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A R E VERS I NG TEM PER AT UR E- DI F FER E N CE M EA SUR EM E N T SY STEM FOR BO W E N R ATI O DET E RM I NATIONJ.H. M c CA U G H E YDepartment of Geography, Queens Unirersity, Kingston, Ontario, Canada, K7L 3N6(Received 10 October, 1980)Atatract. The design and performance of a reversing temperature difference measurement system are reportecl This system employs five-junction copper-constantan thermopilcs for the measurement of A and A Tp, while a linearized thermistor is used to measure Tp. Field performance has been checked against a precision lysimeter as well as against a second temperature difference measurement system in which diode are used for temperature measurement. In both cases, the agreement between the systems is satisfactory for the measurement of hourly values of the Bowen ratio.1. IntroductionThe experimental determination of the Bowen ratio by the psychrometric method requires careful measurement of the gradients of temperature and humidity above a surface. Wherever these gradients become smal( for example over an aerodynamically rough surface such as forest, then very careful and stringent measurement techniques are required in order to achieve satisfactory results. A now commonly used method to determine the Bowen ratio is to measure directly the differences of dry-bulb temperature (AT) and wet-bulb temperature (ATp) over some height interval (Az), and to interchange the psychrometers on a regular basis. This approach was proposed by Tanner (1960), and has been operationalized by Sargeant and Tanner (1967), Black and McNaughton (1971), and Spittlehouse and Black (1980). The rationale of this method is that, if systematic error is present in the measurement and is either constant or slowly changing then interchanging the psychrometers between levels should eliminate or greatly reducu the amount of error in the measurement of the mean temperature differences and in the Bowen ratio (McNeil and Shuttleworth, 1975J The method will not eliminate a systematic error which is height dependent, e.g., different radiational heating errors at different levels (Spittlehouse and Black, 1980).The Bowen ratio (JI), in the case of direct temperature measurements betweentwo levels, is given by Fuchs and Tanner (1970) aswhere s is the sloJu of the saturation vapour pressure curve at T, y is the psychro- metric constant, 6Tp is the wet-bulb temperature difference over Az, Al is the dry-bulb temperature difference over Az, and Tp is the mean wet-bulb temperature of the air layer. The Bowen ratio in association with the surface energy balance equation givesBoundary-Layer 3feieorofogr 21 (1981) 47-55. 0006-8314/81/021 I-4J047S0I.35.Copyright 1981 by D. Reidel Publishing Co. Dordrecht, Holland. and Boston. II.5.Z .48andJ. H. MCCAUGHEY(3)where Qg is the latent heat flux, Qp is the sensible heat flux, Q* is the net radiation flux, and c the soil heat 0ux.In the system described in this paper, AT and 6 Tp are measured directly withfive-junction copper-constantan thermopiles and Tp “n measured with a thermistor. The psychrometers are interchangcd regularly between the two levels of meas- urement. The system is termed the RTDMS reversing temperature-difference measurement system throughout the remainder of this paper. The choice of thermopiles as sensors was made because (1) they are simple to construct, (2) they are direct reading devices which require no auxilliary circuitry, (3) their calibration is stable, and (4) high accuracy is obtainable. Fuchs and Tanner (1970) have provided an analysis of the errors in (1) and suggest that, for satisfactory estimates of latent heat transfer using (2), a precision of 0.01 C is necessary for AT andATj and Tp should be measured to 0. 1 O . The RTDMS achieves this precisionfor Tz for the temperature differences a precision of 0.018 C is applicable (see Equation 42. Construction of the SystemThe thermopiles were constructed from 30 a.w.g. copper-constantan thermocouple wire. For each sensor, each group of five thermojunctions was mounted inside a thin-walled, stainless steel tube (30 cm long, 4.7 mm O.D., and 0.25 mm wall thickness), and they were sealed at the tip of the tube in polyester resin. The tip of the tube had been machined to 4.4 mm O.D. which gave a wall thickness of0.10 mm The wires between the ends of the thermopile were sleeved in plastic tubing to minimize abrasion in the field.The thermopile were calibrated against a platinum resistance thermometer,and the calibration equation, which applies to each sensor, is6 T 0.009 = 4.913 - AT,r 0.999,s 0.018(CJ(4)where AT is the electrical output (mV) of the sensor. All thermopiles were tested for symmetry as part of the calibration If a sensor deviated by more than 0.02 C from symmetry, it was rejected for field use. Also, the time constant of each thermopile was determined, and the average value found was one minute.In the field, the dry- and wet-bulb thermopiles were mounted inside double radiation shields and were held rigidly in place by an acrylic plug (Figure I B). The wet-bulb was mounted 5 cm behind the dry-bulb. The basic design of this housing is due to Lourence and Pruitt (1969) with modifications adapted fromA REVERSING TEMPERATUREDIFFEREIICE MEAS UPEMENT SYSTEM49Fig. 1. Details of reversing temperature difference measurement array. (A) Field placement;(B) Radiation shielding; (C) Aspiration system ; (D) Circuit diagram.Allen (1972 In one of the housingg a linearized thermistor which measured Tp was inserte1 The radiation shields were