UNIVERSITY OF HONG KONGLIBRARY

Hong Kong CollectionGift from

E.K, loyal Observatory

.V \

Page

INTRODUCTION

1. METECrvGLjUGlCAL CbciEKVATION

1 • 1 Euuii^i'iteui: 3

1.1.1 Surface wind Sensor at Tsim Bei Tsui

1.1*2 Upper-air Measurement Systems- at King's Park

1.1.3 Monostatic Acoustic Radar at Junk Bay

1.2 Operation of Meteorological Instruraents 7

1.2.1 Observation Period

1.2.2 normal Operation of Instruments

1.2.3 Calibration ana Maintenance

1.3 Data processing ' 9

1.3.1 vvino Recoras

1.3*2 cloua Observations

1*3.3 Upper-air Radiosonae/Rawin Records

1.3.4 Acoustic Radar Recoras

2* METEOROLOGICAL ASSESSMENTS

2.1 General Climatology of Hong Kong 12

2-2 Meteorological Data Bases ' 14

2.2*1 Surface wind . :

2.2-2 Cloua Data

2«2» 3 Upper-air Wind, arid' Temperature profiles.

2 * 2.4 Mixing Height

2,3 Wind and Temperature Analyses 17

2.3.1 Monthly and Annual Surface Wind Roses

2» 3 « 2 Monthly and Annual Diurnal 'Variationsof Surface Wind

2*3-3 Spells of Light Wind

2.3-4 Seasonal Variation of Vertical windand Comparison with GEPB Data

2*3«5 Seasonal Variation of Vertical Temperatureprofiles and Comparison with GEPB Data

2*4 Stability Analysis 22

2.4.1 Monthly and Annual Variations of Stability

2*4*2 Monthly and Annual Stability-Wind Roses

2«5 Mixing Height Climatology 26

2«6 Model Simulation of Wind Flow overthe Deep Bay Air Shed 28

2.6*1 Use of the Topographic Air PollutionAnalysis System (TAPAS) Models

2*6*2 Model Results and Their Interpretation

3-. CONCLUDING REMARKS 32

ACKNOWLEDGEMENTS 34

REFERENCES 34

FIGURES

Fig* 1*1 Tsim Bei Tsui anemometer locationand the nearby terrain.

Fig* 1*2 Locations of meteorological stationsmentioned in this report*

Fig. 2*1 Typical surface patterns for northeast monsoonin winter and for southwest monsoon in summer.

Fig. 2.2 Annual wind roses for Tsim Bei Tsui (1975-82),Tate8s Cairn (1971-80) and Chiwan (1976-80)*

Fig. 2.3 Monthly wind roses for Tsim Bei Tsui (1975-82),Tate's Cairn (1971-80) and Chiwan (1976-80).

Fig. 2.4 Diurnal variation of surface windat Tsim Bei Tsui (1975-1982).

Fig. 2.5 Monthly mean wind speed at Tsim Bei Tsui (1975-1982)

Fig. 2-6 Cumulative probabilities of light wind spellsat Tsim Bei Tsui and at the Hong Kong InternationalAirport during 1982*

Fig. 2.7 Seasonal percentage distribution of upper-airwind direction at King's Park, 1981-1982.

Fig« 2.8 Seasonal percentage distribution of upper-airwind direction' at Shenzhen, 1981-1982.

Fig. 2.9 Mean seasonal vertical temperature profilesat King's Park, 1981-1982.

Fig. 2.10 Mean seasonal vertical -temperature .profliesat Shenzhen, 1981-1982.

Fig. 2.11 Monthly and annual stability-wind rosesat Tsim Bei Tsui (1975-1982)*

Fig. 2.12 Resultant flow when daytime onshore flow issuperimposed on the prevailing flowduring winter and during summer.

Fig. 2.13 Diurnal variation of the mixing heightestimated from monostatic acoustic radarrecords at Junk Bay (1982/1983).

Fig* 2.14 Wind field simulated with a background flow of 5-knotnorth-northeast winds•

Fig* 2*15 Wind field simulated with a background flow of 5-knotnortheast winds*

Fig. 2*16 Wind field simulated with a background flow of 5-knoteast-northeast winds.

Fig« 2.17 Wind field simulated with a background flow of 5-knoteasterly winds*

Fig. 2.18 Wind field simulated with a background flow of 5-knoteast-southeast winds.

Fig* 2*19 'Wind field simulated with a background flow of 5-knotsouth-southeast winds.

Fig* 2*20 Wind field simulated with a background flow of 5-knotsoutherly winds.

Fig- 2*21 Wind field simulated with a background flow of 5-knotsouth-southwest winds.

Fig* 2*22 Wind field simulated with a background flow of 5-knotwesterly winds*

Notes (Fig* 2,14-2*22) :-

1. A full barb in the wind arrow represents a speed of 10knots. A half barb represents 5 knots*

2. The numerical value, of the wind force is plottedalongside the wind arrow and is expressed to the nearestknot.

TABLES

TABLE 2.1 DIURNAL VARIATION OF WIND {KNOTS}AT TSIM BEI TSUI (1975-1982)

TABLE 2.2 FREQUENCY DISTRIBUTION OF LIGHT WIND SPELLSAT TSIM BEI TSUI DURING THE PERIOD 1975-1982

TABLE 2.3 DIURNAL VARIATION OF ATMOSPHERIC STABILITYAT TSIM BEI TSUI (1975-1982).STABILITY A = 1, B = 2,..., G=7

TABLE 2.4 OCCURRENCES OF INVERSION AT SHENZHENDURING ABOUT 80 OBSERVATION DAYS IN 1981-1982

TABLE 2«5 PERCENTAGE FREQUENCY DISTRIBUTION OF INVERSIONSWITH BASE IN SPECIFIED HEIGHT RANGES ABOVE KING8S PARK(1971-1980)

TABLE 2,6 UPPER-AIR CLIMATOGICAL SUMMARIES OF UPPER-AIR DATAMEASURED AT KINGgS PARK DURING 1971-1980.

TABLE 2*7 LIST OF MEAN' METEOROLOGICAL SITUATIONS SELECTED FORWIND FLOW SIMULATION USING THE 'TAPAS1 MODELS

PHOTOGRAPHS

Plate l.l Anemograph at the Tsim Bei Tsui Police post

plate 1*2 Radiosonde operation at Kingfs Park

Plate 1.3 Monostatic acoustic radar at Junk Bay

Plate 2.1 Topographic features surrounding the -Deep Bay air shed,

INTRODUCTION

As part of a collaborative work plan to protect the

atmospheric and aquatic enivornment of Deep Bay, the Royal

Observatory Hong Kong has examined the following aspects relatinc

to atmospheric transport in the areas-

i) Meteorological Observation

and ii) Meteorological Assessment.

This report covers the work carried out on the above topics*

The essential meteorological observation within the Deep

air shed comes from a weather station at Tsim Bei Tsui . The

station was set up by the Royal Observatory in 1975 to monitor

early indications of arrival of cold surges from the north* Other

supporting data are derived from observations outside tine air

shed. These include upper-air sounding records obtained at Kingrs

Park and monostatic acoustic radar records obtained at Junk Bay«

Section 1 of this report describes in detail the various

instruments, their operation , site conditions and data processing

of the .records. Section 2 presents analyses of various

meteorological parameters carried out with a view to constructing

a data base for further application studies of air pollution ir

Deep Bay, e.g. pollution dispersion and the air shed.8 s dispersive

capability.

Because of the steep and rugged terrain to the .h-outa of Deej

Bay, it is necessary to investigate the effect of topography on

the mean wind flow pattern in the air shed. For this reason/ wind

tlow simulations using an established model were carried out ana

the results are presented at the end of Section 2«

1. METEOROLOGICAL OBSERVATION

1*1 Equipment and Siting

1*1*1 Surface Wind Sensor at Tsim Bei Tsui

A Dines pressure-tube anemograph was installed at the Tsirt

Bei Tsui Police Post (Plate 1.-1) in January 1975. It is at e

height of 17-5 in above ground, or 43-7 ra above mean sea level.

The site condition has been described by Lara (1981)* The locatior

of the anemograph and the nearby terrain are shown in Fig. 1.1.

The site is well exposed to all directions but is slightly

obstructed to the southwest and west-southwest (Pig* 1.1).

The anemograph makes use of the difference of pressure set

up between two pipes * one of which is kept facing the wind by the

action of a wind vane, while the other is connected to a systen

of suction holes on a vertical tube. The ppressure dif f'erenential

causes the movement of a float carrying a pen, the height oi

which above the zero position is proportional to wind speed.

