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Observations of winds above and below a forest canopy located near a clearing
00046981/87 Q 1987 Perymon
WXJ+O.OO hum&
Ltd.
SHORT COMMUNICATIONS OBSERVATIONS
OF WINDS ABOVE AND BELOW A FOREST CANOPY LOCATED NEAR A CLEARING
D. M. LEAHEYand M. C. HANSEN Western Research, Division of Bow Valley Resource Services Ltd, 131344 Avenue N.E., Calgary, Alberta, Canada T2E 6L5 (First received 27 January 1986 and inJ%nfJorm I2 September 1986)
Abstract-Measurements of wind movements have been made within and above a forest canopy located near a 0.5 km’ clearing. They showed the existence of complex wind systems characterized by strong horizontal jets of air and large vertical velocities beneath the canopy during unstable atmospheric conditions. Under stable situations wind flowsseemed to be lesscomplex with both horizontal and vertical winds being greater above the canopy than below. Key word index: Forest canopy, micro-climatology, vertical wind velocities, horizontal jets, atmospheric turbulence.
1. lNTRODUCJ’lON
movements within a forest canopy ate of interest because they govern the local micro-climate by transporting heat, water vapour and gaseous constituents between foliage layers and the u~~tr~n~ atmosphere imm~~teiy above it. Any theory which attempts to estimate this transport must allow for characteristics of the flow. While there have been several published studies of wind measurements within a forest canopy (Petit et at, 1976;Shaw et al., 1974;Droppo and HamiIton,~1973;Szeiczet al., 1979; Leonard and Federer, 1973;Hicks et al., 19751none of these present data on average vertical winds..But studies of wind Rows within sorghum and barley canopies (Shaw and McCartnc~. 1985)found iets of air which because of continuity requirements may be associated with regions of convergence and hence vertical velocities. It was for the above reasons that an observational study was undertaken during the summer of 1985in a forest about 6Om from a 0.5 km’ clearing. The site was located in the Athabasca Oil Sands area of Alberta about 60 km north of Fort McMunay. Trees were primarily pine and aspen with heights ranging from 18to 24 m. Terrain in the vicinity of the study site was ftat. The nearest topognphical feature was Hanky Creek which flows from S to N in a shallow depression of less than IOm about 308 m from the observational site. Wind
2. r~~RUME~TATION
Measurements of horizontal winds, vertical winds and temperature were made at the 10 and 20 m levels on a 24 m tower. The Gill UVW anemometer used for the study was equipped with 23 cm diameter propellers providing response times ranging from 10 to 1 s for wind speeds from 0.1 to IOms-t, respectively. The threshold was about 0.1 ms-‘. Alignment ofanemometen on installation was accomplished using a Weathertronics Model 8297 survey compass with telescope.Special care was taken to ensure proper alignment of each anemometer with crosshairs of the telescope.
Devices used to measure temperatures were manufactured by Handar (sensor model 435 A/B). Response times were 20-30~ for gestation of 90% of ambient conditions. Measured temperature values should be accurate to within 0.2”C. 3. OBSERVATIONS Wind data were analyzed according to whether temperature differencesbetween 20and 10 m levelsindicated stable or unstable atmospheres.There were 594nmd 1281 h of information collected during stable and unstable atmospheric conditions, respectively. Horizontal and vertical wind flows above and below the canopy, at 10and 20 m respectively,were very different. The strongest and most frequent winds below the canopy tended to be directed towards the clearing centre during daytime hours. Winds in excess of 6 m s-’ occurred about 15% of the time. About 90% of these strong winds were associated with inflow towards the clearing. Calms occurred only about 1.8% of the time. At night when the atmosphere was stable the flows below the canopy tended to be away from the cfearing centre. Wind spe& were much lower and calms tended to occur about 16% of the time. Wind patterns above the forest canopy were less organixed with respect to the clearing than winds below canopy kvel. During the daytime winds werepredominantly away from the clearing. Air flow during the night did not show tendencies to be either towards or away from the clearing Calms during unstable and stable periods occurred about 0.2 and 2.3% of the time, res.pectiveIy. Figure 1 shows cumulative fr~uency ~st~butions of hourly average horizontal and vertical wind speeds as observed at the 10 and 20 m levels under unstable atmospheric conditions which occurred during day light. Maximums of both horizontal and vertical wind speeds tended to be much greater below the canopy than above. Under stable atmospheric conditions horizontal and VUtieal
