- Email: [email protected]
Vertical and horizontal air flow above rows of a vineyard
Agricultural Meteorology, 17(1976) 433--452 Q Elsevier Scientific Publishing Company -- Printed in The Netherlands
VERTICAL A N D H O R I Z O N T A L M R F L O W A B O V E R O W S O F ~NEYARD*
A
ALBERT WEISS and L. H. ALLEN Jr.
University of Nebraska, Panhandle Station, Scottsbluff, Nebraska 69361 (U.S.A.) U.S. Department of Agriculture, Agricultural Research Service, University of Florida, Gainesville, Florida 32611 (U.S.A.) (Received May 10, 1976; accepted September 28, 1976)
ABSTRACT Weiss, A. and Allen Jr., L. H., 1976. Vertical and horizontal air flow above rows of a vineyard. Agric. Meteorol., 17: 433--452. Air-flow studies were conducted in a 4.85 ha vineyard. Mean horizontal wind-speed profiles were obtained with cup anemometers. Propeller anemometers measured longitudinal, lateral, and vertical velocities above the rows as a function of height, wind speed, and wind direction. A constant momentum-flux zone above the vineyard, within the measured heights of 227 to 641 cm, was not found. The semilog horizontal wind-speed profiles, obtained over the vineyard, fitted two straight lines, representing an inner and outer zone within the boundary layer. Z0 was nearly constant. The number of uninterrupted updrafts of digitized propeller anemometer records was approximately equal to the number of uninterrupted downdrafts at each height, but the duration of these uninterrupted values was different. The total number of updrafts and downdrafts were, in general, not equal. Spectra from the four vertical anemometers showed that the total variance increased with height, as contributions to the low frequency portions of the spectra increased.
INTRODUCTION
Many micrometeorological studies made over relatively smooth, uniform terrain have shown that a constant-flux layer exists. Since much research has been devoted to relating fluxes to gradients in this layer (see Monin and Yaglom, 1971, chapter 4), flux-gradient relationships are relatively well understood. However, fewer studies have been performed over rough, nonuniform, or wide-row surfaces, such as in a vineyard. In a vineyard study, Graecz (1972) concluded that a constant-flux layer * Contribution from the Agricultural Research Service, U.S. Department of Agriculture, in cooperation with the Cornell University Agricultural Experiment Station, Ithaca, N.Y. Department of Agronomy Series Paper No. 1151.
434
existed, at least between 2.5 and 5.0 m above the ground, and mean vertical velocities were near zero within this layer. Hicks (1973), who also measured flux densities with a fluxatron over this same vineyard, f o u n d the drag coefficient doubled as the wind changed from down-row to cross-row directions, and evapotranspiration rates increased 10 to 20% with cross-row flow as compared with down-row flow. Sadeh et al. (1971), who used a wind tunnel to simulate turbulent flow in a forest, proposed a 10% limiting value of relative roughness (the ratio of canopy height to boundary-layer thickness) above which the log wind profile deviates from a single straight-line relationship. Instead of one log profile, they f o u n d their data fitted two log profiles of different slopes representing an inner and outer layer when the relative roughness was greater than 10%. Measurements of horizontal wind speed behind a series of 8 m wind breaks by Seguin and Gignoux (1974) also fit two semilogarithm straight lines of different slope. The objective of our experiment was to determine the turbulence properties above a vineyard as measured with propeller anemometers. Especially we wanted to investigate the crop boundary layer to determine the nature of the " c o n s t a n t flux layer".
DESCRIPTION OF THE EXPERIMENT
Three individual vertical-component propeller anemometers (W-l, W-2, and W-3) and a three-component, UVW propeller anemometer system were placed above a vineyard canopy at four heights on a 6-m mast. The vineyard was located at Hector, New York, which is on the east side of Seneca Lake about 40 km west of Ithaca. It consisted of 4.85 ha of Delaware and Concord vines trellised in north--south rows with a cross-row fetch of approximately 220 m from the west. The vines were approximately 2 m high and 1 m wide with about a 2-m open space between rows. The heights at which wind measurements were made are given in subsequent tables. Weiss and Allen (1976a) describe the vineyard experimental site in more detail. The vertical c o m p o n e n t anemometers were raised and lowered as a unit to measure the eddy structure over a vineyard as a function of wind speed, wind direction, and height. The UVW anemometer, positioned at the t o p of the array, was always oriented with the U and V propellers facing the mean wind to improve threshold response and avoid shadow effects from the mast. The outputs from the propeller anemometers were recorded on an FM analog tape recorder and zero signals were recorded on alt channels every hour. Later, the data were filtered with a 10 Hz, low-pass filter, digitized at a rate of 5 scans sec -1, and recorded in digital format on magnetic tape for further processing. Cup anemometers were positioned at 2 2 7 , 2 7 7 , 3 2 7 , 3 8 2 , 437, and 497 cm above the ground. Comparisons were made among mean vertical velocities and mean horizontal wind speeds for various heights and between the
435 h o r i z o n t a l wind speed as m e a s u r e d b y c u p and p r o p e l l e r a n e m o m e t e r s . The a n e m o m e t e r array is s h o w n in Fig.1. The c u p a n e m o m e t e r s were placed over
Fig.1. The above-row experimental setup of propeller and cup anemometers facing northward, i.e., with east at the right and west at the left. the leading edge o f t h e d o w n w i n d trellised c a n o p y , whereas the p r o p e l l e r a n e m o m e t e r s were placed over the trailing edge o f t h e same trellised c a n o p y . Detailed m e a s u r e m e n t s were m a d e o n O c t o b e r 11 and o n O c t o b e r 17, 1972. The v i n e y a r d c a n o p y foliage was still green and flourishing because n o killing frosts had o c c u r r e d b e f o r e d a t a were t a k e n in 1972. CALIBRATION OF PROPELLER ANEMOMETERS T h e t a c h o m e t e r g e n e r a t o r o u t p u t of the p r o p e l l e r a n e m o m e t e r s was m e a s u r e d in a wind t u n n e l at t h r e e speeds (54, 194, and 343 cm sec -1 ). T h e p r o p e l l e r was r o t a t e d with r e s p e c t to air flow along the t u n n e l at angles o f a t t a c k f r o m 0 ° t o 180 °, with calibration m e a s u r e m e n t s m a d e at 5 ° intervals. (The angle o f a t t a c k is d e f i n e d h e r e as the angle b e t w e e n the wind v e c t o r and
436 the rotation axis of the anemometer, with 0 ° indicating flow toward the front of the propeller, and 180 ° indicating flow from the rear along the propeller shaft.) At 54 cm sec-1 the propeller anemometers stalled in the region from 70 ° to 110 ° angles of attack, but the anemometers did n o t stall at the two higher speeds except at the 90 ° angle of attack (Fig.2). The mea-
1Q
W I N D S P E E D = 3 4 3 C M SEC 4 • : I D E A L C O S I N E RESPONSE • = MEASURED RFSoONSE ~ll21I * ~e Ao •e Ae ,"
O~
%'.. !
'
:> 0C
| "
1
4
e•
"2;
-05
. oA
"flit
_1001
I
I
I
I
ANGLE
I
1,90 1 1 1 I OF ATTACK
I I
[ '~t£ (~'O
e
(deg)
Fig.2. Comparison of an ideal cosine response with measured response (normalized voltage output, Ve/V180, for angles of attack ~, from 0° to 180°) of the W-1 propeller anemometer for a horizontal velocity of 343 cm sec-i . sured response in a wind tunnel approximated a cosine response over the entire range of angles of attack, with the greatest absolute deviation occurring at about +- 40 ° (at 50 ° and at 130 °) from the stall angle. Correction factors for deviation from cosine response were c o m p u t e d based on the calibration at 343 cm sec -1. The measured response as a function of angle of attack was also nearly linear over the range of 50°--130 ° (Fig.2). Wind velocities were determined by two methods from the propeller anem o m e t e r voltage outputs. For the three-component UVW anemometer, and for a n e m o m e t e r W-3, which was positioned immediately below the UVW unit (and hence used as the vertical-component anemometer of another UVW system), velocity components were c o m p u t e d itemtively using the corrected cosine response. The iteration m e t h o d consists of the following steps. (1) Compute initial estimates of the three velocity components based on tachometer generator outputs. (2) Compute the angles of attack for the three components based on the initial-velocity estimates. (3) Compute new velocities based on correction factors for each angle of attack. These factors corrected for the deviation from an ideal cosine response using the data in Fig.2.