1.1.2 Upper-air Measurement Systems at King8s Park

The upper-air station at King's Park was established in 195]

arid is located on a hill about 1 km north of the 'Royal

Observatory (Fig* 1.2)* Twice a day* a free, unmanned balloor

carrying a radiosonde is launched for the purpose .of rueasurinc

the wind, pressure, temperature and relative humidity . in • the

upper atmosphere. Plate 1*2 shows an upper-air ascent being made

at King * s Park*

.During the period 1971-1980, upper-wind measurements'' were

made by means of a 30-ram Plessey WP2 wind-finding radar tracking

a Vacuum Reflex type 336 W corner reflector attached to the

balloon. The range and angles of elevation and azimuth were

determined at intervals of one minute. The wind at a given height

or pressure level was measured over an interval of 2 or 3 minutes

chosen so that the midpoint almost coincided with the instant at

which the balloon attained that particular height or pressure-1

level. The rate of ascent was approximately 6 m s

Upper-air temperature, pressure and relative humidity were

measured by means of radiosondes* Vaisala RS-13 radiosondes with

pressure, temperature and humidity sensors were used prior to and

until 17 November 1974* after which the RS-18 radiosondes were

brought into use. The RS-18 radiosondes provided a better

resolution in the pressure range of 100 mbar and above* The

various transducers in a radiosonde include a nickel-alloy

aneroid capsule for measuring pressure, a bimetallic thermometer

for temperature and chemically-treated human hair for humidity*

Radiosonde ascents were made using 0«7-kg balloons, while

rawin ascents were made using Q*5~kg balloons.

The Plessey WP2 wind-finding system was replaced by a

Vaisala CORA upper-air sounding system on 1 January 1981• At the

same time, the RS-21-12CN radiosondes were brought into use. The

CORA system makes use of the International OMEGA navigational aid

network which consists of several high-power VLF time signal

transmitters located in different parts of the world* A

transponder inside the radiosonde receives signals from three or

more transmitters and relays them to the ground station.. The

location of the radiosonde is calculated from the phase

differences among the relayed signals. The upper winas are

computed by tracking the radiosonde's flight. Pressure*

temperature ana humidity data from the radiosonde are sampled

every 6 to 9 s and transmitted to the ground station for

processing on a microcomputer equipped with a quality control

program and a 'significant point1 identification program* A

8 significant point8 at a certain level represents temperature

and/or humidity data (at that level) which are required for the

reasonably accurate reproduction of the radiosonde observation *

1.1.3 Monostatic Acoustic Radar at Junk Bay

In 1982i an Aerovironment 300C monostatic acoustic radar

(plate 1*3) was installed at Junk Bay (Fig* 1.2) on the roof top

of the Haven of Hope Sanatorium. The site is about 150 m inland

of the west coast of junk Bay and is about 30 m above mean sea

level. The electronic equipment for the radar system- was 'housed

in an air-conditioned shed, located also on the'roof.

The acoustic radar operates by emitting a sound pulse

vertically upward and then processes the echo which comes back

from the turbulent region aloft. It allows the detection of

turbulence structures, such as the height of the convectively

mixed layer and characterisitics of atmospheric waves and

layering.

The acoustic antenna (Model 302) consists of a vertically

pointing parabolic dish with a speaker mounted on a horn, and is

housed within a dense acoustic enclosure to cut out external

noise. The acoustic sounder operates at 1 600 Hz with a :pulse

repetition rate of once every 15 s* The pulse length is 200 ms,

providing a height resolution of 33 m. The maximum detectable

range is 1 000 m.

The central component of the system is a 300C transceiver-

display unit whose purpose is to continuously record the return

echoes on a chart recorder. In addition to recording at tne site,

the signal is also telemetered via a pair of private telephone

wires to another chart recorder (Model 322A) at the Royal

Observatory.

6

1.2 Operation of Meteorological Instruments

1.2.1 Observation Period

The anemograph at Tsim Bei Tsui has been in operation since

January 1975.

Routine radiosonde and rawin soundings were made at King's

Park since 1 June 1951 and 1 January 1955 respectively*

As part of the Junk Bay air shed study carried out by the

Air Pollution Meteorological Research Unit^ the operation of the

monostatic acoustic radar started on 1 December 1982 and was

terminated on 30 Novemeber 1983* The representativeness of the

observation period is discussed in the Final Report of the Air

Shed Meteorological Study at Junk Bay* prepared by the Unit in

1984.

1*2*2 Normal Operation of the Instruments

The pressure-tube anemograph at Tsim Bei Tsui is manned by

the staff of the Tsim Bei Tsui Police Post and maintained by the

Meteorological Instruments and Equipment Section of the Royal

Observatory* In addition to continuous recording on charts,

routine wind information is passed to the Royal Observatory by

the staff at the Tsim Bei Tsui police Post every three hours

during daylight hours*

The upper-air station at King's Park is also operated by the

Meteorological Instruments and Equipment Section* During the

period 1971-1980, upper-air wind speed; direction, temperature,

humidity and pressure were measured twice daily at 00 Z and 1225.

Two 'additional rawin ascents were scheduled daily at 062 and 18Z.

During the period 1981-1983, four radiosonde ascents were made

daily, at 00, 06, 12 and 182. The 06 and 18Z ascents were

additional ones made during the period.

The monostatic acoustic radar at Junk Bay was operated by

the Air Pollution Meteorological Research Unit, Royal

Observatory.

1.2.3 Calibration and Maintenance

Calibration of the anemograph at Tsira Bei Tsui is conducted

once every quarter* Routine maintenance is carried out once every

month* Emergency repair work is made on receiving reports of

equipment malfunctioning.

The maintenance arid emergency repair of the entire upper-air

measurement equipment are carried out by a technical staff

stationed at King's park. On occasions when the equipment is

unserviceable and the cloud base is sufficiently high, upper-

winds are determined by a pilot balloon with a single theodolite *

Prior to each radiosonde ascent, a 'ground check4 is

performed on the pressuref temperature and humidity sensors of

the radiosonde *

Calibration of the acoustic radar system was conducted

approximately once every month. Routine maintenance was carried

out at a frequency of about twice a week*

1,3 Data Processing

1.3.1 Wind Recoras

Hourly wind information is extracted manually from the

autographic chart of the anemograph and then transferred onto

computer carets* The information on the computer cards is

processed by the computer at the Royal Observatory into a

standard format and then stored on a magnetic tape.

1*3*2 Cloud Ooservations

Visual observations of cloud type and amount, and estimates

of the height of the cloud base are made at the Royal

Observatory. These cloud data are transcribed onto computer cards

before being processed by a computer at the Government Data

processing Agency. The data are then stored on magnetic tapes.

Because of the volume of -data, only the total cloud amount is

tabulate in 'Meteorological Results Part I Surface

Observations8 published annually by the Royal Observatory*

1*3*3 Upper-air Radiosonde/Rawin Records

Prior to 1981, upper-air information obtained at each

radiosonde/rawin ascent was transcribed onto computer cards

before being processed and quality-controlled by a mainframe

computer at the Government Data Processing Division (now the

Government Data processing Agency)• The processed information was

stored on magnetic tapes and published annually by the Royal

Observatory in ' 'Meteorological Results Part II -— upper-Air

Observations1,

Since 1981, upper-air data acquired by the CORA system is

processed and quality-controlled automatically in real time by a

minicomputer. The minicomputer produces coded messages containing

the data and relay them via telephone wires to the Royal

Observatory main computer for direct archival on magnetic tapes*

1*3*4 Acoustic Radar Records

The maximum operable range of the acoustic radar is 1 000 m.

Hourly mixing height values were extracted from the acoustic

radar chart records. During extraction, vertical wind and

temperature data obtained at King *s Park were taken into

consideration *

The mixing height is defined as the thickness of the

atmospheric layer through which pollutants are assumed to mix by

virtue of convection caused by daytime heating at the surface * It

is usually taken to be the height measured from the surface

upward to the lowest elevated temperature inversion. (A

temperature inversion is a stable condition in which the

temperature increases with height.) Temperature inversions

normally showed up on the acoustic radar as ground-based or

elevated echoing layers*

Extraction of data from the acoustic radar records was not

possible under the following meteorological situations:-

i) mixing layers exceeding 1 000 m;

ii) rain or showers; and

iii) windy conditions, with fresh winds or stronger.

As these situations normally result in efficient removal of

10

airborne pollutants from the ambient atmosphere, the mixing

height was given the value of the maximum operable range of the

instrument, namely 1 000 m. This renders a conservative estimate

of the mean mixing height.

11

2. METEOROLOGICAL ASSESSMENTS

2.1 General Climatology of Hong Kong

The climatology of Hong Kong has been described by Malone

(1977). It is summarized in the following paragraphs•

Although Hong Kong lies just within the tropics and is in

the same latitude as Honolulu, it enjoys a variey of weather

unusual in tropical regions. Seasonal changes are well-marked and

are due to Hong Kong's position on the southeast coast of the

Asiatic land mass. The cooling of the continent in winter and its

heating in summer give rise to seasonal monsoon winds on a very

large scale . These winds exert a profound influence on the

climate of south China.

