Vortex Signature Recognition by a Doppler Radar

Abstract

A scanning Doppler radar has capability for measurement of the tangential shear of radial velocity or gradient of the radial component of velocity in a direction normal to the radial vector. Tangential shear of sufficient magnitude in a horizontal direction reveals one term of the expression for vorticity in a horizontal plane. A vortex is reasonably well identified by Doppler radar if the shear is sufficiently steady during the time required for half a revolution of a vortex, and is further confirmed by extension of the shear pattern throughout a depth greater than its diameter, and by invariance of the character of the shear with respect to a change in viewing angle. A vortex in solid rotation has a characteristically uniform signature on a Doppler velocity display which may, however, be smoothed down according to the resolution limitations of the antenna beam. Sharp velocity peaks are rounded down to roughly half the magnitude of the velocity interval encompassed within the half-power antenna beamwidth. An illustration of a persistent vortex at altitudes of 1–4 km in a severe thunderstorm revealed a solidly rotating core averaging 600 m in radius with vorticities as high as 0.06 sec−1, surrounded by a much larger area of considerably smaller vorticity. During a 30-min period, an echo hole was found at a height of 1.5–2 km in the center of the vortex, with a structure quite similar in shape to the echo-free vault which has occurred on a larger scale in several tornadic storms in the Great Plains.