437
(4) Repeat steps (2) and (3) until the difference between the current and previous angles of attack were less than or equal to 1 °, or until five iterations were completed. (Usually it t o o k no more than three iterations before convergence.) This process was similar to a computer algorithm described by Horst (1972). For the two individual vertical anemometers located lower on the mast (W-1 and W-2), the linear portion of the response curve, from approximately 50 ° to 130 ° , was used to yield a linear regression equation based on voltage outputs. Hicks (1972) determined t h a t the difference between the slopes of these straight line segments was less than 5% for the 200--1,000 cm sec-1 range of horizontal velocities. V E R T I C A L VELOCITY AND T U R B U L E N C E
Three runs (13h40--14h10, 14h10--14h40, and 14h44--15h10 EST) with the vertical and UVW a n e m o m e t e r array were made on October 11, 1972, and ten runs (09h29--09h51, 09h51--10h13, 10h15--10h44, 10h45--11h13, 11h13--11h42, 12h15--12h47, 12h48--13h15, 13h19--13h38, 13h38--13h57, and 14h00--14h14 EST) were made on October 17, 1972. The information on October 11 was summarized as a unit since the mean vertical velocities at each respective height were similar. During the first five runs on October 17, the a n e m o m e t e r array was positioned at the upper set of heights (437-641 cm), so the data in the first five runs were summarized as a unit (09h29--11h42 EST). Finally, the data of runs 6--10 on October 17 were averaged together since the a n e m o m e t e r array was at the lower set of heights (227--412 cm) for these five runs (12h15--14h14 EST). Means, standard deviations, and turbulent intensities of the vertical velocities are summarized in Table I. The turbulent intensities of the vertical velocities (the ratio of the rms fluctuations of the vertical velocity to the mean horizontal wind speed) were based on cup a n e m o m e t e r data at the respective vertical anemometer heights. The mean vertical velocities for each run on October 17 when the anemometers were in the upper and lower positions were averaged, and the results plotted in Fig.3. The vine canopies are drawn to scale and it is clear that a distinct vertical-velocity profile was present. Positive vertical velocities (updrafts) dominated between 360 and 560 cm, while negative velocities (downdrafts) dominated above and below this area. The data for vertical anemometer W-4 in the lower position (412 cm, runs 6--10 on October 17) were n o t included in this analysis. A strong d o w n d r a f t was indicated by this a n e m o m e t e r during those runs, which was inconsistent with all other data collected at or near this height. We suspect t h a t either a n e m o m e t e r leveling, signal transmission, or data recording or processing m a y have been faulty. The vertical velocities in Fig.3 showed a consistent divergence from a height of 227 cm to about 450 cm, and a consistent convergence from about
:~g8 TABLE I Mean vertical velocities, standard deviations, and t u r b u l e n t intensities for the f o u r vertic~J a n e m o m e t e r s f o r O c t o b e r 11 and O c t o b e r ] 7 , 1972 Anemometer
Height* (cm)
Velocity (cm sec-1 )
Standard deviation ( c m sec -1 )
Turbulent intensity
Oct. 11, 1972; W-1 W-2 W-3 W-4
1 3 h 4 0 - - 1 5 h 1 0 EST; 227 277 327 437
average o f 3 runs: -13.5 -13.6 - 6.8 i2.4
47.9 51.2 53.5 65.6
0.146 0.135 0.129 0.136
Oct. 17, 1972; 0 9 h 2 9 - - 1 1 h 4 2 EST; W-1 437 W-2 497 W-3 557 W-4 641
average o f 5 runs: 13.6 10.9 0.3 -5.8
44.6 46.9 49.6 57.7
0.118 0.114 ---
Oct. 17, 1972; W-1 W-2 W-3 W-4
average o f 5 runs: -11.9 -10.6 - 6.0 -14.1
39.8 46.9 50.2 58.8
0.149 0,151 0.151 0.152
1 2 h 1 5 - - 1 4 h 1 4 EST; 227 277 327 412
*The h e i g h t is w i t h r e f e r e n c e t o t h e ground.
7OO -
VERTICAL VELOCITY ( c m sec I) -20-10 O 10 2 0 l
600 -
{
{
{
r
>
500 -
~400 l-I 300
200 100 0
-__
1 ~_ L _ _
,, i
Fig.3. Profile o f the m e a n vertical v e l o c i t y based o n the m e a n values for the upper and l o w e r a n e m o m e t e r p o s i t i o n s for O c t o b e r 17, 1 9 7 2 . Relative d i m e n s i o n s o f trellised vineyard r o w s are s h o w n .
450 cm up to 641 cm. Clearly, we did n o t find a layer of constant m o m e n t u m flux immediately above this vineyard canopy. Turbulent intensity of vertical velocity appeared to be relatively invariant with height within a set of runs (Table I). However, the vertical turbulent
439 intensity appeared to be higher during the 12h15--14h14 EST period on October 17 (a cross-row flow period) than during the 0 9 h 2 9 - - 1 1 h 4 2 EST period (an along-row flow period). Also, dual-channel oscilloscope displays of propeller anemometer outputs showed sharper spikes of vertical velocities near the top of the trellised vineyard canopy than at higher elevation. These spikes were probably due to sudden and extreme changes in the vertical c o m p o n e n t near the surface roughness elements (curtains of leaves). Intensity of turbulence of horizontal flow measured with propeller anemometers (I), friction velocity (u.), and drag coefficients (C D ) of the vineyard as related to height, wind speed, and wind direction are included in Table II. The wind direction was measured at the ton of the row and the mean direction was estimated from strip-chart records. The intensity of turbulence, I, is the ratio of the rms value of the fluctuations of the horizontal wind speed (V~2)v2 to the mean horizontal wind speed Vh, i.e.: I = - -
(1)
Turbulence intensity, I, decreased with height (compare October-17 runs at a height of 603 cm with runs at a height of 374 cm in Table H), b u t was n o t related to wind speed. Turbulence intensity was also greater for crossrow flow than for down-row flow (compare October 17 (12h15--14h14 EST) with October 11 ( 1 3 h 0 4 - - 1 5 h 1 0 EST). Runs 1 and 2 (October 17), with a relative down-row c o m p o n e n t (328 ° ) had a lower intensity of turbulence, 0.316, as compared with runs 3, 4, and 5, with a relative cross-row component (303 ° ) which had a turbulence intensity of 0.349 (not shown in Table II). A difference of 25 ° between the two sets of data resulted in a distinct change in the intensity of turbulence. The friction velocity, u. is defined as: U2
,
= -
T
P
_
--u
t
w
t
(2)
where u' and w' are deviations from the mean of horizontal and vertical velocity, respectively, and the over bar is the time average, T is the shearing stxess, and p is the density of the air. In practice, however, it is easier and less costly (in c o m p u t e r time) to calculate it as: -u:,
~- u i w i -
u w
(where the subscript, i, indicates instantaneous values), with little loss of accuracy if the low-frequency c o m p o n e n t s of a trend are removed. Hicks (1970) electronically removed the trend and c o m p u t e d u:, as just the first term on the right hand side of eq.3. Trend was removed in this study by
(3)
440
¢1 o 0 ~
d 0
.-c.
¢,100
°~ 0 - ~ ¢ 9 ¢',,1 u
0
u
m
Q
¢,,1 , ~
0
¢,1 r..4
441
subtracting the product of the mean horizontal and vertical velocities from the mean value of the instantaneous u i w i product. The drag coefficient, CD, is defined as: = 2
(4)
and can be interpreted as a nondimensional measure of the roughness of the underlying surface and its influence on the flow. The values of C D (Table II) were higher for cross-row flow (rough aerodynamic surface) than for downrow flow (smoother aerodynamic surface). COMPARISON OF CUP AND P R O P E L L E R ANEMOMETERS
The horizontal wind speed as measured by cup anemometers was compared with that measured by propeller anemometers (Table II). The heights given are those where the horizontal wind speed, Vh, from the U and V propeller anemometers was determined. The cup anemometer wind speed was from the height closest to the Vh height; 382 cm when the propeller anemometers were in the lower position and 497 cm when the propeller anemometers were in the upper position. Valid comparisons can be made only for runs 1--3 on October 11, and runs 6--10, on October 17, when the distance b e t w e e n V~ and a cup anemometer were - 1 7 and +8 cm, respectively. The cup a n e m o m e t e r indicated a 24% greater horizontal wind speed than the propeller anemometers on October 11, and a 17% greater horizontal wind speed on October 17. Izumi and Barad (1970) compared velocity measurements of cup and sonic anemometers and found that cup anemometers overestimate the wind speed by 10% for uniform terrain. Our cup anemometers may have overestimated mean horizontal wind speeds over the rough vineyard canopy by 17%. Since cup and propeller anemometers b o t h have inertia, this overestimation could partly be due to lack of response of the cup anemometers as compared with the propeller anemometers. H O R I Z O N T A L WIND P R O F I L E S
Graphical comparisons of the cup anemometer data for October 17 showed that a zero-plane displacement, D, of 140 cm gave the best log-linear fit for the lowest four heights. The horizontal wind profiles were n o t corrected for stability, b u t conditions during b o t h days were cool, partially clouded and near neutral conditions were assumed. The data from the 6 anemometers formed t w o distinct linear profiles with different slopes. Run 1 on October 17 ( 0 9 h 2 9 - - 0 9 h 5 1 EST) was typical of the horizontal windspeed profiles (Fig.4). The first segment consisted of the data from the 4 lowest anemometers while the second segment consisted of the data from
442 100C
~-7,1972
0930-0950
RUN 1
EST
100
/ 10
HORIZONTAL WIND SPEED ( C M SEC -1)
Fig.4. H o r i z o n t a l w i n d s p e e d profile illustrating inner and o u t e r z o n e s for run 1 ( 0 9 h 3 0 - 0 9 h 5 0 EST) o n O c t o b e r 1 7 , 1 9 7 2 . D = 1 4 0 cm.
the 2 highest anemometers. These latter two points showed an increase in wind speed larger than that which would be shown by extension of the log profile formed by the 4 lowest anemometers. Fig.5 and 6 relate the horizontal wind speed to the mean vertical velocities at approximately the same (logarithmic) heights for the upper and lower vertical anemometer ~ o n s on October 17, respectively. Two characteristics of the semilog horizontal wind speed profiles were 100(
on
ell~ ee
•
•
enei
•
•
9
euA• ue ell
•
I(X
0CT1,71972 o = RUN 1 • = RUN 2 • = RUN 3 • = RUN4 • "= RUN5
1 ° o ~ HORIZONTAL WIND SPEED
-~o ~ (cm ,sec-1)
I
,o
]
~o
I
~o 40
V EA"flC..z~ VELOCITY
Fig.5. C o m p a r i s o n o f h o r i z o n t a l w i n d s p e e d w i t h vertical v e l o c i t i e s for runs 1 - - 5 o n O c t o b e r 17, 1 9 7 2 w i t h a n e m o m e t e r s in the u p p e r p o s i t i o n a n d D = 1 4 0 cm.