The winter period in Hong Kong is loosely referred to as the

period from mid-October to early April. Starting from October, a

cold anticyclone forms over-Siberia and Mongolia (Fig. 2.1) and

normally reaches its maximum intensity in January, South China

experiences frequent cold outbursts of winter nionsoon during

which winds blow from the northeast quadrant* During monsoon

surges, the radiosonde ascent normally show a dry and stable

atmosphere aloft*

The period from mid-April to mid-May is usually termed the

'spring transition period', and is marked'by very changeable

weather. In March and April the continental anticyclone gradually

weakens and cold surges become indistinct and less frequent.

Incursions of warm moist tropical air from .the southeast become

more frequent* Since the coastal waters are still relatively

cold, the incoming warm moist air, produces widespread stratus,

' ' ' ' • ' ' ' . . - • - ' ' . ' . ' • ' " ' ' • • • 12"..' ' ' '•-" - -.:' : . : ; . . - . : / . . - ' • ' . - ",':

drizzle and fog.

Summer in Bong Kong lasts from approximately mid-May until

mid-September, but may be split into three fairly well defined

periods. From mid-May until June, trough ana light wind

conditions dominate over south China• Rain and thunderstorms

reach a maximum in June. By June, a deep depression forms over

the Indian continent* giving rise to warm and moist west to

southwesterly surface winds over the south China coast (Fig.

2.1) .

From mid-June to raid-August, tropical cyclones affect Hong

Kong with increasing frequency, although the summer monsoon

remains relatively persistent.

During the period raid-August to mid-September, trough and

light wind conditions return. Also, tropical cyclones become more

frequent.

Overall,, summer monsoon is less persistent than winter

monsoon.

The autumn transition period normally lasts from mid-

September to mid-October. During this period there is a rapid

decrease in precipitation* The cold anticyclone starts to develop

again over Siberia and Mongolia and there are weak outbursts of

cool air. The monsoon depression over India generally disappears.

During this period tropical cyclones reach their maximum

frequency, and then rapidly fall off in both frequency and

intensity*

13

2*2 Meteorological Data Bases

2-2*1 Surface Wind

Hourly wind aata obtained at Tsira Bei Tsui during the period

1975-1982 are usea. These are ID-minute means ending on the hour.

Unless otherwise stated,^ all mean winds derived in the analyses

refer to the vector means.

Hourly wind data obtained during 1982 at the Hong Kong

International Airport (Fig * 1*2) are used in the comparison of

light wind conditions* These are chosen because the station

exposure is good and the anemometer mast head is at a comparable

altitude .

The wind flow at Tate8s Cairn, which is 575 m above mean sea

level provides a good representation of unobstructed flow over

Hong Kong. Monthly and annual wind roses for Tate's Cairn are

used in the comparison of mean distribution in wind speed ana

direction.

A description of the site conditions of trie anemometer

stations at the Airport and at Tate8 s Cairn can oe found in Chen

(1975).

Monthly and annual mean speed-direction distributions of

winds measured at a hydroraeteorological station in Chiwan during

the period 1976-1980 have been supplied by the Guangdong

Environmental Protection Bureau. Chiwan is located on a peninsula

to the west of Deep Bay (near Ch1 ih-wan-rniao on plate 2.1).

Available information of the station indicates that it is located

on the slope of a hill and the surface flow there may not be.

adequately represented* . . . . . .

' ' . • • • . ' • • . ' : ' . • • . • • • • • • 14.:' . ;•'• - . . • . - ' • . ' " ' . ' • • ' • ' ' • • ' . . • '•'" . ' • . ' • : . • ' " • .

2.2 * 2 Cloud Data

As no clouu ooservations in the vicinity of Tsim Bei Tsui

are available, hourly cloua data ootaineci at the Royal

Observatory ciuriny the period 1975-1982 are useu. Tne clouci aata

consist of clouci type ana amount, and estimates of the height of

the cloua

2»2*3 Upper-air wind ana Temperature profiles

The Guangdong Environmental Protection Bureau has made

available vertical wind and temperature data ootained at bnenznen

City (Luonu District) during the period 1981-1982. Wind data were

obtained using Model CFJ-II optical theodolites ana temperature

data usin^ tooae! TK-II low-level sorides. Both the wina and

temperature were measured up to an altitude of about: 1 uOU ru.

During each season of the period, observations were made on about

20 days, at a frequency of three times daily: at around local

time 0700, 1300 and 1900 hours respectively.

For comparison purposes, upper-air wind and temperature data

obtained at King's Park on the same observation clays during 1981-

1982 are used.

2.2*4 Mixing Height

Hourly estimates of the mixing height at Junk Bay during the

period December 1982-Noveiuber 1983 are used*

Statistics of inversions detected during 1971-1980 using

radiosondes released at King's Park. (Li 1984)"; are also used. In

15

accordance with practice ^ 'cue worla Meteorological

Organization, an inversion is aefinea as a layer of atmosphere

with a temperature increase with height ana with a thickness of

at least 20 rabar .

16

2.3 Wind and Temperature Analyses

2*3«1 Monthly and Annual Surface Wind Roses

The annual wind rose for Tsim Bex Tsui is shown in Fig. 2.2.

For comparison purposes^ the wind roses for Tate8s Cairn (in 12-

point compass directions) and Chiwan have oeen included in the

figure* The wind rose for Chiwan shows apparent biases

towards occurrences of winds in the direction of the eight major

compass points.

At Tsim Bei Tsui, the prevailing wind comes from the sector

NNE-NE-ENE-E (47% annually). Easterly and northerly winds are

less frequent when compared with the winds at Tate's Cairn, This

is probably related to channelling effects of the roughly

northeast-southwest oriented mountain ridges in China and in Hong

Kong (Plate 2*1).

Winds from the southwest quadrant are relatively less

frequent when compared with those at Tate*s Cairn. As already

discussed in Section 1.1.I/ this is due to partial blocking by a

hill to the southwest of Tsim Bei Tsui* As noted, by Lam (1981)*

the general flow in the Deep Bay area is probably southwesterly

during some of the time when southerly wina is reported at Tsim

Bei Tsui *

Winds from the west-northwest occur as much as 5% of the

time* As will be discussed further in Section 2*4*2f this is

considered to be caused by onshore sea breezes during light wind

conditions* Onshore flow has the effect of recirculatixig

pollutants which otherwise would have been transported.away 'from

land by an offshore prevailing flow*

•'17' . ' • ' : . " ; - , - ' "• . • • • ' ; - . • • ' •

Comparison of the monthly wind roses (Pig. 2.3) suggests

similar observations to the above. Fig* 2*3 further reveals that

the general flow at Deep Bay is probably southerly or west-

southwest south-southeast winds are blowing at Tsim Bei

Tsuif particularly in the months of April to August.

The monthly wind roses in Fig. 2.3 suggest that the

anemometer station at Chiwan is sheltered from southerly winds,

which hardly occur at all*

Also, the average 101 occurrence, of westerly winds annually

at Chiwan is considerably higher than those at Tsim Bei Tsui and

Tatess Cairn. Inspection of the monthly wind roses (Fig* 2«3)

indicates that the westerly winds occurred mostly during the

cooler months with a maximum frequency of about 16% in December.

The reason for this is not immediately apparent * A further look

into the raw data and possibly the site conditions may be

necessary in order to explain this in meteorological terms*

2*3*2 Monthly and Annual Diurnal variations of Surface wind

The monthly and annual diurnal variations of surface wind

measured at Tsira Bei Tsui are presented in Fig* 2*4* The length

of wind arrows in the figure is proportional to the wind

strength* The following observations have been madie:*-

i) It can be seen that during the day in the months March-August,

winds generally veer (i.e. turn clockwise) and that during the

rest of the months the reverse is generally true* In' Section

2.4«2# an attempt is made to explain this phenomenon in terms

of the seasonal prevailing flow and local circulations due to

18

diurnal effects;

ii) The diurnal variation of the mean wind speed is separately

tabulated in Table 2.1f which shows different, distinct trends

between the period from March to September and the period from

October to February. During the first period^ the mean maximurn

wind speed occurs between 2 p.m. and 6 p.m. in the afternoon

and the minimum occurs between around midnight and 7 a«m* in

the morrving. During -the second period, the maximum' wind speed

occurs at around 10 and 11 a.m. before midday and the minimum

occurs at around 8 p*in» in the evening; and

iii) The maximum diurnal variation in the wind speed occurs in

May and June^ when there is a mean increase of about 4 knots

during day time. The minimum variation occurs in January and

February, when there is a mean increase of less than 2 knots

during day time.

In Section 2*4«2* attempts are raade to explain the phenomena

described in i) and ii) in terms of the seasonal prevailing flow

and local circulations due to diurnal effects®

As shown in Fig* 2*5* the maximum mean monthly wind speed of

7*2 knots occurs in March and the minimum of 5*2 knots occurs in

August and December.

