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Richard J. Williams

Abstract

Surface parameters at the location of over 5000 tornadoes from January 1968 through 15 June 1974 were computed objectively at the National Severe Storms Forecast Center. This set of conditions 0–3 h prior to tornado occurrence provides a large sample and was used to determine a variety of tornado related averages. The mean conditions associated with tornadoes include: temperature 74°F, dew point 62°F, sea level pressure 1007 mb, and wind 175° at 7 kt. Significant seasonal and geographical variations from these averages were noted and illustrated.

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John K. Williams and J. Vivekanandan

Abstract

Dual-wavelength ratio (DWR) techniques offer the prospect of producing high-resolution mapping of cloud microphysical properties, including retrievals of cloud liquid water content (LWC) from reflectivity measured by millimeter-wavelength radars. Unfortunately, noise and artifacts in the DWR require smoothing to obtain physically realistic values of LWC with a concomitant loss of resolution. Factors that cause inaccuracy in the retrieved LWC include uncertainty in gas and liquid water attenuation coefficients, Mie scattering due to large water droplets or ice particles, corruption of the radar reflectivities by noise and nonatmospheric returns, and artifacts due to mismatched radar illumination volumes. The error analysis presented here consists of both analytic and heuristic arguments; it is illustrated using data from the Mount Washington Icing Sensors Project (MWISP) and from an idealized simulation. In addition to offering insight into design considerations for a DWR system, some results suggest methods that may mitigate some of these sources of error for existing systems and datasets.

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Richard Williams and Peter J. Wojtowicz

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We have used the basic probability methods of statistical mechanics to derive a droplet-size distribution function for atmospheric clouds. These methods apply to systems that have well-defined constraints, but for which microscopic processes cannot easily be followed in detail. Clouds, in their early stages of evolution, appear to be such a system. The derived expression gives the distribution as a function of the droplet volume, rather than its diameter. This agrees with a wide variety of observations on clouds for which the distribution is not bimodal, and is a convenient way to analyze data. In addition, the theory uniquely relates the width of the droplet-size distribution to the moisture content of the cloud and the concentration of condensation nuclei.

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A. Williams and J. C. Carstens

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No abstract available.

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William J. Williams, Thomas J. Weingartner, and Albert J. Hermann

Abstract

The cross-shelf structure of a buoyancy-driven coastal current, such as produced by a river plume, is modeled in a two-dimensional cross-shelf slice as a “wide” geostrophically balanced buoyancy front. Downwelling-favorable wind stress applied to this front leads to advection in the surface and bottom boundary layers that causes the front to become steeper so that it eventually reaches a steep quasi-steady state. This final state is either convecting, stable and steady, or stable and oscillatory depending on D/δ * and by /f2, where D is bottom depth, δ * is an Ekman depth, by is the cross-shelf buoyancy gradient, and f is the Coriolis parameter. Descriptions of the cross-shelf circulation patterns are given and a scaling is presented for the isopycnal slope. The results potentially apply to the Alaska Coastal Current, which experiences strong, persistent downwelling-favorable wind stress during winter, but also likely have application to river plumes subjected to downwelling-favorable wind stress.

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Dus̆an S. Zrnić and Albert J. Williams III
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A. J. George Nurser and Richard G. Williams

Abstract

The effect of cooling on the separated boundary current predicted by the model of Parsons is studied. The separating current is found to strengthen and to move southwards and eastwards. The model is also robust to limited heating. in which case the separating current weakens and moves northwards.

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William D. Grant and Albert J. Williams III

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No abstract available.

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DANSY T. WILLIAMS and JOEL J. WOODSIDE

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