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Gradient height and velocity profile during typhoons
Journal of Wind Engineering and Industrial Aerodynamics, 13 (1983) 31--41
31
Elsevier Science Publishers B.V., A m s t e r d a m - - Printed in The Netherlands
GRADIENT HEIGHT AND VELOCITY PROFILE DURING TYPHOONS
E.C.C. CHOI Department of Civil Engineering,
University of Hong Kong, Hong Kong
SUMMARY Velocity profile and gradient height have been objects of main concerns in the study of wind engineering for a long time. For normal (non-typhoon) conditions, the shape of the wind profile as well as the gradient height have been well established for different types of terrain. However during severe tropic cyclones and typhoons such information is still insufficient. This paper tries to answer the two questions (i. Is there a gradient height during typhoons; and if so what is its value? 2. How does the velocity vary with height during typhoons?) by analysising the past twenty years typhoon data. Both the upper level wind data from radar sounding and the surface anemometer wind records have been studied. Result of the analysis shows that the gradient height during typhoons is very low; it is only about half the expected value for monsoon winds. Wind profiles for mean and gust wind have also been established. The finding is found to compare favourable with results obtained by Hong Kong Royal Observatory.
i. INTRODUCTION One of the major factor affecting the magnitude of wind loading on structures is the shape of the wind profile.
It has long been recognized that the hori-
zontal wind speed varies with height and that this variation differs under different atmospheric conditions and for different types of terrain.
Extensive
research has been carried out in the past two decades by meteorologists, tists and engineers eg. Panofsky, Harris, Mackey, Davenport and etc. result, valuable data has been gathered from site and laboratories; relationship between velocity and height has been established. of the vital parameters
scien-
As a the general
And the values
(such as the power exponent ~, gradient height Zg, the
roughness length Zo) has also been found for different types of terrain. ever most of the findings are for strong monsoon wind conditions. mation for typhoon wind is scanty and far from being adequate.
How-
Such infor-
Furthermore, as
the wind structure and turbulence mechanism of the typhoon wind is not quite the same as the monsoon wind, these parameters may have a different value. This paper systematically analysed the typhoon wind records for the past twenty years.
There are two types of data.
The first type is the surface wind
records from ten meteorology stations in Hong Kong. upper-level wind data from radar sounding records.
The second type From the analysis,
is the the
gradient height during typhoon conditions is established and also an expression for the variation of velocity with height is being proposed.
0167-6105/83/$03.00
© 1983 Elsevier Science Publishers B.V.
32
2. T O P O G R A P H Y Before
topography Figure
OF SURFACE
looking
STATIONS
at the typhoon
it is important
the topography
the area
1000 m in height. China.
Towards
There
is fairly
are more
important
Royal O b s e r v a t o r y
portion
their height
south and west. and distance
Kowloon
The anemometer
at
NE
at
NNE 8 km
B e a c o n Hill
(451 m)
at
N
Victoria
(553 m)
at
SW
3.9 km
(530 m)
at
SE
6.1 km
is m o u n t e d
station
of
in Hong Kong;
The h a r b o u r
There are many m o u n t a i n s
(576 m)
Peak
the topography
is less
surrounding
the
are as follows
(601 m)
Parker
of
Ocean.
on top of a 32 m hill at
(city centre).
Peak
Mount
to the Pacific
in the area,
Cairn
Tate's
from 500 m to
as follows.
It is situated
Peninsula
of the stations.
ranging
is open
- RO is the oldest
as 1884.
of K o w l o o n
peaks
the genera] stations.
is linked up to the m a i n l a n d
the area
stations
(RO) H e a d q u a r t e r s
i km to its east,
station;
the area
ones are d i s c u s s e d
its data dates b a c k as early the southern
side,
south and west
than ten m e t e o r o l o g y
three of the more
the locations
rugged w i t h m o u n t a i n
On the north
the east,
to understand
of the m e t e o r o l o g y
] shows a map of Hong Kong together w i t h
In general
than
data,
of Hong Kong and also
at a level
29 m above
7 km
5.6 km
ground and 61 m above m e a n
sea
level. Waglan
Island
the southeast the
highest
- WL station
is situated
coast of Hong Kong.
(WL)
The island
point
from most
direction
D'Aguilar
Peak
The a n e m o m e t e r King's
Park
above MSL) with
w i t h only
sea level.
two small
is at a height
peaks
at about
island
far off
than 0.1 km 2 in area and
The island
to its north. Island
is u n o b s t r u c t e d They are the
(230 m) at N 7.5 km.
of 75 m above mean sea level.
(KP) - KP station the centre
is located of K o w l o o n
in all directions.
on top of a small p l a t e a u Peninsula.