2.3*3 Spells of Light wind

The annual mean wind speed of 5*9 knots at Tsim Bei Tsui is

relative low when compared to 7*6 knots at the Hong Kong

International Airport (victoria Harbour area).. ' ' ' ' ••

A comparison has been made of the' frequency of light wind

spells (i«e* duration over which the wind speeds ••/are less than 7

• • • • • • • • . ' ' ' ' ' . • / • ' . ' -19' • ' . ' ' • • • : • • - • > • • . • ' " • • : . - . . • . ' : ' ' • ' . • • ' ' •

knots) using 1982 data obtained at both stations• During data

extraction, a light wind spell is considered to be terminated

when the succeeding hourss wind is 7 knots or higher or when the

data is missing. Plotted on a double-exponential probability

paper, the cumulative probability of light wind spells (Fig. 2.6)

shows clearly that light wind occasions at Tsim Bei Tsui are more

frequent and usually last longer than those at the Airport.

Table 2® 2 presents the frequency distribution of light wind

spells at Tsim Bei Tsui during the period 1975-1982• Light wind

spells lasting more than 50 hours are most frequent during the

period November-January.

2-3*4 Seasonal Variation of Vertical Windand Their Comparison with GEPB Data

The seasonal variation of the vertical wind at King ' s Park

over periods in 1981-1982 coinciding with observations ' carried

out at Shenzhen by GEPB are given in- Fig* 2*7« Those for Shenzhen

are reproduced in Fig** 2«S* Apart from differences which are

discussed later on/ there is broad agreement between these two

sets of data* In generalf the predominant flow in spring, autumn

and winter is easterly and in summer is southwesterly. .The

consistency in the wind direction from surface up to about 200 in

suggests that surface wind data are adequate in describing the

wind direction within this layer.

Compared with data at King' s Park, easterly and east-

northeast winds at the lowest 200 m are relatively infrequent at

Shenzhen. The more predominant flow over Shenzhen is 'east-

southeasterly throughout the year>. While Wutong- Shan (Wu-t'ung.

• : ' . . ' - ' . ' - " • • • ' - - . 20 ' • • '• ' ' . . . ' " : ' : , ' ; / ' . ; - ' ' : ' • • ' - . ' ' . . : : V

Shan In plate 2«1) and a hill to its south may be a reason for

obstruction of- easterly flow towards Shenzhen, further

substantiation of this is required as. these topographic features

are about 10 .km from Shenzhen,

2*3.5 Seasonal Variation of Vertical TemperatureProfiles and Comparison with GEPB Data

The seasonal vertical temperature profiles at King's Park

over periods in 1981-1982 coinciding with observations carried

out at Shenzhen are given in Fig* 2*9. Those for Shenzhen are

reproduced in Fig* 2*10* Allowing for a difference of about one

hour in the observation times (e.g., 7 a..m. at Shenzhen and 8

a«m. at King 's Park), there is broad agreement between these two

sets of data.

It is observed that, throughout the year, the temperature

near the surface at King's Park (66 m) is slightly higher than

those taken at Shenzhen at approximately the same altitude* This

is probably due to the different degrees of urbanization^ hence

different degrees of temperature change in the two places. This

urbanization effect probably also accounts for the absence of an

average inversion at 8 p»rn* at Kinga s park during autumn^ whereas

at Shenzhen, an average nocturnal inversion is observed as early

as 7 p.nu Partly because of increased cloudiness during winter/

such an average inversion is not observed for .the ' winter

temperature profiles at these hours*

In view of similar land uses in Shenahen and in Deep Bay, it

is considered that vertical temperature structures•at, Shenzhen

are representative of the Deep Bay air shed* ' ;

. - - , . - •". "./ • -' ' • • .- - ' - : ' • ' • ' - 21' ' ' . . • : • ,' : • • ' • • ' • • • ' . . • • / ' • • : : - - . • • ' ' . ' :.;-, , "

2e4 Stability Analysis

One of the roost important parameters in assessing the

atmosphere's dispersive capability during a certain time of the

day is the atmospheric stability. Pollutants are readily

dispersed under an unstable atmospheric condition* Dispersion is

still possible under a neutral condition but gets progressively

more difficult as the atmosphere becomes more stable. When an

inversion exists, the vertical mixing of pollutants is severely

limited.

Estimation of the hourly atmospheric stability at Tsim Bei

Tsui is carried out using the conventional Turner's (1964)

method. This method is based on the consideration that stability

is dependent primarily on the net radiation and wind speed*

Without the influence of clouds, the insolation (incoming

radiation) 'during the day depends on the solar elevation! which

is a function of the time of day and time of year. When clouds

exist their amount and thickness reduce incoming and outgoing

radiation• In this method insolation is estimated from the sun's

elevation and modified for existing conditions of total cloud

cover and cloud ceiling height. At night estimates of outgoing

radiation are made by considering cloud cover and wind speed

only*

The sun*s elevation permits differentiation into daytime and

nighttime cases• The solar data for Hong Kong are given by

Peacock (1978)* in which/ for each day of the year, the solar

declination • as'well as the sun's elevation for the time of the

day are given. • :

22'

Turnerfs method divides stability into seven classes A to G,

with A representing the most unstable conditions, D neutral and G

the most stable. Stability classes for day time are A to D* and

for night time D to G« To facilitate computation, these classes

are assigned the values 1 to 7 for A to G respectively.

2«4*1 Monthly and Annual Variations of Stability

The monthly .and annual mean stability variations at Tsira Bei

Tsui are presented in Table 2*3* Over a 24-hour period, the

atmosphere is on the average most unstable at around midday*

Among the different months, it becomes most unstable during the

day , in around August and most stable during night time in around

December. The relatively stronger wind speeds in around March

results in a less unstable atmosphere on the average • for that

month.

2.4*2 Monthly and Annual Stability-Wind Roses

Stability-wind roses are constructed in order to aid, among

other pollution investigations, analysis of worst pollution

cases*

The relationship between the stability and wind (direction

and speed) is best depicted by plotting the stability-wind roses,

as shown in Fig* 2*11. In this analysis, the stability "has been

classified into three broad categories, viz* unstable (A-C),

neutral (D) and stable (E-G). The following observations are

noted s-

i) Under unstable conditions, light winds from the northwest

quadrant become prominent* Also, the occurrence of west-

• ' • • . • • ' ' . - • ' . ' • ' . • • • 23 •'" ' . . - - • . . . . . , ; , - - . • • • • ' • • ; ' ' . V . • : •

northwest winds far exceeds other winds from the same

quadrant•

ii) During the cooler months of November to March, winds from

the south are relatively infrequent unaer unstable

conditions when compared with neutral or stable conditions;

and

iii) During the cooler months, there seems to be more

occurrences of stable condition with winds coining from the

northeast quadrant•

The observation described in i) above indicates an onshore

unstable flow during day time (sea breeze)* with a preferred

direction from the west-northwest. Such flow is also evident from

time sequences of hourly wind at Tsiia Bei Tsui, especially during

light wind conditions- As land-sea circulations are 3-eiimensional

in nature, the impact of such effect on the local air quality

would have to be assessed in full by means of 3-dimensional wind

flow and dispersion modelling. However, this is outside the scope

of this report.

Point ii) above hints at a stable' nocturnal, offshore flow.

However, even if this exists at Tsim Bei Tsui it is not as well-

defined and also not as frequent as the onshore flow during day

time.

The stable conditions noted in iii) above are normally

associated with cool winter monsoon near the surface undercutting

the warmer westerly or southwesterly winds at higher level, thus

creating a temperature inversion*

Having identified a daytime sea-breeze circulation at Tsim

. ' . . • • • ' " . . • - ' '' . . ' - ' 2 . 4 - • - ' • . ' ' ' . ' . • . - ' - . ; V : ' • • : ' ' '

Bei Tsui^ it would be appropriate to offer here an explanation to

the diurnal changes observed in the wind direction, as noted in

observation i) of Section 2*3.2* Fig« 2.12 shows the resultant

wind field when a daytime sea-breeze flow is superimposed on the

prevailing flow at different seasons* The results indeed show

winds veering (i.e. turning clockwise) when the prevailing flow

is from the south or southwest quadrant during the warmer months

and winds backing (i.e. turning counter-clockwise) when the

prevailing flow comes from the northeast during the cooler

months *

Also, it can be s.een from Fig* 2*12 that, with the

establishment of a day-time sea bree.ze circulation, the wind

speed is decreased during the cooler months, but increased during

the warmer months. This, coupled with the fact that winds are

usually stronger during neutral conditions (see Table 2.3 for the

time of day when average neutral conditions occur), generally

explains observation ii) in Section 2*3.2*

25

2*5 Mixing Height Climatology

Pig* 2.13 depicts results of mixing height analysis for Junk

Bay (Koo et al. 1984). The mixing height is primarily derived

from acoustic radar records.

