12.1 Climatological and dynamical evolution of a warm-season coastal jet in the New York Bight region

Wednesday, 19 August 2009: 10:30 AM
The Canyons (Sheraton Salt Lake City Hotel)
Brian A. Colle, Stony Brook University / SUNY, Stony Brook, NY; and D. R. Novak

The coastal regions of New York and New Jersey are often dominated by sea-breeze circulations during the spring and summer months. Occasionally these gentle sea breezes become relatively strong southerly wind events (> 11 m s-1) that may require marine advisories. These strong winds are best described as a jet, since the winds are spatially confined to the New Jersey and Long Island coasts (the New York Bight) and are vertically confined to the boundary layer (< 300 m). The Ambrose Light Station (ALSN6), located at the entrance of New York harbor, often samples this jet. This study investigates the climatology of this New York Bight Jet and the processes responsible for its occurrence.

The New York Bight (NYB) jet event was defined as the occurrence of southerly (160–210 degrees) maximum sustained winds (10 m height) exceeding 11 m s-1 at ALSN6. The sustained wind speed maximum must occur between 18 UTC and 03 UTC. To assure the wind maximum is spatially localized, the sustained wind at buoy 44025, located about 50 km to the east-southeast of ALSN6, must be <85% of the maximum sustained wind at ALSN6. Analysis of wind data during 1997–2006 revealed 134 Ambrose Jet events. There is a seasonal maximum in June and July, with a skew towards the spring months, suggesting that land-sea temperature contrast and the associated local pressure gradient are important to Ambrose Jet occurrence. The jet events exhibit a diurnal cycle, with light southwest winds during the 0600–1500 UTC period, backing to south-southeast and strengthening during the 1500–2100 UTC period. The winds become more southerly and reach a maximum (12.5 m s-1 on average) generally in the 2100–0000 UTC (5-8 PM EDT) period. The wind maximum occurs later in the day after the Coriolis force veers the perturbed south-southeast (early afternoon) flow to a southerly direction by late afternoon. Thus the jet results from a superposition of the diurnally perturbed flow and the background south-southwesterly flow. After the peak intensity, the winds continue to veer to a southwest direction and weaken. A composite analysis shows that the Ambrose Jet is favored on relatively warm days within synoptic-scale southwest flow on the western flank of the Bermuda high.

A NYB jet event on 2 June 2007 was realistically simulated down to 1.33-km grid spacing using the Weather Research and Forecasting (WRFv2.2). A momentum budget illustrates a nearly geostrophic balance in the cross-shore direction in association with the downward sloping marine layer towards the coast, while an antitriptic balance tends to occur in the along-shore direction as the near surface flow approaches ALSN6. A sensitivity experiment with an elongated north-south (N-S) coastline in the mid-Atlantic illustrates that the jet is favored with this N-S coastline configuration rather than the concave coastline of southern New Jersey (NJ) and Delaware. It is also shown that the additional urban heating of the New York City Region does not modify characteristics of the jet. Rather, the jet is part of a larger-scale diurnal pressure response and flow adjustment, and thus the late-day low-level flow enhancement extends nearly a Rossby radius (100-200 km) off the NJ coast.

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