winds always tended to be less Mow
the ~~llopy th
above (Fig. 2). Their behaviour was thus very diRerent from that whichcharacterized unstableatmospheres. Thestrongest
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average hourly horizontal winds recorded at the 10 and 20 m levels were about 3.8 and 5.5 m s- ‘, respectively. Magnitudes of average vertical winds seldom exceeded values of O.O3ms-‘. The distributions of average hourly vertical winds are oi particular interest because of their importance to the vertical flux of heat, moisture and pollutants. A detailed examination of the data showed that magnitudes of the vertical wind speeds at the 10 and 20 m levels exceeded 0.01 m s- ’ more than 95 and 85 y0 of the time, respectively. These frequencies were not sensitive to whether the atmosphere was stable or unstable. The actual time in which hourly average vertical wind speeds exceeded 0.01 m s- ’ was greater than measurements indicate. This is because of the insensitivity of the Gill anemometer to vertical winds of less than 0.1 m s- ‘. Vertical velocities at the IO m level tended to be negative (i.e. towards the ground) 90 and 40”/, of the time under stable and unstable atmospheric conditions, respecjively. There was thus a strong tendency for downdrafts to occur at the 10 m level during the night. During the day there was a less pronounced tendency for updrafts. When these did occur they were often associated with relatively strong vertical speeds (Fig. 1). Vertical velocities at the 20 m level were negative about 55 and 65 “/, of the time under stable and unstable atmospheric conditions, respectively. There was thus a tendency for downdrafts to occur at the 20 m level during both night and day. It is of interest to note that the tendency for downdrafts is greatest during the day when updrafts are more prevalent at the
99 99
1%)
Fig. 1. Cumulative frequency distributions of horizontal and vertical wind speeds under unstable atmospheric ccnditions.
10 m level.
The above analyses demonstrate that downdrafts during the night often extend from the 20 to the 10 m levels. On the other hand, updrafts which occur during the day at the 10 m level are often not experienced at the 20 m level. Wind fluctuation data were analyzed to determine how turbulence levels below and above the canopy compared. Analyses showed that hourly average values of standard deviations of the horizontal wind angle, u8, were about 25 7; greater below the canopy than above. Standard deviations of the vertical velocity a,exhibited an opposite behaviour being almost twice as great above as below the canopy. Differences between values of bg and u, as measured above and below canopy did not show systematrc changes with time ofday. The product O,Q tended to be always about 50% greater above the canopy than below. This means that dispersive forces in the atmosphere were greater above the canopy. This fact would influence the vertical distribution ofmoisture. heat and pollutants. 4.
9 2
m 4.01
STABLE ATMOSPHERES N = 594
I
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1 CUMZATIVE
F?EQUENz
99 99
(x)
Fig. 2. Cumulative frequency distributions of horizontal vertical wind speeds under stable atmospheric
and
This observational study has shown that wind behaviour within forest canopies can exhibit features suggestive of complex flow patterns. The patterns appear to be associated, at times, with strong vertical winds and narrow jets of air. These patterns could have an appreciable effect on the transport of heat, moisture and pollutants within a forest. For these reasons the representativeness of the observations to canopy wind flow should be further investigated. Acknowledgements-This study was financed by Alberta Environment, Research Management Division, Edmonton, Alberta, Canada.
-J 201
ii
CONCLUSIONS
REFERENCES
Droppo J. Cl. and Hamilton, Jr. H. L. (1973) Experimental variability in the determination of the energy balance in a deciduous forest. J. appl. Met. 12, 781-791.
Short Communications Hicks B. B., Hyson P. and Moore C. J. (1975) A study of eddy fluxes over a forest. J. appl. Met. 14, 58-66. Leonard R. E. and Fcdercr C. A. (1973) Estimated and measured roughness parameters for a pine forest. J. appl. Met.
12.302-307.
Petit C., Trinetc M. and Valentin P. (1976) Study of turbulent
1229
diffusion above and within a forest-application in the case of S02. Arnwspheric Environment 10, 1057-1063. Shaw R. H. and McCartney H. A. (1985) Gust penetration into plant canopies. Atmospheric Environment 19.827-830. Szcin G., Petzold D. E. and Wilson R. G. (1979) Wind in the Subarctic forest. J. appl. Mer. 18, 1268-1274.
WXJ+O.OO hum&
Ltd.
