Field Measurement and Experimental Study of Wind Speed Profile During Thunderstorms

Journal of Wind Engineering

and Industrial Aerodynamics 92 (2004) 275–290

Field measurement and experimental study ofwind speed profile during thunderstorms

Edmund C. C. Choi*

School of Civil & Environmental Engineering, Nanyang Technological University, Nanyang Avenue,

Singapore 639798, Singapore

Received 18 September 2003; received in revised form 2 December 2003; accepted 8 December 2003

Abstract

This paper investigates the variation of wind velocity with height during thunderstorms.

Wind velocities during thunderstorms are measured at five levels on a 150 m tall tower.

Velocity profiles for more than 50 thunderstorms are studied. In the analysis, these profiles are

classified into four types according to the profile shape and the height of the highest wind

speed. It is observed that wind profiles during thunderstorms are affected by several factors

with the major ones being distance from the thunderstorm cell centre, intensity of the storm

and ground roughness. Experimental studies on the simulation of thunderstorm wind using an

impinging jet are also carried out to further investigate the effect of these parameters.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Thunderstorm; Downburst; Velocity profile; Wind speed; Wind direction; Terrain effect

1. Introduction

Thunderstorm wind has been a topic of interest to engineers in the past decade.Recently, there are more indications that thunderstorm wind plays a significant partin defining the strong wind characteristics of a place [1,2]. Many investigations arecarried out to study the wind characteristics during a thunderstorm; of particularinterest is the variation of wind speed with height of a downburst which as defined byFujita [3] as ‘‘a strong downdraft which induces an outburst of damaging winds onor near the ground’’. Such investigations included field observations and monitoring[4–6] as well as physical, empirical and numerical modelling [7–10].

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*Tel.: 67905273; fax: 67910676.

E-mail address: [email protected] (E.C.C. Choi).

0167-6105/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jweia.2003.12.001

1.1. Thunderstorm

Thunderstorms are common occurrence around the world. There are nearly 2000thunderstorms in progress at any time over the earth’s surface. Every thunderstormis produced by a cumulonimbus cloud and is always accompanied by lightning andthunder. Although its occurrence is worldwide, the frequency of thunderstormoccurrence varies greatly in different areas. Most thunderstorms are found intropical and sub-tropical regions. A thunderstorm usually lasts no more than 1 h andit begins when a parcel of air, either warmer than the surrounding or being pushedup by colder encroaching air such as a cold front, begins to rise. This unstable up-draught forms the embryo of the thunderstorm. If the rising air is sufficiently moist,condensation will occur to form cloud droplets. A large amount of moisturecondenses and precipitation particles begin to grow as the cell continues to build up.The latent heat released during condensation adds buoyancy to the vapour. Therising air stream forms an updraft that is strong enough to keep the water dropletsand ice crystals suspended in the clouds. The continuous release of latent heatsupplies energy to the system and further accelerates the updraught. As precipitationgathers momentum, the frictional drag induces downdraught. During the maturestage of the thunderstorm, downdraught intensifies. This is the most active period ofa thunderstorm with gusty winds and heavy precipitation. The leading edge of thecold air spreading out over the ground resulting from the downdraft is called the gustfront. In severe thunderstorms, the strong downdraft develops into a downburst,which diverges horizontally as an outburst of destructive winds upon reaching theground. The gust front usually moves ahead of the thunderstorm. Unlike themonsoon wind or even the typhoon wind where the wind moves in the horizontaldirection over a substantial stretch of ground, winds from thunderstorm rushvertically down and spread outwards. Thus, the ground roughness has little effect onthe wind. The wind characteristics of thunderstorm wind, e.g. the mean wind profile,gust profile, gust factor and turbulence intensity, are expected to be quite differentfrom those of the large-scale winds.

2. Field measurement

Singapore is an island situated close to the equator. In general, the climate ofSingapore is made up of two monsoon seasons, northeast monsoon from Decemberto March and southwest monsoon from June to September, and frequentthunderstorms during the inter-monsoon periods. There are more than 100thunderstorm days in a typical year. Thus, Singapore is an ideal place forthunderstorm research.

