TROPICAL CYCLONES IN 2009
1.1 Evolution of tropical cyclone publications
Apart from a short break during 1940-1946, surface observations of meteorological elements since 1884 have been summarized and published in the Observatory's annual publication "Meteorological Results". Upper-air observations began in 1947 and from then onwards the annual publication was divided into two parts, namely "Meteorological Results Part I - Surface Observations" and "Meteorological Results Part II - Upper-air Observations". These two publications were re-titled "Summary of Radiosonde-Radiowind Ascents" and "Surface Observations in Hong Kong" in 1981 and 1987 respectively. In 1993, both of these publications were made obsolete, and since then surface and upper-air data have been included in one revised publication entitled "Summary of Meteorological Observations in Hong Kong".
During the period 1884-1939, reports on some destructive typhoons were printed as Appendices to the "Meteorological Results". This practice was extended and accounts of all tropical cyclones which caused gales in Hong Kong were included in the publication "Director's Annual Departmental Reports" from 1947 to 1967 inclusive. The series "Meteorological Results Part III - Tropical Cyclone Summaries" was subsequently introduced to provide information on tropical cyclones over the western North Pacific and the South China Sea. The first issue, published in 1971, contained reports on tropical cyclones occurring in 1968 within the area bounded by the Equator, 45°N, 100°E and 160°E. With reconnaissance aircraft reports (terminated from August 1987 onwards) and satellite pictures facilitating the tracking of tropical cyclones over the otherwise data-sparse ocean, the eastern boundary of the area of coverage was extended from 160°E to 180° from 1985 onwards. In 1987, the series was re-titled as "Tropical Cyclones in 19YY" but its contents remained largely the same. Starting from 1997, the series was published in both Chinese and English. The CD-ROM version of the publication first appeared in 1998 and the printed version was replaced by the Internet version in 2000.
Tracks of tropical cyclones in the western North Pacific and the South China Sea were published in "Meteorological Results" up to 1939 and in "Meteorological Results Part I" from 1947 to 1967. Before 1961, only daily positions were plotted on the tracks. The time of the daily positions varied to some extent in the older publications but remained fixed at 0000 UTC after 1944. Details of the variation are given in the Observatory's publication "Technical Memoir No. 11, Volume 1". From 1961 onwards, six-hourly positions are shown on the tracks of all tropical cyclones.
Provisional reports on individual tropical cyclones affecting Hong Kong have been prepared since 1960 to meet the immediate needs of the press, shipping companies and others. These reports are printed and supplied on request. Initially, provisional reports were only written on those tropical cyclones for which gale or storm signals had been issued in Hong Kong. From 1968 onwards, provisional reports were prepared for all tropical cyclones that necessitated the issuing of tropical cyclone warning signals.
1.2 Classification of tropical cyclones
To heighten people's alertness of stronger typhoons, the Observatory further categorised 'Typhoon' into 'Typhoon', 'Severe Typhoon' and 'Super Typhoon' starting from the 2009 typhoon season. In this publication, tropical cyclones are classified into the following six categories according to the maximum sustained surface winds near their centres :
| (i) | A TROPICAL DEPRESSION (T.D.) has maximum sustained winds of less than 63 km/h. | |
| (ii) | A TROPICAL STORM (T.S.) has maximum sustained winds in the range 63-87 km/h. | |
| (iii) | A SEVERE TROPICAL STORM (S.T.S.) has maximum sustained winds in the range 88-117 km/h. | |
| (iv) | A TYPHOON (T.) has maximum sustained winds of 118-149 km/h. | |
| (v) | A SEVERE TYPHOON* (S.T.) has maximum sustained winds of 150-184 km/h. | |
| (vi) | A SUPER TYPHOON* (SuperT.) has maximum sustained winds of 185 km/h or more. |
*New categories starting 2009
Throughout this publication, maximum sustained surface winds when used without qualification refer to wind speeds averaged over a period of 10 minutes. Mean hourly winds are winds averaged over a 60-minute interval ending on the hour. Daily rainfall amounts are computed over a 24-hour period ending at midnight Hong Kong Time.
