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J. A. Warburton and L. G. Young

Abstract

Thermal neutron activation methods have been applied to the determination of silver content in samples of hail, rain and snow. Hail and rain samples were collected in South Dakota in regions where AgI cloud seeding was being conducted; the snow was collected in the eastern Sierra Nevada in an area where no such seeding was being conducted. No silver was detected in the snow samples analyzed, indicating concentrations on the average less than 2.5 × 10−11 gm ml−1. Eighty percent of the hail and rain samples analyzed contained measurable quantities of silver up to 70 times the minimum detectable amount. Some of these latter samples were collected in nominally unseeded areas, and although the chemical form in which the silver entered the precipitation is unknown—thereby casting doubt on its consequence—the observations raise important questions which deserve answers, particularly as they may affect statistical evaluations of weather modification experiments.

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J. A. Warburton and L. G. Young

Abstract

Laboratory techniques have been developed for concentrating and isolating silver from aqueous samples. When these concentrated samples are activated in a thermal neutron flux of 4 × 1012n cm−2 sec−1, quantitative measurements down to 10−9 gm masses of silver can be made. When 1-liter samples of water are used, the lower limit of detection of silver in the sample is about 3 × 10−12 gm cm−3 concentration. Typical Ag concentrations being observed in snow collected in the western United States range from this lower limit to 10−8 gm cm−3, the majority being between 10−9 and 10−11. The lower limit does not therefore appear to be a serious factor in silver determinations in snow collected at ground stations. The system described is a non-destructive one, the accuracy of measurement using γ-ray spectrometry being 1, 2, 10 and 80% for masses of silver 10−6, 10−7, 10−8 and 10−9 gm, respectively. By using a 24-sec half-life radioisotope for the Ag determination, the cost of activation and turn-around time on samples is kept to a minimum. Data analysis is computerized and rapid.

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Richard J. Naistat and John A. Young

Abstract

A time-dependent, one-dimensional numerical model of the boundary layer at individual stations is solved by a finite-difference technique for the St. Patrick's Day storm of 1965. Using only surface pressure and temperature gradients at 3-hr intervals as input, the model predicts the changing boundary layer wind profile at each of 225 grid points covering the central and eastern United States. Predicted winds are used to evaluate horizontal flow structure, vertical motion, three-dimensional trajectories, and flow acceleration. Modeled fields are compared to those corresponding to simple steady-state solutions and to observed data.

The solution of the equations of motion retains the local acceleration and baroclinity, omits the nonlinear advection terms, and contains a damping effect in order to control the inertial-like oscillations which have complicated previous models. With the damping included, the model reasonably predicts the location of the low-level jet core, the vertical motion distribution, and, qualitatively, the orientation of the local acceleration in the boundary layer. The model-forecast ascent is well correlated with the observed precipitation rate when adequate surface moisture is available.

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Young-Gyu Park and J. A. Whitehead

Abstract

Convection experiments were carried out in a rectangular tank as a model of oceanic meridional overturning circulation. The objective was finding a relation between the meridional heat flux and thermal forcing. To make the meridional heat flux estimate possible, the heat flux was fixed at one bottom end of the tank using an electrical heater. Temperature was fixed at the other end using a cooling plate. All other boundaries were insulated. In equilibrium, the heat input to the fluid H was the same as the meridional heat flux (heat flux from the source to the sink), so it was possible to find a scaling law relating H to the temperature difference across the tank ΔT and rotation rate f. The experimental result suggests that the meridional heat transport in the experiment was mostly due to geostrophic flows with a minor correction caused by bottom friction. When the typical values of the North Atlantic are introduced, the geostrophic scaling law predicts meridional heat flux comparable to that estimated in the North Atlantic when the vertical eddy diffusivity of heat is about 1 cm2 s−1.

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Michael J. Pecnick and John A. Young

Abstract

The three-dimensional structure and implied dynamics of a strong tropospheric gravity wave event am studied. It is shown that satellite and continuous surface observations reveal the subsynoptic nature of this “wave of depression” to an extent impossible with conventional data. The observations and theory suggest that the gravity wave originated in the upper troposphere near a jet streak, was quasi-hydrostatic and hence relatively nondispersive and long-lived.

The behavior of the wave at upper-tropospheric levels is revealed by sequences of visible and infrared goesynchronous satellite imagery. Quantitative estimates of cloud top temperatures and winds suggest strong subsidence new 300 mb with an isentropic depression as large as 900 m. The upper-level depression and the surface disturbance propagate coherently with a speed of 32 m s−1 indicating that they are part of the same internal gravity wave. The vertical tilt with height is opposite to the propagation direction and thus is consistent with an upper-tropospheric energy source. The negative surface pressure deviation reaches 7 mb and is qualitatively consistent with the field of surface wind divergence.