2.1. Tuas wind measuring station

A wind measuring station is set up at the western end of the Singapore island. It islocated at the Tuas industrial area. The terrain is reasonably flat and surrounded by

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almost homogeneous industrial buildings with the open sea about 2–4 km in the Sand W directions (Fig. 1). Towards north and east, there are isolated taller buildingsfurther away. Three-component ultra sonic high-frequency anemometers are used tomeasure the wind speed. These anemometers are mounted at five levels on a tower, atheights of 35.7, 62, 80, 125, and 152 m: Temperature and humidity sensors are alsoinstalled at these levels. Normally, statistics (mean, r.m.s., maximum, minimum) ofwind speed and direction are recorded continuously every 10 min: When triggeredinto high-speed mode (1 min mean speed exceeding 8 m=s) wind speed and directiondata are recorded at 50 Hz:

2.2. Wind systems

In the present study, weather events are classified into two categories, thethunderstorm (TS) and non-thunderstorm (NTS) events. TS events are identified bythe thunder, inspection of the 10-min mean wind speed, r.m.s. speed, wind direction,temperature, humidity, and pressure data. A typical TS event is characterized by asudden increase in wind speed, a sudden change in wind direction, high r.m.s. value,a rapid drop of temperature and an increase in humidity as shown in Fig. 2. Radarplots are also used to obtain more information and further confirmation of the event.

NTS events are selected from occasions of fine days with a high wind speed. Theseare the monsoon events. It was found that the NE monsoon winds are slightlystronger than that of the SW monsoon winds. Studies on the variation of wind speedwith height are carried out for over 90 NTS events. The averaged power exponents ofthe power-law profile for winds coming over land (N to E) is found to be 0.22 andover sea (W to S) is 0.17. These values reflect the terrain roughness and are used as

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Fig. 1. Site of Tuas wind measuring station.

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datum for TS profile investigation. Furthermore as reported by Choi [2], studies ofextreme wind in Singapore showed that the 10-min mean and the 3-s gust extremewind are governed by small scale storms. The 50-year return wind speeds arerespectively 17 and 29 m=s for the 10-min mean and the 3-s gust. These give anoverall picture of the wind climate in Singapore.

2.3. Thunderstorm wind

While NTS events are considered to be large scale, with winds move more or lesshorizontally over the ground, and wind characteristics largely governed by terrain

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Fig. 2. Typical plots of a thunderstorm event.

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roughness; thunderstorm winds rush vertically down and when the downdraftreaches the ground, it spreads out forming the out-flow and the gust front. It isgenerally accepted that at the gust front and out-flow regions wind speed increaseswith height from the ground upwards, reaching a peak at a height of about 100–200 m and the wind speed starts to decrease. While this is a general statement, it isunderstood that the shape of the wind profile is affected by many factors, e.g. theintensity of the storm, the size of the storm cell, distance of the profile from the cellcentre and the ground roughness.

Out of the 50 over TS events 36 have been selected for analysis in the presentinvestigation. For the thunderstorm velocity profile study, 5-min mean wind speedsare calculated. A 5-min mean is used as TS events are very localized in space andtime and a 10-min mean is observed to be too long to reflect the high windcharacteristics. The variations of the 5-min mean wind speed with height (at the timewhere the 5-min running mean gives the highest value for the storm) during TSevents indicate different shapes of the wind profile. The highest wind speed isobserved sometimes at the lower levels, but sometimes also at the highest level. Usingthe exponent obtained from the NTS events and fitting a power law profile to thedata point at the bottom level and also the data point at the top level, leads to theclassification of the profile shape into four types. For Type I, all data points fitreasonably well with the power profile of the NTS exponent, and with the highestwind speed at the top level. For Type II, the higher wind speeds occur at the lowerlevels. Wind speeds of all levels fall within the NTS power exponent profile fitted tothe lowest level data point (Fig. 3).

Type III is similar to Type II except that wind speeds at other levels exceed theprofiles fitted to lowest level as well as the top level data points. For Type IV, thehighest wind speed is recorded at the top level. All wind speeds are less than theprofile fitted to the top level data point but exceed the profile fitted to the bottompoint.

2.4. Case studies

To gain further insight to the different profiles, the movement and location of thethunderstorm cell(s) is studied. In general, there is a diversity of variations of thethunderstorm cell movement and location within each type of profile; however,certain trend of variations can be observed. The following are discussions of some TSevents.

