Flight Hazards in Thunderstoms Determined by Doppler Velocity Variance

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  • 1 Air Force Cambridge Research Laboratories, Sudbury, Mass.
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Abstract

Vertical velocity variance was measured by means of Doppler radar in a thunderstorm in which no hail fell on the ground. At least 5% of the variance measurements were too large to be accounted for by the spread of precipitation fallspeeds and by vertical gradients of vertical velocity. On this basis, turbulence and/or shear is inferred in the regions of high velocity variance, which were mostly clustered in the front part of the storm, near or on the flanks of major updrafts, at altitudes of 4.5-10 km. The greatest variance value, observed at 7.5 km, was equivalent to a horizontal shear in the updraft in excess of 0.1 sec-1. We conclude that a vertical velocity variance of 2 m2 sec-2 in a storm suggests small hail or light to moderate turbulence, while variances above 4 m2sec-2very likely indicate damaging hail or severe turbulence, or both.

Abstract

Vertical velocity variance was measured by means of Doppler radar in a thunderstorm in which no hail fell on the ground. At least 5% of the variance measurements were too large to be accounted for by the spread of precipitation fallspeeds and by vertical gradients of vertical velocity. On this basis, turbulence and/or shear is inferred in the regions of high velocity variance, which were mostly clustered in the front part of the storm, near or on the flanks of major updrafts, at altitudes of 4.5-10 km. The greatest variance value, observed at 7.5 km, was equivalent to a horizontal shear in the updraft in excess of 0.1 sec-1. We conclude that a vertical velocity variance of 2 m2 sec-2 in a storm suggests small hail or light to moderate turbulence, while variances above 4 m2sec-2very likely indicate damaging hail or severe turbulence, or both.

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