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  • View in gallery

    (a) Topographic map of the region surrounding NYC, with geographic features labeled and elevation gray shaded (m). The NYB jet area of interest is outlined by a box. (b) Surface map at 0000 UTC 1 Apr 2006 for the box in (a) showing station model data including temperature (°C), dewpoint (°C), wind (1 full barb = 5 m s−1), sea level pressure, and cloud cover.

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    The number of NYB jet events per month from January 1997 to December 2006.

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    The number of NYB jet events for various wind speed ranges (m s−1) from January 1997 to December 2006.

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    (a) Wind speed (m s−1) and (b) wind direction evolution at ALSN (at ~30 m ASL) for the 15–19 Jul 1999 period (in UTC and LST). The 4 days of wind directions are contoured using the line shades given by the inset box.

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    The number of NYB jet events with a maximum wind at a particular hour (UTC and LST) from 1800 to 0300 UTC.

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    Hodograph of the 30-m winds (m s−1) at ALSN6 plotted every hour from 12 h before the time of the maximum winds (gray dots) at 2300 UTC and 12 h after the maximum winds (black dots).

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    Sea level pressure difference in hPa between (a) ALSN6 and Central Park, NYC (ALSN6-NYC), and (b) buoy 44025 and EWR (44025-EWR) for the time period in hours relative to the maximum winds at ALSN6.

  • View in gallery

    NARR composite at 1500 UTC for the NYB jet days showing (a) 500-hPa height (every 30 m), (b) sea level pressure (every 1 hPa), and (c) 1000-hPa wind speed (contoured every 1 m s−1, with vectors showing direction). (d)–(f) As in (a)–(c), but for the time closest to the time of the NYB jet wind maximum.

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    (a) The 500-hPa geopotential heights (dashed every 30 m) and sea level pressure (solid every 4 hPa) from the 36-km WRF at 1200 UTC 2 Jun 2007 (forecast hour 12). (b) As in (a), but for 0000 UTC 3 Jun 2007. The boxes in (a) show the locations of the 12-, 4-, and 1.33-km domains, respectively.

  • View in gallery

    Doppler radial velocities (shaded in kt using the inset scale) from the KJFK TDWR and station model data including temperature (°F), dewpoint (°F), wind (1 full barb = 5 m s−1), and sea level pressure at (a) 1400, (b) 1800, and (c) 2300 UTC 2 Jun 2007.

  • View in gallery

    (a) ACARS temperature and wind profile (full barb = 5 m s−1) on a skew T chart from JFK at 1541 UTC 2 Jun 2007. (b) As in (a), but for the 1.33-km WRF at 1600 UTC, and dewpoint temperature is also shown (in gray). (c) As in (a), but at 2206 UTC 2 Jun 2007. (d) As in (b), but at 2200 UTC 2 Jun 2007.

  • View in gallery

    Hodograph of the 30-m winds (m s−1) at ALSN6 plotted every hour for the observations (black) and 1.33-km WRF (gray) from 1200 UTC 2 Jun to 0600 UTC 3 Jun 2007 for WRF and to 1000 UTC 3 June 2007 for the observations.

  • View in gallery

    The 1.33-km WRF pressure (solid every 1 hPa), wind barbs (full barb = 10 kt), wind speed (shaded in m s−1), and temperature (every 3°C) at 100 m ASL at (a) 1400 UTC 2 Jun, (b) 1800 UTC 2 Jun, (c) 2200 UTC 2 Jun, and (d) 0300 UTC 3 Jun 2007. The locations for the cross sections in Figs. 15 and 16 are shown in (a).

  • View in gallery

    As in Fig. 13, but at 500 m ASL.

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    Cross section AA′ from the 1.33-km WRF showing potential temperature (solid every 1 K), and meridional wind speed into the section (shaded in m s−1) at (a) 1800 UTC 2 Jun, (b) 2200 UTC 2 Jun, and (c) 0300 UTC 3 Jun 2007. The location of the cross section is shown in Fig. 13a.

  • View in gallery

    As in Fig. 15, but for along BB′ in Fig. 13a.

