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E. G. MCDOUGALL

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E. BERL and G. A. STERBUTZEL

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A. E. Carte and G. Held

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Detailed climatological studies of hailstorms on the South African plateau have been made using a dense network of voluntary observers. Hailstone structures have been investigated and, since 1971, radar observations have been used to study storm characteristics. Results are given for the frequency of hailstorm occurrence, characteristics of hailfalls at a point, hail paths, types of storms, radar characteristics and hailstone trajectories. Practically all features showed great variability, the magnitude of which is given whenever possible. Application of the results to the design of a hail suppression experiment is briefly discussed.

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A. G. Peyrefitte Jr. and E. G. Astling

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F. A. Huff and G. E. Stout
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G. I. Shapiro and A. E. Hill

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Observed density sections through dense-water pools or lenses on sloping topography typically have an asymmetric structure. One side of the dense lens usually is bounded by isopycnals that slope steeply down to the seabed while, on the other side, the slope of isopycnals is more gentle. A common situation is for the steepest-sloping isopycnals to be on the upslope side of a lens (which is termed the “head-up” state), but the reverse is occasionally true (which is called the “head-down” state). Here a 1½-layer reduced-gravity model, which resolves the bottom boundary layer to provide physical insight into the three-dimensional evolution of these alternative forms, is used. It is found that the head-up state arises when the thickness of the central core of a lens exceeds about two Ekman depth scales, and the head-down state arises when the converse is true. The speed of along- and cross-slope motion of the central, thick core of a dense lens is also investigated, and the results from an ensemble of runs with the three-dimensional reduced-gravity model are found to accord suprisingly well with some approximations derived from bulk dynamics. From a practical point of view, the results concerning the shape of isopycnals bounding dense lenses on slopes can provide valuable information from which to infer important aspects of the underlying dynamics.

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G. R. Bigg and A. E. Gill

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The long period response of sea level on the eastern boundary of the Pacific separates, for linear dynamics, into a remotely forced part mainly due to zonal winds along the equator to the west and a locally driven part where sea level slopes to balance the alongshore wind. Examination of the annual component of sea level on the eastern boundary indicates that the locally forced part dominates, whereas the remotely forced part plays a major role at semiannual and interannual periods.

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G. I. Shapiro and A. E. Hill

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Plumes of dense shelf water cascade down continental slopes in many parts of the world’s oceans and provide a mechanism for shelf–ocean exchange. In this paper a nonlinear process-orientated theory is developed and used to examine the dynamics of cascading. The theory is formulated in terms of a “1½-layer” model and incorporates bottom topography, earth rotation, internal and bottom friction, and entrainment as well as externally imposed pressure gradients. The theory occupies a niche between the stream tube class of model (which considers only bulk properties of a plume) and the full three-dimensional primitive equation approach. The model provides useful insights into the complex interplay between the controlling forces, and it allows one to recover the shape and trajectory of dense plumes as well as the three-dimensional flow field inside the bottom layer. Asymptotic limits are investigated and lead to several basic results. A typical thickness of a fully developed plume is found to be twice the bottom Ekman layer scale, corresponding to reported observations. The relative importance of downslope density-driven cascading and downslope drainage forced by interior currents is assessed. It is found that vertical mixing always assists downslope plume propagation, while an interior current may assist or inhibit cascading. The model is applied to some recent observations at the Hebridean shelf edge west of the British Isles and is used to infer the characteristics of an observed cascade. The model could also be applied to double frontal currents such as the Mediterranean outflow.

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F. A. Huff and G. E. Stout

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An investigation was made of the distribution characteristics of radioactive rainout in convective rain-storms through use of data from 29 storms collected on a network of automatic rainwater samplers in central Illinois during spring and summer 1963. Storm profiles of the concentration of radioactive rainout were correlated with rainfall factors and types of synoptic weather. It was found that the storm profiles of rainout could be grouped into four major and three minor types in which the major types accounted for 90 per cent of the cases and the most frequent type occurred 45 per cent of the time. Tentative explanations of the atmospheric conditions under which the major types prevail are presented.

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G. A. Jacobs and H. E. Ngodock

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In many data assimilation applications, adding an error to represent forcing to certain dynamical equations may be physically unrealistic. Four-dimensional variational methods assume either an error in the dynamical equations of motion (weak constraint) or no error (strong constraint). The weak-constraint methodology proposes the errors to represent uncertainties in either forcing of the dynamical equations or parameterizations of dynamics. Dynamical equations that represent conservation of quantities (mass, entropy, momentum, etc.) may be cast in an analytical or control volume flux form containing minimal errors. The largest errors arise in determining the fluxes through control volume surfaces. Application of forcing errors to conservation formulas produces nonphysical results (generation or destruction of mass or other properties), whereas application of corrections to the fluxes that contribute to the conservation formulas maintains the physically realistic conservation property while providing an ability to account for uncertainties in flux parameterizations. The results suggest that advanced assimilation systems must not be liberal in applying errors to conservative equations. Rather systems must carefully consider the points at which the errors exist and account for them correctly. Though careful accounting of error sources is certainly not an entirely new idea, this paper provides a focused examination of the problem and examines one possible solution within the 4D variational framework.

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