The heights
(64.8 m
It is a city centre and d i s t a n c e s
area
of the nearby
are as follows.
B e a c o n Hill
(451 m)
at
N
Kowloon
Peak
(601 m)
at
NE 6 km
Parker
(530 m)
at
SE 6.8 k m
(553 m)
at
SW 4.8 km
Mount
Victoria
The a n e m o m e t e r
height
From the above data.
is less
(324 m) at NW 6.9 k m and Tung Lung
tall b u i l d i n g s
mountains
is 58 m above m e a n
on top of a small
The other
Peak
4.4 km
is 78 m above m e a n sea level.
discussion, two stations
it is clear
that WL gives
are also selected
because
the best
quality w i n d
RO has the longest
data
3a
history, and KP is the station where balloons for the radar sounding records are being released.
3. UPPER LEVEL WIND DATA Wind measurement is made by using radar sounding technique (RAWIN SONDE and RADIO SONDE ascents).
A balloon released from KP is being tracked by radar.
The wind speed at a given height is measured over an interval of two to three minutes which is chosen so that the midpoint almost coincides with the instant at which the balloon attained that particular height.
Wind speeds are normally
taken at heights of 300 m, 600 m, 900 m, 1500 m, 2100 m, 3000 m and above. Figures 2a to 2i show the RAWIN record of nine occasions of strong typhoon.
It
can be observed that the maximum velocity occurs most frequent at 1500 m level. However from the study of other cases, it is not unusual for the maximum to occur as low as 300 m or as high as 3000 m.
Also there are cases where there
are two or more maxima, one occurring at a low level and the other at 3000 m or above.
Usually the low level one is selected to be the limit of boundary layer
and the higher one is discarded.
Figure 3 shows the average curve for all 48
typhoon occasions being studied.
Before averaging each set of data is being
normallized by the maximum velocity.
A multi variate curvilinear fitting
technique has been used to fit an exponential profile to this curve. exponent and gradient height are treated as unknowns.
The
The result gives an
exponent of 0.204 and a gradient height 1461 m. Unfortunately the above finding cannot be applied to general cases. applies only to KP and that portion of Kowloon Peninsula. earlier, there are several mountains surrounding the site.
It
As pointed out Thus the 300 m and
600 m velocities are strongly affected by topography.
4. SURFACE WIND DATA Since the above approach fails to draw a general conclusion, another method has to be used.
This involves looking at the surface wind speed in conjunction
with the upper level wind data.
The WL data has been selected for this purpose.
Wind record of WL as well as its topographic features has been studied by the author and others [1,2].
It seems that the wind speed recorded by the anemome-
ter at WL approaches the free wind velocity. topographic effect.
There is certain amount of
But the effect is small for mean wind speed and even smaller
for gust speed. In order to compare the surface wind velocity with the gradient wind velocity, the following assumptions are made.
The maximum wind velocity recorded during
a set of RAWIN ascent is taken as the gradient velocity.
The same value of
gradient velocity is assumed as the wind blows over WL which is approximately 20 km from KP.
Thus the mean wind speed recorded at WL can be compared with
34
the gradient wind velocity. of tlle gradient this ratio. believe
speed,
When the WL velocity
(VwL) is expressed
there shows a considerable
The maximum
is 1.0 and the minimum
variation
is 0.35.
for the site).
seems to indicate
relationship
that certain
exists between VWL.
the ratio and VWL.
Although
there is a fair
(which is expected with all the assumptions
clear trend of increasing
ratio with increasing
further postulated
that the ratio would approach
simple exponential
expression
being made),
VWL can be observed.
a
It is
unity at very high VWL.
has been used to represent
curve fitting the following equation
to
(which would be the
A closer look at these ratios
Figure 4 shows a plot of these ratios against amount of scatter
of
It is difficult
that all these points scatter about a single value
case if Zg and ~ are constants
as a ratio
in magnitude
the variation
A
and after
is established.
6.64 VWL
VWL =
e
(I)
Vg
Equation
(l) is also plotted
on figure 4 in dotted line.
ential law is used to represent be evaluated
from equation
the velocity
profile,
Now,
if the expon-
the gradient height
can
i.
6.64
~VwL Zg = ZWL
where
. e
(2)
ZWL
is the anemometer
Zg
is the gradient height
height at WL (74.8 m)
is the power exponent In order to evaluate a)
Zg, one must first
fix the magnitude
The power exponent ~ - In the past decade,
typhoon wind characteristics exponent
for the variation
atmosphere
measurement
b)
and research on
has been carried out by many workers.
of velocity with height at the lower
has been established
and published.
