Reviewed were 2-hr increments of the following data:
- Measured ozone data (Aerometric Information and Retrieval System - AIRS),
- Gridded temperature, wind and precipitation fields (Nested Grid Model [NGM]
calculations), and
- 3 day back trajectories and particle flow / distributions (CAPITA Monte
Carlo simulations).
At the start of this episode (13 July) ozone values were relatively low throughout most of the OTAG region, with some moderately high levels (approximately 90 ppb) in the mid-Atlantic States (NJ/PA). This was primarily due to a rather ordinary meteorological situation: Anti- cyclonic trajectories in the Great Lakes states, some cyclonic movement in the South with unorganized air flow elsewhere. Hot (> 28 C), precipitating weather was evident in the vicinity of this southern low. The particle flow displays indicate good dispersion throughout the region, save for the Southeastern seaboard (FL to DE).
Based upon the wind vector plots, back trajectories and particle flow display maps, the Southern OTAG area was subject to sizeable stretches of unsettled weather that seemed to be driven by the low pressure system over the region for much of the episode. This low appeared to have forced significant convergence of the hot, humid Gulf of Mexico air into the area. Precipitation activity, as indicated by the maps, was energetic enough to keep the region's air from being relatively cloud-free and stagnating, which are necessary conditions for ozone precursors and ozone to build up.
The great bulk of high ozone activity during this episode took place in the Northern half of the OTAG domain, extending from the Great Lakes states (17 - 19 July) eastward to the Mid- Atlantic and New England states (17 - 20 July). At the beginning of the episode this part of the OTAG domain was influenced by anti-cyclonic winds (in the Great Lakes area) and northwesterly flow (in the Northeastern U.S.). The high pressure system that appears to have forced the clockwise winds was centered just above the Great Lakes on 14 July. Good dispersion was evident from the particle flow maps for this period. Consequently, the cool, dry temperatures resulted in average ozone levels at the onset of this episode.
Continuing the assumption that the anti-cyclonic wind fields in the OTAG north are indicative of high pressure, it appears that the center of this system gradually migrated southeastward to somewhere in the Ohio / Pennsylvania region during 14 - 18 July. Concurrently, the back trajectories and wind vectors imply that the high's trailing edge increasingly advected warm (> 28 C), humid Gulf air north-northeastward through the Great Plains states to the Great Lakes states. According to the time lapse temperature maps the region of warmest air (> 28 C) expanded northward then eastward through these states during this period.
The precipitation maps identify some scattered convection throughout portions of the upper Great Lake states (WI / MN) - most noticeably during 18 July. However, there apparently was no large organized frontal activity to translate any surface instability into widespread persistent convective systems. This is visible in the time-lapse ozone maps which show general ozone increases gradually from west to east through the upper midwest during 15 - 19 July. By 19 July, drier, cooler northwesterly flow had begun to influence the upper midwest. High ozone values persisted in this region until 20 July before significantly declining.
Meanwhile, the center of the high pressure system continued to move southeastward into VA and off the coast by 20 July. During 17 - 20 July, the 3 day back trajectories reveal Gulf air increasingly being drawn up the trailing edge of the high through the Appalachians / Ohio River Valley to the Northeast. Additionally, some of the ozone-laden air from the Great Lakes states may also have migrated eastward, pulled into the high's cyclonic flow northeastward.
The consequence was that the northeastern U.S. experienced the highest ozone levels of the entire OTAG area during 16 - 20 July. As in the Midwest 1-2 days earlier, northwesterly flow behind the high began to reduce the ozone levels in the northeast by 21 July. This cooler air combined with some convective activity (visible on the precipitation maps) through the northern OTAG region to terminate this ozone episode.
Reviewed were 2-hr increments of the following data:
- Measured ozone data (Aerometric Information and Retrieval System - AIRS),
- Gridded temperature, wind and specific humidity fields (Nested Grid Model [NGM] calculations), and
- 3 day back trajectories and particle flow / distributions (CAPITA Monte Carlo simulations).