SHORT COMMUNICATIONS OBSERVATIONS
OF WINDS ABOVE AND BELOW A FOREST CANOPY LOCATED NEAR A CLEARING
D. M. LEAHEYand M. C. HANSEN Western Research, Division of Bow Valley Resource Services Ltd, 131344 Avenue N.E., Calgary, Alberta, Canada T2E 6L5 (First received 27 January 1986 and inJ%nfJorm I2 September 1986)
Abstract-Measurements of wind movements have been made within and above a forest canopy located near a 0.5 km’ clearing. They showed the existence of complex wind systems characterized by strong horizontal jets of air and large vertical velocities beneath the canopy during unstable atmospheric conditions. Under stable situations wind flowsseemed to be lesscomplex with both horizontal and vertical winds being greater above the canopy than below. Key word index: Forest canopy, micro-climatology, vertical wind velocities, horizontal jets, atmospheric turbulence.
1. lNTRODUCJ’lON
movements within a forest canopy ate of interest because they govern the local micro-climate by transporting heat, water vapour and gaseous constituents between foliage layers and the u~~tr~n~ atmosphere imm~~teiy above it. Any theory which attempts to estimate this transport must allow for characteristics of the flow. While there have been several published studies of wind measurements within a forest canopy (Petit et at, 1976;Shaw et al., 1974;Droppo and HamiIton,~1973;Szeiczet al., 1979; Leonard and Federer, 1973;Hicks et al., 19751none of these present data on average vertical winds..But studies of wind Rows within sorghum and barley canopies (Shaw and McCartnc~. 1985)found iets of air which because of continuity requirements may be associated with regions of convergence and hence vertical velocities. It was for the above reasons that an observational study was undertaken during the summer of 1985in a forest about 6Om from a 0.5 km’ clearing. The site was located in the Athabasca Oil Sands area of Alberta about 60 km north of Fort McMunay. Trees were primarily pine and aspen with heights ranging from 18to 24 m. Terrain in the vicinity of the study site was ftat. The nearest topognphical feature was Hanky Creek which flows from S to N in a shallow depression of less than IOm about 308 m from the observational site. Wind
2. r~~RUME~TATION
Measurements of horizontal winds, vertical winds and temperature were made at the 10 and 20 m levels on a 24 m tower. The Gill UVW anemometer used for the study was equipped with 23 cm diameter propellers providing response times ranging from 10 to 1 s for wind speeds from 0.1 to IOms-t, respectively. The threshold was about 0.1 ms-‘. Alignment ofanemometen on installation was accomplished using a Weathertronics Model 8297 survey compass with telescope.Special care was taken to ensure proper alignment of each anemometer with crosshairs of the telescope.
Devices used to measure temperatures were manufactured by Handar (sensor model 435 A/B). Response times were 20-30~ for gestation of 90% of ambient conditions. Measured temperature values should be accurate to within 0.2”C. 3. OBSERVATIONS Wind data were analyzed according to whether temperature differencesbetween 20and 10 m levelsindicated stable or unstable atmospheres.There were 594nmd 1281 h of information collected during stable and unstable atmospheric conditions, respectively. Horizontal and vertical wind flows above and below the canopy, at 10and 20 m respectively,were very different. The strongest and most frequent winds below the canopy tended to be directed towards the clearing centre during daytime hours. Winds in excess of 6 m s-’ occurred about 15% of the time. About 90% of these strong winds were associated with inflow towards the clearing. Calms occurred only about 1.8% of the time. At night when the atmosphere was stable the flows below the canopy tended to be away from the cfearing centre. Wind spe& were much lower and calms tended to occur about 16% of the time. Wind patterns above the forest canopy were less organixed with respect to the clearing than winds below canopy kvel. During the daytime winds werepredominantly away from the clearing. Air flow during the night did not show tendencies to be either towards or away from the clearing Calms during unstable and stable periods occurred about 0.2 and 2.3% of the time, res.pectiveIy. Figure 1 shows cumulative fr~uency ~st~butions of hourly average horizontal and vertical wind speeds as observed at the 10 and 20 m levels under unstable atmospheric conditions which occurred during day light. Maximums of both horizontal and vertical wind speeds tended to be much greater below the canopy than above. Under stable atmospheric conditions horizontal and VUtieal
winds always tended to be less Mow
the ~~llopy th
above (Fig. 2). Their behaviour was thus very diRerent from that whichcharacterized unstableatmospheres. Thestrongest
1227
1228
n
Short Communications
./’
12
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0
z 3
,..’