For all months! the minimum mixing height occurs during

night time and the .maximum in the afternoon .

Concerning the seasonal trend of mixing height variation*

the mean maximum' mixing height is as low as 426 m in February and

as high as 1 000 m and beyond in July* The mean mixing height is

low during the cooler months and high during the warmer months *

No attempt is made to' assess the effect of different

topography in Junk Bay and in Deep Bay on the mixing height •

However! for the purpose of modelling pollution dispersion, Aron

il983) noted in his studies of maximum oxidant concentrations in

a number of cities in United States that the mixing height has a

lower correlation to the concentration than other parameters such

as temperature9 inversion characteristics, geopotential height

and pressure gradient. On this consideration, it would appear

that the mixing height data obtained at Junk Bay would suffice

for modelling purposes.

As already discussed in Section 2.3«5# th^ temperature data

obtained at Shenzhen are considered to be representative of the

Deep Bay area. Inversion statistics at Shenzhen for local time

0700, 1300 and 1900 are presented in Table 2.4. At 0700 local

time, inversions are observed during more than 90% of the time,

irrespective of the season* of these, around 40% are ground-based

inversions, which are more frequent during summer and autumn.

26

Also,, at 1900 local time there are more ground-based inversion in

autumn than in other seasons* This probably accounts for the

average inversion appearing as early as 1900 local time in

autumn, as noted' in Section 2®3*5*

Also included are the statistics obtained from 0800H and

2000H radiosonde ascents at King§s park during 1971-1980 (Li

1984) which show a predominance in the occurrence of low-level

inversions in the cooler months (Table 2.5)* Within an altitude

of 600 m the majority of the inversions are concentrated in the

range from 360 m to 600 m.

Apart from ground-based inversions which are normally

results of radiative cooling overnight, inversions observed in

the cooler months are usually caused by the cool winter monsoon

undercutting the warmer westerly or southwesterly aloft* This is

shown in the upper-air climatological summaries at King's Park

(Table 2*6).

27

2*6 Model Simulation of Wind Flowover the Deep Bay Air Shed

2«6*1 The Topographic Air Pollution Analysis System (TAPAS)

To study pollution transport processes in the Deep Bay air

shedf it is necessary to characterize the local wind flow. The

topographic "features surrounding the air shed is such that

deviations from the prevailing broad—scale flow are possible in

the vicinity of hilly terrain• In the absence of a comprehensive

network of anemometer stations in the air shed, model simulation

is often employed to study the effects of terrain on air flow.

Such simulation can either be 'physical modelling., i.e. putting a

scaled model of the topography in a simulated aerodynamic

environraentf or computer modelling! i»e* using using numerical

fnethods to solve the dynamic equations which account for as many

as possible various atmospheric processes required to adequately

describe pollutant transport phenomena. Physical modelling is

normally cost-prohibitive* For this reason, therefore* computer

modelling remains as the only alternative* As in the previous

junk Bay study (Koo et al, 1984), the computer models of the

Topographic Air Pollution Analysis System (TAPAS) organized "by

the Colorado State University are found to be suitable. With the

appropriate input, these models provide- estimates of the terrain-

induced wind flow (speed and direction) and pollution plume

trajectories* These are important factors in pollution

dispersion* The results would allow adjustments to be made when

the more rudimentary straight-line-trajectory dispersion models

a r e -used, ' . . . - • . - . . ' - - " . . ' • . ' ' • • ' • . ' * • • • - : : ' • . • • •

28

Also, the wind simulation results are given at grid points.

It is thus possible to interface them to a topographical

dispersion model*

A full description of the TAPAS models can be found in

Fosberg et al. (1976).

Results of wind field provided by the models have undergone

validation in the United States (Fosberg et al. 1976) which

provides indication of their accuracy* Separate validation using

wind data from a number of surface stations in Hong Kong exhibits

a similar degree of accuracy*

Specific data required by the models are the background wind

flow, temperatures and contour heights at two specific pressure

levels* as well as elevation and roughness length of the

underlying surface for all computational points* The number of

grid points are 1024 (32 x 32) and the' grid size used is 1 km*

2.6.2 Model Results and Their Interpretation

Several meteorological conditions typical of the mean

situations in Hong Kong have been selected for simulation* In

particular, light winds are used because these are normally

associated with poor dispersion conditions and ..because the mean

wind at Tsim Bei Tsui is about 6 knots. pollution transport will

be more efficient in the case of stronger winds*. In-the choice of

cases, due considerations have been given to prevailing flows at

Tsim Bei Tsui. The cases are listed in Table 2.7. Typical

resultant wind fields are presented in Pigs. 2.14 to 2.22 and

represent winds at 10 m above ground, in the Figures, a full barb

29

stands for 10 knots and a half barb stands for 5 knots. The

numerical value of the wind force is also plotted alongside the

wind barb,

In relation to air pollution aspects, the assessment of

topographical effects on the wind flow is based on two main

considerations:-

i) whether there is any reduction in speed; and

ii) whether there, are significant deviations from the mean flow

resulting in localized, circulation.

The former will result in poorer pollution transport and the

latter is conducive to local stagnation conditions *

Resultant wind fields as presented in Figs* 2*14 to 2*22

show virtually undisturbed flow over the relatively flat area in

the vicinity of Tsim Bei Tsui• However s strong topographic

control of the wind pattern is observed near terrain features. In

general, stronger winds 'are observed on the windward sides of

hills and ridgetops, while depressed wind speeds are simulated on

the leeward sides.

During 'the cooler months with light winds from the northeast

quadrant, it can be seen from Figs. 2*14 to 2*18 that over the

Hong Kong side the flow is dominated by Tai Mo Shan and, in

general, areas within the air shed which are affected most by

stagnant flow and reduced wind speeds are• :-

i) the area bounded by Kai Keung Leng, Tai To Yan and

Ma On Kong; and

ii) Tuen Mun.

Less severely affected are Nan Shan (China) and "Tanglang Shan

(China) and the hilly area to the north of Castle Peak*

- ' • • • •-" • ' • " . ' ' . - - ; - : • • " ' • 30',. - • - - ! - ' - . ' ---::••:--. • : • - . •"...:.'': '': .';:/-

During the warmer months with winds from the southern

quadrant, resultant wind fields presentee! on Figs. 2.19-2.21

indicate that the area i) above is also most affected. Nan Shan

(China) is less severely affected, Tuen Mun is affected only by

westerly winds (Fig. 2.22)*

In areas where the effects of topography on the prevalent

flow are appreciable, the above model results will form the basic

input to a variable-trajectory dispersion model. The model will

provide quantitative estimates of the pollution impact due to

identified sources inside or outside the air shed* However, this

is outside the scope of the present report.

31

3. CONCLUDING

Meteorological analyses have been carried out for the Deep

Bay air shed based on available data collected by the Royal

Observatory.

The prevailing flow in the area is found to be different

from that of other locations outside air shed» The flow is

characterized by winds from the northeast quadrant during the

cooler moriths and by southerly winds during the warmer months.

During day time there is an onshore sea-breeze component from the

west-northwest direction at Tsim Bei Tsui.

The onshore flow is a 3-*dimensional phenomenon and has the

effect of recirculating pollutants over which otherwise

would have been transported away from land by an offshore

prevailing flow. The full effect on the local air quality would

have to be assessed by means of appropriate 3-diioensional

modelling• However, this is outside the scope of this report*

Drainage flow during night time is not apparent from the

available data*

Comparison with records at the international Airport

indicates that the mean wind speed is lower and that light wind

spells are more frequent and normally last longer at Deep Bay.

These are considered to be factors that may cause poor

ventilation condition in the air shed,*

In the diurnal cycle, dispersion'of pollutants is especially

limited in the early morning9 when inversions are,observed more

than 90% of the time throughout the'year* Of these, about 40% are

ground-based inversions* Under such conditions, vertical mixing

32

of pollutants from near-ground sources will be severely

restricted.

Findings on the seasonal variation in the atmospheric

stability^ wind speed, mixing height and the number of light wind

spells all indicate less favourable dispersion conditions in the

early winter months than in the summer months* However, the

maximum mean wind speed occurs around March and this suggests

slightly better ventilation during the late winter months*

The rugged terrain in soEie parts of the air shed warrants a

simulation of the year-round spatial, wind distribution by the use

of an established computer model. The model output reveals that

the steep terrain in the vicinity of Tai Mo Shan causes the

dispersive capability of the southeastern part of the air shed

near Ma On Kong and Kara Tin to 'be severely limited as a result of

the generally depressed wind speed and stagnation flow in the

area throughout the year *

It is "believed that the collected data and analyses

presented in this report will form an adequate meteorological

data base and provide a framework for future pollution

investigation and dispersion modelling work*

33

ACKNOWLEDGEMENTS

Thanks are due to the Guangdong Environmental Protection-

Bureau for making available the relevant meteorological.data at

Shenzhen and Chiwan«

The authors are indebted to Prof. W.E. Marlatt, Dept. of

Earth Resources, Colorado State University, who kindly provided

the TAPAS computer models*

REFERENCES

Aronf R,

Chen, T.Y *

Fosberg, M»&«, 1976W. Marlatt andL. Krupnak

Koo, E., 1984B * Y. Lee andC.M* Tarn

Lam, C. Y.