CHOI [3]
from 0.18 to 0.28 and Mackey & Ko [4] postulated present
of ~ and VWL.
Power
i00 m of the
found values
a mean value of 0.19.
ranging In the
study a value of 0.19 is used.
Velocity at Waglan VWL - As can be observed
the value of VWL the lower will be Zg. ponding to certain value of VWL.
from equation
Thus one can only solve
In wind loading applications,
return period and 100 years return period winds are commonly analysis following
for Zg correstile 50 year
used.
of the Waglan data by the Hong Kong Royal Observatory results.
2, the higher
Extreme
[5] gives the
35
VWL (50 year return period)
= 44 m/s
VWL (i00 year return period)= 48 m/s With these figures the gradient height during typhoons over open sea is 164 m for 50 year return period wind and 154 m for i00 year return period wind. The velocity at RO has also been compared with the RAWIN data. VRo/Vg
When the ratio
is plotted against VRO (figure 5) the same trend of increasing
with increasing velocity can also be noted.
Thus it is expected
ratio
that the
gradient height during typhoons also decrease with increasing wind velocity over city centre area. mountains
However since the RO data is affected strongly by surrounding
and buildings,
a detail study has not been carried out.
5. GUST VELOCITY PROFILE Most of the expressions A theoretical approach
describing
Because of its simplicity, exponent,
in nature.
under typhoon conditions.
the power law is often being used.
As for the
some postulated that the value for gust should be half that for mean
wind [6]; however there are others value approaching Observatory
[4] who obtained from on site measurement
that of the mean wind value.
in 1980 carried out an investigation
Wind velocity at four different
levels
150 ft and 200 ft) were studied. the velocities level.
the gust profile are empirical
is still lacking especially
a
In fact the Hong Kong Royal on Typhoon Georgia
(May 1980).
15 m, 30 m, 45 m and 60 m (50 ft, I00 ft,
Two minute running means were calculated
and
of the higher levels were compared with that of the 15 m (50 ft)
From these ratios,
shown in figure 6.
the power exponent was computed.
As can be observed,
Their finding is
firstly the power exponent
increases
with height and secondly it decreases with increasing wind speed (although it seems to approach a constant value at higher velocities). to generalize on the value of the exponent.
Thus it is difficult
In the present paper the gust
profile is to be derived from the mean wind profile. Gust speed is related to the mean wind speed by the gust factor.
Extensive
research on gust factor has been carried out in many part of the world and several expressions have been proposed.
In general these expressions
have
similar forms;
the gust factor is expressed as a function of gust duration
and turbulence
intensity
as proposed by Mackey,
(I).
(t)
In the present study, the following expression
Choi & Lam [7] is used
vt (~--T)Z = I - 0.62(Iz)1"27
in(t/T)
(3)
This expression has been derived from typhoon wind data and also proved to be satisfactory
for subsequent
typhoons.
profile will give the gust profile.
Combining equation 3 with the mean wind However the turbulence
intensity in the
36
equation is also height dependent.
Thus a function of height should be substi-
tuted before it can be of any practical value. The variation of turbulence intensity with height has been studied by many workers.
Most people used an exponential law to describe its variation.
For
typhoon conditions Choi [3] gave an exponent of -0.31, Ho [8] -0.26 and Shiotani
[9] found values ranging from -0.01 to -0.26 for 50 m height and above
and much higher values for levels below 50 m.
Thus it would be natural to
postulate that the absolute magnitude of the exponent decreases with increasing height and it will be equal to zero at gradient height.
Based on the above
argument and using the data of typhoon Freda [3], the following expression is obtained.
-0.25(Zz~) Iz [g
(z_z) Zg
g
(4)
Substituting equation 4 into equation 3 and combining the mean wind profile, one obtains the gust velocity profile.
Ii
(vt)z (vt) a
(zZ____)~
1.27
-- 0.62 Ig
a
(Z/Zg)
in(t/3600)
(5)
Zg-Z a -.32(~g--~---)
1.27 0.62 Ig
Z--Z
--.32 (-Z--~g-)
(Za/Zg)
in(t/3600)
The validity of this expression is checked against the result of Typhoon Georgia (figure 6). Data for the calculation is as follows = 0.19 Zg
= 164 m for 50 year return period wind
t
= 120 sec.
I50, = 0.16
Obtained from on site measurement of Typhoon Freda
Result of the calculation is as listed
level ratio
30/15 m
45/15 m
60/15 m
Velocity ratio
1.098
1.164
1.216
exponent
0.135
0.138
0.141
It can be observed that the computed values of the exponent follow the same trend as in figure 6.