made of polyurethane tubing wrapped on the outside with aluminized tape. The tape has a very high reflectivity and forms an effective barrier to solar radiation (Fuchs and Tanner, 1965). The inner surface of the inner shield was painted with flat black paint, and the inner surface of the outer shield was covered in aJ uminized tape. The sensors were aspirated by small fans (Rotron Nugget NTH4( which were mounted close to the pivot point of the sensor arm (Figure lC). The aspiration rate past the sensors was 3.4 in s . This was determined by placing a hot-wire anemometer (Lambrecht 641N) orthogonal to the airflow at the entrance to the inner shiel1 Given the size of the sensor tips, this ventilation rate ensured virtually 100% wet-bulb depression (Wylie, 1962). Also, the theoretical value of the psychrometer constant 6.60 I0* (I + 0.00115 Tp) C , was applicable (Tanner, 1972).In order to ensure that the double radiation shielding works to maximumefficiency, the temperature of the inner shield should be equal to air temperature.It is important therefore that air is allowed to pass between the two shields. This was achieved by cutting ventilation ports in the outer shield to allow natural ventilation between the shields. To enhance this free air motion, the whole sensor assembly swivelled so that the intakes of the housings faced into the prevailing wind. This was accomplished by mounting the assembly on a rotation bearing (Figure IA), and attaching a vane at the end of the shaft which held the sensor arm The assembly rotated through 350 and was restrained by a stop constructed from a thick rubber strip stretched across a T-shaped support The assembly rotated if the windspeed was above 0.25 in s Distilled water was conducted from individual reservoirs to each wet-bulb by a wick made from cotton shoelace. Before use, each wick was boiled in a mild solution of detergent and bleach to remove any dirt which might be present. The wicks were changed regularly by an operator wearing plastic gloves. Each wet-bulb reservoir, mounted on the side of the acrylic plug had a capacity of50 ml arid, depending upon the saturation deficit of the air, had to be refilled every 1 to 3 days. Ten centimetres of wick, sleeved over the wet-bulb, were ezposed in the airstream inside the radiation shield to ensure that there was no heat con- duction to the sensor tip by the distilled water (Lourencc and Pruitt, 1969). The reservoirs were constructed of clear plastic tubing and were wrapped in aluminized tape in order to mi. nimize radiational heating.The sensor arm was turned through 180 by a 4 r.p.m. Hurst reversing motor (model DB). The motor was mounted next to the shaft which held the sensor arm. A gear wheel, 7.6 cm in diameter, was attached to the end of the motors shaft, and another one was mounted on the sensor arm at its pivot point These two wheels were linked together by a chain.The reversing cycle was controlled by an electronic timer which acted as adown-counter (Figure I D). The time could ie set anywhere from 1 to 99 min by the use of thumbscrews. The count was initiated by a manual switch closure, and the time remaining befpre reversal was shown by an LED display. If for any reason there were an interruption in the experiment, the timer could be reset by means of a reset switch; this instant reset capability virtually eliminated any possibility of timing or co-ordination problems between the RTDMS and the recording system. At present, the timer runs on AC power and consumes about 2tD W. Most of the power is used in the LED display. A totally DC version which will operate from a 12 V battery is being developed and will have exactly the same counting circuit as the AC version but a much smaller power consumption. It is estimated that a regular car battery will operate the DC timer for several weeks on a sin8le charge. The circuit for either timer is available on request.During operation, the timer counted down in seconds to zero when an electricalshort occurred across the output terminals. This changed the mode of the direction relay, and the direction of the reversing motor changed from clockwise (CW) to counter-clockwise (CCW) rotation or vii:e versa At the conclusion of the 180 turn of the sensor arm, a control arm, attached to the shaft which carried the vaneA REVERSING TEMPERATURE-DIFFERENCE MEA 5 UREMENT SYSTEMand sensor arm, ran up against a microswitch which interrupted the power to the motor. If the control arm backed o8 the microswitch, as sometimes happened in windy conditions, power was returned to the motor and the sensor arm was driven once again into a vertical position There were two microswitches in the circuit; one controlled the CW cycle while the other controlled the CCW cycle. Both were mounted on moveable platforms, the horizontal position of which could be adjusted so as to ensure that the sensor arm always stopped in a vertical position. This aspect of the system provided versatility, and it also conserved power and minimized motor wear because the normal condition in the period between actual reversals was to have no power on the reversing motor.3. System Performance TestsTwo field tests have been conducted with the RTDMS. The first was run at Wood- bridge, Ontario, in September 1978 when the performance of the RTDMS was evaluated against the precision lysimeter operated by the Atmospheric Environment Service. The second was conducted in June 1980 in Vancouver, where the RTDMS was compared to another temperature difference measurement system (Black and McNaughton, 1971).3. 