Characterization of nocturnal jets over Philadelphia during air-pollution episodes
Document ID: 169
Verghese, Sachin John
Kizhakkemadam, Sriram N.
Willitsford, Adam H.
Collier, Paul Jason
Unni, Sameer
Philbrick, C. Russell
The Pennsylvania State University, University Park, PA, U.S.A.
Abstract
One of the most significant and interesting processes in the evening boundary layer transition over flat terrain is the development of the Nocturnal Jets or Low Level Jets (LLJ’s). Nocturnal Jet flows, from the mid-west regions of the United States, associated with high-pressure circulation, transports continental dry air into the region at a height near 500 m just above the nocturnal inversion, causing a strong characteristic signature in the meteorological conditions. Investigations have shown that there are many possible causes for the LLJ’s such as inertial oscillations, mountain and valley winds, land and sea breezes, advective accelerations, synoptic-scale baroclinicity associated with weather patterns, baroclinicity associated with sloping terrain and other processes [Stull, 1998]. As a sub-category of LLJ's, these nocturnal jets can be partly explained by frictional decoupling at night, i.e. the action of cooling at the ground in decoupling the flow just above the temperature inversion from surface friction. This decoupling disrupts the daytime balance of forces in the horizontal and produces an acceleration of the flow above the atmospheric surface layer in a manner described by Blackadar [1957]. A thin stream of fast moving air usually referred to as Low Level Jets is produced as a result of this acceleration. LLJ's attain maximum wind speeds between 10 to 20 m/s and are usually located at 400-800m above the ground. These nocturnal jets generally form during the nighttime over land under clear sky conditions but are destroyed just after sunrise, at which time solar heating and vertical mixing erode the wind field. Figure 1 shows the characteristic nose shape that the jet makes in the wind profile at heights between 400-800 m on 1 July 2002. Investigators have associated these Nocturnal Jets with a number of atmospheric processes [Pitchford, 1962; Seaman, 1998; Reitebuch, 2000; Banta, 2001; Clark, 2001]. The importance of nocturnal jets is that significant quantities of ozone and ozone chemistry precursors can be transported at night from upwind urban plumes. Of course, these jets can also transport ozone generated locally during the previous day into other regions such as rural areas that may not substantially contribute to local production during the daytime. An important finding is the intrusion of drier air, often with elevated ozone concentrations, as the LLJ becomes a westerly conveyor of air from the western boundary region in the early morning hours. This paper presents case studies, which show the strong correlation of nocturnal jets with various air-pollution episodes.