Although the values seem a bit smaller, they may
possibly fall on the asymptotic values for very large velocities.
37
6. DISCUSSION In the present paper, typhoon records for the past twenty years have been studied.
The sample size is not very large and furthermore
data is missing because of instrument malfunction. assumptions
have also been taken.
However,
of 150 m - 160 m has been established
several
in spite of all these, there is a
strong indication that the gradient height during typhoons
is very low.
for open sea terrain.
mately half the value for non-typhoon winds.
some of the critical
In the analysis
Expressions
and gust profile have also been proposed and they compared
A value
This is approxi-
for mean wind profile favourable with on
site measurements. Recently, more advanced equipment has been installed Observatory.
More refined data will be available
in the Hong Kong Royal
for more detail analysis and
perhaps solve the problem for city center type of terrain.
REFERENCES i. 2.
3. 4.
5.
6. 7. 8. 9.
M.J. Mikitiuk, P.N. Georgiou, D. Surry, A.G. Davenport, A study of the wind climate for Hong Kong, BLWT-SSI3-1981. W.H. Melbourne, Hong Kong design wind speed estimates and pressure measurements on building on IL8392 Harbour Road and Fleming Road, Department of Mech. Engg,, Monash University, Australia 1981. E.C.C. Choi, Characteristics of typhoons over the South China Sea, Journal of Industrial Aerodynamics, 3(1978), pp.353-365. S. Mackey, P.K.L. Ko, Spacial configuration of gust, Proceedings, 4th International Conference, Wind effects on buildings and structures, 1975, pp.41-52. T.Y. Chen, Comparison of surface winds in Hong Kong, Royal Observatory Hong Kong, Technical note No. 41, 1975. P. Sachs, Wind forces in engineering, Pergamon, 1978. S. Mackey, E.C.C. Choi, R. Lam, gust factors, Proceedings, seminar on wind loads on structures, Hawaii, 1970, pp.191-202. J.K.C. Ho, The nature of gustiness of typhoon winds and gust loading on buildings, Ph.D. Thesis, University of Hong Kong, 1976. M. Shiotani, Turbulence measurement at the sea coast during high wind, Report No. i, Research Institute of Industrial Technology, Nihon University, 1976.
38
DINES CUP BOTH
-VP-
u I z ~ 4 5 6 MILES L
I
*
I
.__.
Figure I
Location of Meteorology Stations in Hong Kong
3000
3000 fANDA 62
I
3000
:REDA 71
ROSE 71
2OOO
2000
200(
I000
1000
IOOC
•
I
#
25
50
Velocity (m/s)
"
0
Figures 2a-2c
I
25
(m/s)
•
I
50
0
Upper Level Wind Data
~5
(m/s)
i
I
i
39
3000
•
3000
3000 LSIE 75.2
ZLSIE 75.1
)OT 73
2000
200(
42000 E
100{
I000
2~
#o
2~
Velocity (m/s)
•
(m/s)
"
300(
~GNE 78
DORIS 75
2000
2000
200C
i000
1000
I000
o
I
25
•
I
Velocity (m/s)
50
0
Figure 2g-2i
I
5O
Upper Level Wind Data
3000
FLOSSIE 75
I
2~
~o o
(m/s)
Figures 2d-2f
3000
1000
o
•
!
25
(m/s)
50
0
Upper Level Wind Data
!
I
25
5O
(m/s)
40
3000 x Observed --
F i t t e d curve = 0.204 Zg = 1461
2000
/ /
I
I000 /
J i
J
I
I
....
I
.4
.2
.6
i
j
.8
i .0
Normalized Velocity Figure 3
Mean p r o f i l e
from upper l e v e l wind data
1.0
0.8
7
0.6
•
VW___kL
f /
/
Vg
/
d
• • g
/ /
0.4 /
,
/ I I
0.2
/
I
/
/ 0 Figure 4
I
I
5
I0
I
VWL
15/s)(m
i
I
I
20
25
30
V a r i a t i o n o f v e l o c i t y r a t i o w i t h v e l o c i t y a t WL
41 0.6
•|
0.4
~eo
VRO Vg
oo
Oo
o•
•Q
0.2
e •
i
I
I
I
I
5
10
15
20
25
VRO (m/s)
Figure 5 Variation of velocity ratio with velocity at RO 0.5
~le
v200 • v50 x v150 v50
0.4
\\
vlO~0
o
\
V5o
0
~o.3
L~J ~=
o o
o\~-
o •
0 X~O. •
0.2 o o ~
~~.~.Li;~.o~x_ o - o - ' - ~
o
o~ oo~-~7~ ~*
0.i
I
10
V5o (m/s)
15
Figure 6 Variation of power exponent for Typhoon Georgia (from ROHK)
I
2O
31
Elsevier Science Publishers B.V., A m s t e r d a m - - Printed in The Netherlands
GRADIENT HEIGHT AND VELOCITY PROFILE DURING TYPHOONS
E.C.C. CHOI Department of Civil Engineering,
University of Hong Kong, Hong Kong
SUMMARY Velocity profile and gradient height have been objects of main concerns in the study of wind engineering for a long time. For normal (non-typhoon) conditions, the shape of the wind profile as well as the gradient height have been well established for different types of terrain. However during severe tropic cyclones and typhoons such information is still insufficient. This paper tries to answer the two questions (i. Is there a gradient height during typhoons; and if so what is its value? 2. How does the velocity vary with height during typhoons?) by analysising the past twenty years typhoon data. Both the upper level wind data from radar sounding and the surface anemometer wind records have been studied. Result of the analysis shows that the gradient height during typhoons is very low; it is only about half the expected value for monsoon winds. Wind profiles for mean and gust wind have also been established. The finding is found to compare favourable with results obtained by Hong Kong Royal Observatory.
