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- Author or Editor: A. J. Dyer x
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Abstract
It is suggested that Eulerian mean velocities and eddy fluxes are not necessarily identical with those observed by (quasi-Lagrangian) GHOST balloons. Although a general relationship between the two sets of variables has not been established, it is argued, on the basis of a number of simple examples, that differences can emerge that are related to the turbulence statistics of the flow. Such differences are known to exist in other fluid systems.
Abstract
It is suggested that Eulerian mean velocities and eddy fluxes are not necessarily identical with those observed by (quasi-Lagrangian) GHOST balloons. Although a general relationship between the two sets of variables has not been established, it is argued, on the basis of a number of simple examples, that differences can emerge that are related to the turbulence statistics of the flow. Such differences are known to exist in other fluid systems.
Abstract
An evaluation is made of the typical variation with height and time of mean and eddy flow of momentum in the atmospheric boundary layer using carefully determined wind profile data obtained over a separation of 200 m in a downwind direction. Twenty-three half-hour runs were used comprising almost the whole of three days of daylight hours under conditions of no cloud. The measurements were recorded up to a height of 16 m at a site specially selected for horizontal uniformity.
The overall average wind differences observed were approximately constant with height, implying a linear dependence with height of the mean vertical velocity w̄. The average value of w̄ at the 16-m level was −0.54 cm sec−1, with individual values ranging from −1.49 to 0.81 cm sec−1.
The departure of the eddy flux of momentum from the corresponding surface value was also found to be approximately proportional to height. The overall average departure of the eddy flux at the 16-m level was 16.8%, with excursions extending from −44 to +55%.
The enforced neglect of certain terms in the relevant equations may lead to overestimates of the quoted w̄ values and eddy flux variations.
The importance of these results to basic studies of flux-gradient relations is discussed. In fundamental experiments the need to obtain an extensive body of data at good quality sites is emphasized.
Abstract
An evaluation is made of the typical variation with height and time of mean and eddy flow of momentum in the atmospheric boundary layer using carefully determined wind profile data obtained over a separation of 200 m in a downwind direction. Twenty-three half-hour runs were used comprising almost the whole of three days of daylight hours under conditions of no cloud. The measurements were recorded up to a height of 16 m at a site specially selected for horizontal uniformity.
The overall average wind differences observed were approximately constant with height, implying a linear dependence with height of the mean vertical velocity w̄. The average value of w̄ at the 16-m level was −0.54 cm sec−1, with individual values ranging from −1.49 to 0.81 cm sec−1.
The departure of the eddy flux of momentum from the corresponding surface value was also found to be approximately proportional to height. The overall average departure of the eddy flux at the 16-m level was 16.8%, with excursions extending from −44 to +55%.
The enforced neglect of certain terms in the relevant equations may lead to overestimates of the quoted w̄ values and eddy flux variations.
The importance of these results to basic studies of flux-gradient relations is discussed. In fundamental experiments the need to obtain an extensive body of data at good quality sites is emphasized.
Abstract
The Evapotron, an instrument designed to measure vertical fluxes of heat and water vapor in the lower atmosphere, is described. Completely transistorized, the instrument consumes 50–100 watts and can be operated by a scientific technician. By means of an energy balance comparison the accuracy of the instrument has been demonstrated to be within about 5%. A detailed technical description is to be published separately.
Abstract
The Evapotron, an instrument designed to measure vertical fluxes of heat and water vapor in the lower atmosphere, is described. Completely transistorized, the instrument consumes 50–100 watts and can be operated by a scientific technician. By means of an energy balance comparison the accuracy of the instrument has been demonstrated to be within about 5%. A detailed technical description is to be published separately.
Abstract
During the summer of 1961, the vertical transfer of water vapor and sensible heat was determined at a height of 4 m over an irrigated field at Davis, Calif. Simultaneous determinations of evaporation at the surface were obtained with a 20-ft weighing Iysimeter, and the corresponding sensible heat transfer computed using an energy balance approach.
