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Persistence of wind direction
Discussion
661
Paulson and Potter suggest that Fro. 8 (p. 732) demonstrates the dependence of e.m.v. on an operational factor, namely the voltage. The fact that here the operating voltage is also the &shover voltage is not coincidental since it was the flashover voltage that determined the operating voltage. FIGURE3 (p. 726) and FIG. 8 (p. 732) together demonstrate that &shover voltage is strongly influenced by some properties of the ash, i.e. that operational factors can also be dependent on ash properties, Never-theless, it would be wrong to associate e.m.v., or efficiency, only with the ash properties and we believe that any tendency to do this to the exclusion of plant factors has now been countered by Barrett’s computer analysis, which presents e.m.v. as functions of total electrode area, electrode area per unit volume of gas flow, the ratio of collector area to ht. set capacity and collector area per zone. J. DALMON D. TIDY
Central Electricity Research Laboratories, Kelvin Avenue,
Leatherhead Surrey.
DISCUSSION PERSISTENCE
OF WIND DIRECTION*
A~rrroucm the contents of the paper by Shirvaikar are quite interesting they do not introduce any new or startling concepts on the behavior of wind direction persistence as a function of wind speed, averaging time or sector width. However, the linear plot fit, of the cumulative probability curves on log pro~bility paper (FIG. l), would be difficult to explain from meteorolo~ theory alone. Perhaps the most sign&cant thing illustrated by Shirvaikar’s work is that probabilities for wind direction persistence times can be calculated rather easily if the wind records are available, and the related parameter R*(T), which is defined to be the return period time, can be derived. I feel that these two parameters are of considerable importance in air pollution studies. For example, the design of FAPS (Fate of Atmospheric Pollutants Study) conducted by NCAR is critically dependent on some understanding of the behavior of these parameters. The decrease of U,,,.. and the increase of Umlnwithincreasingpersistenceshouldcertainlybeexpected from the basic concepts of synoptic meteorology. Maximum surface wind speeds usually occur with strong surface pressure gradients, and this type of situation is generally associated with unstable synoptic patterns, i.e. deep low pressure cells and frontal activity causing frequent changes in wind direction. On the other hand, minimum wind speeds are a result of stable synoptic patterns, where the surface pressure gradients are usually weak and diurnal heating and cooling effects are more prevalent. When this type of environment exists, wind direction persistence would be expected to decrease because the diurnal cycle is more apt to effect changes in the low level wind flow, and the geostrophic wind flow is too light to counteract it. The discussion about averaging time and sector width, presented in Section (5), verifies what most surface wind traces would tend to ind~~~nsiderable short period oscillations and a meandering of the average wind direction over several sector widths within a specified period of time. The applications of persistence analysis discussed by Shirvaikar in Section (7) puts much significance on U,,,raand its relationship to Pasquill’s stability classi.Rcation. Unfortunately, small values of U,,,,, and high values of stability usually produce strong wind shear with height. Unless we assume that the plume cannot mix upward at all, the values shown in Fro. 4 might not necessarily be correct. Finally, it is of some interest to note that preliminary analyses of the plume measurements, taken recently by FAPS near St. Louis, show that the best defined and most narrow plume existed in an unstable environment. This would probably lead to disagreement with Pasquill’s diffusion categories; however, a complete analysis has not yet been made, and therefore, final conclusions are still pending. National Center for Atmospherk Research, Lzboratmy of Atmospherk Scknce, P.0. 30x 1470, &d&r, Cokwab 8030& U.S.A.
661
Paulson and Potter suggest that Fro. 8 (p. 732) demonstrates the dependence of e.m.v. on an operational factor, namely the voltage. The fact that here the operating voltage is also the &shover voltage is not coincidental since it was the flashover voltage that determined the operating voltage. FIGURE3 (p. 726) and FIG. 8 (p. 732) together demonstrate that &shover voltage is strongly influenced by some properties of the ash, i.e. that operational factors can also be dependent on ash properties, Never-theless, it would be wrong to associate e.m.v., or efficiency, only with the ash properties and we believe that any tendency to do this to the exclusion of plant factors has now been countered by Barrett’s computer analysis, which presents e.m.v. as functions of total electrode area, electrode area per unit volume of gas flow, the ratio of collector area to ht. set capacity and collector area per zone. J. DALMON D. TIDY
Central Electricity Research Laboratories, Kelvin Avenue,
Leatherhead Surrey.
DISCUSSION PERSISTENCE
OF WIND DIRECTION*
A~rrroucm the contents of the paper by Shirvaikar are quite interesting they do not introduce any new or startling concepts on the behavior of wind direction persistence as a function of wind speed, averaging time or sector width. However, the linear plot fit, of the cumulative probability curves on log pro~bility paper (FIG. l), would be difficult to explain from meteorolo~ theory alone. Perhaps the most sign&cant thing illustrated by Shirvaikar’s work is that probabilities for wind direction persistence times can be calculated rather easily if the wind records are available, and the related parameter R*(T), which is defined to be the return period time, can be derived. I feel that these two parameters are of considerable importance in air pollution studies. For example, the design of FAPS (Fate of Atmospheric Pollutants Study) conducted by NCAR is critically dependent on some understanding of the behavior of these parameters. The decrease of U,,,.. and the increase of Umlnwithincreasingpersistenceshouldcertainlybeexpected from the basic concepts of synoptic meteorology. Maximum surface wind speeds usually occur with strong surface pressure gradients, and this type of situation is generally associated with unstable synoptic patterns, i.e. deep low pressure cells and frontal activity causing frequent changes in wind direction. On the other hand, minimum wind speeds are a result of stable synoptic patterns, where the surface pressure gradients are usually weak and diurnal heating and cooling effects are more prevalent. When this type of environment exists, wind direction persistence would be expected to decrease because the diurnal cycle is more apt to effect changes in the low level wind flow, and the geostrophic wind flow is too light to counteract it. The discussion about averaging time and sector width, presented in Section (5), verifies what most surface wind traces would tend to ind~~~nsiderable short period oscillations and a meandering of the average wind direction over several sector widths within a specified period of time. The applications of persistence analysis discussed by Shirvaikar in Section (7) puts much significance on U,,,raand its relationship to Pasquill’s stability classi.Rcation. Unfortunately, small values of U,,,,, and high values of stability usually produce strong wind shear with height. Unless we assume that the plume cannot mix upward at all, the values shown in Fro. 4 might not necessarily be correct. Finally, it is of some interest to note that preliminary analyses of the plume measurements, taken recently by FAPS near St. Louis, show that the best defined and most narrow plume existed in an unstable environment. This would probably lead to disagreement with Pasquill’s diffusion categories; however, a complete analysis has not yet been made, and therefore, final conclusions are still pending. National Center for Atmospherk Research, Lzboratmy of Atmospherk Scknce, P.0. 30x 1470, &d&r, Cokwab 8030& U.S.A.