The four display panels that express AIRS-derived ozone air quality measurements all combine to emphasize that the southeastern United States ("SE US", i.e., primarily Tennessee east to the Carolinas, centering on Northern Georgia) witnessed the great bulk of high ozone levels. The only exceedances of the ozone national ambient air quality standard (NAAQS) in the OTAG region during this episode occurred in the SE US.
Some residual high ozone (> 0.100 ppm, no NAAQS exceedances) appears to have occasionally advected northward to the lower mid-Atlantic states (Pennsylvania, New Jersey), as well as the lower portions of the Great Lakes states (through southern Wisconsin and Michigan on 26 July). Otherwise, this period looks to have witnessed mostly moderate in ozone concentrations throughout the OTAG region, save for the SE US.
A review of the displays of time-lapse meteorological fields generated by the Nested Grid Model (NGM) offers an initial understanding of why the high ozone was mostly restricted to the SE US. High temperatures (> 28 C), short anti-cyclonic wind vectors (i.e., weak winds, stagnation) and relatively moist conditions (> 16 g/kg specific humidity), persisted through much of the episode in the SE US. One can also reasonably speculate large scale subsidence in this region greatly reduced mixing heights - further restricting dilution.
The gradients in the scalar quantities (temperature, specific humidity) from SE US northward and westward were fairly significant most of the time, indicating that weather conditions were much less conducive to high ozone in much of the other OTAG areas for this episode. On occasion (24-25 July) the northern states did witness advection of the hot, humid Gulf of Mexico air mass. However, these circumstances did not persist for long enough periods to create weather conditions favorable for ozone exceedances in these states.
The NGM-derived kinematic and thermodynamic fields strongly suggest that a semi-stationary high pressure system prevailed over the SE US for most of the episode. Furthermore, this apparent high pressure slowly migrated off the SE coast of the US. This would have likely drawn the hot, humid air mass from the Gulf of Mexico directly into the SE US. The SE US's position on the high's trailing edge was most apparent on 28 July, when the high ozone values were most widespread throughout this part of the country. Assuming the presence of a strong high pressure system would also signify poor dilution and a buildup of pollutants in the region.
The low-level (500 meter height) particle flow simulations also point to stagnation during most of the episode in the SE US. Namely, large mass concentrations as well as no "clearing out" of the hypothetical particles predominated in the area during this episode. An organized thinning or absence of the tracer particles signifies a frontal passage. This frontal feature was not evident in the tracer particle plots for the SE US.
The displays of the 3 day back trajectories, Acalculated with the CAPITA Monte Carlo model, include features that could be evidence of some long-range transport of air pollution, including ozone and ozone precursors. Namely, there were persistent, anti-cyclonically curved trajectories emanating from the Lake Michigan states / Pennsylvania into the SE US during 21 July and 28 July. On these days the Lake Michigan states may also have been subject to a high pressure system, albeit not as strong as that in the SE US. These kinematics might partially explain that the buildup to high ozone was greatest for the SE US during this episode on these two days.
As a "control" scenario, when the back trajectories came straight (i.e., no anti-cyclonic curve) from the Lake Michigan states into the SE US, then the ozone concentrations, as well as temperature and moisture were relatively low (30 July). This could be the result of a modified outbreak of Canadian air entering directly into the region. Additionally, shorter 3 day back trajectories originating in the Gulf, as well as short anti- cyclonically curved trajectories had a strong presence in the SE US. This fact underscores the kinematics represented in the region's NGM wind fields.
10 June 96 (revised) Wis DNR
A brief analysis of the time-lapse "movie" displays of data for the 1995 OTAG episode (7 - 18 July). Reviewed were 2-hr increments of the following data:
- Measured ozone data (Aerometric Information and Retrieval System - AIRS),
- Gridded temperature, wind and precipitation fields (Nested Grid Model [NGM] calculations), and
- 3 day back trajectories and particle flow / distributions (CAPITA Monte Carlo simulations).