A comparison of the two methods of determining these vertical fluxes showed excellent agreement early in the summer, but significant differences appeared as the season progressed. These could be attributed to severe contrasts developing between the heavily irrigated site and the surrounding areas, which, with no rain occurring, gradually dried out.
The fetch available was of the order 50–190 m depending on wind direction. The results clearly indicated that under the conditions of this experiment, the vertical fluxes were not constant with height, and a condition of horizontal uniformity did not exist.
Abstract
During the summer of 1961, the vertical transfer of water vapor and sensible heat was determined at a height of 4 m over an irrigated field at Davis, Calif. Simultaneous determinations of evaporation at the surface were obtained with a 20-ft weighing Iysimeter, and the corresponding sensible heat transfer computed using an energy balance approach.
A comparison of the two methods of determining these vertical fluxes showed excellent agreement early in the summer, but significant differences appeared as the season progressed. These could be attributed to severe contrasts developing between the heavily irrigated site and the surrounding areas, which, with no rain occurring, gradually dried out.
The fetch available was of the order 50–190 m depending on wind direction. The results clearly indicated that under the conditions of this experiment, the vertical fluxes were not constant with height, and a condition of horizontal uniformity did not exist.
Abstract
Ice particle concentrations have been measured from the NOAA/NCAR Explorer sailplane in unmixed and mixed updraft regions within northeast Colorado cumulus congestus clouds, and compared with the concentrations predicted from measured ice nucleus spectra. The clouds investigated were “cold, continental” cumulus with droplet populations of ∼1000 cm−3 and cloud base temperatures between +7 and −7°C. The concentrations of ice particles within unmixed updraft regions, exclusive of ice particles so large that they almost certainly entered the unmixed region by sedimentation or recycling, are consistent with those expected on the basis of the ice nucleus spectra, suggesting that primary ice nucleation is the dominant mechanism active within these regions. Millimetric size ice particles found within unmixed updraft cores presumably enter through sedimentation or recycling. The data do not indicate an important role for ice multiplication in these clouds.
Abstract
Ice particle concentrations have been measured from the NOAA/NCAR Explorer sailplane in unmixed and mixed updraft regions within northeast Colorado cumulus congestus clouds, and compared with the concentrations predicted from measured ice nucleus spectra. The clouds investigated were “cold, continental” cumulus with droplet populations of ∼1000 cm−3 and cloud base temperatures between +7 and −7°C. The concentrations of ice particles within unmixed updraft regions, exclusive of ice particles so large that they almost certainly entered the unmixed region by sedimentation or recycling, are consistent with those expected on the basis of the ice nucleus spectra, suggesting that primary ice nucleation is the dominant mechanism active within these regions. Millimetric size ice particles found within unmixed updraft cores presumably enter through sedimentation or recycling. The data do not indicate an important role for ice multiplication in these clouds.
Abstract
Abstract
Abstract
As a result of experience gained with the Evapotron in the measurement of eddy fluxes, a new instrument called the Fluxatron has been developed. The computing efficiency has been improved by filtering out slow eddies which do not contribute to the eddy flux.
The Fluxatron employs a propeller anemometer to detect the vertical wind component, and the response time of this device (0.3 sec) is suitable for measurements to be made at a height of 4 m. Only 2 W of battery power are consumed, in contrast to the Evapotron which uses 50N–100 W.
The new instrument is extremely simple to operate in the field, and is thus suitable for use by relatively unskilled personnel.
In its present form, the Fluxatron measures only sensible heat transfer. The measurement of the evaporative flux presents no difficulty in principle, but it is hoped that an alternative humidity sensor may be found other than a fine-wire wet bulb as in the Evapotron.
Abstract
As a result of experience gained with the Evapotron in the measurement of eddy fluxes, a new instrument called the Fluxatron has been developed. The computing efficiency has been improved by filtering out slow eddies which do not contribute to the eddy flux.
