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N. K. JOHNSON

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Harley N. Johnson
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HARLEY N. JOHNSON

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HARLEY N. JOHNSON

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SYNOPSIS

The climate of western South Dakota is especially favorable for raising alfalfa, as 73 per cent of the annual rainfall of 15 to 20 inches is received during the period fro April 1 to September 30, the percentage of sunshine is high, the rate of evaporation is comparatively low, and moderate temperatures usually prevail during well. Alfalfa needs considerable moisture while growing, but fair weather while the hay crop is being harvested.

Alfalfa seed is usually produced from the second crop when conditions are such as to retard the maturing of the first hay crop. Alfalfa seed is produced in paying quantities in South Dakota only when there is a comparative shortage in the moisture supply, hence the weather conditions determine whether the second crop shall be cut for hay or left for seed. If there is considerable rainfall, the second crop is usually cut for hay, and a third crop is frequently possible.

As alfalfa hay is damaged by rain when curing, special weather forecasts are issued and widely distributed during harvest. The growing season is usually of sufficient length to mature seed from the second crop, but if it is too dry after the first crop has been harvested, and the growth of the second crop checked, there is danger of frost injury to seed in the early fall. A frost-warning service is therefore maintained and is widely utilized by seed growers.—J. W. S.

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C. J. Biter and Peter N. Johnson

An operations center was designed for the Cooperative Convective Precipitation Experiment to facilitate the task of coordinating research activities. This paper describes the facilities of the center and the role of the operations team—consisting of a director and supporting instrument systems coordinators—in its development and daily use. Features of this center should be of interest to personnel planning large meteorological field experiments involving complex, integrated research operations.

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N. Robb McDonald and E. R. Johnson

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A one-parameter family of exact solutions describing the bifurcation of a steady two-dimensional current with uniform vorticity near a gap in a thin barrier is found. The unsteady evolution of source-driven flows toward these steady states is studied using a version of contour dynamics, appropriately modified to take into account the presence of a barrier with a single gap. It is shown that some of the steady solutions are realizable as large-time limits of the source-driven flows, although some are not owing to persistent eddy-shedding events in the vicinity of the gap. For the special case when there is zero net flux through the gap, numerical experiments show that the through-gap flux of vortical fluid increases with the width of the gap and that this flux approaches a steady limit with time.

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Peter N. Johnson and Jack L. Fink

The Multiple Aircraft Position System (MAPS) was developed by the Convective Storms Division of the National Center for Atmospheric Research, in response to the need for aircraft position in the coordination of large field projects involving several research aircraft. Radio interferometer techniques are used to provide rapid, accurate determination of positions for up to 10 aircraft furnished with special airborne radio beacons. Three remote interferometer array sites receive the signals from the airborne beacons, each transmitting at a unique frequency. Each remote site is tuned to one beacon frequency at a time according to a programable polling sequence; several phase difference measurements are made from the received signal. These data then are telemetered to a central control station where they are transferred to a computer that calculates the direction cosines from each remote site to the airborne beacon, and thus determines the position of the beacon. The aircraft positions then are immediately available for display. The deployment of MAPS near Miles City, Mont, for the 1981 Cooperative Convective Precipitation Experiment (CCOPE) field program is described.

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Gary N. Johnson and Paul L. Smith Jr.

A T-28 aircraft was specially modified to permit gathering in situ cloud physics and dynamics data from the interior regions of hailstorms. Since its first use in 1969, the T-28 instrumentation and data acquisition system has undergone evolutionary changes as newer instruments and more sophisticated electronic products have become available. A unique complement of instruments that measure and record hydrometeor sizes and concentrations has been assembled to provide observations of particle size spectra over the full range from cloud droplets to hailstones. The instrumentation and the data recording system are described in this paper, and planned future enhancements that will entail greater use of on-board computers are discussed.

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Andrew J. Heymsfield, Peter N. Johnson, and James E. Dye

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The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements.

The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region from entrainment. A turbulent wake would be expected to occur in the down-shear portion of the cell, producing increased turbulence and mixing in that region.

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O. R. Southwick, E. R. Johnson, and N. R. McDonald

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Recent studies show that vertical eddy diffusivity is sufficient on its own to introduce intense horizontal shear layers at sloping ocean margins (Molemaker et al.; Gula et al.; Dewar et al.). These layers influence mesoscale energy and potential vorticity budgets but cannot be fully represented in models without sloping boundaries, no-slip boundary conditions, and sufficiently high resolution. This paper investigates the detachment of these shear layers and their subsequent rolling up into concentrated eddies. These shed eddies, or “sheddies,” may have significant oceanographic impacts. Their growth is considered using a simple point vortex model that adapts the Brown–Michael model of vortex shedding to quasigeostrophic flow and allows detailed consideration of the vorticity fluxes. The model shows good qualitative agreement with observations and experimental and numerical results. It is applied to a number of examples of well-known cases of sheddy formation, including the Agulhas cyclones, California Undercurrent, and Canary Eddy Corridor, and also is used to investigate the effects of shed vorticity in the growth of the Cook Strait eddy and the interaction of the North Brazil Current rings with the islands of the Lesser Antilles.

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