1.3 Naming of tropical cyclones
Over the western North Pacific and the South China Sea between 1947 and 1999, tropical cyclone names were assigned by the U.S. Armed Forces' Joint Typhoon Warning Center according to a pre-determined but unofficial list. However, with effect from 2000, the Japan Meteorological Agency assigns names from a new list to tropical cyclones attaining tropical storm strength. Table 1.1 shows the name list effective from 1 January 2009. The name list was adopted by the Typhoon Committee. It consists of a total of 140 names contributed by 14 countries and territories. Apart from being used in forecasts and warnings issued to the international aviation and shipping communities, the names will also be used officially in information on tropical cyclones issued to the international press. Besides, Japan Meteorological Agency has been delegated since 1981 with the responsibility of assigning to each tropical cyclone in the western North Pacific and the South China Sea of tropical storm strength a numerical code of four digits. For example, the first tropical cyclone of tropical storm strength or above as classified by Japan Meteorological Agency which occurred within the region in 2009 was assigned the code "0901". In this publication, the appropriate code immediately follows the name of the tropical cyclone in bracket, e.g. Typhoon Kujira (0901).
Mean sea level pressure and surface wind data presented in this report were obtained from a network of meteorological stations and anemometers operated by the Hong Kong Observatory. Details of such stations are listed in Tables 1.2 and 1.3.
Maximum storm surges caused by tropical cyclones were measured by tide gauges installed at several locations around Hong Kong. The locations of anemometers and tide gauges mentioned in this report are shown in Figure 1.1.
Rainfall data presented in this report were obtained from a network of meteorological and rainfall stations operated by the Hong Kong Observatory and raingauges operated by the Geotechnical Engineering Office (GEO).
In Section 2, an overview of all the tropical cyclones over the western North Pacific and the South China Sea in 2009 is presented.
The reports in Section 3 are individual accounts of the life history of tropical cyclones affecting Hong Kong in 2009. They include the following information :-
| (a) | the effects of the tropical cyclone on Hong Kong; | |
| (b) | the sequence of display of tropical cyclone warning signals; | |
| (c) | the maximum gust peak speeds and maximum hourly mean winds recorded in Hong Kong; | |
| (d) | the lowest mean sea level pressure recorded at the Hong Kong Observatory; | |
| (e) | the daily amounts of rainfall recorded at the Hong Kong Observatory and selected locations; | |
| (f) | the times and heights of the maximum sea level and maximum storm surge recorded at various tide stations in Hong Kong; | |
| (g) | satellite and radar imageries. |
Statistics and information relating to tropical cyclones are presented in various tables in Section 4.
Six-hourly positions together with the corresponding estimated minimum central pressures and maximum sustained surface winds for individual tropical cyclones are tabulated in Section 5.
In this publication, different times are used in different contexts. The official reference times are given in Co-ordinated Universal Time and labelled UTC. Times of the day expressed as "a.m.", "p.m.", "morning", "evening" etc. in the tropical cyclone narratives are in Hong Kong Time which is eight hours ahead of UTC.
1.6 Hong Kong's Tropical Cyclone Warning System
Table 1.4 shows the meaning of tropical cyclone warning signals in Hong Kong.
Starting from 2007, the reference for the issue of No.3 and No.8 signals has been expanded from the Victoria Harbour to a network of eight near-sea level reference anemometers covering the whole of Hong Kong as depicted in Figure 1.1.
The reference anemometers were selected on account of their good exposure and geographical distribution, taking into account the natural separation by Hong Kong's mountain ranges. Together, they provide a broad picture of the wind condition in Hong Kong.
The No. 3 or No. 8 signal, as the case may be, will be issued when half or more anemometers in the reference network register or are expected to register sustained strong winds or gale/storm force winds and the wind condition is expected to persist. The wind speed range of the No.3 signal is 41-62 km/h and that of the No.8 signal is 63-117 km/h.
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