ON THE USE OF TETROONS FOR THE ESTIMATION OF ATMOSPHERIC DISPERSION ON THE MESOSCALE

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  • 1 U.S. Weather Bureau, Woshinqton, D.C.
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Abstract

Atmospheric dispersion in the crosswind direction from a continuous point source is estimated as a function of downwind distance through the use of running-mean-velocity variance statistics obtained from radar-tracked constant volume balloon (tetroon) flights over ocean and desert. On the average, the tetroon data indicate that, at a height of 2,000–3,000 ft. and in the range 0.4–1.4 n. mi., the lateral dispersion is proportional to the 0.8 power of the downwind distance, whereas the vertical dispersion is proportional to the 0.5 power of the downwind distance. In both dimensions the power becomes less at larger distances and with increasing atmospheric stability. The relatively small power in the case of vertical dispersion is partly associated with vertical oscillations of period near 10 min., a period which appears to vary with lapse rate in the theoretically prescribed manner. In a few cases this periodicity is sufficiently pronounced so as to yield a small decrease in vertical dispersion with downwind distance and time (pinching effect) but this may be due to insufficient flight duration. A comparison of the dispersion estimated from tetroon flights, and the dispersion determined by fluorescent dye techniques, suggests that the tetroons yield reasonable dispersion statistics. If further experiments show that this is indeed the case, it is proposed that the relatively cheap tetroons be utilized to develop a dispersion climatology and that they be made available for an immediate estimate of trajectory and dispersion in the case of an accidental release of contaminant.

Abstract

Atmospheric dispersion in the crosswind direction from a continuous point source is estimated as a function of downwind distance through the use of running-mean-velocity variance statistics obtained from radar-tracked constant volume balloon (tetroon) flights over ocean and desert. On the average, the tetroon data indicate that, at a height of 2,000–3,000 ft. and in the range 0.4–1.4 n. mi., the lateral dispersion is proportional to the 0.8 power of the downwind distance, whereas the vertical dispersion is proportional to the 0.5 power of the downwind distance. In both dimensions the power becomes less at larger distances and with increasing atmospheric stability. The relatively small power in the case of vertical dispersion is partly associated with vertical oscillations of period near 10 min., a period which appears to vary with lapse rate in the theoretically prescribed manner. In a few cases this periodicity is sufficiently pronounced so as to yield a small decrease in vertical dispersion with downwind distance and time (pinching effect) but this may be due to insufficient flight duration. A comparison of the dispersion estimated from tetroon flights, and the dispersion determined by fluorescent dye techniques, suggests that the tetroons yield reasonable dispersion statistics. If further experiments show that this is indeed the case, it is proposed that the relatively cheap tetroons be utilized to develop a dispersion climatology and that they be made available for an immediate estimate of trajectory and dispersion in the case of an accidental release of contaminant.

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