3. OVERVIEW OF TRANSPORT REGIMES

Transport of ozone and precursors is affected by flow regimes that differ with altitude. The characteristics of these flow regimes were examined for the July 14-15 and July 31-August 1, 1995 regional ozone episodes in the Northeast. The flows can be classified into three regimes for discussion purposes: (1) At higher altitudes, the flow is driven by synoptic pressure gradients and tends not to exhibit diurnal effects. (2) Below about 2000 m, mesoscale effects can redirect or channel the flows. These effects include jets that form above the surface layer at night, thermal effects, channeling by river valleys, and the lee trough that forms east of the Appalachians. (3) At the surface, the flow is also driven by local effects, but it decouples from the other two regimes at night. The surface winds tend to become stagnant at night during episode conditions. They can also remain light through much of the day, although they couple to the aloft winds when the mixing layer deepens during the day and brings momentum down from aloft. Surface winds can also transport accumulated emissions offshore where they can react and subsequently be transported back onshore. While offshore, the lack of surface heating means that the surface layer stays decoupled from the aloft winds and from ozone transported aloft.

The flow regimes described above were examined through time-height wind cross sections, trajectory analyses, and resultant wind vectors at three altitudes over the radar profiler sites. Figure 3-1 shows the time-height cross section for the winds over the Rutgers University radar profiler at New Brunswick, NJ from midday July 31 through midday August 1. The winds above about 800-1000 m agl in this figure were driven by a large high-pressure system that was centered over southwestern Pennsylvania on July 31 and had moved southeastward by the morning of August 1. The mixing layer on these days extended to about 1600-1800 m agl and was limited by the subsidence inversion noted in the figure. The mixing-layer structure is described in more detail in Section 7. Between about 800 m and the top of the mixed layer, the synoptically-driven winds were from the southwest through the northwest (when they were not essentially calm). This is typical of ozone episodes as will be discussed in Section 4.

Below about 600-800 m, the winds diverged during parts of the day from the synoptic flow farther aloft. From about 1800-0800 EST, a nocturnal jet is evident in the figure below the synoptic flow and above the nocturnal surface radiation inversion. The flow in the jet started out southerly and turned to westerly late at night. The jet flow had different speeds and directions than the flow farther aloft. Jet flows occur frequently during episode conditions and are discussed in Section 5. Even in cases when the jet is not evident, however, the flow between the near surface layer and the synoptic flow farther aloft can differ from the higher and lower flows due to other mesoscale effects. For example, westerly flow across the Appalachians aloft can cause a lee trough east of the Appalachians that generates southwesterly flow east of the trough below the level of the ridges (Gaza, 1996).

The surface radiation inversion provides a barrier at night that decouples the flow a few hundred meters above the surface from the surface. The surface flow at night and through the morning during episode conditions is often calm or very light as seen in Figure 3-1. This surface stagnation allows emissions to accumulate near the surface during the night and morning hours while transport continues aloft. In the morning hours, the mixing layer over land deepens from surface heating; and the surface emissions start to mix with the air transported aloft overnight. As the surface and upper air mix, the surface air is accelerated. By midafternoon, air from all three flow regimes can be mixed together and jointly contribute to the ozone concentrations downwind. If the surface air is transported over water, there is no heating mechanism to drive the mixing, and the emissions in the near-surface layer can stay confined to that layer until the air mass hits a shoreline.

The relationship between the flow regimes and the vertical ozone distribution at Gettysburg, PA in early morning on August 1, 1995 is shown in Figure 3-2. This figure shows over 80 ppb of ozone transported from the southwest at 10 m/s in the jet. The radar profiler data from that period showed that this transport continued all night from the south to southwest at similar speeds. The ozone seen in the jet could have been transported a few hundred kilometers overnight if the jet was widespread and persistent. At higher elevations, over 70 ppb was being transported in the synoptic flow from the west. Also seen at about 1200 m is an area of depleted ozone and increased NO_y, indicating transport of some fresh emissions in addition to ozone above the surface layer. At the surface, the wind was very light from the south, and the ozone was depleted by fresh emissions. When the mixing layer increased on August 1, the ozone and precursors from all three regimes would have mixed together.