i. INTRODUCTION One of the major factor affecting the magnitude of wind loading on structures is the shape of the wind profile.
It has long been recognized that the hori-
zontal wind speed varies with height and that this variation differs under different atmospheric conditions and for different types of terrain.
Extensive
research has been carried out in the past two decades by meteorologists, tists and engineers eg. Panofsky, Harris, Mackey, Davenport and etc. result, valuable data has been gathered from site and laboratories; relationship between velocity and height has been established. of the vital parameters
scien-
As a the general
And the values
(such as the power exponent ~, gradient height Zg, the
roughness length Zo) has also been found for different types of terrain. ever most of the findings are for strong monsoon wind conditions. mation for typhoon wind is scanty and far from being adequate.
How-
Such infor-
Furthermore, as
the wind structure and turbulence mechanism of the typhoon wind is not quite the same as the monsoon wind, these parameters may have a different value. This paper systematically analysed the typhoon wind records for the past twenty years.
There are two types of data.
The first type is the surface wind
records from ten meteorology stations in Hong Kong. upper-level wind data from radar sounding records.
The second type From the analysis,
is the the
gradient height during typhoon conditions is established and also an expression for the variation of velocity with height is being proposed.
0167-6105/83/$03.00
© 1983 Elsevier Science Publishers B.V.
32
2. T O P O G R A P H Y Before
topography Figure
OF SURFACE
looking
STATIONS
at the typhoon
it is important
the topography
the area
1000 m in height. China.
Towards
There
is fairly
are more
important
Royal O b s e r v a t o r y
portion
their height
south and west. and distance
Kowloon
The anemometer
at
NE
at
NNE 8 km
B e a c o n Hill
(451 m)
at
N
Victoria
(553 m)
at
SW
3.9 km
(530 m)
at
SE
6.1 km
is m o u n t e d
station
of
in Hong Kong;
The h a r b o u r
There are many m o u n t a i n s
(576 m)
Peak
the topography
is less
surrounding
the
are as follows
(601 m)
Parker
of
Ocean.
on top of a 32 m hill at
(city centre).
Peak
Mount
to the Pacific
in the area,
Cairn
Tate's
from 500 m to
as follows.
It is situated
Peninsula
of the stations.
ranging
is open
- RO is the oldest
as 1884.
of K o w l o o n
peaks
the genera] stations.
is linked up to the m a i n l a n d
the area
stations
(RO) H e a d q u a r t e r s
i km to its east,
station;
the area
ones are d i s c u s s e d
its data dates b a c k as early the southern
side,
south and west
than ten m e t e o r o l o g y
three of the more
the locations
rugged w i t h m o u n t a i n
On the north
the east,
to understand
of the m e t e o r o l o g y
] shows a map of Hong Kong together w i t h
In general
than
data,
of Hong Kong and also
at a level
29 m above
7 km
5.6 km
ground and 61 m above m e a n
sea
level. Waglan
Island
the southeast the
highest
- WL station
is situated
coast of Hong Kong.
(WL)
The island
point
from most
direction
D'Aguilar
Peak
The a n e m o m e t e r King's
Park
above MSL) with
w i t h only
sea level.
two small
is at a height
peaks
at about
island
far off
than 0.1 km 2 in area and
The island
to its north. Island
is u n o b s t r u c t e d They are the
(230 m) at N 7.5 km.
of 75 m above mean sea level.
(KP) - KP station the centre
is located of K o w l o o n
in all directions.
on top of a small p l a t e a u Peninsula.
