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William M. Porch and David B. Smith
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William M. Porch and Dale A. Gillette

Abstract

Fast-response light scattering measurements at two heights during a Texas dust storm are combined with horizontal and vertical wind data to derive and compare aerosol flux estimates using three techniques. The major result of this study is that a relative equivalence exists between the fine-particle (0.1 μm < radius < 1 μm) exchange coefficient and the eddy viscosity of the wind. The data also shed some light on the complex dependence of wind speed threshold for suspension and aerosol flux in high winds for different surface conditions and soil types. These results show the value of the experimental technique to studies of toxic particulate suspension and deposition by wind.

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William M. Porch, William E. Clements, and Richard L. Coulter

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This paper describes a regular oscillation observed in nighttime drainage airflow in a valley under relatively light upper-level wind conditions. The period of these oscillations is about 20 minutes with at least one harmonic at about 10 minutes. A strong coherence between tributary flow and main valley fluctuations was observed, with the phase of the tributary flow leading the valley oscillation; this indicates the importance of tributaries as major contributors to the dynamics of cold air flow in valleys.

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Richard L. Coulter, Timothy J. Martin, and William M. Porch

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The characteristics of tributary drainage flow in stable, nocturnal conditions in three closely located tributaries are compared. The orientation of the tributaries with respect to Kimball Creek, into which they drain, appears to be a controlling factor in the tributary flow. In particular, oscillations in the drainage flow are found to be weakest and drainage mass per unit area greatest in the tributary most closely aligned with the main canyon.

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William M. Porch, Richard B. Fritz, Richard L. Coulter, and Paul H. Gudiksen

Abstract

Field experiments measuring nocturnal tributary flows have shown complex internal structure. Variations in the flow range from short-term (8–16 min) oscillations (related to tributary/valley flow interactions) to long-term flow changes throughout the night (related to upper ridge slope and tributary sidewall cooling rate changes). The mean vertical structure in the tributary flow shows a three layer structure. Outflow winds are observed near the surface and in an elevated jet up to several hundred meters height. A flow minimum or counterflow exists at about the height of the drainage flow maximum in the main valley. Comparisons of flow volumes and variations from a single large tributary show that 5%–15% of the nocturnal flow in the main valley may be contributed through one tributary. This implies that tributaries may dominate main valley sidewall and midvalley subsidence contributions to valley drainage flows.

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David S. Ensor, William M. Porch, Michael J. Pilat, and Robert J. Charlson

Abstract

The possible climatic effects of the secular increase of aerosols from man's activities have been coupled with the microphysics of the aerosol properties. The magnitude of the critical aerosol absorption coefficient to backscatter coefficient, (b abs/ b bs)critical, was estimated for a model atmosphere corresponding to cooling or heating of the earth with increasing aerosol concentration. The b abs/b bs ratio was calculated with Mie theory assuming a Junge particle size distribution and spherical particles as a function of the imaginary part of the particle refractive index (particle light absorption) and the size distribution slope. Comparing the b abs/b bs ratio calculated from Mie theory to the critical b abs/ b bs, cooling might ensue if the imaginary part is less than 10−3 while heating may result if it is greater than 0.1.

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