Li, T.S.

Malone, D.J.

peacock, J.E*

Turner, D.B.

1983 Mixing height — an inconsistentindicator of potential air pollutionconcentrations, A tin. Environ,, vol. 17,pp 2193-2197*

1975 Comparison of surface winds in Hong:Kong, Royal Observatory Technical NoteNo, 41*

Estimating airflow patterns over complexterrain,. Forest Service Reseach PaperRM-162, U*S* Dept* of Agriculture.

Final report of the air shedmeteorological study at Junk Bay,Royal Observatory Occasional paper(to be published).

1981 A preliminary report on themeteorological conditions in the DeepBay area, Royal Observatory OccasionalPaper No. 47 (restricted).

1984 Hong Kong upper-air climatologicalsummaries :•1971-198O, RoyalObservatory Climatological Note(to. be published)*

1977 Hong Kong Forecasters' Manual/ RoyalObservatory Forecasters1 Note .No* 2.

1978 Solar data for Hong Kong, RoyalObservatory Technical Mote No'. 14.

1964 A diffusion model for an • urban area/;J. Appl, Met;, Vol. 3, pp 83-91V

34

Fig. 1*1 Tsim Bei Tsui anemometer locationand the nearby terrain*

*' ^ ~

.' '* 4 •, i« Vf / , ,P- «' '"'i '-'"

^ Cf ^"A. '.JsTarfftlN.. *'''

t*fmww&

J IlHong KongInteratLtiona

ROYAL OBSERVATORY HONG KONG

Fig. 1.2 Locations of meteorological stationsmentioned in this report.

r Hof 10 I 20

H I G H

a* LOW

type (HE)6 IBB. 1948

Southerly type(S)27 JOKE 1948

Fig. 2.1 Typical surface patterns for northeast monsoonin winter and for southwest monsoon in summer.

s

7-16 1 7 - 2 7 ^ 2 8 IN KNOTS

Bei

w-Chiwan

-"- E

Fig. 2.2 Annual wind roses for Tsim Bei Tsui (1975-82),-n^.u^'1.^ r»^4V^ /.I .Q*71-.fl'fl \ an/i nHi^yan M

Tote's Cairn

Tsim Bei Tsui

T f T

C hi wan

V

0 10 20 30 40 50 */,

2&3 Monthly wind roses for Tsim Bei Tsui (1975-82)fTatess Cairn (1971-80) and Chiwan (1976-80).

IN IN 9N 4N 8N 6N 7M SH OH ION i«4 I8H S» 14N S« SSH |7N tiff SON f«l

JAN

FEB

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T T / t ^ ^ ^ ^ ^ ^ ^ ^ \AUG

SEP

OCT

NOV>s >s s s s ^ ^ / / / / / ^ ^ • ^ ^ ^ ^

DEC

10 20 knots

Fig. 2.4 Diurna l variat ion of surface windat Tsim Bei Tsui (1975-1982).

oc

J£~ 4•o

XIC

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

Fig. 2.5 Monthly mean wind speed at Tsim Bei Tsui (1975-1982).

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Seasonal percentage distribution of upper-air.wind direction at King ' s Park, 1981-1982.

Spring Slimmer

h( m )950

850

750

650

550

450

350

250

150

50

h( m )950

850

750

650

550

450

350

250

150

50

WNW NW NNW N NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW N NNE NE ENE E ESE SE SSE S SSW SW WSW W

Autumnh

(m)950

850

750

650

550

450

350

250

150

SO

Winter

WNW NW NNW N NNE NE ENE E ESE SE SSE S SSW SW WSW W

17 16 ,1 IS 15 i*"f3 19 2

WNW NW NNW N NNE NE ENE E ESE SE SSE S SSW SW WSW W

Fig, 2.88 Seasonal percentage distribution of upper-airwind direction at Shenzhen, 1981-1962-

SPRING

020

960

900

840

780

720

660

BOO

540

480

420

360

300

240

180

120

66

0

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12 13 14 15 16 17 18 19 20 21 22 23 24 T(*C)

z(m)1020

960

900

840

780

720

660

600

540

480

420

360

300

240

180.

120

66

0

SUMMER

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2m)020 -

960

900

B40

780

720

560

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540

480

420

360

300

240

180

120

66

0

AUTUMN WINTER

\08H 20H 14H

16 17 18 19 20 21 22 23 24 25 26 27 28 T(*C) 19 20 21 TC*C)

Fig. 2.9 Mean seasonal vertical temperature profilesat King's Park, 1981-1982.

h(m)

1000

900

800

700

600

500

400

300

200

100

Surface

h(m)

1000

900

800

700

600

500

400

300

200

100

Surface

h(m)

1000

900

800

700

600

500

400

300

200

100

- Surface

07:0013:0019:00

Autumn Winter

13 14 15 16 17 18 19 20 21 22 23 24 (eO T 21 22 23 24 25 26 27 28 29 30 31 32 33 ( e C) Th

(m)1000

900

800

700

600

500

400

300

200

100

Surface17 18 19 20 21 22 23 24 25 26 27 28 29 30 POT 11 12 13 14 IS 16 17 18 19 20 21 PC

Fig* 2.10 Mean seasonal vertical temperature profilesat Shenzhen, 1981-1982*

WinterPrevailingNE flow

/ Onshore flowfrom WNW

Resultant

SummerPrevailingSW flow

Onshore flowfrom WNW

Resultant

Fig. 2.12 Resultant flow when daytime onshore flow issuperimposed on the prevailing flowduring winter and during summer.

DEC1982

JAN1983

FEB

MAR

APR

MAY

JUN

JUL

Itsr 400H200

AUG

SEP

OCT

NOV

1 2 3 4 5 8 7 8 9 10 11 12 13 U IS 16 17 16 19 20 21 22 23 24 IHOURIJ-J~-J-JL~J--a^—L-JL-JL.JL-J l^J^±^J^

•827

371

529

,422

MONTHLY MEANHEIGHT Cm)

366

258

211

267

218

342

427

441

418

437

420

331

MEAN DAILY

Fig. 2.13 Diurnal variation of the mixing heightestimated from monostatic acoustic radarrecords at Junk Bay (1982/1983).

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

y

y y

y, yJ J J

(\y3§9 /

J , x20Q300^

^3"

,300

f200

y

<10Q-

14

,/ ,/ a/ a/ a/ a/ a/ a/ a/ 7

,/ ./ aTV a/ =/ a/ a/ a/ a/ a/ a/ /<_

s/ a/ a/ A/ a/ a/ / a/ / ,/ a/ a/

y a/././

a/ y y y

y,/ y

100

6/ 6/ 5/ 4/ 5/ 5/ 4/ 6/

5/ y y ,/,/ y

5/ 4/ 4/ 4/ 4/ 4/ 4/

4/ 4/ 4/ 4/ 4/ 4/ ^

/ / >/ /'a/v

y / j y-y/ y

4/ 4/ 4/ 5/ 5/ J *f- J- h h f- f*F

4/ 4/ Z !/ 5/ 4/ ,*5/

V

/y

y y

r/VY

Oa//

y

y.

/ / / BEEP BAY/ / /4/ 4/ 4/ 4/ 4/ 4/ 4/ 4/ 4/ 4/ X< y ,/L/

shear4/ 5/ Jq)

XtaonartShanr?

£/ 4/ 4/ 4/

4/ 4/ 4/

y / y y y

4/ 4/ 4/ 4/ 4/ /4/ 4/ 4/

4/ SW 4/ 3/( 3/ 3/ 3\

\C^hV N100 ; ,X, 4Xxf?/fring/ T O O -

/La/4/ y 4.

JOO

4/ 4/ 4/ 4/ 4/ 4/ 4/ 4/ >

4/ 4/ 4/ 4/ 4/ 4/ 4/

4/ 4/ 4/ J J 3/X

24 (

25 (

/ Z - J /5/ 5^«i0h4* ar«4iri

a/ a/ a/ ./y y j jy a/ aA J

t f f""fir*« 4/ 4/ 4/

4/ 4/ 4/ 4/

4/ 3/ 2/

'/ /fM" / A4 / 4 / 1 4/ 4/ 3/ 2/

,A . A- i y

/K7 a/),/ AfC^

A/ .AA.XlU a/

^y xy.ng/v-5/ y / 3/\ (

200'

rti

\i10 0>

T

7

a/

u- X

?£!<

F« Tuan Mb

\)2t)0

M/t

>4A ,/

y j ia A , /

a / \

y,

r20Cj ,x.

y

3ng

*n^an^

Cil't

aoq13/

15/

900oF'

700feoo^

11\ fjl' ' 1 0 / 12'

,%e>S^orf/9/ TH \45\ /ytf

/30p

"Voq

NHedla.y/ / 4-. .. ,">y;11*//^7

s/ ./v,/./ /y yt

iLrfiCh^ I 13/ ^12/iOO

\30Q,

71 18 *" y r>6o z.1

a/^7^V/IUIj;H'H9

Fig. 2,14 Wind field simulated with a background flow of 5-knotnorth-northeast winds.

