5. CHANNELED FLOWS

Below about 2000 m msl, mesoscale and topographic effects can redirect or channel the flows, resulting in transport directions and distances different from the synoptically-driven winds above or the surface winds below. 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. Near the coast, land-sea breezes are also important. The flow in this transport regime was examined for the July 14-15 and July 31-August 1 episodes. The characteristics observed for the episodes are described below. The general characteristics of the jet flows and the lee trough are also described in this section.

5.1 TRANSPORT IN THE CHANNELED-FLOW REGIME ON JULY 14-15 AND JULY 31-AUGUST 1

The nature of the channeled flows can be seen in the time-height cross sections of the upper-air winds. Figures 5-1 and 5-2 show the winds at the Rutgers radar profiler site at New Brunswick, NJ for the July 14-15 and July 31-August 1 episodes. In these figures, the flows above the surface up to about 800-1000 m decoupled from the surface flow and accelerated each night. The flows in this region also had different speeds and/or directions than the flow farther aloft. During the day, the winds throughout the boundary layer became coupled and were similar up through 1000-1500 m agl. Also during the day, frictional effects slowed the winds aloft below the nighttime speeds. On most nights in these figures, the jet can be seen in a layer just above the surface, where the speeds are higher than those above or below. On the night of July 15, at the end of the episode, the winds aloft did not form a jet (i.e., they were not faster than the winds both above and below). They were, however, faster than the winds below. This is a normal occurrence on most summer nights. When the frictional effects are decoupled, the winds aloft will be faster than the surface winds.

On the nights before the regional exceedance days of July 14, 15, and August 1, the nighttime winds in the lower boundary layer were generally from the southwest in the early night hours and rotated to westerly or even northwesterly later at night. On the nights before July 16 and July 31, which were not widespread exceedance days, the flow directions became easterly or northeasterly in this layer.

The acceleration of the flow in the lower boundary layer aloft at night and the variation of flow with altitude were common features throughout the region. Figures 5-3 to 5-10 (Figure 5-3, 5-4, 5-5, 5-6, 5-7, 5-8, 5-9, 5-10) show the upper-air winds at 0300 and 1500 EST on July 14-15 and July 31-August 1. These feature can be seen throughout the region on each of the days except the early morning of July 31 when the winds aloft were essentially calm over much of the region.

The potential for transport overnight in the lower boundary layer is reflected in back trajectories at 500 m agl ending at 0500 EST on the episode days. These trajectories are shown in Figures 5-11, 5-12, and 5-13. The trajectories show transport of roughly 200-400 km (125-250 miles) in 12 hours overnight.

5.2 CHARACTERISTICS OF LOW-LEVEL JETS

The occurrence and characteristics of low-level jets in the Northeast during the summer of 1995 have been examined by Ray et al. (1997). Their results are excerpted below.

Low-level jets form at night and consist of aloft layers of air that have higher wind speeds than air located above and below the jet. Such jets have been observed and studied extensively in the central United States (Parrish et al., 1988), but are not as well understood in the northeastern United States. As discussed by Blackadar (1957), during daytime, winds within the mixed layer that respond to a geostrophic pressure gradient become sub-geostrophic due to the frictional effects of the ground. At night, a low-level jet can form when the stable conditions in the nocturnal boundary layer decouple aloft winds from surface frictional effects, causing the aloft winds to accelerate in response to the geostrophic pressure gradient. Under certain conditions an inertial oscillation develops, causing the winds to overshoot and become super-geostrophic, generating a jet in a shallow layer residing above the nocturnal boundary layer. While aloft wind speeds usually increase after sunset with the decoupling of surface frictional forces, the atmospheric conditions are not always such that a low-level jet is formed. Aloft wind maxima can also be caused by approaching fronts, convergence, sloping terrain, lake effects, etc.

A typical example of the evolution of the low-level jet is illustrated in Figure 3-1 shown earlier. That figure shows the temporal evolution of the low-level jet at Rutgers University, NJ on the night of July 31-August 1, 1995. At 1800 EST, winds below 900 m agl were about 3 m/s, which was similar to the winds measured earlier in the afternoon and comparable to the surface winds. By 1900 EST, the nocturnal boundary layer had formed, which decoupled the surface and aloft air causing the aloft winds to accelerate. The aloft winds increased in speed throughout the night and by 0200 EST, they reached about 15 m/s. Below the jet, the surface winds at night were less than 2 m/s.

During the summer of 1995, these low-level jets were most likely to develop when the entire OTR was under the influence of a large surface high-pressure system with weak pressure gradients, centered to the south or just offshore of the OTR. These were also the conditions favorable for the production of high ozone concentrations. Low-level jets occurred on six out of nine nights preceding a regional ozone episode day in 1995. Also, low-level jets were two to three times more likely to be observed on nights preceding ozone episode days than on other nights. In addition, they were more frequently observed at sites located on the coastal plain east of the Appalachians (Gettysburg, PA; Rutgers University, NJ; and Millstone Point, CT) than at the site located west of the Appalachians (Holbrook, PA) or at the site located in the Hudson River Valley (Redhook, NY).

Aloft winds throughout the NARSTO-Northeast study area were examined for the nights of the July 12-15 and July 31-August 2, 1995 ozone episodes to determine the characteristics of these low-level jets. These characteristics are summarized below.

• Low-level jets formed after sunset and lasted from 3 to 15 hours.
• Average wind speeds were between 5 and 15 m/s (with wind speed maxima of about 20 m/s or more) and usually peaked after midnight.
• Wind directions exhibited an inertial turning in the clockwise direction from south south-westerly to westerly throughout the night.
• The thicknesses of the jets were between 200 and 900 m agl.
• Low-level jets were observed at one time or another at all the upper-air sites examined. However, they were more frequently observed, had stronger winds, and lasted longer at sites located in the coastal plain east of the Appalachians than at sites located west of the Appalachians.

5.3 CHARACTERISTICS OF THE APPALACHIAN LEE TROUGH

The Appalachian lee trough is a region of lower pressure that forms to the east of the Appalachian Mountains (Pagnotti, 1987; Gaza, 1996). Gaza (1996) examined the occurrence of the trough during the NARSTO-Northeast study period. Westerly to northwesterly synoptic winds flowing perpendicular to the mountains produce the trough with an axis parallel to the mountains that extends from eastern Maine southwestward into New Jersey and eastern Pennsylvania. The trough is characterized by southwesterly winds in advance of (east of) the trough axis and westerly to northwesterly winds behind (west of) the trough. Such troughs have been observed on 40 percent of the days from May through September (Weisman, 1990). However, Pagnotti (1987) reported that the lee trough was observed on 70 percent of the cases when ozone exceeded 200 ppb in New York, New Jersey, or Connecticut from 1978-1983. This is not surprising given the predominance of westerly to northwesterly synoptic flow during ozone episodes.

Gaza (1996) reports that the afternoon position of the trough can affect the location of the zone of maximum ozone. The location of the trough is dependent on the speed of the synoptic winds. The trough moves farther east (near the coast) with stronger winds and closer to mountains with weaker winds. The trough tends to stay at inland locations if the winds at 850 mb (about 1500 m msl) are less than about 10 m/s (Taylor, 1997). This is the case on most exceedance days in the New York City area (Gaza, 1996).

The importance of the trough is not well understood, but the southwesterly flow ahead of the trough might help align the flow with the urban corridor emissions and contribute to day-to-day carryover along the corridor as well as affect the direction of the surface flows in the afternoon. The southwesterly flow ahead of the trough may also reinforce similar flow in the jet.