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    Backward trajectories 1–8 starting at 30 m ASL at 2200 UTC 2 Jun. The height of the trajectory is given by the width of the ribbon (inset scale). The trajectory is divided into 1-h segments. The 30-m wind speed (shaded in m s−1) at 2200 UTC is also shown.

  • View in gallery

    Momentum terms at model σ = 34 (~150 m above surface) at (a) 1900 UTC 2 Jun and (b) 2300 UTC 2 Jun showing the pressure (every 0.5 hPa) and the total acceleration (thick black; A), pressure gradient force (thick gray, P), Coriolis force (thin black to the right of the wind barb; C), and boundary layer drag and mixing (thin black; F). The terms A, P, C, and F are shown for two representative locations for reference. The box in (b) shows the location of the average momentum budget traces in Fig. 19.

  • View in gallery

    (a) Time series of the total (black) and geostrophic (gray) wind speed (in m s−1) averaged for the black box in Fig. 18b within the 1.33-km domain for hours 12–30 (1200–0600 UTC). (b) As in (a), but for the υ momentum budget (m s−2) showing the total acceleration (vaccel; gray solid), frictional and mixing processes (vpbl; dotted), pressure gradient (pgy; short dashed), Coriolis (cory; long dashed), and the meridional wind (vtotal: thick black in m s−1). (c) As in (b), but for the u momentum budget.

  • View in gallery

    The 1.33-km WRF pressure (solid every 1 hPa), winds barbs (full barb = 10 kt), wind speed (shaded in m s−1), and temperature (every 3°C) at 100 m ASL for the (a) NORAD and (b) and NSCOAST runs at 2200 UTC 2 Jun.

  • View in gallery

    The 12-km WRF pressure (solid every 2 hPa), winds barbs (full barb = 10 kt), wind speed (shaded in m s−1), and temperature (every 3°C) at 100 m ASL for the (a) control and (b) NORAD runs at 0000 UTC 3 Jun.

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The New York Bight Jet: Climatology and Dynamical Evolution

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  • 1 School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York
  • | 2 NOAA/NWS/NCEP/Hydrometeorological Prediction Center, Camp Springs, Maryland
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Abstract

This paper describes the southerly New York Bight (NYB) jet (11–17 m s−1) that develops primarily during the warm season just above the surface offshore (east) of the northern New Jersey coast and south of Long Island (the NYB). Observations from two offshore buoys are used to develop a 9-yr climatology of 134 jet events from 1997 to 2006. There is a seasonal maximum (2.5 events per month) during June and July, with a skew toward the spring months. The wind directions for the jet trace out a nearly elliptical orbit for the 24-h period around the time of jet maximum at ~2300 UTC [1900 eastern daylight time (EDT)] on average. Composites reveal that the NYB jet occurs on days with southwesterly synoptic flow, and the jet is part of a larger-scale (200–300 km) wind enhancement offshore of the mid-Atlantic and northeast U.S. coasts during the early evening hours.

High-resolution observations (surface mesonet, aircraft soundings, and a terminal Doppler weather radar) and Weather Research and Forecasting (WRF) model simulations down to 1.33-km grid spacing are used to diagnose the evolution of the NYB jet on 2 June 2007. The NYB jet at ~150 m MSL occurs within the sloping marine inversion near the coast. Low-level trajectories illustrate low-level diffluence and weak subsidence within the jet. A WRF momentum budget highlights the evolving pressure gradient and accelerations during jet formation. The maximum jet winds occur 1–2 h after the peak meridional pressure gradient is established through a geostrophic adjustment process. Sensitivity experiments show that jet occurrence is dependent on diurnal heating and that the concave bend in the southern New Jersey coast limits the southern extent of the jet.