The heights
(64.8 m
It is a city centre and d i s t a n c e s
area
of the nearby
are as follows.
B e a c o n Hill
(451 m)
at
N
Kowloon
Peak
(601 m)
at
NE 6 km
Parker
(530 m)
at
SE 6.8 k m
(553 m)
at
SW 4.8 km
Mount
Victoria
The a n e m o m e t e r
height
From the above data.
is less
(324 m) at NW 6.9 k m and Tung Lung
tall b u i l d i n g s
mountains
is 58 m above m e a n
on top of a small
The other
Peak
4.4 km
is 78 m above m e a n sea level.
discussion, two stations
it is clear
that WL gives
are also selected
because
the best
quality w i n d
RO has the longest
data
3a
history, and KP is the station where balloons for the radar sounding records are being released.
3. UPPER LEVEL WIND DATA Wind measurement is made by using radar sounding technique (RAWIN SONDE and RADIO SONDE ascents).
A balloon released from KP is being tracked by radar.
The wind speed at a given height is measured over an interval of two to three minutes which is chosen so that the midpoint almost coincides with the instant at which the balloon attained that particular height.
Wind speeds are normally
taken at heights of 300 m, 600 m, 900 m, 1500 m, 2100 m, 3000 m and above. Figures 2a to 2i show the RAWIN record of nine occasions of strong typhoon.
It
can be observed that the maximum velocity occurs most frequent at 1500 m level. However from the study of other cases, it is not unusual for the maximum to occur as low as 300 m or as high as 3000 m.
Also there are cases where there
are two or more maxima, one occurring at a low level and the other at 3000 m or above.
Usually the low level one is selected to be the limit of boundary layer
and the higher one is discarded.
Figure 3 shows the average curve for all 48
typhoon occasions being studied.
Before averaging each set of data is being
normallized by the maximum velocity.
A multi variate curvilinear fitting
technique has been used to fit an exponential profile to this curve. exponent and gradient height are treated as unknowns.
The
The result gives an
exponent of 0.204 and a gradient height 1461 m. Unfortunately the above finding cannot be applied to general cases. applies only to KP and that portion of Kowloon Peninsula. earlier, there are several mountains surrounding the site.
It
As pointed out Thus the 300 m and
600 m velocities are strongly affected by topography.
4. SURFACE WIND DATA Since the above approach fails to draw a general conclusion, another method has to be used.
This involves looking at the surface wind speed in conjunction
with the upper level wind data.
The WL data has been selected for this purpose.
Wind record of WL as well as its topographic features has been studied by the author and others [1,2].
It seems that the wind speed recorded by the anemome-
ter at WL approaches the free wind velocity. topographic effect.
There is certain amount of
But the effect is small for mean wind speed and even smaller
for gust speed. In order to compare the surface wind velocity with the gradient wind velocity, the following assumptions are made.
The maximum wind velocity recorded during
a set of RAWIN ascent is taken as the gradient velocity.
The same value of
gradient velocity is assumed as the wind blows over WL which is approximately 20 km from KP.
Thus the mean wind speed recorded at WL can be compared with
34
the gradient wind velocity. of tlle gradient this ratio. believe
speed,
When the WL velocity
(VwL) is expressed
there shows a considerable
The maximum
is 1.0 and the minimum
variation
is 0.35.
for the site).
seems to indicate
relationship
that certain
exists between VWL.
the ratio and VWL.
Although
there is a fair
(which is expected with all the assumptions
clear trend of increasing
ratio with increasing
further postulated
that the ratio would approach
simple exponential
expression
being made),
VWL can be observed.
a
It is
unity at very high VWL.
has been used to represent
curve fitting the following equation
to
(which would be the
A closer look at these ratios
Figure 4 shows a plot of these ratios against amount of scatter
of
It is difficult
that all these points scatter about a single value
case if Zg and ~ are constants
as a ratio
in magnitude
the variation
A
and after
is established.
6.64 VWL
VWL =
e
(I)
Vg
Equation
(l) is also plotted
on figure 4 in dotted line.
ential law is used to represent be evaluated
from equation
the velocity
profile,
Now,
if the expon-
the gradient height
can
i.
6.64
~VwL Zg = ZWL
where
. e
(2)
ZWL
is the anemometer
Zg
is the gradient height
height at WL (74.8 m)
is the power exponent In order to evaluate a)
Zg, one must first
fix the magnitude
The power exponent ~ - In the past decade,
typhoon wind characteristics exponent
for the variation
atmosphere
measurement
b)
and research on
has been carried out by many workers.
of velocity with height at the lower
has been established
and published.
CHOI [3]
from 0.18 to 0.28 and Mackey & Ko [4] postulated present
of ~ and VWL.
