52

INTRODUCTION

(a) G E O G R A P H I C A L ASPECTS

The Manawatu Gorge is a narrow passagecut by the Manawatu River through the mainmountain chain o f the North Island o f NewZealand. The mountain chain is relatively lowin this area and although it forms a ridge over300m high (Fig. 1) few parts exceed 500m. Incontrast, the Ruahine Range to the north andthe Tararua Range to the south rise to well over1000m

The 300m contour in Fig. 1 is somewhatirregular because of small watercourses whichdrain off the ridge. The crest of the ridge is alsoirregular i n places. However, i n comparisonwith most mountain terrain in New Zealand theridge i s qui te simple w i t h the prevailingwestnorthwest wind approaching a t almostright angles to i t over nearly flat land. Thesecharacteristics are of considerable interest forthey allow the application of two-dimensionalflow models to the ridge and approach path.(b) I N D U C E M E N T FOR STUDY

In 1981, t h e Manawatu-Oroua ElectricPower Board requested the cooperation of theNew Zealand Meteorological Service in makingsome wind measurements at a potential wind

MANAWATU WIND STUDIES:SPEED-UP OVER A RIDGE

S. J. ReidNew Zealand Meteorological Service

Weather and Climate (1985) 5: 52-63

ABSTRACT

The area around the Manawatu Gorge has a high frequency o f strongwinds. The addition of 4 temporary anemometers to a network of another 4instruments which have operated over a longer period has allowed the windvariations due to terrain to be investigated in some detail. The speed-up due toa ridge lying across the most frequent airflow direction has been obtained fromthese observations and compared to theoretical calculations. The agreementappears satisfactory.

power site within the Board's region. A site onthe ridge, about 12 km south of the Gorge, wasselected and an anemograph was installed. Inaddition, the Meteorological Service undertookto investigate the spatial variations of the windclimate through t h e use o f short-periodanemometer installations t o supplement thelonger-term stations. This was partly to refineknowledge of the extent of the high wind areasand partly to develop techniques which couldbe applied to similar studies in the future.(C) W I N D POWER ASPECTS

The wind power which can be obtained froma site depends primarily on the characteristicsof the turbine. The height above ground andthe area swept by the blades are especiallyimportant. I n addit ion, t h e w i n d speedthreshold below which there is no output, thespeed at which the power generated reaches thecapacity of the system (the rated power), andthe cut-out speed where the machine shutsdown to prevent damage, are parameters moreof the turbine than of the wind climate. Wind-power technology is advancing rapidly at themoment and assumptions about the magnitudesof the parameters would be quickly out of date.Nevertheless, useful work o f a prospectingnature can be conducted because i t seemsreasonably certain that areas with high wind

Manawatu Wind Studies

• Palmerston North

Whitson Farm

ManawatuGorge

0

Fig. 1: Location map o f the area studied and the positions of anemometers used.

Pahiatua Track

300m Contour

• Owen Farm

North Range Road N

North Range Road S

15 km 1 0

53

54

speeds are desirable for wind power because therated power will then be available for a largerproportion of the time.(d) SCOPE OF THE PRESENT REPORT

For reasons given in the last paragraph, i tappears that any work on wind power whichcan be presently undertaken i s on ly o f apreliminary nature. However, in addition tomeasuring wind at more places to provide basicwind climate information for wind energy andother applications, i t is clearly important toisolate the factors which affect the wind speedon a ridge, because o f the enhanced windspeeds found in such situations. The presentstudy provides an opportunity to compare thewind speeds predicted i n a comparativelysimple situation w i th those observed. Thefactors, i f known, can be used not only toidentify the particular characteristics o f a sitewhich make it desirable for wind power or someother application, but also to estimate winds atsites which are between wind stations.

INSTRUMENT D E P L O Y M E N TThe locations o f the anemometers used in

this study have been marked in Fig. 1 . I naddition to the instruments on the ridge, windrecordings have been collected for a number ofyears at the airfields at Ohakea and PalmerstonNorth. Both lie in the direction that is usuallyupwind of the ridge. Ohakea is about 40 kmfrom the ridge and Palmerston North about 10km. T h e exposure o f the anemometer a tOhakea departs from the ideal because of theproximity o f a radar antenna and the limitedclearance between the sensor and the roof ofthe building on which the instrument stands.Buildings t o t h e northeast a n d a smal lescarpment to the west may also affect thereadings. At Palmerston North the exposure ismostly open but increased roughness over thecity lying to the south and southeast from theanemometer may reduce the speeds from thesedirections a little, and airport buildings andareas of trees in other directions may also havesome effect.