Corresponding author address: Dr. Brian A. Colle, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000. Email: brian.colle@stonybrook.edu

Abstract

This paper describes the southerly New York Bight (NYB) jet (11–17 m s−1) that develops primarily during the warm season just above the surface offshore (east) of the northern New Jersey coast and south of Long Island (the NYB). Observations from two offshore buoys are used to develop a 9-yr climatology of 134 jet events from 1997 to 2006. There is a seasonal maximum (2.5 events per month) during June and July, with a skew toward the spring months. The wind directions for the jet trace out a nearly elliptical orbit for the 24-h period around the time of jet maximum at ~2300 UTC [1900 eastern daylight time (EDT)] on average. Composites reveal that the NYB jet occurs on days with southwesterly synoptic flow, and the jet is part of a larger-scale (200–300 km) wind enhancement offshore of the mid-Atlantic and northeast U.S. coasts during the early evening hours.

High-resolution observations (surface mesonet, aircraft soundings, and a terminal Doppler weather radar) and Weather Research and Forecasting (WRF) model simulations down to 1.33-km grid spacing are used to diagnose the evolution of the NYB jet on 2 June 2007. The NYB jet at ~150 m MSL occurs within the sloping marine inversion near the coast. Low-level trajectories illustrate low-level diffluence and weak subsidence within the jet. A WRF momentum budget highlights the evolving pressure gradient and accelerations during jet formation. The maximum jet winds occur 1–2 h after the peak meridional pressure gradient is established through a geostrophic adjustment process. Sensitivity experiments show that jet occurrence is dependent on diurnal heating and that the concave bend in the southern New Jersey coast limits the southern extent of the jet.

Corresponding author address: Dr. Brian A. Colle, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000. Email: brian.colle@stonybrook.edu

1. Introduction

a. Background

In many coastal areas the sea breeze is a ubiquitous phenomenon during the warm season. Along the mid-Atlantic and northeast U.S. coasts, sea breezes have been well studied and modeled (Frizzola and Fisher 1963; Bornstein and Thompson 1981; Colle et al. 2003; Novak and Colle 2006; Colby 2004; Zhang et al. 2006; Holt and Pullen 2007). However, there has been little investigation of coastal diurnal flows in this region on a scale larger than a typical sea breeze (i.e., 100–200 km). Such coastal flows may be influenced by differential surface heating and coastal geometry, as well as the Appalachian terrain farther inland (Fig. 1a).

The goal of this study is to investigate a coastal low-level jet that develops primarily during the spring and summer months, in which relatively strong (>11 m s−1) southerly winds occur in the New York Bight (NYB; the offshore region bounded by the northern New Jersey and Long Island coasts) around buoy 44065 (previously called the Ambrose Light Station or ALSN6) during the late afternoon or evening hours. Figure 1b shows an example surface map during an NYB jet event occurring at 0000 UTC 1 April 2006. At this time the offshore winds at buoy 44025 were 15–17 kt (7–9 m s−1), while the 30-m winds at ALSN6 were gusting to 32 kt (~16 m s−1).

A coastal LLJ can be defined as a well-defined wind speed maximum that exists parallel to the coast in the lower troposphere (e.g., Chao 1985). These winds can be enhanced by both terrain and diurnal forcing. The most common type of coastal jet that has been investigated is the barrier jet (Parish 1982), which develops along steep coastal topography when the low-level flow is blocked and it is then accelerated down the pressure gradient, with the terrain located generally to the right of the wind direction in the Northern Hemisphere (e.g., Olson et al. 2007). For example, northeasterly LLJs have been documented to the east of the Appalachians in response to cold-air damming (Bell and Bosart 1988), which can be modified (weakened) during the day by an increase in the frictional stress and vertical mixing (Doyle and Warner 1991).

Diurnally forced LLJs have been documented extensively in the central United States (Wexler 1961; Bonner 1968; Mitchell et al. 1995; Whiteman et al. 1997), but little attention has been given to their development along the U.S. east coast. Using profiler data, Zhang et al. (2006) documented an LLJ over the mid-Atlantic states, which was found to be around 700 m AGL and occurred 90% of the time between 0000 and 0600 UTC during the warm season. As in the central plains, the role of the terrain is important, with the mountains (Appalachians for the mid-Atlantic) tending to produce a sloping mixed layer to the east. This, combined with the relatively cool coastal waters, favors a northeast to southwest thermal wind vector and, thus, an enhanced southwest or southerly flow at low levels. Low-level jets over northern Florida have both inertial and baroclinic contributions associated with the ocean–land surface horizontal temperature gradient (Karipot et al. 2009).

