetland Park station, is a well exposed station and there has been no significant change in the mean wind speed over the past years. Therefore, it is a suitable replacement of the Wetland Park station.
The Observatory has also analysed the trend of mean winds of the other reference anemometers. Statistics over the years indicated that there were no significant changes. Hence, there is no need to make other replacement at present. The Observatory will closely monitor the long-term trend of wind speed of all reference anemometers and changes, if any, at their surroundings, and review the layout of the anemometer network regularly.
The aim of changing the reference anemometer is to better reflect the general wind condition over Hong Kong. The current change is only a technical adjustment of the reference anemometer network and does not involve any change to the tropical cyclone warning signal system. During the approach of tropical cyclone, a number of meteorological factors have to be considered in the issuance of tropical cyclone warning signal. These include, inter alia, the size and structure of the tropical cyclone, the forecast track and intensity change, as well as the variations of local wind condition. Thus the replacement of one reference anemometer would have minimal impact on whether the tropical cyclone warning signal needs to be issued or not.