Wind data for Wharite Peak, at the southernend of the Ruahine Range, have been obtainedsince 1966. O f the earlier measurements themost useful were obtained from a position 6mabove the ground on the western side o f thecrest. Since 1979 the anemometer has been ontop of a 122m television transmitter tower, the

Manawatu Wind Studiesbase of which is itself 914m above sea level. Thewind data from these two positions may beviewed as representing separate sites eventhough the horizontal distance between thepositions is only 50m.

The four temporary anemometer stationswhich were set up to supplement the othermeasurements operated during t h e periodJanuary to March 1983. They were at the placeslabelled: Owen Farm, Nor th Range RoadNorth, North Range Road South, and WhitsonFarm in Fig. 1.

At Owen Farm the anemometer was locatedon river flats near the eastern end of the Gorge.It is not known to what extent the Gorge hasaffected the winds recorded. However, i t isthought that the larger part of the flow past theanemometer is across the range and the windsmeasured are similar to the winds which wouldbe measured on other sites just in the lee of therange.

The North Range Road anemometers werelocated beside a road running northward alongthe range from the Pahiatua Track anemometerand descending the side of the ridge to near theeastern end o f the Gorge. The northernmostanemometer site was on a part of the ridge witha generally smooth profile (Fig. 2). The landclose to the anemometer was flat (Fig. 3) so thatlocal effects would have been minimal. A t thesouthern site, the anemometer was just on thewestern side of the crest of the ridge (Fig. 4) atthe head of a shallow gully leading up from thewest. This site was approximately equidistantfrom the Pahiatua Track and the northernanemometer on North Range Road.

The final site was on the western slopes ofthe range a t Whitson Farm, o n a localpromontory. To the south a number of largervalleys cut in to these slopes producing anincreased steepness t o westerly airstreamsapproaching the range. Fig. 5 shows the terrainnear the Pahiatua Track anemometer as anexample. Fig. 6 shows the Pahiatua Trackanemometer on the crest of the range.

ANALYSIS OF T H E D ATA

The chart records from the anemometerswere analysed manually, and for each hour amean speed and direction were determined. Theprocedures f o r analysing t he charts wereslightly different f o r t h e f o u r temporary

Manawatu Wind Studies 5 5

Wharite Peak. North Range Road North.

Fig. 2: General views of the ridge looking northeast towardsNorth Range Road North.

56 M a n a w a t u Wind Studies

,

Fig. 3: North Range Road North anemometer site (lookingnorth).

Fig. 4: North Range Road South anemometer site (lookingnortheast).

Manawatu Wind Studies 5 7

North Range Road South Pahiatua Track

Fig. 5 : View o f the ridge from the west showing thepositions of some of the anemometers.

Fig. 6: Pahiatua Track anemometer site (looking east).

58

CD C:) C : ) C D c )M C N0]3cIS ONIM

C )

C;)CD

Manawatu Wind Studies

0 0

021LU1.1-\ 0

CZ)

COLULI_

I—< 7: Variations of wind speed at the project anemometers over a period of 5 days during the study.

NORTH RANGE S

NORTH RANGE NPALMERSTON N

Manawatu Wind Studies

stations than f o r the longer-term stationsbecause different anemometers were installed.For the Lambrecht anemographs at the formersites an average over one hour was obtained,whereas a t t h e l a t t e r p laces M u n r oanemographs were used and ten-minute meansderived. The averages over ten-minute periodsbefore each hour were used as hourly values forthe latter instruments. These different practicesare not expected to produce large systematicdifferences in the final data.

An i n i t i a l comparison w a s made b yconsidering the variations of wind speed over aperiod of 5 days in February 1983 (Fig. 7). Theindividual hourly wind speeds are averagedover 5-hour periods and plotted as a function oftime. The winds at 600m from radar tracking ofballoons released at Ohakea are also shown.During most of the period winds were very highand from the west or northwest. The speeds atthe Pahiatua Track instrument were par-ticularly high, exceeding the 600m wind a tOhalcea for some of the time.

