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  • Author or Editor: JAMES C. FANKHAUSER x
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Chester W. Newton
and
James C. Fankhauser

Abstract

In typical squall-line situations wherein the wind veers strongly with height, individual convective storms move as much as 60 deg right or 30 deg left of the direction of the mean wind in the cloud layer. It is shown that, on the average, the radar echoes having largest diameters move farthest to right of the wind.

This behavior is consistent with physical considerations and with supply-and-demand requirements of the storm water budget. For a given rainfall intensity, the amount of water precipitated by a storm is proportional to its area or to the diameter squared. The amount of water vapor intercepted is proportional directly to the diameter, and to the velocity of the storm relative to the winds of the lower-tropospheric moist layer. A large storm must intercept more vapor in proportion to its diameter than a small one, requiring a larger migration velocity relative to the moist layer. This requirement is satisfied if (wind veering with height) large storms move toward the right of the mean wind.

Based on these considerations, a simple expression is derived for the direction of storm motion as related to storm diameter. This describes the mean behavior fairly well, but there is considerable residual scatter. With this taken into account, an expression is given for the probability of storm passage over a given point as related to the initial storm location and size.

Some characteristic patterns of development are illustrated. New convective elements tend to form on or amalgamate with the right-hand side or end of an existing storm cluster or squall line, somewhat on the up-wind side relative to the mean wind. This pattern of generation contributes to the movement of large storm clusters strongly toward the right of the winds, and also to make large storms move consistently more slowly than small ones.

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G. Brant Foote
,
Charles G. Wade
,
James C. Fankhauser
,
Peter W. Summers
,
Edwin L. Crow
, and
Mark E. Solak

Abstract

An analysis of the seeding operations during the National Hail Research Experiment 1972–74 randomized seeding program is carried out for the purpose of critiquing the seeding procedures and establishing the actual rates at which seeding material was dispensed as opposed to the prescribed rates. The seeding coverage, a parameter defined in the paper, is found to be only about 50% on the average. The reasons for the low seeding coverage are discussed in terms of seeding logistics and storm evolution, and three case studies are presented to illustrate the problems that can arise. Some results on the rate at which storm cells can develop and on the duration of convective activity over a fixed target area are presented. It is concluded that seeding convective clouds using aircraft flying near cloud base is more difficult than is widely acknowledged.

Since the seeding operations were more thorough on some days than on others, one might reasonably expect that seeding effects, if they exist, would be more marked on the days with the higher coverage. Post hoc analyses that stratify the surface hail and rain data according to seeding coverage are presented. The results do not allow one to reject the hypothesis that seeding had no effect on surface precipitation.

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