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- Author or Editor: LARS E. OLSSON x
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Abstract
The lake-breeze circulation on the Great Lakes is often as vigorous as its oceanic counterpart. This paper shows that lake breezes frequently exert drastic control on mesoscale air pollution patterns in urbanized shore-line areas, in this case, Chicago, Ill. Observational data were gathered from a surface mesonetwork, surface and satellite cloud photography, a chain of pilot balloons normal to the shore, optically tracked constant-level balloons, and aircraft measurements of suspended particulate concentrations in several size ranges. On the 2 late summer days studied, the lake breezes were extremely well developed. Inflow depths ranged from 500 to 1000 m, with peak inflow velocities of 6–7 m/s. Beginning at the shore-line between 0800 and 0900 LST, the breezes penetrated inland over 40 km. Clearly defined return flow layers were present both days. Eulerian wind field measurements from serial pilot, balloon releases were used to make cross sections of the u wind component. Computed one-dimensional divergence values defined the approximate u, w streamline patterns with time. Convergence values in a narrow (1–2 km) zone at the lake-breeze front exceeded 200×10−3 s−1, and calculated upward motions reached 125 cm/s. Optically tracked tetroons yielded Lagrangian trajectory data that confirmed the basic pattern. Most importantly, the tetroons recirculated within the lake-breeze cell, describing a helical trajectory roughly centered on the shoreline. This strongly suggests that air pollutants will likewise be at least partially recirculated over the shoreline, accumulating to levels higher than would otherwise be expected.
An NCAR Queen Air instrumented aircraft took continuous cross sections of particulate concentrations and temperature through the lake-breeze life cycle. The smaller suspended particulates (0.5–3.0 µm), which essentially float with the air, clearly suggest that a significant, fraction of the pollutants released from nearshore sources move inland within the inflow, rise aloft, at the front, advect lakeward in the return flow layer, and then sink back down into the inflow layer offshore. By contrast, larger particles (7–9 µm), having significant terminal velocities, fall out of the cell while over the lake and do not appear to take part in the recirculation phenomena. The role of continuous fumigation of plumes from elevated point sources is also discussed. A schematic model of the lake breeze and its effects on pollutant transport is presented.
Abstract
The lake-breeze circulation on the Great Lakes is often as vigorous as its oceanic counterpart. This paper shows that lake breezes frequently exert drastic control on mesoscale air pollution patterns in urbanized shore-line areas, in this case, Chicago, Ill. Observational data were gathered from a surface mesonetwork, surface and satellite cloud photography, a chain of pilot balloons normal to the shore, optically tracked constant-level balloons, and aircraft measurements of suspended particulate concentrations in several size ranges. On the 2 late summer days studied, the lake breezes were extremely well developed. Inflow depths ranged from 500 to 1000 m, with peak inflow velocities of 6–7 m/s. Beginning at the shore-line between 0800 and 0900 LST, the breezes penetrated inland over 40 km. Clearly defined return flow layers were present both days. Eulerian wind field measurements from serial pilot, balloon releases were used to make cross sections of the u wind component. Computed one-dimensional divergence values defined the approximate u, w streamline patterns with time. Convergence values in a narrow (1–2 km) zone at the lake-breeze front exceeded 200×10−3 s−1, and calculated upward motions reached 125 cm/s. Optically tracked tetroons yielded Lagrangian trajectory data that confirmed the basic pattern. Most importantly, the tetroons recirculated within the lake-breeze cell, describing a helical trajectory roughly centered on the shoreline. This strongly suggests that air pollutants will likewise be at least partially recirculated over the shoreline, accumulating to levels higher than would otherwise be expected.
An NCAR Queen Air instrumented aircraft took continuous cross sections of particulate concentrations and temperature through the lake-breeze life cycle. The smaller suspended particulates (0.5–3.0 µm), which essentially float with the air, clearly suggest that a significant, fraction of the pollutants released from nearshore sources move inland within the inflow, rise aloft, at the front, advect lakeward in the return flow layer, and then sink back down into the inflow layer offshore. By contrast, larger particles (7–9 µm), having significant terminal velocities, fall out of the cell while over the lake and do not appear to take part in the recirculation phenomena. The role of continuous fumigation of plumes from elevated point sources is also discussed. A schematic model of the lake breeze and its effects on pollutant transport is presented.
Abstract
Intense daytime heating in an interior valley and cold upwelled water offshore combine to produce a strong horizontal temperature gradient and the development of a baroclinic field. This leads to an onshore component of the wind in the lower layers with the main features guided by the local topography. Penetration rates of 5 m/s were found. A layer of winds with an easterly component was also found between the low-level westerlies and an upper level westerly current. This easterly current showed signs of being a return flow and was found to have higher small-particle concentrations over the shore than either layer above or below it.
Abstract
Intense daytime heating in an interior valley and cold upwelled water offshore combine to produce a strong horizontal temperature gradient and the development of a baroclinic field. This leads to an onshore component of the wind in the lower layers with the main features guided by the local topography. Penetration rates of 5 m/s were found. A layer of winds with an easterly component was also found between the low-level westerlies and an upper level westerly current. This easterly current showed signs of being a return flow and was found to have higher small-particle concentrations over the shore than either layer above or below it.
Wind sensors mounted on towers and smokestacks do not always indicate the true free-air flow. To determine the probable errors in measurements of wind speed and direction around such structures, quarter-scale models have been tested in a large wind tunnel. Data on changes in wind speed and direction were obtained by using smoke, very small wind vanes, and a scale model propeller anemometer. Most emphasis has been placed on a relatively open lattice-type tower, but a solid tower and a stack were also studied.
The analysis shows that in the wake of lattice-type towers disturbance is moderate to severe, and that in the wake of solid towers and stacks there is extreme turbulence, with reversal of flow. Recommendations for locating wind sensors in the wind field relative to the supporting structure are given for each of the three structures studied. Guidelines are suggested regarding probable errors in measurements of wind speed and direction around different supporting structures, as outlined below.
For an open triangular tower with equal sides D, the wake is about 1-1/2D in width for a distance downwind of at least 6D. Sensors mounted 2 D out from the corner of such a tower will usually measure speeds within ± 10° of that of the undisturbed flow for an arc of about 330°. The disturbance by very dense towers and stacks is much greater. Wind sensors mounted 3 diameters out from the face of a stack will measure wind speeds within ± 10%, and directions within ± 10° of the undisturbed flow for an arc of about 180°.
Wind sensors mounted on towers and smokestacks do not always indicate the true free-air flow. To determine the probable errors in measurements of wind speed and direction around such structures, quarter-scale models have been tested in a large wind tunnel. Data on changes in wind speed and direction were obtained by using smoke, very small wind vanes, and a scale model propeller anemometer. Most emphasis has been placed on a relatively open lattice-type tower, but a solid tower and a stack were also studied.
The analysis shows that in the wake of lattice-type towers disturbance is moderate to severe, and that in the wake of solid towers and stacks there is extreme turbulence, with reversal of flow. Recommendations for locating wind sensors in the wind field relative to the supporting structure are given for each of the three structures studied. Guidelines are suggested regarding probable errors in measurements of wind speed and direction around different supporting structures, as outlined below.
For an open triangular tower with equal sides D, the wake is about 1-1/2D in width for a distance downwind of at least 6D. Sensors mounted 2 D out from the corner of such a tower will usually measure speeds within ± 10° of that of the undisturbed flow for an arc of about 330°. The disturbance by very dense towers and stacks is much greater. Wind sensors mounted 3 diameters out from the face of a stack will measure wind speeds within ± 10%, and directions within ± 10° of the undisturbed flow for an arc of about 180°.