Although the major features in Fig. 7 appearsimilar at all stations, the details do vary. Thusthe speeds at the Whitson Farm and NorthRange Road anemometers are quite similar butsometimes one is higher and sometimes theother. Similarly, the speeds a t PalmerstonNorth Airport and Owen Farm do not bear aconstant relationship to each other or to theridge-top anemometers. This behaviour is notunexpected and several reasons such as stabilityeffects, mesoscale weather systems, a n dpropagation effects could contribute. Randomvariations can be easily eliminated by averaginga large enough sample. However, stabilityeffects a r e l i ke ly t o produce systematicdifferences depending on the time of day andweather conditions.

A more comprehensive comparison using allavailable data was also performed. This was thebasis for the analysis of the observations in thenext section. I t was carried out on computerfiles o f the data, calculating differences o fspeeds and directions for simultaneous winds atpairs o f stations. The means and standarddeviations o f the differences were formed forthe different classes of speed and direction andalso for classes of all speeds for each directionand all directions for each speed. The meansand standard deviations were determined foreach of four six-hour periods during the day aswell as for all hours, and seasonally as well asover the year as a whole.

59

SPEED-UP OVER A RIDGE

(a) DEFINITIONS AND THEORYSpeed-up is the ratio of the wind speed over

a hill-top to that found at the same absoluteheight over flat ground upwind of the hill at asufficient distance for the flow to be unaffectedby the hill. The winds must be measured in thesame vertical plane and the flow pattern in thisplane can be used to describe the f low two-dimensionally. This is particularly appropriatein the case of flow over a ridge, for which thereis little divergence out of the plane. The speed-up, S, is

S V(z) /V6(H + z)where H is the height of the hill, z is the heightabove the local surface, and V and Vo representthe winds observed at the hill and in the freeatmosphere upwind, measured relative to localground level.

Jackson and Hunt (1975) recommend the useof the fractional speed-up, AS, where the windspeeds are referred to a fixed height above localground level. This parameter is defined by

A S (V(z)--V8(z))/1/8(z)where the variables have the same meanings asbefore. T h e fractional speed-up has t h eadvantages o f being more easily determined(surface winds are commonly measured at afixed height above local ground level) and is lessvariable with height than the speed-up S.

(b) EVALUATION OF OBSERVED SPEED-UPSValues of the fractional speed-up, AS, have

been evaluated using the mean speed differencefrom the comparison between each of the ridge-top anemometers and one or other of the low-level stations: Ohakea and Palmerston North.The comparison has been confined to west andnorthwest winds between 250 and 335 degreestrue at the ridge site but uses all speeds. Duringthe comparison period, January to March 1983,directions i n the above range were morefrequent than usual occurring between 60%and 70% o f the total time depending on thesite.

The observed values of AS are given in Table1 for all hours and for the afternoon (1200-1700hours) when the stability is usually lowest.Using Turner's (1964) classification of stabilityconditions, an analysis based on wind, cloudand solar elevation angle at Ohakea shows thatfor all hours during the 1983 comparison period

60

Station Com- Observed A S Calculated

West and northwestwinds only

parison All 1200- using

61

station hours 1700hr Bowen'stheory

Whitson Frm P'ton N 0.6 0.4 0.6

North Rge Rd N P'ton N 0.9 0.5 0.8North Rge Rd S P'ton N 0.9 0.6 0.8Pahiatua Track Ohakea 1.3 1.0 1.1Wharite Pk 6m Ohakea 1.0 0.8 1.9Wharite Pk 122m Ohakea 1.4 1.0 1.2

Wind directions Neutral Stable Unstable

West and northwestwinds only 81 9 10

Al l wind directions 61 26 13

TA B L E 1: VA L U E S OF THE F R A C T I O N A L SPEED-UPS O B T A I N E D U S I N G A L L A V A I L A B L E D A T A

C O M PA R E D W I T H C A L C U L AT E D VALUES.