The three flow regimes can also be visualized by examining the resultant winds at reference altitudes for the episode days. Figures 3-3, 3-4, and 3-5 show resultant wind vectors ending at 1700 EST at 10, 500, and 1000 m agl for the five radar profiler sites for July 14 and August 1. These vectors are the vector sum of the hourly wind vectors at the noted altitudes and times. Figures 3-3 shows the 12-hour resultant vectors for July 14. The flows vary between the sites, but the surface winds were generally light compared to those aloft. At Holbrook (HBK), Rutgers, Millstone Point (MSP), and Redhook (RHK), the aloft flows came from more northerly (i.e., clockwise) directions than the surface flows. Similar conditions can be seen in Figure 3-4 for August 1. Figure 3-5 is also for August 1, but shows the 24-hour resultant vectors. Figure 3-5 shows the effect of the jet at all sites. In Figure 3-4, which did not cover the nighttime hours, the upper two vectors are similar lengths. In Figure 3-5, the lengths of the 500 m vectors are substantially longer than those of the other two heights. They are also much more than twice as long as the same vectors in Figure 3-4, indicating that most of the transport occurred overnight. This shows the potential for long-range transport in the channeled flows below 2000 m msl. Both Figures 3-4 and 3-5 show strong shear between the three layers. In the urban corridor, the low-level flows were southerly or southwesterly along the corridor, while the upper-level flows would have brought air from more westerly locations.

Back-trajectory analyses were performed to evaluate the transport distances and the origins of the air for the three transport regimes. Analyses were performed for July 14, 15, and August 1. Some NWS data were not immediately available for some of July 31, but the winds at all levels were very light on that day and were not conducive to significant transport until evening. The portions of back trajectories from August 1 going into July 31 may be less reliable than for other days due to the reduced data set. These trajectories will be revised for subsequent drafts of this report, but the general findings from them are not expected to change.

Wind fields for the episode days were calculated with the CALMET (version 3) diagnostic wind model (Scire et al., 1995) using the NARSTO-Northeast upper-air and surface data for input. This model provides a means to interpolate the data in time and space between the measurements. The resulting wind fields are derived directly from the measurements. The available upper-air data included six rawinsondes per day at seven sites, standard two/day soundings at National Weather Service sites, and hourly radar profiler and sodar data. The upper-air measurement sites are shown in Figure 1-2.

The model produced hourly-averaged wind fields for a 196-by-200 square-cell mesh with 5 km cell resolution and 14 layers within the bottom 3300 m of the atmosphere. CALMET is terrain following, with layer tops at 20, 50, 100, 200, 400, 600, 800, 1000, 1400, etc. meters. The winds were linearly interpolated in time between hours to provide 15­minute time steps. The wind field was interpolated in space to the location of each trajectory from the four nearest cell centers.

Trajectories were calculated using 15-minute time steps for the layers that included 10 m (layer 1), 500 m (layer 6), and 1000 m (layer 8) above ground level. The layers are referenced to ground level, and the trajectories were calculated for constant model layers. Back trajectories were calculated for several sites throughout the study region ending at 1500 EST on the episode days. Figures 3-6, 3-7, and 3-8 show the back trajectories at all three altitudes for each of the three days. The square boxes on the figures represent the end points at 1500 EST and the beginning of the 2300-2400 EST segment (i.e., 2300 EST) along the trajectories. Symbols are shown every three hours along the trajectories. Flows that occur in thin layers such as the near-surface flow offshore may not be well represented by these trajectories.

Figures 3-6 to 3-8 show transport near the surface was generally from the southwest through westsouthwest. The straight-line transport distances at the surface ranged from roughly 150 to 300 km (90-200 miles) in 24 hours and from 75-200+ km (50-125+ miles) during the day.

The 500 m transport was from the southwest through northwest on July 14 and August 1 and from the northwest on July 15. The transport distances at 500 m agl were on the order of 200-300 km (125-200 miles) for 12 hours during the daytime and as much as 600-800 km (375-500 miles) for 24 hours, with much of the transport occurring at night. Many of the trajectories at 500 m cross the Appalachians. This may be an artifact of the model which calculates the trajectories above the terrain. It might be more instructive to calculate constant density or isentropic trajectories. However, examination of these trajectories superimposed on terrain shows that many of them are indeed channeled through gaps in the mountains.

The 1000 m transport was from the west through northwest on July 14 and 15 and from the southwest through west on August 1. The transport distances at 1000 m ranged roughly from 300 to 800 km (200-500 miles) over 24-hour periods, with the faster speeds to the north.