Theory is applied to estimate and explain gravity wave properties throughout the troposphere: vertical tilt (decreasing upward) as large as 1:9 in the lower troposphere; maximum wave energy at upper levels, with maximum wind deviation ∼ 15 m s−1, horizontal divergence ∼ 4×10−4 s−1, vertical parcel displacement ∼ 1 km, local potential temperature deviations of several degrees, pressure perturbations ∼ 7 mb, and the time to completely establish the wave throughout the troposphere ∼ 4 h. Further improvement in the description may demand development of “solitary” wave theory for deep depression waves in shear flow.

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K. C. Young and A. J. Warren

Abstract

In this derivation of the equilibrium supersaturation curve for soluble particles, Köhler treated the van't Hoff factor as a constant. McDonald points out that this factor is actually a function of the droplet molality. In this paper, we have rederived equilibrium supersaturation relation following Köhler's approach, except that the van't Hoff factor is treated as a variable in the derivation. The resulting equation is considerably more complicated than the result obtained by Köhler. A simple substitution of a variable van't Hoff factor into Köhler's familiar equation that was derived assuming a constant van't Hoff factor results in a fairly significant error.

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A. J. Manfroi and W. R. Young

Abstract

The authors study the stability of a barotropic sinusoidal meridional flow on a β plane. Because of bottom drag and lateral viscosity, the system is dissipative and forcing maintains a basic-state velocity that carries fluid across the planetary vorticity contours; this is a simple model of forced potential vorticity mixing. When the Reynolds number is slightly above the stability threshold, a perturbation expansion can be used to obtain an amplitude equation for the most unstable disturbances. These instabilities are zonal flows with a much larger length scale than that of the basic state.

Numerical and analytic considerations show that random initial perturbations rapidly reorganize into a set of fast and narrow eastward jets separated by slower and broader regions of westward flow. There then follows a much slower adjustment of the jets, involving gradual meridional migration and merger. Because of the existence of a Lyapunov functional for the dynamics, this one-dimensional inverse cascade ultimately settles into a steady solution.

For a fixed β, the meridional separation of the eastward jets depends on the bottom drag. When the bottom drag is zero, the process of jet merger proceeds very slowly to completion until only one jet is left in the domain. For small bottom drag, the steady-state meridional separation of the jets varies as (bottom drag)−1/3. Varying the nondimensional β parameter can change the instability from supercritical (when β is small) to subcritical (when β is larger). Thus, the system has a rich phenomenology involving multiple stable solutions, hysteritic transitions, and so on.

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J. R. Holton, J. M. Wallace, and J. A. Young

Abstract

No abstract available.

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D. N. Sikdar, J. A. Young, and V. E. Suomi

Abstract

Time series of areal cloud coverage over the central Pacific are studied to determine the large-scale variations of tropical disturbance activity for a four-month period in 1967. The persistent and fluctuating components are found to be more prevalent in regions of high and low mean cloudiness, respectively, with major longitudinal variations in activity most evident in the Southern Hemisphere. Normalized power spectra show that the transient activity occurring in the equatorial zone is dominated by shorter periods of about four days, while longer periods are dominant away from the equator. Coherency magnitudes and phases between different locations indicate that propagating cloud systems are most identifiable at lower frequencies in the Southern Hemisphere, and at somewhat higher frequencies in the Northern Hemisphere. Activity with periods in excess of five days consists largely of both westward and eastward propagation on the planetary wave scale. The shorter periods appear to consist of both synoptic-scale and planetary-scale modes, with westward propagation dominant near the equator.

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J. A. Warburton, L. G. Young, and R. H. Stone

Abstract

Trace chemical analysis techniques have been used in a series of cloud-seeding experiments in the central Sierra Nevada with the ultimate purpose of distinguishing whether the submicron-sized aerosol particles used for seeding are removed by nucleation or by scavenging in snowfall. The research programs used submicron-sized seeding aerosols with different nucleating characteristics. When winter storms were seeded with silver iodide in the Lake Tahoe and Lake Almanor watersheds, positive correlations were observed between silver concentrations and precipitation amounts in both catchment areas.This is considered to be evidence that the AgI aerosols are not being removed in the snowfall entirely by scavenging processes. When two separate aerosols of silver iodide and indium sesquioxide were released simultaneously from the same ground locations during winter snowstorms in the Lake Almanor watershed, it was found that considerably more of the ice-nucleating aerosol particles (AgI) were removed by the snowfall than the non-ice-nucleating ones (In203). Under the experimental conditions employed, scavenging alone of the two aerosols would lead to a chemical ratio of Ag:In in the snowfall of 0.83:l. Ratios as high as 17.2:l were observed, the mean ratio being 4: I. These results are considered to be evidence of the removal of substantial numbers of the AgI aerosol particles through direct nucleation of ice crystals.

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