443 100C
tern t u
•
•
ow
•
ii
•
£~ 10C
•
OCT. 17,,1972 • = RUN 6 • = RUN 7 • = RUN 8 • : RUN 9 o : RUNIO O HORIZONTAL WIND SPEED
( c m sec-I)
IO
2O
VERTICAL VELOCITY
Fig.6. C o m p a r i s o n o f h o r i z o n t a l w i n d s p e e d w i t h vertical velocities for r u n s 6 - - 1 0 o n O c t o b e r 17, 1 9 7 2 w i t h a n e m o m e t e r s in the l o w e r p o s i t i o n a n d D = 1 4 0 cm.
evident. First, the roughness length (Z0), based on the data from the four lowest anemometers, was nearly constant and averaged 6.0 cm. Second, the two linear segments of the log profiles intersected at approximately the same height where the mean vertical velocity changed from net downdraft to net updraft. The horizontal wind speed profile in Fig.4 is similar to those measured by Sadeh et al. (1971) over a model forest, and by Seguin and Gignoux (1974) over a series of windbreaks. The two uppermost data points which do not lie on the same profile as the four lower data points would normally not be used in the analysis of the wind profiles. Usually these higher data points have been eliminated based on the assumption that they lie above the aerodynamic boundary layer and are not representative of the underlying surface. Furthermore, flux divergence in the boundary layer, where the log profile was used, has been assumed implicitly to be zero. The mean vertical velocities presented in Fig.3 showed a clear pattern at each height in relation to other heights, although there was scatter between runs. TOTAL AND UNINTERRUPTED AND STALLS
VALUES FOR UPDRAFTS, DOWNDRAFTS,
Desjardins (1972) placed two propeller anemometers above a barley crop at the same height but separated horizontally by 4 m. He found that, in general, updrafts did not equal downdrafts at both locations as measured by the propeller anemometers. In this study, we extended the study of updrafts, downdrafts, and stalls into the vertical. Table III shows data separated into numbers of updrafts, downdrafts, and
444
~
o
e~
~.'~
e~
I l t l
I l l l
e~ 0
0
0
Lo
e~
e~
L',-
0 0
445
stalls for the three sets of runs. These values are a function of the digitizing rate and could also be represented in terms of time. The total vertical air passage for each anemometer was calculated by determining the velocity at each scan and multiplying it by the digitizing period and accumulating the sum. The mean velocities were calculated for both the updrafts and the downdrafts. As the distance from a surface increases, the number of stalls would be expected to decrease. However, Table III shows more stalls at anemometers W-3 and W-4 than at W-1 and W-2. These differences were due to the two methods used to determine velocities. The differences in the n u m b e r of updrafts and downdrafts and in the length of air passage, obtained from the data presented in Table III reflect the mean (net) vertical velocities, presented in Table I. The mean velocities for downdrafts were generally greater than those for updrafts when the anemometers were in the lower position. However, when the anemometers were in the upper position, on October 17, the mean vertical velocities associated with updrafts, for the lower two heights, were greater than velocities associated with downdrafts. This relationship was reversed for the upper two heights. Table IV presents data from the same runs on October 11 and October 17 in terms of uninterrupted updrafts, downdrafts, and stalls. The number of uninterrupted updrafts was generally slightly lower than of uninterrupted downdrafts at the same heights. The mean durations were n o t equal, and updraft mean durations usually varied in height inversely with respect to d o w n d r a f t mean durations. In Table III, the total number of u p d r a f t and d o w n d r a f t counts were, in general, very different. The mean velocity for uninterrupted updrafts and downdrafts increased with height in all cases, but the mean air passage was a function of both the mean duration and the mean vertical velocity. The number of uninterrupted stalls almost equaled the number of total stalls. Fig.7 shows histograms of uninterrupted occurrences of updrafts and downdrafts of each vertical anemometer at 1-sec intervals for run 1 {09h29-09h51 EST) on October 17. This frequency distribution is representative of the data taken on both days. Values to the left of zero on the time axis represent uninterrupted duration in seconds of downdrafts, while those to the right represent uninterrupted duration for updrafts. For run 1 (09h29--09h51 EST) on October 17, when the vertical anemometers were in the upper position and the wind direction was 331 ° , the lower two anemometers had distributions which tailed to the right (updrafts) with uninterrupted occurrences of updrafts that lasted up to 15 sec. Anemometer W-3 came closest to a normal distribution, while the anemometer W-4 distribution tailed to the left (downdrafts) with uninterrupted downdrafts lasting up to 15 sec. Fig.8 shows the frequency distribution of mean velocities associated with uninterrupted updrafts and downdrafts for run I on October 17. The abscissa is divided into velocity intervals of 10 cm sec-' . Two c o m m o n properties
446 0
0 0 0 0
IIII
IIII
0 ,-~ ,.~
"0 0 e~.
e~
e~
2
• 0
'5
0
oO v-~ o o [-.-
0
0
e=
I ~4 , ~ ¢ 0 ¢'0 ¢ 0 , ¢ 0
0 0
m
Zo
447
OCT 17,1972 0929~0951 EST
RUN 1
~z
100~
641 cm
Or200
3
D
100
0
O[
o o
557cm --
42+7m 100~}_
downJ L updrafts j_[- L L _ ~ f t s
200
1
106~
4:37cm
0 j
llllilillillIIililiJllJlJli+Itl I©
5
0
5
10
TIME (sec) Fig.7. U n i n t e r r u p t e d occurrences o f updrafts and d o w n d r a f t s at 1-sec intervals for four heights above ground over a vineyard. R u n 1, 0 9 h 2 9 - - 0 9 h 5 1 E S T , o n October 17, 1 9 7 2 .
RUN 1
0CT17,1972 0929-0951 EST
Ld 100 I~
U 0 a
4
r ©~-
lcm
downdrafts
updrafts 557cm
0~
I00I
Tcm
+F
cg
©i
_z 100 0 z D
~
3
7
cm
j,,l,,+,i,,,,l,,,,l,,,,l,,
10 5 0 5 10 VELOCITY (cm sect1)
,,
.10
Fig.8. Mean velocities for uninterrupted updrafts and d o w n d r a f t s at intervals o f 10 c m sec -1 for four heights above ground over a vineyard. R u n 1, 0 9 h 2 9 - - 0 9 h 5 1 E S T o n October 17, 1 9 7 2 .
were found in all runs and are illustrated by Fig.8. As height increased, vertical velocities related to both uninterrupted updrafts and downdrafts increased. Also, as a corollary, as height increased, the distributions became flatter. The second property has been considered previously, i.e., the large number of stalls associated with angles of attack near 90 ° when the iterative
448
direction cosine technique was used to compute vertical velocities. These large numbers of stalls are illustrated by the histograms of anemometers W-3 and W-4 at heights of 557 cm and 641 cm, respectively. The numbers of uninterrupted occurrences between the interval 0--10 cm sec -1 for both updrafts and downdrafts were much lower than those for neighboring intervals. This result was different from the findings for the same intervals for anemometers W-1 and W-2. Therefore, it can be concluded that the current correction factors employed in the iterative direction cosine technique overestimate the number of stalls. Furthermore, it has been shown by McBean (1974) for momentum and heat flux densities as measured by a sonic anemometer, and by Weiss and Allen (1976b) for momentum flux density using a UVW propeller anemometer, that over half of the momentum flux associated with downdrafts occurred between 80 ° and 90 ° angles of attack. However, the momentum flux associated with updrafts occurred in a broader band between 95 ° to 125 ° . Nevertheless, this downdraft momentum flux near stall angles indicates that great care must be taken in calibrating propeller anemometers and computing wind speeds near these angles of attack. SPECTRA FROM THE PROPELLER ANEMOMETERS
Spectra were computed from the four vertical anemometers, the U and V anemometers, and the horizontal wind speed (Va) for run 1 on October 17 (Fig.9), using the fast Fourier transform (FFT) developed by Cooley and
~f.,.~%. 4 •. " 641 cm
1000 100 10
OCT 17,1972 RUN 1 0929-0951 EST
1 10000
1 ,,/....,
IOO0
%
•"'~
100 10
3 ":
1 1 .,,...,. 2
1000
.-,"
100
•
10 ,i-
%
600 c~
•. 557 cr~
1
1
.