The Fluxatron employs a propeller anemometer to detect the vertical wind component, and the response time of this device (0.3 sec) is suitable for measurements to be made at a height of 4 m. Only 2 W of battery power are consumed, in contrast to the Evapotron which uses 50N–100 W.
The new instrument is extremely simple to operate in the field, and is thus suitable for use by relatively unskilled personnel.
In its present form, the Fluxatron measures only sensible heat transfer. The measurement of the evaporative flux presents no difficulty in principle, but it is hoped that an alternative humidity sensor may be found other than a fine-wire wet bulb as in the Evapotron.
Abstract
The results of a heat flux comparison experiment carried out at Hay, New South Wales, Australia, during May 1966 using a sonic anemometer thermometer (SAT), Fluxatron and Evapotron are reported. The instruments agree with each other to within a factor of 2 for individual runs. The large fluctuations from run to run of the individual estimates are mainly caused by the fact that the Eulerian point average does not provide an adequate statistical sample of the heat flux. This point is illustrated by the non-stationary behavior of the instantaneous product of vertical wind and temperature. As auxiliary results, values of σ w /u * and σ T /T * have been obtained which are somewhat higher than, but in general agreement with, observations reported by Mordukhovich and Zwang.
Abstract
The results of a heat flux comparison experiment carried out at Hay, New South Wales, Australia, during May 1966 using a sonic anemometer thermometer (SAT), Fluxatron and Evapotron are reported. The instruments agree with each other to within a factor of 2 for individual runs. The large fluctuations from run to run of the individual estimates are mainly caused by the fact that the Eulerian point average does not provide an adequate statistical sample of the heat flux. This point is illustrated by the non-stationary behavior of the instantaneous product of vertical wind and temperature. As auxiliary results, values of σ w /u * and σ T /T * have been obtained which are somewhat higher than, but in general agreement with, observations reported by Mordukhovich and Zwang.
Abstract
Experiments have been conducted to determine the effects, within cumulus clouds, of seeding with silver iodide; such effects are determined by correlating the presence of ice particles with the presence of silver iodide. This correlation is made possible by the use of a sailplane, which has the capability of remaining in the cloud updraft and the seeded plume for relatively long periods of time. The correlation method provided a very sensitive test for distinguishing seeding effects from natural ice development. Case studies using this technique are discussed.
Measurements in one small cloud having a base at −12°C showed concentrations at −15°C in the plume of up to 400 ice crystals per liter about 4 min after seeding. Aggregation of stellar dendrites occurred for particles as small as 400–500 μm diameter and about half of the available crystals were in aggregates after about 15 min. The output efficiency of the TB-1 pyrotechnic flares used in the experiment is estimated to be at least 1011 ice particles per gram of silver iodide in the temperature range −12 to −17°C which is close to previously reported values. Significant nucleation of ice by silver iodide occurred for at least 20 min after seeding. Linear ice crystal growth rates of stellar dendrites of the order of 0.15 mm min−1 were determined from these observations.
Abstract
Experiments have been conducted to determine the effects, within cumulus clouds, of seeding with silver iodide; such effects are determined by correlating the presence of ice particles with the presence of silver iodide. This correlation is made possible by the use of a sailplane, which has the capability of remaining in the cloud updraft and the seeded plume for relatively long periods of time. The correlation method provided a very sensitive test for distinguishing seeding effects from natural ice development. Case studies using this technique are discussed.
Measurements in one small cloud having a base at −12°C showed concentrations at −15°C in the plume of up to 400 ice crystals per liter about 4 min after seeding. Aggregation of stellar dendrites occurred for particles as small as 400–500 μm diameter and about half of the available crystals were in aggregates after about 15 min. The output efficiency of the TB-1 pyrotechnic flares used in the experiment is estimated to be at least 1011 ice particles per gram of silver iodide in the temperature range −12 to −17°C which is close to previously reported values. Significant nucleation of ice by silver iodide occurred for at least 20 min after seeding. Linear ice crystal growth rates of stellar dendrites of the order of 0.15 mm min−1 were determined from these observations.