UNSTABLE N 1281 :
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ATMOSPHERES
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8
;i 2 g4 a =0 001
01
1
10
50
CUMULATIVE
90
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FREQUENCY
9999
(X)
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01-T-
10
50
CUMULATIVE
FREQUENCY
1
.:’
I 20 m/---i
_--_.__ _--
90
95
average hourly horizontal winds recorded at the 10 and 20 m levels were about 3.8 and 5.5 m s- ‘, respectively. Magnitudes of average vertical winds seldom exceeded values of O.O3ms-‘. The distributions of average hourly vertical winds are oi particular interest because of their importance to the vertical flux of heat, moisture and pollutants. A detailed examination of the data showed that magnitudes of the vertical wind speeds at the 10 and 20 m levels exceeded 0.01 m s- ’ more than 95 and 85 y0 of the time, respectively. These frequencies were not sensitive to whether the atmosphere was stable or unstable. The actual time in which hourly average vertical wind speeds exceeded 0.01 m s- ’ was greater than measurements indicate. This is because of the insensitivity of the Gill anemometer to vertical winds of less than 0.1 m s- ‘. Vertical velocities at the IO m level tended to be negative (i.e. towards the ground) 90 and 40”/, of the time under stable and unstable atmospheric conditions, respecjively. There was thus a strong tendency for downdrafts to occur at the 10 m level during the night. During the day there was a less pronounced tendency for updrafts. When these did occur they were often associated with relatively strong vertical speeds (Fig. 1). Vertical velocities at the 20 m level were negative about 55 and 65 “/, of the time under stable and unstable atmospheric conditions, respectively. There was thus a tendency for downdrafts to occur at the 20 m level during both night and day. It is of interest to note that the tendency for downdrafts is greatest during the day when updrafts are more prevalent at the
99 99
1%)
Fig. 1. Cumulative frequency distributions of horizontal and vertical wind speeds under unstable atmospheric ccnditions.
10 m level.
The above analyses demonstrate that downdrafts during the night often extend from the 20 to the 10 m levels. On the other hand, updrafts which occur during the day at the 10 m level are often not experienced at the 20 m level. Wind fluctuation data were analyzed to determine how turbulence levels below and above the canopy compared. Analyses showed that hourly average values of standard deviations of the horizontal wind angle, u8, were about 25 7; greater below the canopy than above. Standard deviations of the vertical velocity a,exhibited an opposite behaviour being almost twice as great above as below the canopy. Differences between values of bg and u, as measured above and below canopy did not show systematrc changes with time ofday. The product O,Q tended to be always about 50% greater above the canopy than below. This means that dispersive forces in the atmosphere were greater above the canopy. This fact would influence the vertical distribution ofmoisture. heat and pollutants. 4.
9 2
m 4.01
STABLE ATMOSPHERES N = 594
I
o.c/
00101
&__?___+I
W
1 CUMZATIVE
F?EQUENz
99 99
(x)
Fig. 2. Cumulative frequency distributions of horizontal vertical wind speeds under stable atmospheric
and
This observational study has shown that wind behaviour within forest canopies can exhibit features suggestive of complex flow patterns. The patterns appear to be associated, at times, with strong vertical winds and narrow jets of air. These patterns could have an appreciable effect on the transport of heat, moisture and pollutants within a forest. For these reasons the representativeness of the observations to canopy wind flow should be further investigated. Acknowledgements-This study was financed by Alberta Environment, Research Management Division, Edmonton, Alberta, Canada.
-J 201
ii
CONCLUSIONS
REFERENCES
Droppo J. Cl. and Hamilton, Jr. H. L. (1973) Experimental variability in the determination of the energy balance in a deciduous forest. J. appl. Met. 12, 781-791.
Short Communications Hicks B. B., Hyson P. and Moore C. J. (1975) A study of eddy fluxes over a forest. J. appl. Met. 14, 58-66. Leonard R. E. and Fcdercr C. A. (1973) Estimated and measured roughness parameters for a pine forest. J. appl. Met.
12.302-307.
Petit C., Trinetc M. and Valentin P. (1976) Study of turbulent
1229
diffusion above and within a forest-application in the case of S02. Arnwspheric Environment 10, 1057-1063. Shaw R. H. and McCartney H. A. (1985) Gust penetration into plant canopies. Atmospheric Environment 19.827-830. Szcin G., Petzold D. E. and Wilson R. G. (1979) Wind in the Subarctic forest. J. appl. Mer. 18, 1268-1274.