Along the West Coast, a northerly LLJ occurs when there is synoptic high pressure located ~1000 km offshore of the northern California coast while there is also strong diurnal heating inland (Burk and Thompson 1996). The northerly wind maxima have been observed along the Oregon coast (Elliott and O’Brien 1977), as well as along the California coast during the Coastal Ocean Dynamics Experiment (CODE; Zemba and Friehe 1987; Beardsley et al. 1987). The core of the jet, typically at 300–700 m above mean sea level (MSL) and reaching speeds of ~15 m s−1, exists within a steeply sloped inversion at the top of the marine boundary layer (MBL). The combination of the cool marine layer to the west and the heated continent enhances the west–east temperature and pressure gradients, which via thermal wind results in a northerly LLJ. Thermal wind arguments suggest that the jet should reside near the surface, but the jet core is located within the inversion, capping the MBL inversion as a result of strong vertical mixing of momentum (Beardsley et al. 1987). The interaction of the flow with various coastal bends as well as the coastal terrain can result in localized enhancements of the jet along the West Coast (Rogers et al. 1998; Pomeroy and Parish 2001).

b. Objectives

Although the diurnally forced northerly LLJ has been well documented along the West Coast, there has been little documentation of a similar LLJ along the coastal waters of the eastern United States. The East Coast has some similarities to the West Coast, namely, the presence of relatively cool sea surface temperatures (12°–20°C) to the northwest of the Gulf Stream from the mid-Atlantic northward during the late spring and early summer, which are comparable to the relatively cool (10°–15°C) sea surface temperatures along the West Coast. There is also a semipermanent high pressure area located off the West and East Coasts, with interior heating over the continent. Thus, the East Coast has a potential large-scale setup for enhanced southerlies that are a mirror image of the diurnally forced northerlies along the West Coast. However, there are complexities in the mid-Atlantic and the Northeast, such as the gradual coastal line curvature over the mid-Atlantic, major estuaries and bays (e.g., the Chesapeake), and a major coastal bend eastward associated with Long Island and southern New England (the NYB). Although there is limited steep terrain near the East Coast (Fig. 1a), there are numerous large urban areas that may impact the diurnal heating and flows.

This paper will present evidence of a southerly LLJ over a portion of the NYB region, which may expand spatially later in the diurnal cycle. In addition to obtaining a better understanding of the structure of the jet and comparisons with the West Coast LLJ, this study will address the following motivational questions:

  • What are the climatological characteristics of the NYB jet?
  • What are the synoptic conditions favoring the development of the NYB jet?
  • How does the NYB jet form and evolve?
  • What are the impacts of the coastline curvature, inland terrain, and urban centers on the development of the jet?

Section 2 discusses the climatological characteristics of the NYB jet and the large-scale composites favoring such flows. A detailed case study of an NYB jet will be presented in section 3. Comparison with the U.S. west coast jet and the potential impacts of the NYB jet will be discussed in section 4. Conclusions are provided in section 5.

2. Jet climatology

a. Data and methods

A seasonal climatology of the NYB jet was developed from 1997 to 2006 using the Ambrose Light Station (ALSN6) to the south of western Long Island (Fig. 1b). ALSN6 has an anemometer height of 29 m MSL. Thus, in order to compare this station’s wind measurement with nearby stations, the winds were reduced to a 10-m height using a logarithmic wind profile [see Holton (2004), Eq. (5.33)], assuming neutral stability conditions. The roughness length (z0) was taken as 0.0005 m, which is a value consistent with open water (Stull 1988) and is the standard value used by the National Data Buoy Center (NDBC). The resultant ratio is 0.903; thus, the archived 29-m wind speed was multiplied by this value to obtain the 10-m wind speed. No change in wind direction was made. All wind speed values cited hereafter use the 10-m sustained wind speed unless otherwise noted.

