Hkmets Bulletin, Volume 9, Numbers 1/2, 1999
Total Page：16
File Type：pdf, Size：1020Kb
o G ME EO OLOG CAL E • VOLUME 9, NUMBER 1/2, 1999 ISSN: 1024-4468 ISSN 1024·4468 The Hong Kong Meteorological SocietY Bulletin is the official organ of the SocietY, devoted to articles, editorials, news and views, activities and announcements of the SocietY. Members are encouraged to send any articles, media items or information for publication in the Bulletin. For guidance see the "information for contributors" in the inside back cover. EDITOR-in-CHIEF EDITORIAL BOARD Advertisements for products and/or Bill Kyle Johnny CL. Chan services of interest to members of Y.K. Chan the SocietY are accepted for W:L. Chang publication in the BULLETIN. Edwin S. T. Lal For information on formats and rates please contact the SocietY secretary at the address opposite. SUBSCRIPTION RATES (Two issues per volume) The BULLETIN is copyright material. Institutional rate: HK$ 300 per volume Views and opinions expressed in the articles or any correspondence are Individual rate: HK$ 1SO per volume those of the author(s) alone and do not necessarily represent the views Published by and opinions of the SocietY. Permission to use figures, tables, and brief extracts from this publication in any scientific or educational work is hereby granted provided that the source is properly acknowledged. Any other use of the material requires the prior written permission of the Hong Kong Meteorological Society. The mention of specific products and/or companies does not imply The Hong Kong Meteorological Society there is any endorsement by the SocietY or Its office bearers in c/o Hong Kong Observatory preference to others which are not 134A Nathan Road so mentioned. Kowloon, Hong Kong ------ HONG KONG METEOROLOGICAL SOCIETY etln• Volume 9, Numbers 1/2, 1999 CONTENTS Editorial 2 East and West The Weather Version 3 Edwin S.T. Lai Hong Kong's Active Typhoon Season 16 Norman K.W. Cheung and W.). Kyle 1998 Tropical Cyclone Summary for the Western 46 North Pacific Ocean (west of 180 degrees) W.]. Kyle Hong Kong Weather Reviews 67 Editorial This issue of the Bulletin combines two issues in one volume and contains three papers. The first paper by Edwin S.T. tai of Hong Kong Observatory provides yet another view concerning the heavy rainstorms in early July 1997. It examines the atmospheric instability which appears to have been maintained and enhanced by the strong confluence of competing weather systems from east and west and which provided the principal mechanisms at play in the rainstorm episodes. In the second paper, Norman K.W. Cheung of the School of Geography, University of Oxford, UK, and Bill Kyle of the Department of Geography, The University of Hong Kong, use tropical cyclone data for the western North Pacific Ocean over the past 30 years along with the associated synoptic patterns to provide a reliable working definition of the time periods of the Typhoon season and Active Typhoon Season in Hong Kong. The third paper continues the annual review of tropical cyclone activity in the western North Pacific Ocean by Bill Kyle of the Department of Geography, The University of Hong Kong, which documents all storms occurring in the region and provides information about tracks, weather conditions, and the resulting impacts on areas affected during the 1998 season. The remainder of the issue contains the regular feature, Hong Kong Weather Reviews. The Editorial Board, as always, look forward to receiving any opinions, suggestions or contributions sent in by readers. Bill Kyle, Editor-in-Chief 2 HKMetS Bulletin Vol 9 Nos. //2, /999 - Edwin S. T. Lai Hong Kong ObselVatory Nathan Road, Kowloon, Hong Kong Email: [email protected] East and West - The Weather Version Abstra~t The historic handover of sovereignty of Hong Kong on 1 July 1997 will be indelibly linked with one of the worst rainstorm episodes to hit the territory in recent years. The outbreak of heavy rain was unseasonal and yet tenaciously persistent. Excessive rainfall led to widespread flooding and landslips, creating havoc for the week-long festive celebrations that followed the handover ceremonies. Atmospheric instability appeared to be maintained and enhanced by strong confluence generated by competing weather systems from east and west. Through analysis of synoptic data, satellite and radar images, as well as observations from local monitoring networks, this paper highlights the various mechanisms at play on different spatial scales that combined to produce the infamous "Handover Rain". Introduction Persistent heavy rain affected Hong Kong from 30 June to 4 July in 1997. Rainfall at the Hong Kong Observatory during the period amounted to 372.4 mm. Elsewhere, Shatin had more than 700 mm with a day on 2 July (Figure 