Power
i00 m of the
found values
a mean value of 0.19.
ranging In the
study a value of 0.19 is used.
Velocity at Waglan VWL - As can be observed
the value of VWL the lower will be Zg. ponding to certain value of VWL.
from equation
Thus one can only solve
In wind loading applications,
return period and 100 years return period winds are commonly analysis following
for Zg correstile 50 year
used.
of the Waglan data by the Hong Kong Royal Observatory results.
2, the higher
Extreme
[5] gives the
35
VWL (50 year return period)
= 44 m/s
VWL (i00 year return period)= 48 m/s With these figures the gradient height during typhoons over open sea is 164 m for 50 year return period wind and 154 m for i00 year return period wind. The velocity at RO has also been compared with the RAWIN data. VRo/Vg
When the ratio
is plotted against VRO (figure 5) the same trend of increasing
with increasing velocity can also be noted.
Thus it is expected
ratio
that the
gradient height during typhoons also decrease with increasing wind velocity over city centre area. mountains
However since the RO data is affected strongly by surrounding
and buildings,
a detail study has not been carried out.
5. GUST VELOCITY PROFILE Most of the expressions A theoretical approach
describing
Because of its simplicity, exponent,
in nature.
under typhoon conditions.
the power law is often being used.
As for the
some postulated that the value for gust should be half that for mean
wind [6]; however there are others value approaching Observatory
[4] who obtained from on site measurement
that of the mean wind value.
in 1980 carried out an investigation
Wind velocity at four different
levels
150 ft and 200 ft) were studied. the velocities level.
the gust profile are empirical
is still lacking especially
a
In fact the Hong Kong Royal on Typhoon Georgia
(May 1980).
15 m, 30 m, 45 m and 60 m (50 ft, I00 ft,
Two minute running means were calculated
and
of the higher levels were compared with that of the 15 m (50 ft)
From these ratios,
shown in figure 6.
the power exponent was computed.
As can be observed,
Their finding is
firstly the power exponent
increases
with height and secondly it decreases with increasing wind speed (although it seems to approach a constant value at higher velocities). to generalize on the value of the exponent.
Thus it is difficult
In the present paper the gust
profile is to be derived from the mean wind profile. Gust speed is related to the mean wind speed by the gust factor.
Extensive
research on gust factor has been carried out in many part of the world and several expressions have been proposed.
In general these expressions
have
similar forms;
the gust factor is expressed as a function of gust duration
and turbulence
intensity
as proposed by Mackey,
(I).
(t)
In the present study, the following expression
Choi & Lam [7] is used
vt (~--T)Z = I - 0.62(Iz)1"27
in(t/T)
(3)
This expression has been derived from typhoon wind data and also proved to be satisfactory
for subsequent
typhoons.
profile will give the gust profile.
Combining equation 3 with the mean wind However the turbulence
intensity in the
36
equation is also height dependent.
Thus a function of height should be substi-
tuted before it can be of any practical value. The variation of turbulence intensity with height has been studied by many workers.
Most people used an exponential law to describe its variation.
For
typhoon conditions Choi [3] gave an exponent of -0.31, Ho [8] -0.26 and Shiotani
[9] found values ranging from -0.01 to -0.26 for 50 m height and above
and much higher values for levels below 50 m.
Thus it would be natural to
postulate that the absolute magnitude of the exponent decreases with increasing height and it will be equal to zero at gradient height.
Based on the above
argument and using the data of typhoon Freda [3], the following expression is obtained.
-0.25(Zz~) Iz [g
(z_z) Zg
g
(4)
Substituting equation 4 into equation 3 and combining the mean wind profile, one obtains the gust velocity profile.
Ii
(vt)z (vt) a
(zZ____)~
1.27
-- 0.62 Ig
a
(Z/Zg)
in(t/3600)
(5)
Zg-Z a -.32(~g--~---)
1.27 0.62 Ig
Z--Z
--.32 (-Z--~g-)
(Za/Zg)
in(t/3600)
The validity of this expression is checked against the result of Typhoon Georgia (figure 6). Data for the calculation is as follows = 0.19 Zg
= 164 m for 50 year return period wind
t
= 120 sec.
I50, = 0.16
Obtained from on site measurement of Typhoon Freda
Result of the calculation is as listed
level ratio
30/15 m
45/15 m
60/15 m
Velocity ratio
1.098
1.164
1.216
exponent
0.135
0.138
0.141
It can be observed that the computed values of the exponent follow the same trend as in figure 6.
Although the values seem a bit smaller, they may
possibly fall on the asymptotic values for very large velocities.