8 9 10 11 I'd 13 14 15 16 17 18 19 20- 21 22 23 24 25 26 27 28 29 30 31

, 5/5/5XI CJ • >-J' *•!'

X9XaX8 /5/X5X5X5/5/5/5// Bx ax ax ax ax sx

X5X5X5/5X5X5X

X X^l5/ 4/ .' (b a

X X X XX 4X 4X 4X 4

/ / x x^ 4/1 4/f 44

O 4/ 4/ 4/ 4/

4/ 4/ 4/ 4/

Fig. 2.15 Wind f ie ld s imulated wi th a background f low of 5-knotnortheast winds .

8 9 10 11 12 13 14 15 18 17 18 19 20 21 22 23 24 25 28 27 28 29 30 31

Fig. 2.16 Wind field simulated with a background flow of 5-knoteast-northeast winds.

10 ii 12 13 14 15 16 17 13 19 20 21 22 23 24 25 26 27 29 29 30 31

Fig. 2.17 Wind field simulated with; a. background floweasterly winds.

of 5-knot

5 0

10 11 12 13 14 15 24 25 26 27 23 23 30

Lau Fan Shan

Fig. 2.18 Wind field simulated with a background flow of 5-knoteast-southeast winds.

10 li 12 13 14 15 18 17 IB jg 20 21 22 23 24 25 26 27 ij«

( i , ()v

hoo , ' ,G i ) 5v 4 v

5 f5 v

o

"1M \°\ YA4 x vjf' - »

•\ ^tvV'\•\ \ \ r\ \\ ;\ 5\ \ \ s\ ;\\v\ v\5\\ \ \ \

10.,

B \ r \ H v R V

x 2 Q O .30Q

<&

,300 2£Q

20(3"tanoyShciF

\ \

<\ -\ A A ^ -7

\ \ \ !\ 'I )\ \ \ 5\ '\ '1 }\ \ "\ '\ \ \ 1

S 300 X

^o^r- looA

V)oo1 ' F

\ \T

\'\:\ \ \ \ \' XO.4

\ V'*\i/?;,"\6\ 5\ YN..:>, '•A M ( !" , » *> ' f') 1

\

\\

\

I A\•\5\

v\*\ ^ «\ «^ <\ >v

\ \ \ \ \ \ \ '\h

' ' \ \V\V

-\ ;\ ;\ ;\ ;\ ;\\V\\v\

\ i.-

, 3 - / ™ 9 < -\T

\ \ \ \ \ \ \ \ X A X \ \4\ 4\ 4\ 4\ 4\ 4\ 4\ 4\ 4\ 4\ Vv. X^5 4\ / 4\\ \ \ ^DE^P ^>AY\ \ \ \ T A i ^

\ \ \ \ \ \ \ \ \ \ y v\!\ \ \ \ \ \ \A \ Y\ 5\ "T\A A \ '» A \ fc ._. A •. • A .. • _ . A A A A5y 5v 4v 4x 4» 4v 4,7 ^\"?^ \X \ >5\ 4 \ j ^\

x x x x x VQU Pori SiXon ^i/ ^ ^' A X A

3, r)

16

-200") .N\ 5l '**\ A A \ \ \ \ .swjusM . * :4 A A A

\ \ \ \ \ \ Y V\ \X\\

24 c

25 (

29 t

\ \ \ji artgin mt^rti

A A A

\ \ V'\ \s\\ 7

\ \ \\ .\"R Congi

V \<a<n TirX-ry

\5, 5V 5X

5\ 5\ 6,\ \ 5\ \ ;\ 13, Ja"

VTueri Mur

'' ieak

-700reooj

#0 ^'

Tap Shek ^00300 5.

31 o\ \ \ \c\ \ \ \ \ \ \ \

0 30

Fig. 2,19 Wind field simulated with a background flow of 5-knotsouth-southeast winds.

10 11 12 13 14 15 16 17 IB 19 20 21 22 23 24 25 26 27 28 30 31

5, \ 4, } \A 4 5/ \4\ 4 4 4, „ 5,

hl' 1 ;\U, in4 i 'JaJ

" } 1 J 1 ]

] "j ] 1 17? 1

i

/ > / < >/ 7 (>zo°/ / W

J J

31 (

Fig, 2.20 Wind field simulated with a background flow of 5-knotsoutherly winds.

10 12 13 14 15 18 17 IB 19 20 21 22 23 24 25 28 27 28 29 30 31

7 ; °

n ) 1 1\ 5/ 5/ 5/ 6/ 5/ >\ 7 7 7 7/0 /•'

/;;;;; 7 ; ;v; j7 ;; 7 ) 7; 2.2, s )

o5. 5yH*iflhls are^in mt*r*s 4

7 7 * /

rrrrj

W i n d f i n l d n l i T W l n t r > d w i t h n bnak . j rmmd f l a w o f '5-knotsouth-southwest winds.

6 7 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Fig. 2.22^ simulated with a background flow of 5-knot

Hour

Month

Jan

Feb

Mac

Apr

May

Jun

Jul

Aug

Sep

Dot

Nov

Dec

Tear

10 11 12 15 14 15 16 1? 18 19 20 21 22 23 24

5.4 5.5 5.4 5.8 5.6 5.8 5.9^6.0 6.1 6.4 6.1\5.4 5.5 5.4 5.6 5.8

5.a/ClN\5.9/C?\5.9 5.9/6.2 6.1 6.6/7~72\6.6 6.1 6.1 6.3 6.2 6.1

5.5 5.6 5.3 5.3

t.5 6.2_6.0 5.9 5.9 6.0"1>. 7 s 7.8 8.0 8.1/7

'"57J) 4.2 4.3 4.1 4.3 4.3 4.7 \.1 5.4" 3 6.1 6.4 6.6\7.1 7.4 7.2

4.7 4.7 4.3 4.Q 8)4.3 4.3 4.?) 5.4 5.3\6.1 6.4 6.8/7.5 7.5 7.7

5.2 5«8 5.6 5.5 5.8 5.7

4.1 4.4 4.0-^5794.2 4.1 4.5 4.5

4.9 4.9 4.7 4.9 4.9 4.8 5.0 6.

5.2 5.3^5.04.8 5.0 4.8 4.9 6.0 6.7

7.4 7.5 7.5 7.5

5.2 5 . 5 7 ^ 6 . 3 6.5 6.7

.1 6.8 6.7 6.7 6.4 6.7 6.97.~ 6.8 6.9 6.6 6.6 6.3

5.0 5.5 5.3 5.5 5.6 5.9 5.6 (6.1 6.8(7.3/6.5/5.5 5.2 5.5 5.6 5.9

5.4 5.1 5.0 5.2 5.1 5.5 5.0 5.9\6.4 6.7 6.5(5.8 5.2 5.2 5.1 5.3

5.1 5.3 5.2 5.3 5.2 5.4 5.4 5.7 6.1 6.6 6.7 6.5 6.6 6.7 6.8 6.8 6.9 6.6 6.2 5.7 5.6 5.5 5.5 5-4

TABLE 2.1 DIURNAL VARIATION OF WIND SPEED (KNOTS)AT TSIM BEI TSUI (1975-1982)

Month

No. of Hours

0-10 11-15 16-20 21-25 26-30 51-35 56-40 41-45 46-50 ^51 Total

Jan

Peb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Mov

Dec

65

82

77

77

85

75

77

75

87

81

86

76

29

51

22

2?