I0 0
1
v
",".'-..,.. vh
1000
1000
•
":
I O0
497 cr~
t0
""
: - ' > ~ . . . .% .v 604 cm
i 1
1 ........ 1 " '.
1000 IO0 t0 •
°°
~"
1000
437c~
1
I0C
..bU %
596 cm
1
11
1
001 01 i 1 i0
00101 i 1 I0 f(Hz)
Fig.9. Variance spectra from t h e f o u r vertical anemometers, the U and V anemometers, and the horizontal wind speed (Vb) for run 1, 09h29--09h51 EST, October 17, 1972.
449 Tukey (1965). A 5-min running mean was used as a high-pass filter but was n o t compensated for low-frequency distortion. Also, the times of the runs are slightly different from the same numbered runs previously discussed, since the filter was symmetrical, i.e., the data for the first and last 2.5 min of each run were not used in computing the spectra. As height increased, the contribution to the total variance from lower frequencies increased. Also, the four spectra appear to have major contributions to the total variance centered around the same frequencies. These peak frequencies are more a function of height than of wind direction or wind speed. When the anemometers were in the lower position, the peak frequency was around 0.25 Hz, whereas in the upper position, the peak frequency shifted to approximately 0.12 Hz. The spectra from the U and V anemometers and the horizontal wind speed (Vh) indicate that they are influenced more by lower frequencies than the vertical anemometers and have about 5 to 8 times as much variance as do the vertical velocities. These three spectra (U, V, and Vh ) all appear similar. The peak frequencies for the horizontal wind speed for the lower and upper set of anemometer heights was about 0.1 Hz and 0.04 Hz, respectively. ~ The coherence between anemometers W-1 and W-2, W-2 and W-3, and W-3 and W-4 is shown in Fig.10 for run 1 on October 17. 0CT17,,1972
1.0
Ill
:
•
RUN 1
t t, $ ~ ] L I
0929-0951
.
• ANEM. 1 - 2 • ANEM.2- 3 ,, A N E M + 3 - 4
.
,'i ~St+- l!i.m ~,2 "
•
L3 Z Ld
EST
•
.I, "l
T
•
+ o
.001
I
i
01
i
.1 f(Hz)
~
1
,; _
10
F i g . 1 0 . Coherence b e t w e e n vertical a n e m o m e t e r s W-1 a n d Wo2, W-2 a n d W-3, a n d W-3 a n d W-4 f o r r u n 1, 0 9 h 2 9 - - 0 9 h 5 1 E S T , O c t o b e r 17, 1 9 7 2 .
The data in the first decade (0.001 to 0.01 Hz) showed some scatter which was probably due to the effects of filtering. When the anemometers were in the upper position as in run 1 on October 17, the coherence between all anemometers was above 0.9 from approximately 0.005Hz to 0.1 Hz. For frequencies greater than 0.05 Hz, the coherence was always higher between anemometers W-1 to W-2 and W-2 to W-3 than between W-3 to W-4. The reason for this difference was t h a t the distances separating anemometers
450
W-1 to W-2 and W-2 to W-3 were between 50 and 60 cm, whereas the distance between anemometers W-3 to W-4 was 85 cm for these runs. In general, overall values of coherence were lower between all anemometers when the anemometers were in the lower position. As height increased, coherence between all anemometers increased until higher frequencies were reached and then t h e y all decreased rapidly.
CONCLUSIONS
Three individual vertical-component propeller anemometers and a threec o m p o n e n t UVW propeller a n e m o m e t e r system were placed at four heights on a 6-m mast. These anemometers were raised and lowered as a unit to collect data at two sets of heights. Data were collected from three basic wind directions; 156 ° , 313 ° and 283 ° . Horizontal wind speed was measured simultaneously by cup anemometers. The following conclusions can be made. (1) A constant flux zone was n o t f o u n d over the vineyard within the heights measured, 227--641 cm, which contrasted with Graetz's {1972) finding. (2) Intensity of turbulence indicated t h a t cross-row flow was aerodynamically rougher than down-row flow. (3) Over the vineyard, the horizontal wind as measured by cup anemometers was about 17% greater than t h a t measured by propeller anemometers. (4) The semilog horizontal wind speed profiles obtained over the vineyard indicated an inner and outer zone within the boundary layer consistent with the findings of Sadeh et al. {1971) and Seguin and Gignoux (1974). (5) The n u m b e r of uninterrupted updrafts was approximately equal to the number of uninterrupted downdrafts at each height, but their durations were different. The mean durations of uninterrupted updrafts and downdrafts were greater at anemometer heights where net updrafts and downdrafts, respectively, were observed. The total numbers of updrafts and downdrafts were, in general, n o t equal. (6) The frequency distributions of uninterrupted occurrences of updrafts and downdrafts for the four vertical anemometers, as a function of height, were generally skewed, Incidents of uninterrupted updrafts and downdrafts lasting up to 15 sec were observed. (7) The frequency distributions of uninterrupted velocities associated with updrafts and downdrafts for the four vertical anemometers illustrated that as height increased the distributions became flatter and velocities increased. (8) Spectra from the four vertical anemometers indicated that contributions to the total variance from lower frequencies increased as height increased. (9) Spectra from the U and V anemometers and the horizontal wind speed (V~) were all similar. They contained 5 to 8 times the variance as the vertical anemometer spectra and had contributions to the total variance
451
centered around lower frequencies than the vertical anemometers. (10) Coherence between anemometers was a function of the distance separating anemometers and the height above the surface. The closer that anemometers were to each other and the greater the height above the surface, the greater was the value of coherence. Above 0.2 Hz, the coherence between all anemometers fell off rapidly. (11) There was disagreement b e t w e e n this study and the results of Graetz {1972) concerning the existence and height of a constant flux layer over a vineyard. The constant flux layer is important because in this zone transfer of heat, mass and m o m e n t u m can be treated as a one-dimensional (vertical) problem. Therefore, further studies in vineyards are needed. ACKNOWLEDGEMENTS
We wish to thank the Don J. Wickham family of Hector, New York, who allowed us to use their vineyard for this study; M. N. Johnson, USDA-ARS Engineering Technician, Ithaca, New York, for constructing the propeller anemometers; Dr. G. A. McBean, Atmospheric Environment, Canada, who supplied the c o m p u t e r program which was used to calculate spectra and coherence; Ms. Barbara Babcock, who t y p e d the manuscript. REFERENCES Cooley, J. W. and Tukey, J. W., 1965. An algorithm for the machine calculation of complex Fourier series. Math. Comput., 19: 297--301. Desjardins, R. L., 1972. A Study of Carbon Dioxide and Sensible Heat Fluxes Using the Eddy Correlation Technique. Ph.D. Thesis, Cornell University, Ithaca, N.Y., 175 pp. (Dissertation Abstracts No. 73--341.) Graetz, R. D., 1972. A Micrometeorological Study of the Contributions of Individual Elements of a Community of Plants to the Momentum, Heat, and Vapour Fluxes from that Community as a Whole. Ph.D. Thesis, Australian National University, Canberra, A.C.T., 118 pp. Hicks, B. B., 1970. The measurements of atmospheric fluxes near the surface. A generalized approach. J. Appl. Meteorol., 9: 386--388. Hicks, B. B., 1972. Propeller anemometers as sensors of atmospheric turbulence. Boundary-Layer Meteorol., 3: 214--228. Hicks, B. B., 1973. Eddy fluxes over a vineyard. Agric. Meteorol., 12: 203--215. Horst, T. W., 1972. A Computer Algorithm for Correcting Non-Cosine Response in the Gill Anemometer. Pacific Northwest Laboratory Annual Report for 1971 to the USAEC Division of Biology and Medicine, Vol. II: Physical Sciences, Part 1 : Atmospheric Sciences, BNWL-1651-1. Battelle, Pacific Northwest Laboratories, Richland, Wash. (Available from NTIS, Springfield, Va.) Izumi, Y. and Barad, M. L., 1970. Wind speeds as measured by cup and sonic anemometers and influenced by tower structure. J. Appl. Meteorol., 9: 851--856. McBean, G. A., 1974. The turbulent transfer mechanisms: a time domain analysis. Q. J. R. Meteorol. Soc., 100: 53--66. Monin, A. S. and Yaglom, A. M., 1971. Statistical Fluid Mechanics. MIT Press, Cambridge, Mass., 769 pp.
452 Sadeh, W. Z., Cermak, J. E. and Kawatani, T., 1971. Flow over high roughness elements Boundary-Layer Meteorol., 1: 321--344. Seguin, B. and Gignoux, N., 1974. Etude experimentale de l'influence d ' u n reseau de brise-vent sur le profil vertical de vitesse de vent. (An experimental study of wind profile modification by a network of shelterbelts.) Agric. Meteorol., 13: 15--23. Weiss, A. and Allen Jr., L. H., 1976a. Air-flow patterns in vineyard rows. Agric. Meteorol., 16: 329--342. Weiss, A. and Allen Jr., L. H., 1976b. The flux angle distribution of m o m e n t u m as determined by propeller anemometer measurements. Q. J. R. Meteorol. Soc0, 102: 749--753.
