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P. Hyson

Abstract

For the simple Delta I mounting an analysis is presented for the difference between thermistor and air temperature. For an alternative mounting, e.g., the STS-I, using a Mylar film sheet to isolate the sensor from the support posts, the same problem is treated by an extension of this analysis. It is concluded that the Delta I mounting has the advantage of only a small variation due to solar radiation, i.e., between daytime and nighttime measurements. Its disadvantage is that the difference between thermistor and air temperature is greater, i.e., a greater correction needs to be applied. On the other hand, this correction can be calculated with less uncertainty, because (i) the analysis is simpler, (ii) the effect of uncertainty in the support post temperature is largely removed once the thermistor leads exceed a certain length, and (iii) the coefficient of heat convection is more reliably computed for spheres and cylinders than for a flat plate, which has a varying angle of incidence with the direction of flow.

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P. Hyson

Abstract

An investigation is made into cup anemometer response to fluctuating wind speeds. The anemometer overruns under these conditions and the percentage overrun depends on the wind speed, and the frequency and amplitude of the fluctuations. An estimate of this overrun in the real atmosphere, i.e.,the “μerror” is made and found to be of the order of 1% for a typical micrometeorological anemometer.

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P. Hyson and B. B. Hicks

Abstract

Two instruments are described which serve as humidity sensors in conjunction with existing eddy correlation techniques. The first instrument is an infrared absorption device, with a 40 cm path length, operating in a water vapor vibrational band at 6.3 μm. The second instrument is a development of this, operating at 2.7 μm with a 20 cm path length. Both devices have been successfully field-tested in a latent heat flux format, using a propeller anemometer as a vertical velocity sensor. Satisfactory energy balances at the surface have been obtained. In the case of the 2.7 μm instrument, a specific advantage is its lack of sensitivity to ambient humidity levels, while both instruments are insensitive to slow variations of optical and electronic performance.

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P. Hyson, J. R. Garratt, and R. J. Francey

Abstract

Measurements of the uw covariance over the sea using the eddy correlation technique are used to compare two methods of correction when the mean “vertical” wind measured by the turbulence sensor is nonzero. A real-time continuous electronic correction is introduced and its performance assessed by evaluation of hourly mean values of the uw covariance and comparison with subsequent algebraically corrected values from a neighboring instrument. The validity of the algebraic correction is independently assessed by comparison of results from the two sets of instruments.

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B. B. Hicks, P. Hyson, and C. J. Moore

Abstract

Eddy correlation instruments mounted above a plantation of Pinus radiato near Mt. Gambier, South Australia, have been operated during two periods of intensive effort, in May and October, 1972. Measurements of the Reynolds stress and of wind speed gradients show that the zero plane for momentum is located at about d = 0.8h (where h is the height of the trees), and that the roughness length of the surface is about 30% of the difference (hd).Sensible heat fluxes and temperature gradients give a displacement length not significantly different from that applicable in the momentum case, but the roughness length for sensible heat transfer is smaller than that for momentum, by about a factor of 3.Advective effects are found to be important, particularly when the fetch across the canopy is less than about 0.8 km (corresponding in our case to an effective fetch/height ratio of between 100 and 200). Long-fetch cases allow an evaluation of the heat storage (S) in the canopy and in the air below the height of eddy flux measurement. The rate of heat storage is found to be about 60 ± 20 W m−2 per °C h−1 of canopy temperature change (for a densely packed forest with trees about 13 m high), which is compatible with measurements of the biomass and assumed specific heats. The residual heat energy at about 6 m above the effective zero plane, unaccounted for by the various measured fluxes, is found to be related to the difference in net radiation over grassland and forest.During daytime, the forest is found to lose heat by turbulence in much the same manner as pasture, with fluxes of similar magnitude (although possibly differing to the extent of differences in ground flux, albedo and emissivity, for example) and giving similar Bowen ratios. At night, however, the evaporation from the forest tends to continue as heat is supplied by the cooling canopy. This is in direct contrast to the usual situation over pasture, where the heat storage is not of sufficient magnitude to result in this behavior.

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