To define jet occurrence, thresholds for both the 10-m wind speed and width of the enhanced winds were needed. To determine the wind threshold, the climatological distribution of the late afternoon (1800–0300 UTC time period) southerly wind maximum (160°–210° window centered on 185°, since this is parallel to the central New Jersey coast) was calculated at ALSN6 from April to August of 1997–2006. The threshold wind speed defining the NYB jet (11 m s−1) was chosen as the speed that is one standard deviation above the mean southerly sustained wind speed.

To ensure the strong wind is spatially confined as a jet, buoy 44025, located ~50 km to the east-southeast of ALSN6 (Fig. 1b), served as a reference for determining the width of the jet. Buoy 44025 has an anemometer height of 5 m and, thus, the same logarithmic wind profile as for ALSN6 was used to convert the wind to 10 m. Assuming a jet width of 100–200 km, which will be shown in a case study in the next section, a cross-jet e-folding decrease would yield a 44025 wind speed that is less than 85% of ALSN6. Thus, the final criteria required that the winds at buoy 44025 be at least 15% weaker than ALSN6 for a ±1-h period around the time of maximum sustained winds at ALSN6. Archived hourly wind data from ALSN6 and buoy 44025 were obtained from the National Data Buoy Center for 1997–2006. Buoy 44025 data were missing during the July–October 2005 time period, and thus an analysis was not conducted during 2005, resulting in a 9-yr dataset.

To gain insight into the synoptic patterns supporting NYB jet events, a spatial composite was created using 3-hourly North American Regional Reanalysis (NARR) data at a 32-km grid spacing (Mesinger et al. 2006). The NARR time closest to the NYB jet was used in the spatial composites.

b. Results

Using the above criteria, 134 Ambrose jet events were identified for the January 1997–December 2006 time period. There is a seasonal maximum (2.5 events per month) during June and July (Fig. 2), with a skew in the distribution toward the spring months. Using monthly mean maximum temperatures from the National Climatic Data Center (NCDC), the largest climatological temperature difference between New York City’s (NYC) Central Park (1970–2000 mean) and the sea surface temperatures at ALSN6 (November 1984–December 2001 mean) is in May at 9.6°C (not shown), although differences greater than 5°C are maintained from March through August. The correspondence between this climatological temperature difference and the NYB jet occurrence suggests that land–sea temperature contrasts are important.

Figure 3 shows the frequency distribution of the magnitude of the maximum sustained winds at ALSN6. More than half of the defined NYB jet events have wind speeds of 11–12 m s−1. Meanwhile, about 28% of the events have winds greater than 13 m s−1 (25 kt), which meets or exceeds the small craft advisory wind conditions for the National Weather Service (NWS 2008). Only ~4% of events have winds greater than 15 m s−1 (30 kt).

Sequential NYB jet events have been observed at ALSN6. There have been seven periods of two consecutive jet events, three periods of three consecutive jet events, and two periods with four consecutive jet events. For example, consider wind observations at ALSN6 during 15–19 July 1999 (Fig. 4). During this period, the winds increased rapidly from 2–4 to 12–14 m s−1 between 1500 and 2100 UTC each day (Fig. 4a). The winds reached a peak around 2300 UTC (1800 LST) and, subsequently, decreased several meters per second during the late evening. Meanwhile, the wind directions at ALSN6 rapidly backed from southwesterly-westerly (220°–270°) shortly after sunrise at 1200–1400 UTC to southerly (170°–180°) by 1800 UTC (Fig. 4b). The winds then slowly veered to southwesterly (220°) by 0600 UTC.

The sequential events suggest that there is a diurnal regularity to these jet events. The time of the maximum sustained wind for all jet events at ALSN6 is shown in Fig. 5. There is a frequency maximum at 2300 UTC (1800 LST), with the distribution slightly skewed toward earlier times. The earliest (latest) time of maximum sustained wind was 1800 UTC (0100 UTC). The average inland maximum temperature is reached at 2000 UTC [1600 local daylight time (LDT), 1500 local standard time (LST)] using hourly data from the NCDC during the 1970–2000 time period (not shown). Since the sea surface temperature does not exhibit a significant diurnal temperature range, the maximum land–sea temperature contrast would be expected around 2000 UTC, well before the time of the NYB jet maximum winds.