VERTICAL A N D H O R I Z O N T A L M R F L O W A B O V E R O W S O F ~NEYARD*
A
ALBERT WEISS and L. H. ALLEN Jr.
University of Nebraska, Panhandle Station, Scottsbluff, Nebraska 69361 (U.S.A.) U.S. Department of Agriculture, Agricultural Research Service, University of Florida, Gainesville, Florida 32611 (U.S.A.) (Received May 10, 1976; accepted September 28, 1976)
ABSTRACT Weiss, A. and Allen Jr., L. H., 1976. Vertical and horizontal air flow above rows of a vineyard. Agric. Meteorol., 17: 433--452. Air-flow studies were conducted in a 4.85 ha vineyard. Mean horizontal wind-speed profiles were obtained with cup anemometers. Propeller anemometers measured longitudinal, lateral, and vertical velocities above the rows as a function of height, wind speed, and wind direction. A constant momentum-flux zone above the vineyard, within the measured heights of 227 to 641 cm, was not found. The semilog horizontal wind-speed profiles, obtained over the vineyard, fitted two straight lines, representing an inner and outer zone within the boundary layer. Z0 was nearly constant. The number of uninterrupted updrafts of digitized propeller anemometer records was approximately equal to the number of uninterrupted downdrafts at each height, but the duration of these uninterrupted values was different. The total number of updrafts and downdrafts were, in general, not equal. Spectra from the four vertical anemometers showed that the total variance increased with height, as contributions to the low frequency portions of the spectra increased.
INTRODUCTION
Many micrometeorological studies made over relatively smooth, uniform terrain have shown that a constant-flux layer exists. Since much research has been devoted to relating fluxes to gradients in this layer (see Monin and Yaglom, 1971, chapter 4), flux-gradient relationships are relatively well understood. However, fewer studies have been performed over rough, nonuniform, or wide-row surfaces, such as in a vineyard. In a vineyard study, Graecz (1972) concluded that a constant-flux layer * Contribution from the Agricultural Research Service, U.S. Department of Agriculture, in cooperation with the Cornell University Agricultural Experiment Station, Ithaca, N.Y. Department of Agronomy Series Paper No. 1151.
434
existed, at least between 2.5 and 5.0 m above the ground, and mean vertical velocities were near zero within this layer. Hicks (1973), who also measured flux densities with a fluxatron over this same vineyard, f o u n d the drag coefficient doubled as the wind changed from down-row to cross-row directions, and evapotranspiration rates increased 10 to 20% with cross-row flow as compared with down-row flow. Sadeh et al. (1971), who used a wind tunnel to simulate turbulent flow in a forest, proposed a 10% limiting value of relative roughness (the ratio of canopy height to boundary-layer thickness) above which the log wind profile deviates from a single straight-line relationship. Instead of one log profile, they f o u n d their data fitted two log profiles of different slopes representing an inner and outer layer when the relative roughness was greater than 10%. Measurements of horizontal wind speed behind a series of 8 m wind breaks by Seguin and Gignoux (1974) also fit two semilogarithm straight lines of different slope. The objective of our experiment was to determine the turbulence properties above a vineyard as measured with propeller anemometers. Especially we wanted to investigate the crop boundary layer to determine the nature of the " c o n s t a n t flux layer".
DESCRIPTION OF THE EXPERIMENT
Three individual vertical-component propeller anemometers (W-l, W-2, and W-3) and a three-component, UVW propeller anemometer system were placed above a vineyard canopy at four heights on a 6-m mast. The vineyard was located at Hector, New York, which is on the east side of Seneca Lake about 40 km west of Ithaca. It consisted of 4.85 ha of Delaware and Concord vines trellised in north--south rows with a cross-row fetch of approximately 220 m from the west. The vines were approximately 2 m high and 1 m wide with about a 2-m open space between rows. The heights at which wind measurements were made are given in subsequent tables. Weiss and Allen (1976a) describe the vineyard experimental site in more detail. The vertical c o m p o n e n t anemometers were raised and lowered as a unit to measure the eddy structure over a vineyard as a function of wind speed, wind direction, and height. The UVW anemometer, positioned at the t o p of the array, was always oriented with the U and V propellers facing the mean wind to improve threshold response and avoid shadow effects from the mast. The outputs from the propeller anemometers were recorded on an FM analog tape recorder and zero signals were recorded on alt channels every hour. Later, the data were filtered with a 10 Hz, low-pass filter, digitized at a rate of 5 scans sec -1, and recorded in digital format on magnetic tape for further processing. Cup anemometers were positioned at 2 2 7 , 2 7 7 , 3 2 7 , 3 8 2 , 437, and 497 cm above the ground. Comparisons were made among mean vertical velocities and mean horizontal wind speeds for various heights and between the
435 h o r i z o n t a l wind speed as m e a s u r e d b y c u p and p r o p e l l e r a n e m o m e t e r s . The a n e m o m e t e r array is s h o w n in Fig.1. The c u p a n e m o m e t e r s were placed over
Fig.1. The above-row experimental setup of propeller and cup anemometers facing northward, i.e., with east at the right and west at the left. the leading edge o f t h e d o w n w i n d trellised c a n o p y , whereas the p r o p e l l e r a n e m o m e t e r s were placed over the trailing edge o f t h e same trellised c a n o p y . Detailed m e a s u r e m e n t s were m a d e o n O c t o b e r 11 and o n O c t o b e r 17, 1972. The v i n e y a r d c a n o p y foliage was still green and flourishing because n o killing frosts had o c c u r r e d b e f o r e d a t a were t a k e n in 1972. CALIBRATION OF PROPELLER ANEMOMETERS T h e t a c h o m e t e r g e n e r a t o r o u t p u t of the p r o p e l l e r a n e m o m e t e r s was m e a s u r e d in a wind t u n n e l at t h r e e speeds (54, 194, and 343 cm sec -1 ). T h e p r o p e l l e r was r o t a t e d with r e s p e c t to air flow along the t u n n e l at angles o f a t t a c k f r o m 0 ° t o 180 °, with calibration m e a s u r e m e n t s m a d e at 5 ° intervals. (The angle o f a t t a c k is d e f i n e d h e r e as the angle b e t w e e n the wind v e c t o r and
436 the rotation axis of the anemometer, with 0 ° indicating flow toward the front of the propeller, and 180 ° indicating flow from the rear along the propeller shaft.) At 54 cm sec-1 the propeller anemometers stalled in the region from 70 ° to 110 ° angles of attack, but the anemometers did n o t stall at the two higher speeds except at the 90 ° angle of attack (Fig.2). The mea-
1Q
W I N D S P E E D = 3 4 3 C M SEC 4 • : I D E A L C O S I N E RESPONSE • = MEASURED RFSoONSE ~ll21I * ~e Ao •e Ae ,"
O~
%'.. !
'
:> 0C
| "
1
4
e•
"2;
-05
. oA
"flit
_1001
I
I
I
I
ANGLE
I
1,90 1 1 1 I OF ATTACK
I I
[ '~t£ (~'O
e
(deg)
Fig.2. Comparison of an ideal cosine response with measured response (normalized voltage output, Ve/V180, for angles of attack ~, from 0° to 180°) of the W-1 propeller anemometer for a horizontal velocity of 343 cm sec-i . sured response in a wind tunnel approximated a cosine response over the entire range of angles of attack, with the greatest absolute deviation occurring at about +- 40 ° (at 50 ° and at 130 °) from the stall angle. Correction factors for deviation from cosine response were c o m p u t e d based on the calibration at 343 cm sec -1. The measured response as a function of angle of attack was also nearly linear over the range of 50°--130 ° (Fig.2). Wind velocities were determined by two methods from the propeller anem o m e t e r voltage outputs. For the three-component UVW anemometer, and for a n e m o m e t e r W-3, which was positioned immediately below the UVW unit (and hence used as the vertical-component anemometer of another UVW system), velocity components were c o m p u t e d itemtively using the corrected cosine response. The iteration m e t h o d consists of the following steps. (1) Compute initial estimates of the three velocity components based on tachometer generator outputs. (2) Compute the angles of attack for the three components based on the initial-velocity estimates. (3) Compute new velocities based on correction factors for each angle of attack. These factors corrected for the deviation from an ideal cosine response using the data in Fig.2.