1. WOODBRIDGE TEST RESULTSOn September 7, 8, and 9, 1978, the RTDMS was operated at the Woodbridge experimental site. This site is grass covered, and during the experiment, the grass was 0.07 in high. The RTDMS was set up adjacent to the lysimeter, and temperature differences were measured between 0.20 and 1.20 in The values were recorded continuously on a potentiometric recorder (Honeywell, Model 194). The psychro- meters were reversed every 30 min; the first 5 min after reversal was treated as an adjustment period. Samples of AT and Adp were extracted every minute from the traces for the next 25 min, and the hourly mean Bowen ratio was found from the data from two half-hour periods. Net radiation was measured by a Funk net pyrradiometer, and soil heat flux was measured by a flux plate placed at a depth of 0.01 in in the lysimeter. The latent heat flux was found from (2). The sensitive lysimeter is 6 in in diameter and it was covered with the same grass as the sur- rounding area The sensitivity of this lysimeter for evapotranspiration is 10 W in A description of the lysimeter is given in Mukammal et at. (1971).During the experiment, a variety of atmospheric conditions prevailed: September 7 was characterized by broken cloud for the whole day accompanied by brisk winds; September 8 was overcast and rain occurred from Ol tD to 04tD h; September 9 was cloudless all day. On each day, the RTDMS was operated from 07tD to 1810 h. Two hours of data were lost on the afternoon of September 9The system is owned by Dr T. R Oke, Department of Geography, University of British Columbia.Q (Lysimeter) (Wm)Fig. 2. Comparison of Queens system with lysimeter, Woodbridge, September 1978.because the temperature difference signals exceeded the range Of the recorder. The maximum values for dry- and wet-bulb temperature differences were 0.90 and 0.98 C in , respectively, and the corresponding minima were 0.02 and 0.05 C in . No data were collected under inversion conditions. The perform-ance of the RTDMS is summarized in Figure 2 where the value of latent heat flux from the RTDMS is shown as Qg (Queens). The data points are well distributed about the 1 : 1 line, and there is no evidence of a systematic underestimation or overestimation of the latent heat flux by the RTDMS.3.2. VANCOUVER TEST RESULTSIn June, 1980 at a site at Vancouver International Airport, a comparative test was run between the RTDMS and another proven reversing temperature difference system from UBC (Kalanda ei of., 1980). The UBC system had undergone modi- fication to the aspiratim components before beginning the experiment. Indi- vidual fans (Rotron Nugget NTH4) had been installed in a similar fashion to that shown for the RTDMS (Figure 1). These provided a minimum aspiration rate of 3.5 in s for each pair of sensors. Both systems were set up side-by-side over a grass surface; the supporting data (Q* and QG) to solve the energy balancewere collected from a Swissteco net pyrradiometer (Typ: S-1) and three Middletonsoil heat flux plates. The net pyrradiometer was positioned at a height of 1 in, and the soil heat flux plates, connected in series, were inserted at a depth of 0.01 inA REVERSING TEMPERATURE-DIFFERENCE MEASUREMENT SYSTEM 53and separated to obtain a spatial sample. The lowest level of measurement for dT and A dp was 0.55 in, and the separation between levels was l in (Figure 3). The sensor heads on both systems reversed every 15 miii The first 5 min after a reversal was an adjustment period when no data were recorded, and for the next 10 min the signals were integratel The half-hour mean Bowen ratio was calculated from the data from two consecutive sample periods. All data, with the exception of the temperature differences from the UBC system, were recorded on a data system (Campbell Scientific, Model CR5). The UBC system had its own separate data 1o8ger which also contained the logic circuits that controlled the reversal and sampling sequences.Fig. 3. Reversing temperature difference apparatus in position at Vancouver Airport site, June 1980.The comparison began at 15tD h on June 19 and continued until 0830 h on June 23. In tota( 71 pairs of hourly average values of latent heat flux were available for comparative purposes. During the experimental period, both strong inversion(AT - 3.18 C in , dip - 2.95 FC in ) and strong lapse (d F - 0.98 FC in ,Adp - 1.17 C in ) conditions occurred. The weather conditions varied from clear skies with light southwest winds on the 19th, through scattered cumulus with southeast winds on the 20th, to overcast conditions with southeast winds on54 J. H. MCCAUGHEYthe 21st. Intermittent rain occurred on the morning of the 22nd, and it was overcast with southeast winds from then until the comparison was terminated. The results are shown in Figure 4. There is a slight tendency for the values from the RTDMS to Ie less than those from the UBC system, and the average difference is 7 %. It is not known whether the values of Qp from the RTDMS are systematically low or whether the UBC system values are too high However, the high correlation between the data sets and the low standard error of estimate indicate that the performance of the RTDMS is quite adequate for hourly estimates of the Bowen ratio.Q (Queens) (Win“)Date |u ne 1980Fig. 4. Comparison of UBC and Queens systems, Vancouver Airport, June 1980.AcknowledgmentsThis work was supported by grants from the Natural Sciences and Engineering Research Council of Canada. The author is grateful to Dr R I. Mukamma( AES, for permission to use the Woodbridge lysimetric data,