37
6. DISCUSSION In the present paper, typhoon records for the past twenty years have been studied.
The sample size is not very large and furthermore
data is missing because of instrument malfunction. assumptions
have also been taken.
However,
of 150 m - 160 m has been established
several
in spite of all these, there is a
strong indication that the gradient height during typhoons
is very low.
for open sea terrain.
mately half the value for non-typhoon winds.
some of the critical
In the analysis
Expressions
and gust profile have also been proposed and they compared
A value
This is approxi-
for mean wind profile favourable with on
site measurements. Recently, more advanced equipment has been installed Observatory.
More refined data will be available
in the Hong Kong Royal
for more detail analysis and
perhaps solve the problem for city center type of terrain.
REFERENCES i. 2.
3. 4.
5.
6. 7. 8. 9.
M.J. Mikitiuk, P.N. Georgiou, D. Surry, A.G. Davenport, A study of the wind climate for Hong Kong, BLWT-SSI3-1981. W.H. Melbourne, Hong Kong design wind speed estimates and pressure measurements on building on IL8392 Harbour Road and Fleming Road, Department of Mech. Engg,, Monash University, Australia 1981. E.C.C. Choi, Characteristics of typhoons over the South China Sea, Journal of Industrial Aerodynamics, 3(1978), pp.353-365. S. Mackey, P.K.L. Ko, Spacial configuration of gust, Proceedings, 4th International Conference, Wind effects on buildings and structures, 1975, pp.41-52. T.Y. Chen, Comparison of surface winds in Hong Kong, Royal Observatory Hong Kong, Technical note No. 41, 1975. P. Sachs, Wind forces in engineering, Pergamon, 1978. S. Mackey, E.C.C. Choi, R. Lam, gust factors, Proceedings, seminar on wind loads on structures, Hawaii, 1970, pp.191-202. J.K.C. Ho, The nature of gustiness of typhoon winds and gust loading on buildings, Ph.D. Thesis, University of Hong Kong, 1976. M. Shiotani, Turbulence measurement at the sea coast during high wind, Report No. i, Research Institute of Industrial Technology, Nihon University, 1976.
38
DINES CUP BOTH
-VP-
u I z ~ 4 5 6 MILES L
I
*
I
.__.
Figure I
Location of Meteorology Stations in Hong Kong
3000
3000 fANDA 62
I
3000
:REDA 71
ROSE 71
2OOO
2000
200(
I000
1000
IOOC
•
I
#
25
50
Velocity (m/s)
"
0
Figures 2a-2c
I
25
(m/s)
•
I
50
0
Upper Level Wind Data
~5
(m/s)
i
I
i
39
3000
•
3000
3000 LSIE 75.2
ZLSIE 75.1
)OT 73
2000
200(
42000 E
100{
I000
2~
#o
2~
Velocity (m/s)
•
(m/s)
"
300(
~GNE 78
DORIS 75
2000
2000
200C
i000
1000
I000
o
I
25
•
I
Velocity (m/s)
50
0
Figure 2g-2i
I
5O
Upper Level Wind Data
3000
FLOSSIE 75
I
2~
~o o
(m/s)
Figures 2d-2f
3000
1000
o
•
!
25
(m/s)
50
0
Upper Level Wind Data
!
I
25
5O
(m/s)
40
3000 x Observed --
F i t t e d curve = 0.204 Zg = 1461
2000
/ /
I
I000 /
J i
J
I
I
....
I
.4
.2
.6
i
j
.8
i .0
Normalized Velocity Figure 3
Mean p r o f i l e
from upper l e v e l wind data
1.0
0.8
7
0.6
•
VW___kL
f /
/
Vg
/
d
• • g
/ /
0.4 /
,
/ I I
0.2
/
I
/
/ 0 Figure 4
I
I
5
I0
I
VWL
15/s)(m
i
I
I
20
25
30
V a r i a t i o n o f v e l o c i t y r a t i o w i t h v e l o c i t y a t WL
41 0.6
•|
0.4
~eo
VRO Vg
oo
Oo
o•
•Q
0.2
e •
i
I
I
I
I
5
10
15
20
25
VRO (m/s)
Figure 5 Variation of velocity ratio with velocity at RO 0.5
~le
v200 • v50 x v150 v50
0.4
\\
vlO~0
o
\
V5o
0
~o.3
L~J ~=
o o
o\~-
o •
0 X~O. •
0.2 o o ~
~~.~.Li;~.o~x_ o - o - ' - ~
o
o~ oo~-~7~ ~*
0.i
I
10
V5o (m/s)
15
Figure 6 Variation of power exponent for Typhoon Georgia (from ROHK)
I
2O