437
(4) Repeat steps (2) and (3) until the difference between the current and previous angles of attack were less than or equal to 1 °, or until five iterations were completed. (Usually it t o o k no more than three iterations before convergence.) This process was similar to a computer algorithm described by Horst (1972). For the two individual vertical anemometers located lower on the mast (W-1 and W-2), the linear portion of the response curve, from approximately 50 ° to 130 ° , was used to yield a linear regression equation based on voltage outputs. Hicks (1972) determined t h a t the difference between the slopes of these straight line segments was less than 5% for the 200--1,000 cm sec-1 range of horizontal velocities. V E R T I C A L VELOCITY AND T U R B U L E N C E
Three runs (13h40--14h10, 14h10--14h40, and 14h44--15h10 EST) with the vertical and UVW a n e m o m e t e r array were made on October 11, 1972, and ten runs (09h29--09h51, 09h51--10h13, 10h15--10h44, 10h45--11h13, 11h13--11h42, 12h15--12h47, 12h48--13h15, 13h19--13h38, 13h38--13h57, and 14h00--14h14 EST) were made on October 17, 1972. The information on October 11 was summarized as a unit since the mean vertical velocities at each respective height were similar. During the first five runs on October 17, the a n e m o m e t e r array was positioned at the upper set of heights (437-641 cm), so the data in the first five runs were summarized as a unit (09h29--11h42 EST). Finally, the data of runs 6--10 on October 17 were averaged together since the a n e m o m e t e r array was at the lower set of heights (227--412 cm) for these five runs (12h15--14h14 EST). Means, standard deviations, and turbulent intensities of the vertical velocities are summarized in Table I. The turbulent intensities of the vertical velocities (the ratio of the rms fluctuations of the vertical velocity to the mean horizontal wind speed) were based on cup a n e m o m e t e r data at the respective vertical anemometer heights. The mean vertical velocities for each run on October 17 when the anemometers were in the upper and lower positions were averaged, and the results plotted in Fig.3. The vine canopies are drawn to scale and it is clear that a distinct vertical-velocity profile was present. Positive vertical velocities (updrafts) dominated between 360 and 560 cm, while negative velocities (downdrafts) dominated above and below this area. The data for vertical anemometer W-4 in the lower position (412 cm, runs 6--10 on October 17) were n o t included in this analysis. A strong d o w n d r a f t was indicated by this a n e m o m e t e r during those runs, which was inconsistent with all other data collected at or near this height. We suspect t h a t either a n e m o m e t e r leveling, signal transmission, or data recording or processing m a y have been faulty. The vertical velocities in Fig.3 showed a consistent divergence from a height of 227 cm to about 450 cm, and a consistent convergence from about
:~g8 TABLE I Mean vertical velocities, standard deviations, and t u r b u l e n t intensities for the f o u r vertic~J a n e m o m e t e r s f o r O c t o b e r 11 and O c t o b e r ] 7 , 1972 Anemometer
Height* (cm)
Velocity (cm sec-1 )
Standard deviation ( c m sec -1 )
Turbulent intensity
Oct. 11, 1972; W-1 W-2 W-3 W-4
1 3 h 4 0 - - 1 5 h 1 0 EST; 227 277 327 437
average o f 3 runs: -13.5 -13.6 - 6.8 i2.4
47.9 51.2 53.5 65.6
0.146 0.135 0.129 0.136
Oct. 17, 1972; 0 9 h 2 9 - - 1 1 h 4 2 EST; W-1 437 W-2 497 W-3 557 W-4 641
average o f 5 runs: 13.6 10.9 0.3 -5.8
44.6 46.9 49.6 57.7
0.118 0.114 ---
Oct. 17, 1972; W-1 W-2 W-3 W-4
average o f 5 runs: -11.9 -10.6 - 6.0 -14.1
39.8 46.9 50.2 58.8
0.149 0,151 0.151 0.152
1 2 h 1 5 - - 1 4 h 1 4 EST; 227 277 327 412
*The h e i g h t is w i t h r e f e r e n c e t o t h e ground.
7OO -
VERTICAL VELOCITY ( c m sec I) -20-10 O 10 2 0 l
600 -
{
{
{
r
>
500 -
~400 l-I 300
200 100 0
-__
1 ~_ L _ _
,, i
Fig.3. Profile o f the m e a n vertical v e l o c i t y based o n the m e a n values for the upper and l o w e r a n e m o m e t e r p o s i t i o n s for O c t o b e r 17, 1 9 7 2 . Relative d i m e n s i o n s o f trellised vineyard r o w s are s h o w n .
450 cm up to 641 cm. Clearly, we did n o t find a layer of constant m o m e n t u m flux immediately above this vineyard canopy. Turbulent intensity of vertical velocity appeared to be relatively invariant with height within a set of runs (Table I). However, the vertical turbulent
439 intensity appeared to be higher during the 12h15--14h14 EST period on October 17 (a cross-row flow period) than during the 0 9 h 2 9 - - 1 1 h 4 2 EST period (an along-row flow period). Also, dual-channel oscilloscope displays of propeller anemometer outputs showed sharper spikes of vertical velocities near the top of the trellised vineyard canopy than at higher elevation. These spikes were probably due to sudden and extreme changes in the vertical c o m p o n e n t near the surface roughness elements (curtains of leaves). Intensity of turbulence of horizontal flow measured with propeller anemometers (I), friction velocity (u.), and drag coefficients (C D ) of the vineyard as related to height, wind speed, and wind direction are included in Table II. The wind direction was measured at the ton of the row and the mean direction was estimated from strip-chart records. The intensity of turbulence, I, is the ratio of the rms value of the fluctuations of the horizontal wind speed (V~2)v2 to the mean horizontal wind speed Vh, i.e.: I = - -
(1)
Turbulence intensity, I, decreased with height (compare October-17 runs at a height of 603 cm with runs at a height of 374 cm in Table H), b u t was n o t related to wind speed. Turbulence intensity was also greater for crossrow flow than for down-row flow (compare October 17 (12h15--14h14 EST) with October 11 ( 1 3 h 0 4 - - 1 5 h 1 0 EST). Runs 1 and 2 (October 17), with a relative down-row c o m p o n e n t (328 ° ) had a lower intensity of turbulence, 0.316, as compared with runs 3, 4, and 5, with a relative cross-row component (303 ° ) which had a turbulence intensity of 0.349 (not shown in Table II). A difference of 25 ° between the two sets of data resulted in a distinct change in the intensity of turbulence. The friction velocity, u. is defined as: U2
,
= -
T
P
_
--u
t
w
t
(2)
where u' and w' are deviations from the mean of horizontal and vertical velocity, respectively, and the over bar is the time average, T is the shearing stxess, and p is the density of the air. In practice, however, it is easier and less costly (in c o m p u t e r time) to calculate it as: -u:,
~- u i w i -
u w
(where the subscript, i, indicates instantaneous values), with little loss of accuracy if the low-frequency c o m p o n e n t s of a trend are removed. Hicks (1970) electronically removed the trend and c o m p u t e d u:, as just the first term on the right hand side of eq.3. Trend was removed in this study by
(3)
440
¢1 o 0 ~
d 0
.-c.
¢,100
°~ 0 - ~ ¢ 9 ¢',,1 u
0
u
m
Q
¢,,1 , ~
0
¢,1 r..4
441
subtracting the product of the mean horizontal and vertical velocities from the mean value of the instantaneous u i w i product. The drag coefficient, CD, is defined as: = 2
(4)
and can be interpreted as a nondimensional measure of the roughness of the underlying surface and its influence on the flow. The values of C D (Table II) were higher for cross-row flow (rough aerodynamic surface) than for downrow flow (smoother aerodynamic surface). COMPARISON OF CUP AND P R O P E L L E R ANEMOMETERS
The horizontal wind speed as measured by cup anemometers was compared with that measured by propeller anemometers (Table II). The heights given are those where the horizontal wind speed, Vh, from the U and V propeller anemometers was determined. The cup anemometer wind speed was from the height closest to the Vh height; 382 cm when the propeller anemometers were in the lower position and 497 cm when the propeller anemometers were in the upper position. Valid comparisons can be made only for runs 1--3 on October 11, and runs 6--10, on October 17, when the distance b e t w e e n V~ and a cup anemometer were - 1 7 and +8 cm, respectively. The cup a n e m o m e t e r indicated a 24% greater horizontal wind speed than the propeller anemometers on October 11, and a 17% greater horizontal wind speed on October 17. Izumi and Barad (1970) compared velocity measurements of cup and sonic anemometers and found that cup anemometers overestimate the wind speed by 10% for uniform terrain. Our cup anemometers may have overestimated mean horizontal wind speeds over the rough vineyard canopy by 17%. Since cup and propeller anemometers b o t h have inertia, this overestimation could partly be due to lack of response of the cup anemometers as compared with the propeller anemometers. H O R I Z O N T A L WIND P R O F I L E S
Graphical comparisons of the cup anemometer data for October 17 showed that a zero-plane displacement, D, of 140 cm gave the best log-linear fit for the lowest four heights. The horizontal wind profiles were n o t corrected for stability, b u t conditions during b o t h days were cool, partially clouded and near neutral conditions were assumed. The data from the 6 anemometers formed t w o distinct linear profiles with different slopes. Run 1 on October 17 ( 0 9 h 2 9 - - 0 9 h 5 1 EST) was typical of the horizontal windspeed profiles (Fig.4). The first segment consisted of the data from the 4 lowest anemometers while the second segment consisted of the data from
442 100C
~-7,1972
0930-0950
RUN 1
EST
100
/ 10
HORIZONTAL WIND SPEED ( C M SEC -1)
Fig.4. H o r i z o n t a l w i n d s p e e d profile illustrating inner and o u t e r z o n e s for run 1 ( 0 9 h 3 0 - 0 9 h 5 0 EST) o n O c t o b e r 1 7 , 1 9 7 2 . D = 1 4 0 cm.
the 2 highest anemometers. These latter two points showed an increase in wind speed larger than that which would be shown by extension of the log profile formed by the 4 lowest anemometers. Fig.5 and 6 relate the horizontal wind speed to the mean vertical velocities at approximately the same (logarithmic) heights for the upper and lower vertical anemometer ~ o n s on October 17, respectively. Two characteristics of the semilog horizontal wind speed profiles were 100(
on
ell~ ee
•
•
enei
•
•
9
euA• ue ell
•
I(X
0CT1,71972 o = RUN 1 • = RUN 2 • = RUN 3 • = RUN4 • "= RUN5
1 ° o ~ HORIZONTAL WIND SPEED
-~o ~ (cm ,sec-1)
I
,o
]
~o
I
~o 40
V EA"flC..z~ VELOCITY
Fig.5. C o m p a r i s o n o f h o r i z o n t a l w i n d s p e e d w i t h vertical v e l o c i t i e s for runs 1 - - 5 o n O c t o b e r 17, 1 9 7 2 w i t h a n e m o m e t e r s in the u p p e r p o s i t i o n a n d D = 1 4 0 cm.