On 23 March 2001, storm clouds were observed over the southern part ofSingapore. In Fig. 4(a), the radar picture shows the more intense rain cloud is morethan 10 km to the south east of Tuas Station. The velocity profile is shown in Fig.4(b) and it belongs to Type I. The envelope of the maximum gust (dotted line) is alsoshown in the figure.

As shown in Fig. 5, the radar picture showed that on 25 December 2000 the stormclouds were located almost directly over Tuas. The radar picture was recorded at15:46. The velocity profile for the higher wind speed at 16:02 showed a Type IIvelocity profile.

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For event 29 January 2001, the general weather system was observed to moveslowly from east to west. Fig. 6(a) shows the radar picture at 11.21 am with the moreintense rain cloud almost directly over the Tuas station. Higher wind speedsoccurred at 11.29 and the velocity profile is shown in Fig. 6(b). It can be seen to be aType III profile.

On 21 May 2002, storm clouds were approaching Singapore from the west asshown in Fig. 7. Velocity profile for the higher wind speed which occurred at 4.43 ispresented in the figure. The profile is classified to be Type IV.

Sometimes the situation is more complicated. Fig. 8 shows the radar picture for 8April 2001 at 16.31 and 17.01. From the wind records, the general wind directionbefore the storm was from the south with a wind speed of about 4 m=s recorded atTuas. Wind speed started to decrease at 16.00 and decreased to almost zero at 16.30

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Fig. 3. Wind profiles during TS events.


(at which time the wind direction started to change to the north). After this the windspeed started to increase and reached values of more than 8 m=s at 16.50 before itstarted to decrease again. Fig. 9(a) shows the variation of the 10-min mean windspeed and mean direction with time for the event. For this event, the storm cell wasmore than 15 km away from the Tuas station. The velocity profile, also shown inFig. 9(b), belongs to Type IV.

It seems for this event, a storm cell occurring at the north of Tuas during aboundary layer wind blowing from the south. The outflow from the storm, reachingTuas from the north, counter acted the opposing boundary layer wind andsubsequently with the northerly out flow velocity bigger than the southerly wind, thewind direction changed to north. This event has a Type IV profile with the higher

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Fig. 4. (a) TS 23Mar01 Radar picture and (b) 5-min mean and gust profiles.


recorded wind speed at the top level. This might be due to that the highest elevationof measurement is not high enough or some other unknown factor.

3. Experimental studies

From the field measurement reported in the previous section, it seems the windprofile observed for a thunderstorm with cell centre far away from the monitoring

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Fig. 5. (a) TS 25Dec00 Radar picture and (b) 5-min mean and gust profiles.


location approached that of the large-scale wind profile. However, the wind profileobserved for a thunderstorm with cell centre close to the monitoring locationexhibited the feature where higher wind speeds were observed at the lower elevations.In order to investigate how the distance of the wind profile from the cell centreaffects its shape, velocity measurements with experimental thunderstorm simulationwere carried out.

Various techniques were used in the experimental simulation of thunderstormdowndraft, e.g. Holmes [11] using an impinging jet and Mason et al. [12] using apulsed jet. Other works using similar method was carried out by Wood et al. [8],

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Fig. 6. (a) TS 29Jan01 Radar picture and (b) 5-min mean and gust profiles.


Letchford and Illidge [13] and the study of pressure distribution due to downburst onobstacles by Chay and Letchford [14,15]. In the present study, a continuous streamof air generated by a blower unit was directed to a flat plane through a long circularpipe of 100 mm diameter. The velocity at the jet exit was more or less uniform butcurved slightly close to the pipe wall. The turbulence intensity was about 5%. Theexperiment was carried out with the jet exit placed at distances five times to eighttimes the jet diameter from the testing surface. Velocity at various heights from theplane of the surface and at various distance from the jet axis were measured usingtwo-dimensional hot wire anemometer. The jet velocity at the exit of the pipe wasalso monitored. The study was carried out with three different types of surfaceroughness, ply-wood board (smooth), carpet (rough) and coarse grain particles (veryrough) fitted to the plane surface (Fig. 10).

Horizontal velocity profiles for different testing conditions are shown in Figs.11(a),(b) and (c), respectively, for the smooth, carpet and coarse grain surfaces. Thehorizontal velocities normalized by the jet exit speed are plotted for locations withdistances 1D (jet diameter) to 5.5D from the jet axis. It can be seen that the peakwind speed decreases with increasing distance and the height at which the peak

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Fig. 7. (a) TS 21May02 Radar picture and (b) 5-min mean and gust profiles.


