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Jack H. Shreffler

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Jack H. Shreffler

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A model is formulated which predicts the flux of a gaseous pollutant to a vegetational surface provided the surface concentration is known. The effect of canopy structure on the flux is accounted for in terms of height, zero-plane displacement, roughness and leaf area density. The predictions of the model are shown to be consistent with the data of A. C. Chamberlain for deposition and evaporation at an artificial grass-like surface.

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Jack H. Shreffler

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Hourly averaged winds are computed from a one-year record taken atop five 10 m towers and four 30 m towers distributed over 4000 km2 of typical agricultural land. Vertical temperature differences are available from three of the 30 m towers. The ratio of spatially averaged speeds at the two levels is plotted against a mean bulk Richardson number. The range of stability where measurements are possibly consistent with the presence of established similarity profiles is limited and dependent on wind speed. However, the speed ratios follow smoothly varying functional relations over a wide range of stability for each classification of data by season and wind speed.

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Jack H. Shreffler

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From Regional Air Monitoring System (RAMS) tower data recorded in St. Louis during 1976, time series of convergence, heat island intensity and solar radiation are presented for five calm periods each exceeding 12 h. The records demonstrate that heat-island-induced convergence is markedly stronger during the day than at night. There are indications of a periodicity of 1.5-2 h in the nighttime convergence, while the daytime convergence is more variable and cannot be characterized as periodic. An event recorded by one series is interpreted as storm cell development associated with the daytime convergence.

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Jack H. Shreffler

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Hourly averaged winds from 30 m towers of the RAMS network (St. Louis region) are analyzed to determine systematic differences between urban and rural wind speed and direction. Previous studies of tower winds in London and New York have advanced the notion of a critical wind speed (∼4 m s−1) below which speeds are higher in the city than in the adjacent countryside. This apparent acceleration of low-speed flows has been explained as resulting from the dominance of heat island effects over roughness effects. Analyzing observations from all of 1976, this study finds slightly higher speeds in central St. Louis only under nearly calm conditions, typified by a weak heat island and convective instability. This result raises doubts about the universal applicability of the concept of a critical wind speed as previously formulated.

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Jack H. Shreffler and Francis S. Binkowski

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Strong thunderstorm activity over Iowa on two successive afternoons was the apparent source of pressure-jump lines (PJL's) which moved SSE at 50 km h−1 through the nocturnal boundary layer and were detected by National Weather Service (NWS) stations as far away as Paducah, Kentucky. Rainshowers and thunder were reported at many NWS stations as the PJL's passed.

The Regional Air Monitoring System (RAMS) network at St. Louis provided detailed information on the PJL'S. Arrival there was indicated by an abrupt pressure rise of 1.5 mb, a near reversal of the surface flow, and a vertical displacement of 750 m extending through the lower 4 km of the atmosphere. The passage of each PJL was coincident with the turbulent collapse of the nocturnal jet. The observations of the PJL events seem indicative of an internal bore and are similar to those of the Morning Glory seen in northern Australia. We speculate that the bore originates from a late afternoon convergence produced by thunderstorm outflow and opposing low-level winds involving the nocturnal jet.

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