443 100C
tern t u
•
•
ow
•
ii
•
£~ 10C
•
OCT. 17,,1972 • = RUN 6 • = RUN 7 • = RUN 8 • : RUN 9 o : RUNIO O HORIZONTAL WIND SPEED
( c m sec-I)
IO
2O
VERTICAL VELOCITY
Fig.6. C o m p a r i s o n o f h o r i z o n t a l w i n d s p e e d w i t h vertical velocities for r u n s 6 - - 1 0 o n O c t o b e r 17, 1 9 7 2 w i t h a n e m o m e t e r s in the l o w e r p o s i t i o n a n d D = 1 4 0 cm.
evident. First, the roughness length (Z0), based on the data from the four lowest anemometers, was nearly constant and averaged 6.0 cm. Second, the two linear segments of the log profiles intersected at approximately the same height where the mean vertical velocity changed from net downdraft to net updraft. The horizontal wind speed profile in Fig.4 is similar to those measured by Sadeh et al. (1971) over a model forest, and by Seguin and Gignoux (1974) over a series of windbreaks. The two uppermost data points which do not lie on the same profile as the four lower data points would normally not be used in the analysis of the wind profiles. Usually these higher data points have been eliminated based on the assumption that they lie above the aerodynamic boundary layer and are not representative of the underlying surface. Furthermore, flux divergence in the boundary layer, where the log profile was used, has been assumed implicitly to be zero. The mean vertical velocities presented in Fig.3 showed a clear pattern at each height in relation to other heights, although there was scatter between runs. TOTAL AND UNINTERRUPTED AND STALLS
VALUES FOR UPDRAFTS, DOWNDRAFTS,
Desjardins (1972) placed two propeller anemometers above a barley crop at the same height but separated horizontally by 4 m. He found that, in general, updrafts did not equal downdrafts at both locations as measured by the propeller anemometers. In this study, we extended the study of updrafts, downdrafts, and stalls into the vertical. Table III shows data separated into numbers of updrafts, downdrafts, and
444
~
o
e~
~.'~
e~
I l t l
I l l l
e~ 0
0
0
Lo
e~
e~
L',-
0 0
445
stalls for the three sets of runs. These values are a function of the digitizing rate and could also be represented in terms of time. The total vertical air passage for each anemometer was calculated by determining the velocity at each scan and multiplying it by the digitizing period and accumulating the sum. The mean velocities were calculated for both the updrafts and the downdrafts. As the distance from a surface increases, the number of stalls would be expected to decrease. However, Table III shows more stalls at anemometers W-3 and W-4 than at W-1 and W-2. These differences were due to the two methods used to determine velocities. The differences in the n u m b e r of updrafts and downdrafts and in the length of air passage, obtained from the data presented in Table III reflect the mean (net) vertical velocities, presented in Table I. The mean velocities for downdrafts were generally greater than those for updrafts when the anemometers were in the lower position. However, when the anemometers were in the upper position, on October 17, the mean vertical velocities associated with updrafts, for the lower two heights, were greater than velocities associated with downdrafts. This relationship was reversed for the upper two heights. Table IV presents data from the same runs on October 11 and October 17 in terms of uninterrupted updrafts, downdrafts, and stalls. The number of uninterrupted updrafts was generally slightly lower than of uninterrupted downdrafts at the same heights. The mean durations were n o t equal, and updraft mean durations usually varied in height inversely with respect to d o w n d r a f t mean durations. In Table III, the total number of u p d r a f t and d o w n d r a f t counts were, in general, very different. The mean velocity for uninterrupted updrafts and downdrafts increased with height in all cases, but the mean air passage was a function of both the mean duration and the mean vertical velocity. The number of uninterrupted stalls almost equaled the number of total stalls. Fig.7 shows histograms of uninterrupted occurrences of updrafts and downdrafts of each vertical anemometer at 1-sec intervals for run 1 {09h29-09h51 EST) on October 17. This frequency distribution is representative of the data taken on both days. Values to the left of zero on the time axis represent uninterrupted duration in seconds of downdrafts, while those to the right represent uninterrupted duration for updrafts. For run 1 (09h29--09h51 EST) on October 17, when the vertical anemometers were in the upper position and the wind direction was 331 ° , the lower two anemometers had distributions which tailed to the right (updrafts) with uninterrupted occurrences of updrafts that lasted up to 15 sec. Anemometer W-3 came closest to a normal distribution, while the anemometer W-4 distribution tailed to the left (downdrafts) with uninterrupted downdrafts lasting up to 15 sec. Fig.8 shows the frequency distribution of mean velocities associated with uninterrupted updrafts and downdrafts for run I on October 17. The abscissa is divided into velocity intervals of 10 cm sec-' . Two c o m m o n properties
446 0
0 0 0 0
IIII
IIII
0 ,-~ ,.~
"0 0 e~.
e~
e~
2
• 0
'5
0
oO v-~ o o [-.-
0
0
e=
I ~4 , ~ ¢ 0 ¢'0 ¢ 0 , ¢ 0
0 0
m
Zo
447
OCT 17,1972 0929~0951 EST
RUN 1
~z
100~
641 cm
Or200
3
D
100
0
O[
o o
557cm --
42+7m 100~}_
downJ L updrafts j_[- L L _ ~ f t s
200
1
106~
4:37cm
0 j
llllilillillIIililiJllJlJli+Itl I©
5
0
5
10
TIME (sec) Fig.7. U n i n t e r r u p t e d occurrences o f updrafts and d o w n d r a f t s at 1-sec intervals for four heights above ground over a vineyard. R u n 1, 0 9 h 2 9 - - 0 9 h 5 1 E S T , o n October 17, 1 9 7 2 .
RUN 1
0CT17,1972 0929-0951 EST
Ld 100 I~
U 0 a
4
r ©~-
lcm
downdrafts
updrafts 557cm
0~
I00I
Tcm
+F
cg
©i
_z 100 0 z D
~
3
7
cm
j,,l,,+,i,,,,l,,,,l,,,,l,,
10 5 0 5 10 VELOCITY (cm sect1)
,,
.10
Fig.8. Mean velocities for uninterrupted updrafts and d o w n d r a f t s at intervals o f 10 c m sec -1 for four heights above ground over a vineyard. R u n 1, 0 9 h 2 9 - - 0 9 h 5 1 E S T o n October 17, 1 9 7 2 .
were found in all runs and are illustrated by Fig.8. As height increased, vertical velocities related to both uninterrupted updrafts and downdrafts increased. Also, as a corollary, as height increased, the distributions became flatter. The second property has been considered previously, i.e., the large number of stalls associated with angles of attack near 90 ° when the iterative
448
direction cosine technique was used to compute vertical velocities. These large numbers of stalls are illustrated by the histograms of anemometers W-3 and W-4 at heights of 557 cm and 641 cm, respectively. The numbers of uninterrupted occurrences between the interval 0--10 cm sec -1 for both updrafts and downdrafts were much lower than those for neighboring intervals. This result was different from the findings for the same intervals for anemometers W-1 and W-2. Therefore, it can be concluded that the current correction factors employed in the iterative direction cosine technique overestimate the number of stalls. Furthermore, it has been shown by McBean (1974) for momentum and heat flux densities as measured by a sonic anemometer, and by Weiss and Allen (1976b) for momentum flux density using a UVW propeller anemometer, that over half of the momentum flux associated with downdrafts occurred between 80 ° and 90 ° angles of attack. However, the momentum flux associated with updrafts occurred in a broader band between 95 ° to 125 ° . Nevertheless, this downdraft momentum flux near stall angles indicates that great care must be taken in calibrating propeller anemometers and computing wind speeds near these angles of attack. SPECTRA FROM THE PROPELLER ANEMOMETERS
Spectra were computed from the four vertical anemometers, the U and V anemometers, and the horizontal wind speed (Va) for run 1 on October 17 (Fig.9), using the fast Fourier transform (FFT) developed by Cooley and
~f.,.~%. 4 •. " 641 cm
1000 100 10
OCT 17,1972 RUN 1 0929-0951 EST
1 10000
1 ,,/....,
IOO0
%
•"'~
100 10
3 ":
1 1 .,,...,. 2
1000
.-,"
100
•
10 ,i-
%
600 c~
•. 557 cr~
1
1
.
