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  • Author or Editor: D. Atlas x
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R. C. Srivastava
and
D. Atlas

Abstract

Analytical solutions for the growth and vertical and horizontal motion of a precipitation particle growing by coalescence with cloud drops are derived under simplified steady-state assumptions. An equation is also developed for the concentration density of a continuous distribution of growing particles.

Assuming that the cloud water content varies linearly with height, and that the fall speed of a drop is proportional to the square root of its diameter, it is shown that the combination of a linearly increasing updraft surmounted by a sharply decreasing one sets a sharp upper limit to the particle size, and sorts the particles horizontally. Particles which spend their entire life in regions of horizontal convergence associated with increasing updraft are packed into a narrower shaft than that in which they originated. Initially smaller particles are carried above into the region of horizontal divergence associated with decreasing updraft and are displaced far to the sides of the cloud core. It is found that when the updraft increases sharply there is a very small range of initial sizes which can grow to fall-out size. These facts are used to suggest that a steady “balance level” (equal reflectivity in rising and falling particles) may be maintained at a height near and below an updraft maximum. Particle size spectra computed from the concentration density equation are continuous and well-behaved for rising, floating and falling particles alike, without necessarily even maximizing for the floating size.

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J. T. Willis
,
K. A. Browning
, and
D. Atlas

Abstract

Simultaneous measurements of the radar cross section and fallspeed of 5 cm (and larger) ice spheres falling in free air have been obtained using a high-precision tracking radar operating at a wavelength of 5.47 cm. While they were dry, the spheres fell with supercritical Reynolds numbers and drag coefficients of only 0.24 to 0.30. These coefficients are much smaller than those normally attributed to hailstones under any conditions. The surface of one sphere, 5.1 cm in diameter, became wet during its fall. This was accompanied by a 5 db decrease in its normalized radar cross section and a twofold increase in its drag coefficient. The implications of these observations are discussed.

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D. Atlas
,
J. I. Metcalf
,
J. H. Richter
, and
E. E. Gossard

Abstract

Ultra-high resolution (2m) radar observations show the amplification of unstable Kelvin-Helmholtz (KH) waves, the development of roll vortices, their breaking and the resulting turbulence, and appear to represent our first view of the life cycle of clear air turbulence. The KH waves are initiated at the base of an inversion at which the Richardson number, Ri, is slightly positive just prior to wave action, and above which Ri≫0. Accordingly, only a small enhancement of the wind shear at the interface will reduce Ri to the critical value (0–0.25) required to trigger KH waves. The KH waves also trigger stable waves in the dynamically stable stratum immediately above. Quantitative measurements indicate reflectivities typically 10 times greater, and occasionally 300 times greater, than the previously recorded maximum, but in strata of only a few meters vertical extent. Large-volume averaging by the prior low-resolution radars accounts largely for the discrepancy. The thinness of some of the scatter layers and the smoothness of the reflectivity contours precludes turbulent eddies exceeding a few meters, but the high reflectivities require major centimetric scale perturbations in refractivity. Direct measurements of microscale perturbations of the required magnitude by Lane, though rare, support the deductions. The origin of this microscale turbulence, especially in layers of large dynamic stability, is a mystery deserving attention. The intermittency of the KH wave activity and the undulations of the layer of large refractivity variance explain the previously reported patchiness of turbulence in and near stable strata, but raise serious questions as to the validity of long-path (duration) measurements of turbulence spectra. Both the form and intensity of the turbulence spectrum are also strongly dependent on height and the “age” of CAT.

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