At the start of this episode (7 July) the back trajectories and NGM wind vectors reveal a sizeable northwesterly flow pattern (relatively cool dry air) throughout much of the OTAG region, save for the Texas/Gulf states/Georgia area. The region of warm temperatures (> 28 C) was also restricted to the deep South, with a strong gradient towards cooler values northward. There appeared to be an adequate degree of dispersion - based upon the generally "thin" spread in the particle distribution maps for this day.
Dry conditions prevailed save for a little precipitation in the lower plains and PA/NJ/DE coastal area during the first few days of this episode. The weather in the northern two-thirds of the region was largely dominated by a massive high pressure system centered north of the Great Lakes at this time.
With the exception of modestly high ozone readings (75 - 100 ppb) ozone levels were ordinary throughout the Eastern half of the country during 7 - 8 July. This consequence would be expected from the above-described meteorological conditions.
During 8 - 12 July 95 the changing synoptic-scale weather dynamics dominated the distribution of ground-level ozone throughout most of the OTAG area. Namely, the high pressure system in Canada slowly migrated eastward. There was increasing stagnation through the central region of the domain - at the trailing edge of the high as well as behind it. Moist, warm air form the Gulf of Mexico initially advected straight north through the central plains states and then turned anti-cyclonically towards the Lake Michigan states. This air mass movement resulted in rather warm (> 35 C) hazy, ozone-conducive weather entering the Lake Michigan states on the 12th via westerly flow. Unorganized convective activity and scattered precipitation resulted from these very hot conditions. July 12 was the general peak ozone day in the upper midwest states for the episode. Many sites in the this region measured exceedances to the ozone national ambient air quality standard (NAAQS) on this day.
Relatively weak air flows were evident throughout most of the OTAG area during 12 - 14 July, based on the short 3 day back trajectories and abbreviated wind vectors. Concurrently, the significantly warm area continued to shift eastward into the Mid-Atlantic states. It appears that some type of frontal activity was building ahead of this heat. This phenomena is indicated by the convergence patterns in the particle flow displays for the eastern seaboard during these days.
A fair amount of precipitation was also evident for this time and area. Warm gulf air was being advected into the Southeast US during this time. Northwest flow of drier, cooler air was slowly beginning to influence the western half of the OTAG region by 14 July. Scattered, unorganized rain activity occurred throughout much of the region on this day. An exception was the organized precipitation pattern in the convergance zone along the eastern seaboard.
The warm (> 28 C) air that settled into the Northeast region during 13 - 14 July, this stagnating volume apparently was laden with pollutants. The consequence was highest average daily ozone values of the entire were generally measured in this area on 14 July. High ozone was also observed throughout much of the Southeast US.
The general weather / ozone pattern advected slightly to the east by 15 July with high ozone still being witnessed in the eastern third of the OTAG region. Small pockets of exceedances to the ozone NAAQS were recorded at sites in the Eastern Ohio River Valley eastward to the PA/NY/NJ/CT region.
The wind flow vectors and back trajectories indicated a small high pressure system with significant stagnation through the border states (Arkansas east to Virginia). Increasing presence of northwest flow, possibly associated with the leading edge of a large western high pressure system, into the Plains and Great Lake states. The region of warm air (> 28 C) had begun to shrunk. Most of the OTAG area was witnessing declining ozone levels at this time.
During the remaining 3 days of the episode (16 - 18 July inclusive), the areas subject to northwest flow sizeably increased and the high pressure system in the center part of the domain weakened. These kinematics contributed to a dramatic shrinking of those areas that had warm temperatures and high ozone.
A substantial portion of the weather scenario at the beginning of the episode (i.e., northwest flow dominating much of the OTAG area) had returned by the end of the episode.