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Fig. 8. (a,b) TS 08Apr01 radar pictures.


occurs increase with distance from the jet axis. From the shape of the profile, it canbe seen that close to the jet axis, the ground roughness effect is small. At largerdistances from the jet axis, the effect of ground roughness is stronger and velocitygradually develops into the boundary layer profile. It can also be seen that therougher the surface the higher is the elevation of the peak wind speed. Fig. 12 shows

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Fig. 9. (a) TS 08Apr01 wind speed, wind direction and (b) velocity profile.


a plot of the variation of the height of peak wind speed with distance to the jet axis.It clearly indicates that height of the peak wind speed increases with the distancefrom the jet axis and also increases with ground roughness.

Further study was carried out on the profiles. A power-law profile was fitted to theportion of the profile below the peak wind speed. The power exponents obtained forprofiles at various distances from the jet axis are listed in Table 1. Although thevalues of the exponent are scattered, an increasing trend of value with distance canbe observed. The experimental findings confirm the on-site observations that as themeasuring point gets further away from the thunderstorm cell, the observed windprofile is more influenced by the terrain effect and the peak wind speed (of theprofile) is pushed to higher elevations.

4. Discussion

Field measurement of wind speed at different heights during thunderstorms isreported in this paper. Wind profiles obtained for TS events are compared with windprofiles for large-scale wind over the site. For discussion, the TS profiles are classifiedinto four types based on the comparison. Investigations are carried out in attempt tofind out the reasons for the different types of profile. Study on the wind speed,direction in conjunction with the radar picture for each TS event reviews certaintrend of variations. In general, Type I profile is observed when the storm cell centre isfar away (10 or more kilometres) from the measuring station. The out flow has tomove over a considerable fetch before reaching the measuring station and thus isstrongly affected by terrain effect. When the storm cell is very close to the station, aType II or Type III profile is observed. The profile is less affected by terrainroughness and more dominated by the out spread of the downdraft. The occurrencesof Type IV profiles are more random, sometimes they are further away from themeasuring station, but other times they are close. It seems that Type IV profiles arefrom the more intense storms. This may result in a thicker out flow layer with thehighest wind speed occurring at or above the top observation level.

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Fig. 10. Experimental set-up for thunderstorm wind simulation.


Another factor that needs to be considered is the wind condition just before and atthe arrival of the gust front. The measured velocity reflects the vector sum of thewind speed due to the thunderstorm downburst and that of the overall weather

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Fig. 11. Velocity profile for (a) smooth surface; (b) carpet surface; (c) coarse grain surface.


system. Preliminary observations also indicate that with the storm cell moving withthe weather system, the measured wind profile suggests that wind speeds are thevector sum of the thunderstorm outflow wind and the boundary layer profile wind ofthe weather system, and not the translational speed of the storm cell.

To gain further insight to the field measurements, physical simulation ofthunderstorm wind using an impinging jet is also carried out. The experimentalresult confirms that close to the jet axis (i.e., cell centre of thunderstorm storm) theheight of the peak wind speed of the profile is lower and the shape is less affected byterrain roughness. For wind profiles monitored at a larger distance from the jet axis,the elevation of the peak wind speed becomes higher and the profile is more affectedby terrain roughness effect. The profile gradually develops into the boundary layerprofile appropriate for that terrain roughness. These are in line with the fieldobservations of thunderstorm wind profiles.

Acknowledgements

The author wishes to express his gratitude to Meteorological Services Singaporefor providing radar image data for this study.

References

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for the next generation wind tunnel, J. Wind Eng. Ind. Aerodyn. 90 (2002) 1415–1433.

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Fig. 12. Variation of the height of peak wind speed with distance to jet axis.

Table 1

Power-law exponent for profiles at different distances from the jet axis

Distance to jet axis (jet diameter) 1D 2D 3D 4D 5D 5.5D

Smooth NA NA NA 0.046 0.059 0.074

Carpet NA 0.110 0.085 0.111 0.085 0.199

Coarse grain NA 0.134 0.169 0.162 0.209 0.198


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Field Measurement and Experimental Study of Wind Speed Profile During Thunderstorms

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