The average diurnal wind evolution for the NYB jet events was constructed using a composite hodograph at ALSN6 (Fig. 6). The center time (t = 0) is defined as the time of jet maximum, and the winds are averaged each hour from 12 h before (−12 h) and after (+12 h) this time. At −12 h, the winds are southwesterly (220°) at ~5 m s−1. From −3 to −8 h, the winds back to south-southeasterly (~170°) and increase to 7–9 m s−1, which is occurring during the period of maximum diurnal heating and a developing sea-breeze circulation orientated slightly to the west toward the New Jersey coast. From −9 to −12 h, the winds increase further to 12.5 m s−1 and slowly veer to southerly (183°), while for the 10-h period after the jet maximum, the winds steadily veer to southwesterly and decrease to 6 m s−1.

The clockwise wind rotation with time at ALSN6 suggests an inertial component to the wind evolution; however, the pressure gradient also evolves near the coast. For example, the approximate north–south pressure difference near the coast (ALSN6-NYC) peaks ~5 h before the jet maximum at ~1800 UTC (Fig. 7a), which is near the time of the maximum temperature difference between these two stations (not shown). Meanwhile, the pressure difference directed from east-southeast to west-northwest [buoy 44025 to Newark, New Jersey (EWR) in Fig. 1] peaks 1–3 h before the jet maximum (Fig. 7b). As will be highlighted below [section 3d(2)], the Coriolis force acting on this imposed west–east pressure gradient through geostrophic adjustment (Gill 1982) can lead to the southerly wind maximum at 2200–2300 UTC, and then as the pressure gradient relaxes during the evening the winds veer to the ambient southwesterlies.

Figure 8 shows the NARR composite of the mid- and low-level flow patterns during the NYB jet evolution. At 1500 UTC (late morning) during the NYB jet events (Figs. 8a–c), there is a broad ridge centered just inland of the U.S. east coast and a trough over the central United States (Fig. 8a). A 1022-hPa surface high is present about 1000 km to the east of the southeast U.S. coast, while a weak trough is present over the Great Lakes (Fig. 8b). There is a relatively weak pressure gradient near the NYB coast region, with the isobars orientated nearly zonal in this region. Much of the NYB area had southwesterly 1000-hPa winds of less than 3 m s−1 at this time.

Near the time of the jet maximum at 0000 UTC (Figs. 8d–f), the 500-hPa ridge axis had moved slightly (100–200 km) eastward to the East Coast (Fig. 8d). The surface high had moved eastward to near Bermuda, with southwesterly flow along the East Coast (Fig. 8e). The surface pressure gradient is largest near the coast at this time, while it is weaker inland and farther offshore. This enhanced pressure gradient is associated with an enhancement of the 1000-hPa winds (>8 m s−1) from the southern Virginia coast northward to Long Island (Fig. 8f), and there is another wind maximum from Cape Cod, Massachusetts, to coastal Maine. Thus, there was a twofold enhancement of the coast-parallel winds during the day. The wind and pressure gradient enhancements occur over a relatively large region offshore (east) of the New Jersey coast, which is consistent with the largest observed pressure difference between buoy 44025 and EWR near this time (Fig. 7b). This suggests that the NYB jet is part of a larger-scale surface diurnal wind response from the coastal mid-Atlantic to coastal New England.

3. The 2 June 2007 NYB jet event

a. Data and methods

The 2 June 2007 jet event was analyzed in more detail, since it was representative of the jet climatology. Conventional surface and rawinsonde observations and Aircraft Communications Addressing and Reporting System (ACARS) data, as well as the radial velocity data from the terminal Doppler weather radar (TDWR) at John F. Kennedy International Airport, New York, New York [JFK in Fig. 1b; see Allan et al. (2004)] were used. The data sources allow for a unique vertical sampling of the NYB jet winds.