35

52

52

32

28

52

55

24

20

15

16

15

15

14

6

13

10

15

18

22

7

9

3

12

6

6

6

7

6

8

10

10

4

5

5

4

5

3

1

7

3

2

3

2

5

5

0

0

4

1

1

5

3

2

5

1

2

3

0

0

1

1

0

0

1

1

4

3

1

1

1

0

1

0

0

4

0

1

1

1

2

0

0

0

0

1

0

1

1

2

1

4

3

0

1

0

0

0

1

0

0

3

3

3

138

147

123

133

152

133

124

142

139

147

166

146

Year 941 359 177 90 40 50 16 11 12 14 1690

TABLE 2.2 FREQUENCY DISTRIBUTION OF LIGHT WIND SPELLSAT TSIM BEI TSUI DURING THE PERIOD 1975-1982

Hour

South

Jan

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

HOT

Dec

Year

8

5.4 5.5

5.1 5.1

5.0 4.9

5.0 5.1

5.4 5.4

5.4 5.4

5.4 5.5

5.6 5.5

5.5 5.5

5.4 5.5

5.4 5.5

5.5 5.5

5.2 5.5

5.1 5.0

4.9 4.9

5.0 5.0

5.4 5.4

5.4 5.5

5.4 5.4

5.6 5.5

5.5 5.5

5.4 5.5

5.5 5.5

5.5 5.5

5.2 5.2

5.1 5.1

4.9 5.0

5.1 5.0

5.4 5.5

5.5 5.4

5.5 5.5

5.5 5.5

5.5 5.4

5.5 5.4

5.5 5.5

5.5 5.5

10 11 12 13 14 15 16 1? 18 19 20 21 22 25 24

41 5.2JJ5.5 5.5 5.1 5.0 5.0 5.0 3.1 3.4 5.

1 5.'0 /5.6 5.7 5.4 3.5 3-5 5.5 5.5 3.5 3.

/4.3/3.7 5.7 5.6 5.5 5.5 5.6 3.7 5.7 3.7 \.1 4.8 47

/%7 5.5 5.5 5.1 5.1 3.1 5.4 3.5 5.7 5.8 5.8\4«8 4.0

4

5.6 5.4

5.5 5.5

5.5 5.1/f f \

5.5 5.8 5.7 5.8

2.8 2.9 2.8 3.0 5^0 3.5 5.7 5.7 5.8

2.6 2.7 2.7 2.8 2.8 \5.2 5.6 5.7 5.8

5.5 5.'0 2.6 2.4 2.5 2.4 2.7/3.1 3.5 3.5 3.7

3.4 5;0-^S7ls\2.6 2.6 2.5/5T2 3.2 3.5 5.6

4.8 5.

4.8 510

.5 5.2 3.1 3\0 2.7 5.'o—5.0 3.1 5.2 3.6

3.5 5.0 /2.9 2.7 2.6 2.7\5.5 5.2 3.7.

5 5.5U5.4 5.0 5.0 2.8 2.6 2.7«>" ly X.<«

5.1 5.1 5.6

,575 5.6 5.6

5.4 5.5 5.6

5.7 5.9 5.9

4.9 5.0

5.0^5.2

5.

5.5 5.5 5.5 5.5 5.5 5.5 5.5 4.1 5.4 5.1 5.0 2.9 2.9 3.0 3.5 5.5 5.6 4.5 5.1 5.2 5.5 5.5 5.5 5.5

TABLE 2 .3 DIURNAL VARIATION OF ATMOSPHERIC STABILITYAT TSIM BE! TSUI (1975-1982).STABILITY A = 1, B = 2 , . . . , G=7

^-\TiiaeHt (mj\.

0-49

50-99

100-149

150-199

200-249

250-299

500-549

350-599

400-449

450-499

500-599

600-699

700-799

800-899

900-999

1000-1099

Total

Ho. of Oba

Bo. of Obewiifa ianrecgi

Spring

4331

11

5110

4

5

1

1

0

0

29

21

n19

07 h

Sassier

10

60

510

0

1101

1

1

2

1

0

28

19

18

tt*feUBD

11552

1

0

0

2

1

42

2

5

0

2

0

58

21

21

Winter

7411

2

2

1

2

2

1

5

1

4

2

1

0

54

20

20

pring

1

0

01

0

0

11401

1

5

0

0

0

15

19

10

13h

SdSffi^E

012

0

1

300

0

0

2

1

1

0

0

0

11

18

7

totaeaia

0

0

0

0

0

0

0

10

0

0

0

1

1

1

0

4

H

5

f

Winter

0

0

0

0

0

0

10

110

1

1

1

4

1

11

17

10

Spring

12

0

1

501

552

1

2

2

1

0

0

24

20

16

19b

Summer

0

0

111010

0

0

5

0

1

2

0

0

10

22

6

tataim

11

1

1

1

1

1

1

2

1

1

0

0

1

1

0

1

24

19

17

Vdneber

3551

0

01501

3

2

1

1

1

0

23

19

15

TABLE 2.4 OCCURRENCES OF INVERSION OBSERVED AT SHENZHENDURING ABOUT 80 OBSERVATIONS IN 1981-1982

OF ! 0600 AND 2000f 0000 AND 1?00 H

JAM FFH HAY JIIN JHL AMG SFP NOV OFC YEAR

6b«i19 "i-1NO. OF ocPupKFNrt

1PO-179 HMO, -CiF OLtU&KFI-jnt

NO.' OF OCfUPHFhCE

£4 0-2*) 9 H

300-5*19 I-,NO. OF 0£ruPNFi,itiLi<i00 fl'F A^PEMTS

fjO.-uF OCPUPRFMTE

NO. UF OcCURhFuCE

4^0-519 h

NO I UF .ACCENTS *

540*600 N

66-bOO MNO, OF OCCURRFNPfc.NO A UF A SCfc. NT $

.00 „ 0 0 .000 0 If

bPO 5 6 t»PO

S0(; .00 *lb0 U 1

9 3 1 ^ "PbPO bbh 6?0

1«13 .71 1.777 4 ! 1

bPG 56b 6?0

1 a^4 1 P^l P ?blif 7 1X1

6PO 5fcb b?0

"J? 6i^ s,'j!

*?b "13 "l26?U S66 b?U

6?U 566 620

°?9 e?H 812t>PO 566 6?0

142 112 157b?0 56b 6?0

v)

,000

efbOO

SbOO

bOO

s.so13

bOO

oOO

S1bbOO

600

lfl.0010dbOO

eoo0

b P U

0 0U

16?0

1

1 .137

@146?0

*ia

?J!

eii

0S96?0

0

.171

.00

eoo0

596

.67a

3

3

2

.50596

*16596

0616

65 o

,00U

616

eoo0616

.16616

3616

e^22

616

e a616

9616

.0819

616

.000

6?0

0

0b?0

.000

,000

6PO

a6PO

" 8

.656?0

1.136?0

3.7123

B o599

,00599

.00599

,000599

.000

5^9

a599

599

3

599

3.1719

599

.000

0

.000

6?0

."000

6?0

6?0

1 .6110

6?0

eio6?0

1.6110

620

b?0

,000

600

.000

600

,000

600

600

.17

600

B15bOO

@12600

3.0018

bOO

S15bOO

10.5063

bOQ

,000

6PO

0

06?0

.65a

6?0

1.137

0pq

6PO

*ln

3*M

120

,000

72^7

,03

9

.47

"§6

20472^7

2.66194

7297

72^7

19772^7

905

TABLE 2.5 PERCENTAGE FREQUENCY DISTRIBUTION OF INVERSIONSWITH BASE IN SPECIFIED HEIGHT RANGES ABOVE KING'S PARK(1971-1980)

Bai^ft

Dw Feist1000 8*lstiv*

lixlEc iatlo (Via* Bl»3ti«MM

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77.0(14.7)7.6( £.4)063

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117.2(27.2)2l.0( 3,0*)18.7( 5.3)87.0( 8,9U.i( 3.1)093

59.9(26.3,

,2.3}, .87. 7( 6.5)17.5( 2.6

. . ,27.7 1.2

84. <»( 5.6)20.3( 1.2)

215

85, Lf e.o)16. 0( 2.3)040

123.X 34.023.1(ia.i( 4.574.9 13.313.7 3057

10

162. 5(28.518. 5( 3»212.41 3.969.4(19.1

9.7( 3.3044

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176,5(28.515.0( 3.59.6( 6.7

72.«(17.78.2( 3.0052

115.5(56.019.91 5.7i«-a( 7.380.3(14.0)I2.i( 5.2071

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TABLE 2.6 UPPER-AIR CLIMATOGICAL SUMMARIES OF UPPER-AIR DATAMEASURED AT KING!S PARK DURING 1971-1980...

TABLE 2*7 LIST OF METEOROLOGICAL SITUATIONSSELECTED FOR WIND FLOW SIMULATIONUSING ^APAS1 MODELS

Background Flow Applicable Situation

Direction Speed (knots)

1)

2)

3)

4)

5)

6)

7)

8)

9)

NNE

NE

ENE

E

ESE

SSE

S

SSW

W

5 Autumn, winter, spri

5

5 Winter, spring

5

5

5 Spring, summer

5 Summer, autumn

5

5

Plate 1.1 Anemograph at the Tsim Bei Tsui Police Post(viewed from southwest)

Plate 1.2 Radiosonde operation at King's ParkPlate 1.3 Monostatic acoustic radar at Junk Bay

"•i-fing-Mqg• "%

^ •'%

Plate 2.1 Topographic features surrounding the Deep Bay air shed

ft*

TOG