1). Black rainstorm warning was issued on the morning of t July, with two red warnings following on successive days. Landslip and flood warnings were also in effect at the time. A summary of the main meteorological elements during the period, as well as conditions before and after for comparison purposes, is given in Table t. Climatologically, the wet spell was unusual in the sense that it occurred outside the perennial June flood period in southern China. The monsoon trough should have migrated northwards to central China by then, leaving the southern parts under the dominance of the sub-tropical high. Such unseasonal happenings were set against the HKMetS Bulletin Vol. 9 Nos. //2, /999 3 background of a momentous occasion in the history of Hong Kong - the handover of sovereignty from the British administration to the Chinese government. Numerical simulation of the rain episode was discussed in Li and Ma (1998). Presenting the observational findings from climatic and synoptic analyses, satellite and radar images, as well as local wind and rainfall data, this paper highlights the events on different atmospheric scales that combine to trigger and sustain the now notorious "Handover Rain". Figure 1. Daily Rainfall Distribution Maps over Hong Kong (rainfall unit mm) r (0) 2 July, 1997 ~ I \ ~;? General Circulation and Synoptic Anomalies Data from the "Annual Report on Climate System" for 1.997 compiled by Japan Meteorological Agency (]MA, 1998) were extracted to depict the significant synoptic evolution affecting the general circulation and weather patterns in Asia and the western Pacific. Mean fields and anomalies at 850-hPa level in Figures 2 and 3 respectively showed an unusually prominent ridge extending towards southern India during the Handover pentad (HOP) of 30 June - 4 July in 1997. Cross equatorial flow was very active over the Indian Ocean. But instead of spreading out towards the Indian sub continent, the airstream swung around the southern India anticyclone and formed a southwesterly jet along the north flank of the extended ridge, bringing a continuous supply of moisture influx' towards southern China and sustaining convective development downstream. 4 HKMetS Bulletin Vol. 9 Nos. //2, /999 - Table 1. Extract of Meteorological Observations (at Hong Kong ObselVatory headquarters except for winds which were recorded at Waglan Island) Mean Pressure Total Rainfall Wind Direction Mean Wind Date (hPa) (mm) (deg/bearing) Speed (km/h) June 25 1002.1 14.9 160/SSE 7.4 June 26 1002.6 57.5 070/ENE 6.3 June 27 1001.9 - 230/SW 8.5 June 28 1001.5 1.1 080/NofE 8.5 June 29 1001.9 4.8 ISO/SSE 9.5 June 30 1002.7 27.6 190/WofS 16.9 July 1 1004.7 122.6 140/SE 19.0 July 2 1005.4 115.5 240/WSW 28.5 July 3 1006.5 83.3 909/E 15.6 1006.4 23.4 180/S 29.7 - July 4 July 5 1006.6 Trace 200/SSW 27.3 July 6 1005.3 15.1 190/WofS 21.1 July 7 1004.6 Trace 200/SSW 25.0 July 8 1004.3 13.3 210/SSW 33.8 July 9 1003.9 3.4 200/SSW 30.8 ~- -~~ Figure 2. 850 hPa mean winds and Figure 3. 850 hPa streamfunction and height fields during HOP wind anomalies during HOP HKMetS Bulletin Vol. 9 Nos. //2/ /999 5 ---- -------- Defining "pre-HOP" to be the pentad of 25 - 29 June and "post-HOP" for 5 - 9 July, mean height differences at 850-hPa level before HOP (i.e. HOP - pre-HOP) and after HOP (i.e. post-HOP - HOP) are shown in Figure 4 and 5 respectively. The main feature was the re-intensification of the Pacific anticyclone following the passage and recurvature of Severe Tropical Storm Peter over the Ryukyus and Japan in late June. The height gradient between 1200 E and 125°E suggested that the intensifying Pacific ridge was on the verge of the South China Sea but not quite able to establish a foothold inside. As a result, cyclonic curvature increased within a broad band of southwesterlies that were being swept along towards the south China coast. In streamline analyses, a region of strong confluence between the southeasterlies and the southwesterlies persisted over the coastal waters of Guangdong. Figure 4. 850 hPa geopotential Figure 5. 850 hPa geopotential height differences(in m): height differences(in m): HOP - pre-HOP HOP post-HOP On the daily rainfall distribution maps in Figure 1, if the major axis of isohyet maxima (heavy dashed line) could be interpreted as an area of enhanced convection generated by strong confluence effect, then this line of confluence was found to move ever so slowly across Hong Kong. It covered a mere distance of about 25 kilometres in 4 days (a speed of only about 0.2 - 0.3 km per hour) and was effectively almost stationary along the longitude of 114°E. Despite the lack of speed, a consistent movement towards the northwest in response to the encroaching Pacific ridge from the east was quite unmistakable on a day-to-day basis.