TA B L E 2 : F R E Q U E N C I E S (07o) O F N E U T R A L ,STABLE, A N D U N S T A B L E C O N D I T I O N S A TO H A K E A D U R I N G JAN. -MAR. 1983.

neutral conditions occurred for 61% of the time(Table 2). Including only west and northwestdirections, the frequency is increased to 81%,with stable and unstable conditions havingsimilar frequencies (o f about 10%). I n theafternoon stable conditions do not occur andthe frequency of unstable conditions with windsfrom the west and northwest is increased toabout 20%. T h e differences between thecolumns of observed AS can be ascribed to thefrequency o f unstable conditions. I n theseconditions the difference between surface windsand those at higher levels tends to becomesmaller because of the increased transport o fmomentum caused by the more active con-vection between the two levels, implying asmaller

At the two positions on Wharite Peak thecomparison periods were different. Also at thelower position the majority of the data were forthe hours 0800-1800 hours and the observedvalue o f A S for all hours would have beenbiased towards unstable conditions. This wouldalso be the reason f o r the small differencebetween the values for the afternoon and allhours. Other factors, discussed later, wouldalso have been important at this site.

Manawatu Wind Studies

(C) C A L C U L A T E D VA L U E S OF SPEED-UP

The theory of speed-up has been reviewed byHunt and Simpson (1982). I t appears that forsimple hil l shapes the fractional speed-up inneutral conditions can be calculated using theheight and width of the hill. For realistic hillshapes, the surface can be approximated bysimple analytic functions from which a shapeparameter, cr(z), can be derived. As an example,at the crest of a bell-shaped hill with the profileH/(1+ (x/L)2)

a(z) = 1/(1 + (z/L))2where H is the height o f the hil l, x is thehorizontal coordinate, z the height above localground level and L is the distance between thecrest o f the h i l l and the place where theelevation above the surrounding terrain is halfits maximum value. The highest value of a(Z) is1 and occurs at the ground. However, in thepractical cases examined here z< <L and themaximum value c a n b e applied t o t h eanemometer height.

The final column of values of AS in Table Ihas been calculated using the equation given byBowen (1983):

AS= 0-(z)(H/L)[In ( o ) I n (i-6)

for the layer near the ground under neutralconditions, where the shape parameter, cr(z),has already been defined for a bell-shaped hill;L and z have also been defined, zo is theroughness length, and h the depth of the innerlayer is defined by:

hOn 0 . 3 2 L

The cross-sections o f the ridge a t thelocations of the anemometers are given in Fig.8. They are constructed along an azimuth o f300 degrees true which is close to the mostfrequent wind direction in the region. They arebased on contours on New Zealand Map Series1 (sheet N149 4th edition 1969, published by theDepartment of Lands and Survey) with heightstaken at 1 km intervals and at turning pointsand major changes of slope. The contours were

Manawatu Wind Studies

SECTION ALONG

_ h (m) 1

300 IWharite Peak

1

1 1 0 8

- h(m) 2

6 1 4 , 2 0 2 4

North Range Road NorthI

- - N . . ) : / e n ' sFarm

i

- h (m) 3

- Palmerston North Airport--___I

6 2

North Range Road SouthWhitson Farm

12 1 0

- h(m) 4

6Pahiatua Track

1000

800

600

400

800600

400200

800600

400

200

800600

400

200

12 1X km

2

Fig. 8: Sections across the ridge in the direction 300' true-I20' true in the vicinity of the anemometer positions labelled.

6 4 2 2 4 km 6 x

61

Fig 9: The form o f the unsymmetrical inverse polynomial function with a half-width on the windward side of 4 km and aleeward half-width o f 2 km.

62

Stationkm

1,2km

Whitson Farm 1.0 3.0 250North Range Road N 1.5 1.3 350North Range Road S 3.0 1.0 350Pahiatua Track 0.7 0.9 350Wharite Peak 1.5 1.1 700

not available over the upper part o f WharitePeak and the profile has been estimated. Theposition of each anemometer is shown on theappropriate section. In some cases the section isdrawn approx ima te l y b e t w e e n t w oanemometers and the positions are projectedonto i t . T h u s , t h e Palmerston N o r t hanemometer is shown in section 3. Ohakea,which cannot be shown because it is almost 40km away f rom the ridge, i s between thewestward extensions of sections 2 and 3.

Hunt and Simpson (1982) use the term Li inplace of the half-width L in expressions similarto the above. One o f the h i l l shapes theyconsider is unsymmetrical and provides a muchbetter fit to the cross-sections in Fig. 8. In thiscase, the term L is a characteristic of the sideof the hill from which the wind approaches andL, applies to the lee side. The profile of the hillis given by:

f(x) H / ( 1 x i < 2 )

where i = 1 f o r x 0 and i = 2 for x > 0, anexample is given in Fig. 9. The shape parameterat the crest o f the hill is the maximum valuea max:

(Tmax 0 . 5 ( 1 + )

The values used for each of the ridge sites isgiven in Table 3. The assumed bases of the hillsare denoted by the dashed lines in Fig. 8 and theheight H is measured above these lines.

