Doppler Radar Observations of Dust Devils in Texas

Analyses of a dust-devil dataset collected in northwest Texas are presented. The data were collected just above the ground at close range with a mobile, W-band (3-mm wavelength) Doppler radar having an azimuthal (radial) resolution of 3-5 m (30 m) at the range of the dust devils. Most dust devils appeared as quasi-circular rings of relatively high radar reflectivity. Four dust-devil vortices were probed, three of which were cyclonic and one anticyclonic. Documentation was obtained of a pair of adjacent cyclonic vortices rotating cyclonically around each other. Approximate radial profiles of azimuthal and radial wind components and of radar reflectivity are detailed and discussed. The diameters of the core of the dust devils ranged from 30 to 130 m; the latter diameters are much wider than that of typical dust devils in a homogeneous environment. The widest vortex was cyclonic and exhibited evidence of a two-cell structure (i.e., sinking motion near the center and rising motion just outside the radius of maximum wind), a broad, calm eye, and an annulus of maximum vorticity just inside the radius of maximum wind. As the vortex widened, it developed an asymmetry, and some evidence was found that two waves propagated cyclonically around it. The narrowest dust devil had the structure of a Rankine combined vortex, that is, a central core of constant vorticity surrounded by potential flow. Owing to very strong radial shear of the azimuthal wind, the vorticity in the dust-devil cores ranged from 0.5 to 1 s-1, which is as high as the vorticity in some tornadoes. However, the maximum ground-relative wind speeds in each dust devil were only 6.5-13.5 m s-1. The location of the highest radar reflectivity was located at or within the radius of maximum wind. In the widest dust devil, the vorticity estimated from the Doppler shear associated with its vortex signature was much less than the smaller-scale vorticity ring estimated from the azimuthal wind profile. It is therefore suggested that the vorticity estimated from the Doppler shear in tornadoes may be underestimated significantly when the tornado vortex exhibits a two-cell structure and that Doppler shear alone may not be a good indicator of vortex intensity.