The Weather Research and Forecasting (WRF) model (version v2.2; Skamarock et al. 2005) was used to provide additional data for diagnosing the structural evolution and dynamics of the NYB jet. For this simulation, stationary 1.33-, 4-, and 12-km domains were nested within a 36-km domain using a one-way nest interface (Fig. 9). This study will focus primarily on the 1.33-km results. The model top was set at 100 hPa. Thirty-eight unevenly spaced full-sigma levels were used in the vertical, with the maximum resolution in the boundary layer. Five-minute-averaged terrain data were analyzed to the 36- and 12-km model grids using a Cressman analysis scheme and filtered by a two-pass smoother–desmoother. For the 4- and 1.33-km domains (Fig. 9a), a 30-s topography dataset was interpolated to the grid in order to better resolve the inland hills and valleys. A 30-s land-use dataset from the National Center for Atmospheric Research (NCAR) was used to initialize 25 surface categories for all domains. Initial atmospheric conditions at 0000 UTC 2 June 2007 were generated by interpolating the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis (1° grid spacing) to the WRF grid. The 3-hourly GFS forecasts were linearly interpolated in time in order to provide the evolving lateral boundary conditions for the 36-km domain.

The U.S. Navy Optimum Thermal Interpolation System (OTIS) sea surface temperature analyses (~30-km grid spacing) were used to initialize the WRF surface temperatures over water at 0000 UTC 2 June 2007. The control (CTL) simulation used the Thompson microphysical scheme (Thompson et al. 2004), and the Grell convective parameterization (Grell et al. 1994) was applied, except for the 4- and 1.33-km domains. The Yonsei University (YSU) boundary layer scheme (Hong et al. 2006) and a long- and a shortwave atmospheric radiation scheme (Dudhia 1989) were used, as well as a radiative upper boundary condition (Klemp and Durran 1983).

b. Synoptic evolution

At 1200 UTC 2 June (Fig. 9a), both the observed (not shown) and 36-km WRF results indicate a 500-hPa ridge extending northwestward from offshore (east) of the southeast U.S. coast to the Great Lakes region, while a cutoff low was located over the northern plains. Meanwhile, surface high pressure extended eastward from near Bermuda to the eastern United States. This resulted in generally light (2–5 m s−1) south-southwesterly surface winds along the mid-Atlantic and southern New England coasts. For this relatively short (12 h) forecast, the WRF was within 10 m and 1–2 hPa for the 500-hPa geopotential heights and sea level pressures, respectively, across the domain. The surface temperatures across the northeast United States at this time ranged from 21°–22°C near the coast to 17°–20°C over the coastal waters (not shown).

By 0000 UTC 3 June (Fig. 9b), the 500-hPa low was approaching the western Great Lakes, while the ridge along the mid-Atlantic was narrowing due to the northward movement of Tropical Storm Barry into southeast Georgia and northeast Florida. The sea level pressures in the WRF along the East Coast were predicted to within 2 hPa of the observed. During the prior 12 h, the sea level pressures fell 2–4 hPa over the eastern United States and 1–2 hPa over the western Atlantic, which increased the pressure gradient and southerly winds along the coast.

c. Mesoscale evolution

1) Observations

A series of mesoscale analyses in the NYB region are used to describe the diurnal changes in the coastal winds and temperatures during this event. At 1400 UTC 2 June (Fig. 10a), southwesterly low-level winds across the region are generally less than 5 m s−1 at the surface and 7 m s−1 at ~500 m MSL, given the inbound velocities ~20 km southwest of the JFK TDWR (Fig. 10). The surface temperatures range from 20.5°C at ALSN6 to 27°C around NYC. By 1800 UTC 2 June (Fig. 10), the surface temperatures over the inland areas warmed to near 32°C around NYC, while the temperatures at ALSN6 decreased slightly to around 19°C given the relatively cool sea surface temperatures in this area (~15°C). The surface winds over the interior areas north and west of NYC were still southwesterly, while 10 m s−1 southerly winds had developed at ALSN6. The maximum inbound radial velocities (from the south) of 10–15 m s−1 occurred at 100–200 m above the surface, while 7–10 m s−1 southerlies (inbound velocities) were ~50 km to the south of JFK (up to 1 km MSL).

