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Atmospheric Lateral Diffusion Estimates from Tetroons

J. K. AngellU.S. Weather Bureau, Washington, D.C.

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D. H. PackU.S. Weather Bureau, Washington, D.C.

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Abstract

Successive tetroon releases are utilized to provide an estimate of the lateral diffusion from a continuous point source as a function of downwind distance. On the average the tetroons yield a lateral standard deviation proportional to the 0.85 power of the downwind distance. The mean ratio of lateral standard deviation to downwind distance varies from 0.40 (5 km downwind) to 0.31 (40 km downwind) for tetroon releases over time intervals of 3 hours and from 0.64 to 0.49 for tetroon releases ova time intervals of 21 hours. These mean ratios are large because of the preponderance of tetroon flights within the Los Angeles Basin. The data also imply a lateral standard deviation proportional to about the 0.2 power of the time interval of tetroon release and hence, to a first approximation, to about the 0.2 power of the sampling time.

The lateral standard deviations obtained from successive tetroon releases are compared to estimates derived from individual tetroon flights through evaluation of running means of the lateral velocity. The agreement is good at a downwind distance of 5 km but becomes progressively worse with increase in down-wind distance so that, at a downwind distance of 20 km, the average lateral standard deviation derived from the individual trajectories is only 70 per cent of that derived from the series.

Abstract

Successive tetroon releases are utilized to provide an estimate of the lateral diffusion from a continuous point source as a function of downwind distance. On the average the tetroons yield a lateral standard deviation proportional to the 0.85 power of the downwind distance. The mean ratio of lateral standard deviation to downwind distance varies from 0.40 (5 km downwind) to 0.31 (40 km downwind) for tetroon releases over time intervals of 3 hours and from 0.64 to 0.49 for tetroon releases ova time intervals of 21 hours. These mean ratios are large because of the preponderance of tetroon flights within the Los Angeles Basin. The data also imply a lateral standard deviation proportional to about the 0.2 power of the time interval of tetroon release and hence, to a first approximation, to about the 0.2 power of the sampling time.

The lateral standard deviations obtained from successive tetroon releases are compared to estimates derived from individual tetroon flights through evaluation of running means of the lateral velocity. The agreement is good at a downwind distance of 5 km but becomes progressively worse with increase in down-wind distance so that, at a downwind distance of 20 km, the average lateral standard deviation derived from the individual trajectories is only 70 per cent of that derived from the series.

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