I0 0
1
v
",".'-..,.. vh
1000
1000
•
":
I O0
497 cr~
t0
""
: - ' > ~ . . . .% .v 604 cm
i 1
1 ........ 1 " '.
1000 IO0 t0 •
°°
~"
1000
437c~
1
I0C
..bU %
596 cm
1
11
1
001 01 i 1 i0
00101 i 1 I0 f(Hz)
Fig.9. Variance spectra from t h e f o u r vertical anemometers, the U and V anemometers, and the horizontal wind speed (Vb) for run 1, 09h29--09h51 EST, October 17, 1972.
449 Tukey (1965). A 5-min running mean was used as a high-pass filter but was n o t compensated for low-frequency distortion. Also, the times of the runs are slightly different from the same numbered runs previously discussed, since the filter was symmetrical, i.e., the data for the first and last 2.5 min of each run were not used in computing the spectra. As height increased, the contribution to the total variance from lower frequencies increased. Also, the four spectra appear to have major contributions to the total variance centered around the same frequencies. These peak frequencies are more a function of height than of wind direction or wind speed. When the anemometers were in the lower position, the peak frequency was around 0.25 Hz, whereas in the upper position, the peak frequency shifted to approximately 0.12 Hz. The spectra from the U and V anemometers and the horizontal wind speed (Vh) indicate that they are influenced more by lower frequencies than the vertical anemometers and have about 5 to 8 times as much variance as do the vertical velocities. These three spectra (U, V, and Vh ) all appear similar. The peak frequencies for the horizontal wind speed for the lower and upper set of anemometer heights was about 0.1 Hz and 0.04 Hz, respectively. ~ The coherence between anemometers W-1 and W-2, W-2 and W-3, and W-3 and W-4 is shown in Fig.10 for run 1 on October 17. 0CT17,,1972
1.0
Ill
:
•
RUN 1
t t, $ ~ ] L I
0929-0951
.
• ANEM. 1 - 2 • ANEM.2- 3 ,, A N E M + 3 - 4
.
,'i ~St+- l!i.m ~,2 "
•
L3 Z Ld
EST
•
.I, "l
T
•
+ o
.001
I
i
01
i
.1 f(Hz)
~
1
,; _
10
F i g . 1 0 . Coherence b e t w e e n vertical a n e m o m e t e r s W-1 a n d Wo2, W-2 a n d W-3, a n d W-3 a n d W-4 f o r r u n 1, 0 9 h 2 9 - - 0 9 h 5 1 E S T , O c t o b e r 17, 1 9 7 2 .
The data in the first decade (0.001 to 0.01 Hz) showed some scatter which was probably due to the effects of filtering. When the anemometers were in the upper position as in run 1 on October 17, the coherence between all anemometers was above 0.9 from approximately 0.005Hz to 0.1 Hz. For frequencies greater than 0.05 Hz, the coherence was always higher between anemometers W-1 to W-2 and W-2 to W-3 than between W-3 to W-4. The reason for this difference was t h a t the distances separating anemometers
450
W-1 to W-2 and W-2 to W-3 were between 50 and 60 cm, whereas the distance between anemometers W-3 to W-4 was 85 cm for these runs. In general, overall values of coherence were lower between all anemometers when the anemometers were in the lower position. As height increased, coherence between all anemometers increased until higher frequencies were reached and then t h e y all decreased rapidly.
CONCLUSIONS
Three individual vertical-component propeller anemometers and a threec o m p o n e n t UVW propeller a n e m o m e t e r system were placed at four heights on a 6-m mast. These anemometers were raised and lowered as a unit to collect data at two sets of heights. Data were collected from three basic wind directions; 156 ° , 313 ° and 283 ° . Horizontal wind speed was measured simultaneously by cup anemometers. The following conclusions can be made. (1) A constant flux zone was n o t f o u n d over the vineyard within the heights measured, 227--641 cm, which contrasted with Graetz's {1972) finding. (2) Intensity of turbulence indicated t h a t cross-row flow was aerodynamically rougher than down-row flow. (3) Over the vineyard, the horizontal wind as measured by cup anemometers was about 17% greater than t h a t measured by propeller anemometers. (4) The semilog horizontal wind speed profiles obtained over the vineyard indicated an inner and outer zone within the boundary layer consistent with the findings of Sadeh et al. {1971) and Seguin and Gignoux (1974). (5) The n u m b e r of uninterrupted updrafts was approximately equal to the number of uninterrupted downdrafts at each height, but their durations were different. The mean durations of uninterrupted updrafts and downdrafts were greater at anemometer heights where net updrafts and downdrafts, respectively, were observed. The total numbers of updrafts and downdrafts were, in general, n o t equal. (6) The frequency distributions of uninterrupted occurrences of updrafts and downdrafts for the four vertical anemometers, as a function of height, were generally skewed, Incidents of uninterrupted updrafts and downdrafts lasting up to 15 sec were observed. (7) The frequency distributions of uninterrupted velocities associated with updrafts and downdrafts for the four vertical anemometers illustrated that as height increased the distributions became flatter and velocities increased. (8) Spectra from the four vertical anemometers indicated that contributions to the total variance from lower frequencies increased as height increased. (9) Spectra from the U and V anemometers and the horizontal wind speed (V~) were all similar. They contained 5 to 8 times the variance as the vertical anemometer spectra and had contributions to the total variance
451
centered around lower frequencies than the vertical anemometers. (10) Coherence between anemometers was a function of the distance separating anemometers and the height above the surface. The closer that anemometers were to each other and the greater the height above the surface, the greater was the value of coherence. Above 0.2 Hz, the coherence between all anemometers fell off rapidly. (11) There was disagreement b e t w e e n this study and the results of Graetz {1972) concerning the existence and height of a constant flux layer over a vineyard. The constant flux layer is important because in this zone transfer of heat, mass and m o m e n t u m can be treated as a one-dimensional (vertical) problem. Therefore, further studies in vineyards are needed. ACKNOWLEDGEMENTS
We wish to thank the Don J. Wickham family of Hector, New York, who allowed us to use their vineyard for this study; M. N. Johnson, USDA-ARS Engineering Technician, Ithaca, New York, for constructing the propeller anemometers; Dr. G. A. McBean, Atmospheric Environment, Canada, who supplied the c o m p u t e r program which was used to calculate spectra and coherence; Ms. Barbara Babcock, who t y p e d the manuscript. REFERENCES Cooley, J. W. and Tukey, J. W., 1965. An algorithm for the machine calculation of complex Fourier series. Math. Comput., 19: 297--301. Desjardins, R. L., 1972. A Study of Carbon Dioxide and Sensible Heat Fluxes Using the Eddy Correlation Technique. Ph.D. Thesis, Cornell University, Ithaca, N.Y., 175 pp. (Dissertation Abstracts No. 73--341.) Graetz, R. D., 1972. A Micrometeorological Study of the Contributions of Individual Elements of a Community of Plants to the Momentum, Heat, and Vapour Fluxes from that Community as a Whole. Ph.D. Thesis, Australian National University, Canberra, A.C.T., 118 pp. Hicks, B. B., 1970. The measurements of atmospheric fluxes near the surface. A generalized approach. J. Appl. Meteorol., 9: 386--388. Hicks, B. B., 1972. Propeller anemometers as sensors of atmospheric turbulence. Boundary-Layer Meteorol., 3: 214--228. Hicks, B. B., 1973. Eddy fluxes over a vineyard. Agric. Meteorol., 12: 203--215. Horst, T. W., 1972. A Computer Algorithm for Correcting Non-Cosine Response in the Gill Anemometer. Pacific Northwest Laboratory Annual Report for 1971 to the USAEC Division of Biology and Medicine, Vol. II: Physical Sciences, Part 1 : Atmospheric Sciences, BNWL-1651-1. Battelle, Pacific Northwest Laboratories, Richland, Wash. (Available from NTIS, Springfield, Va.) Izumi, Y. and Barad, M. L., 1970. Wind speeds as measured by cup and sonic anemometers and influenced by tower structure. J. Appl. Meteorol., 9: 851--856. McBean, G. A., 1974. The turbulent transfer mechanisms: a time domain analysis. Q. J. R. Meteorol. Soc., 100: 53--66. Monin, A. S. and Yaglom, A. M., 1971. Statistical Fluid Mechanics. MIT Press, Cambridge, Mass., 769 pp.
452 Sadeh, W. Z., Cermak, J. E. and Kawatani, T., 1971. Flow over high roughness elements Boundary-Layer Meteorol., 1: 321--344. Seguin, B. and Gignoux, N., 1974. Etude experimentale de l'influence d ' u n reseau de brise-vent sur le profil vertical de vitesse de vent. (An experimental study of wind profile modification by a network of shelterbelts.) Agric. Meteorol., 13: 15--23. Weiss, A. and Allen Jr., L. H., 1976a. Air-flow patterns in vineyard rows. Agric. Meteorol., 16: 329--342. Weiss, A. and Allen Jr., L. H., 1976b. The flux angle distribution of m o m e n t u m as determined by propeller anemometer measurements. Q. J. R. Meteorol. Soc0, 102: 749--753.