( d ) D ISCUSSION

The values of AS in the right-hand columnof Table I are quite sensitive to the magnitudeof the roughness length zo. The value of zo forshort grass (0.03m) is used for most stations butis increased to 0.1m for Wharite Peak which

TABLE 3: PARAMETERS TAKEN FOR CALCULAT-ING A S AT EACH OF THE STATIONS, BASED ONFIG. 8.

Manawatu Wind Studies

has scrub on the upper slopes and for NorthRange Road South which is bordered by aplantation o f young pines. The larger valuesincrease the calculated value o f A S a littleabove what would be obtained with lowerroughness values.

It is also apparent that A S is extremelydependent on the stability. The differencesbetween the columns o f observed values i nTable 1 is caused by a swing of about 10% ofoccasions f rom the stable t o the unstableclasses. The values for each particular stabilityclass would therefore be quite different. In themost stable classes the low level wind becomesdecoupled from the flow at ridge-top level andAS would be very large. However, theseconditions were most infrequent during thestudy period and the values obtained for allhours are probably not too far f rom thosewhich would be obtained in neutral stability.The agreement with the calculated values whichare for neutral conditions seems, in most cases,to be about as good as can be expected in a realsituation i n which the stability cannot becontrolled.

At the lower anemometer on Wharite Peakthe expected speed-up is very high but theobserved speed-ups are much less. This i sprobably partly due to the predominantly day-time observations a t this site which couldexplain 20% o f the discrepancy. Using asmaller roughness length could reduce theexpected speed-up by 30%. There is also somedoubt as to the applicability o f the theory insuch places (Bowen, 1983). Another factor islikely to be deficiencies in the exposure of theanemometer which was adjacent to a steepslope where the airflow would tend to separateon the windward side o f the crest in somesituations. Only instruments which are highabove t h e g round c a n g i v e a correctmeasurement in these circumstances.

The high speed-up observed at the PahiatuaTrack anemometer is an interesting result. It islikely to be localised in the section of ridge nearthe Pahiatua Track which has an especiallynarrow section but could be repeated at otherplaces where the ridge narrows in a similarmanner. An additional factor may be providedby a lateral concentration of the flow up thevalley lying northwest from the site.

Manawatu Wind Studies

CONCLUSIONSThe wind data obtained i n a short field

experiment have been combined wi th dataobtained in the same area over a longer timespan. An analysis programme has been appliedto compare the wind speeds and directions foreach hour at pairs of stations, and to examinethe mean differences between the simultaneouswinds. The changes of wind speed as airflowsfrom the west and northwest approach and passover a ridge have been examined and have beencompared with airflow theory. There is anapproximate doubling o f speed (increase o f9007o-14007o) between t h e f l o w ove r t h eManawatu Plains towards the range and thecrest of the range. The variation found betweenstations appears to be as much related to thewidth of the ridge as to its height. However, theheight o f the measurements above ground isalso an important factor. The increase of windspeed varies strongly with time of day due tochanges o f stability in the lower atmosphere.The agreement between the predictions o fairflow theory and the observations appear onthe whole to be satisfactory and the calculationof speed-ups based on simple parameters of ahill provides a valuable t o o l f o r t h einterpretation of wind data and many practicalproblems.

63ACKNOWLEDGMENTS

The operation o f the anemometers wascarried out by M r A . M . Bromley and hisassistants. T h e Meteorological Service i sgrateful f o r assistance received f r o m t h eManawatu-Oroua Power Board, and to land-owners and farm managers who cooperatedwith the project.

REFERENCESBowen, A . J. , 1983: Discussion o f session 21 — Wind

characteristics: Structure and Statistics (b). J. WindEng. Ind. Aero., 15, 253-255.

Hunt, J . C. R. and J. E. Simpson, 1982: Atmosphericboundary layers over non-homogeneous terrain. I nEngineering Meteorology, E. Plate, Ed., Elsevier.

Jackson, P. S. and J. C. R. Hunt, 1975: Turbulent flowover a low hill. Quart. J. Roy. Met. Soc., 101, 929-955.

Turner, D. B., 1964: A diffusion model for an urban area.J. Appl. Meteor., 3, 83-91.