Search Results

You are looking at 1 - 10 of 15 items for

  • Author or Editor: Perry J. Samson x
  • Refine by Access: All Content x
Clear All Modify Search
Perry J. Samson

Abstract

This paper presents a technique for quantifying the relationships between observed concentrations of atmospheric sulfate aerosol and their corresponding upstream history of sulfur dioxide emissions, wind speed and mixing height. Using reported sulfate concentrations from several sampling sites in the northeastern United States, 72 h upstream trajectories have been computed for winds in the mixed layer of the atmosphere over the duration of their respective sampling periods.

Trajectories from one site were computed for four sublayers, each 400 m thick, extending from the surface to 1600 m. The deviations in along-trajectory and cross-trajectory directions of each of the sublayers from the position of the whole mixed layer were computed. From this the functions σ y(t) and σ x(t) for travel times of 6 to 72 h were derived for each layer individually and collectively for the whole layer. The values of σ y(t) and σ x(t) for the whole mixed-layer were found to be roughly equivalent over this time period and to grow linearly in time. The growth can be described by the relationship σ y(t) = 5.4t , where σ y is in kilometers and t is in hours.

Using these statistics to describe the potential impact from upstream sources, each trajectory was integrated over finite time steps to estimate the potential emissions loading along that trajectory as a function of time upstream. Correspondingly, estimates were made as a function of time upstream of the wind speed in the layer and the depth of the mixed layer.

It was found that sulfate concentrations were insensitive to upstream mixing height, as determined in this study, but were almost always positively correlated with the inverse of wind speed occurring 24 h or more upstream of the sampling point. No consistent relationship was evident between sulfate concentrations and potential upstream SO2 emissions loading.

The ratio of observed to potential sulfate generation, as determined from the total upstream sulfur dioxide input, indicates an average net efficiency of roughly 15–30% conversion of sulfate dioxide to sulfate before deposition. These values vary dramatically with increasing efficiency corresponding to increasing resultant sulfate concentration.

Full access
Mitchell J. Small and Perry J. Samson

Abstract

Methods are presented for generating an ensemble of synthetic atmospheric trajectories. These include methods for a set of independent trajectories, and methods for a correlated set of sequential trajectories. The models incorporate first-order autocorrelation of successive zonal and meridional displacement, and in the case of the sequential set, correlations between trajectories due to spatial patterns of air flow.

The model for independent trajectories is applied and validated using three years (1976–78) of observed forward trajectories originating from southeast Michigan. The synthetic trajectories are statistically consistent with observed wind fields, and result in large-scale diffusion which is comparable to existing estimates. Limitations in the assumption of spatial homogeneity of wind statistics are explored using backward trajectories arriving at six locations in eastern North America during July 1978. The applications highlight the problems which may arise when observed trajectories are used in atmospheric transport modeling, and support the use of synthetic trajectories, at least in a complementary analysis, to obtain a more accurate long-term representation. The procedure for correlated trajectories is not implemented; however, it may prove useful for transport problems of shorter time scales where wind persistence is important.

Full access
Jonathan D. Kahl and Perry J. Samson

Abstract

Routine and supplemental rawinsonde observations collected during the Preliminary Regional Experiment for Storm-Central (PRE-STORM) were analyzed to assess the uncertainty in boundary-layer trajectory calculations due to imprecise interpolation of the horizontal wind field. This study was designed to complement our earlier analysis of rawinsonde data collected during the Cross Appalachian Tracer Experiment (CAPTEX; Kahl and Samson 1986). The present study is representative of widespread convective conditions, while our previous study was representative of fairly persistent, undisturbed flow.

Spatial autocorrelation analysis revealed significant wind field variability on scales less than 100 km. Evaluation of several spatial and temporal interpolation techniques yielded mean absolute errors in estimation of u and v wind components ranging from 3.3–6.5 m −1. Spatial interpolation accuracy improved only slightly when supplemental measurements were included in the interpolation procedure.

Estimates of trajectory errors were obtained using the “trajectory of errors” model of Kahl and Samson (1986). Mean horizontal errors of 493 km were found after 72 h of travel. Contributions of spatial and temporal interpolation to the overall trajectory error were equivalent. Trajectory errors wore 40% greater than those estimated using CAPTEX interpolation statistics.

The presence of predominant mesoscale circulations during PRE-STORM is responsible for the elevated small-scale wind variability as compared to CAPTEX conditions, thus leading to larger interpolation errors and, in turn, larger trajectory errors. Our results suggest that data resolution finer than that considered in this study is necessary to significantly improve trajectory accuracy during meteorological conditions similar to PRE-STORM.

Full access
Mark E. Fernau and Perry J. Samson

Abstract

The precipitation chemistry associated with the flow patterns of transport-derived clusters has been examined. Cluster analysis was applied to transport vectors, derived from there years of daily trajectories arriving at monitoring sites, in order to define a synoptic climatology of representative three-day periods of air mass movement. The resulting clusters were successful in defining wet, dry, polluted and nonpolluted clusters, as shown by the spatial patterns of median deposition and by statistical testing. The highest pollutant depositions over the widest areas resulted from mean transport patterns with large areas of slow air mass movement over the regions of high sulfur emissions and which were frequently persistent over several periods or followed persistent clusters. There was a large amount of overlap among the chemistry distributions and large variation within most of the clusters. Seasonal differences exist within each cluster, with sulfur deposition within a given cluster generally being higher in the warmer months. Cluster analysis was shown to be useful in the computer-assisted classification of spatial patterns of weather and pollution data.

Full access
Jonathan D. Kahl and Perry J. Samson

Abstract

Meteorological observations conducted during the Cross Appalachian Tracer Experiment (CAPTEX) were utilized to quantify the uncertainty in boundary layer trajectory calculations due to low-resolution meteorological data [the current National Weather Service (NWS) rawinsonde network). Evaluation of several spatial and temporal interpolation techniques against high-resolution measurements revealed mean absolute errors of 2–4 m s−1 in estimation of horizontal wind components.

A trajectory of errors procedure is introduced that allows the quantification of probable errors in transport calculation due to imprecise interpolation. Our results, based on the observed distributions of spatial and temporal interpolation errors during CAPTEX, indicate dust boundary layer trajectories calculated using the current NWS network with 12 h resolution contain a 50% chance of exceeding horizontal displacement errors of 350 km after 72 h travel time. An increase in spatial resolution is shown to improve the accuracy of trajectory calculations more than an increase in temporal resolution. These results are representative of relatively undisturbed flow in the northeastern United States and southern Canada and do not include the possible effects of nonindependent trajectory errors.

Full access
Teresa M. Schulz and Perry J. Samson

Abstract

Nonprecipitating low cloud frequencies for 1982 have been extracted from the U.S. Air Force 3-Dimensional Nephanalysis archives for central North America. These data were compiled from satellite, surface and pilot reports and contain, among other parameters, the low cloud type and present weather code for each 3-h time period for grid points in the Northern Hemisphere. The overall average precipitating to nonprecipitating low cloud ratio for central North America was 0.25, indicating that nonpredpitating low clouds were very common. Seasonal frequencies of these nonprecipitating low clouds are presented. The persistence of clear and nonprecipitating cloudy skies from one time period to the next was also calculated on a seasonal basis. Annual average values for transitions between clear and cloudy skies as well as between nonprecipitating and precipitating low cloudy time periods are also presented. A cluster analysis was performed on the seasonal nonprecipitating low cloud frequency data. It was found that the original 4096 grid points could be described well by seven robust clusters. Thew clusters are identified and their distinguishing characteristics are discussed. It was found that not only were these clusters distinct from one another, but they exhibited considerable within-cluster consistency.

Full access
Jonathan D. Kahl and Perry J. Samson

Abstract

The dependence of wind interpolation accuracy on vertical shear was investigated using routine and supplemental rawinsonde data collected during the Cross Appalachian Tracer Experiment. Spatial interpolation error distributions for horizontal wind components were stratified by the “bulk” shear within the mixed layer. Results indicated that interpolation errors were approximately proportional to vertical wind shear.

Full access
Peter Nowacki, Perry J. Samson, and Sanford Sillman

Abstract

It is shown that Urban Airshed Model (UAM-IV) calculated air pollutant concentrations during photochemical smog episodes in Atlanta, Georgia, depend strongly on the numerical parameterization of the daytime vertical diffusivity. Results found suggest that vertical mixing is overestimated by the UAM-IV during unstable daytime conditions, as calculated vertical diffusivity values exceed measured and comparable literature values. Although deviations between measured and UAM-IV calculated air pollutant concentrations may only in part be due to the UAM-IV diffusivity parameterization, results indicate the large error potential in vertical diffusivity parameterization. Easily implemented enhancements to UAM-IV algorithms are proposed, thus improving UAM-IV modeling performance during unstable stratification.

Full access
Perry J. Samson and Kenneth W. Ragland

Abstract

For June and July 1975, ozone concentrations throughout the Midwest showed a consistent dependence onwind direction with the highest concentrations associated with winds from the southeast. This systematicpattern suggests that large-scale transport rather than local sources control the general trends of the ozonelevel in the study area.Ozone and meteorological data covering an episode period 29 July-2 August have been analyzed usingweather maps, visibility reports, upper air soundings, trajectories and aircraft measurements to ascertainthe scale of the ozone problem. Results indicate that the highest ozone concentrations occurred within regions of reported haze, and that these regions had definite bounds which could be followed over the country.Moreover, distinct areas of reported obscured sky formed in the vicinity of St. Louis and Pittsburgh whichcould be followed over thousands of kilometers reducing visibilities substantially even in upper Ontario.Trajectories indicate that adverse conditions occurred when stagnant air over the Ohio River valley wastransported northward on the western half of a high pressure system. The haze region was preceded to thenorth and east by extremely high temperatures (>35"C). Vertical ozone profiles obtained by aircraft measurements show that ozone concentrations above the surface inversion in the haze remain quite high over-night, but above the subsidence inversion the ozone was low. It is suggested that the episode was due primarily to large-scale transport of ozone produced from precursors accumulated during the stagnant periodover the eastern Midwest.

Full access
Mark E. Fernau and Perry J. Samson

Abstract

Cluster analysis has been applied to transport vectors, derived from three years of daily backwards trajectories, in order to define a synoptic climatology of representative three-day periods of air mass movement. The resulting clusters represent groups whose mean air mass transport fields are statistically different from one another and correspond to the types of high and low pressure patterns seen on daily weather maps. Seasonal differences were evident in the frequency of occurrence of each cluster. The clusters were relatively insensitive to changes in number of sites or years used; however, different clustering methods yielded somewhat different clusters. Ward's method yielded clusters with more or less equal numbers while other methods tended to produce one large cluster and a series of “outlier” clusters. Cluster analysis was useful in the computer-assisted classification of spatial patterns of weather data and should be considered for use along with more widely used synoptic climatological tools such as principal component analysis.

Full access