The low-level southerly jet near ALSN6 reached its peak at 2200–2300 UTC 2 June (Fig. 10c). The 30-m winds at ALSN6 at 2300 UTC were 13 m s−1 (gusting to ~17 m s−1), while the surface winds at other locations were 5–10 m s−1. The inbound velocities at JFK indicate a southerly maximum of ~17 m s−1 at 70 m MSL but, unfortunately, no useful velocity data are available to the south of this point (above this level). The surface temperatures ranged from 21°C at Ambrose to around 30°C in the NYC region.

The shallow nature of the jet was observed during an ACARS sounding at JFK airport on 2 June 2002 (Fig. 11). At 1541 UTC 2 June (Fig. 11a), there was a relatively shallow marine layer in the lowest 300 m MSL, which was associated with a nearly isothermal layer of around 23°C. The winds were generally southwesterly at ~5 m s−1 in this layer at this time. By 2200 UTC 2 June (Fig. 11c), a maximum in south-southwesterly winds (~17 m s−1) had developed at 200 m, with the winds rapidly weakening to ~5 m s−1 and veering to more southwesterly at 1000 m. The location of the NYB jet was at the top of the marine layer inversion, in which the temperature increased from 22°C at the surface to 27°C at 200 m MSL.

A time series hodograph at ALSN6 illustrates the observed surface wind evolution during the full event (Fig. 12). The surface winds were south-southwesterly at 5 m s−1 at 1200 UTC 2 June, and then increased to 8 m s−1 and became more southerly by 1700 UTC 2 June. The southerly winds increased to 12.5 m s−1 by 2200 UTC 2 June. Subsequently, the winds weakened and rotated 10° more to the southwest. Overall, the wind evolution is similar to the composite hodograph of all events (cf. Fig. 6).

2) WRF simulations

The 30-m wind evolution at ALSN6 in the 1.33-km WRF simulation was similar to that observed (Fig. 12). The simulated winds increased and backed from south-southwesterly to more southerly during the late morning and early afternoon, and then reached their peak around 2200 UTC 2 June. The simulated peak winds were about 0.5 m s−1 weaker than observed, and the WRF winds veered to more southwesterly than observed during the evening hours (0000–0600 UTC 3 June).

The 1.33-km WRF wind and temperature profiles were also similar to the ACARS sounding at 2200 UTC 2 June (Figs. 11b and 11d). The WRF had a well-defined inversion in the lowest 300 m; however, the temperature in this layer was 2°–3°C cooler than observed. The WRF had a peak wind of 15 m s−1 at the top of the inversion, which is ~2 m s−1 weaker than observed at this time. As observed, the WRF winds weakened above the jet to ~7 m s−1 by 900 hPa (~1000 m).

The 1.33-km WRF run provided a high-resolution dataset to better understand the three-dimensional structures associated with the NYB jet. The horizontal evolution is highlighted near the jet height (100 m) and just above the jet (500 m). Figures 13 and 14 show the temperatures, pressures, and winds at 100 and 500 m, respectively. At 1400 UTC, the inland 100-m temperatures were 26°–27°C at this time (Fig. 13a), while at the surface they were 27°–28°C (not shown), which is similar to the observed (Fig. 10a). Meanwhile, the temperatures over the water were 18°–21°C at 100 m and 17°–18°C at the surface (not shown). The 100-m winds were <5 m s−1 over the interior and varied from westerly to southwesterly (Fig. 13a). The strongest pressure difference at 100 m was located over the Atlantic as the pressure increased 3–4 hPa from ALSN6 to 400 km to the south-southwest. At 500 m (Fig. 14a), the winds were 2–5 m s−1 weaker and more westerly than 100 m over the coastal waters, and there was only a ~3°C temperature difference between the inland areas and the ocean.

By 1800 UTC 2 June (Figs. 13b and 14b), the WRF 100-m temperatures had warmed to 29°–30°C over some of the urban areas (Fig. 13b), while the surface temperatures were ~1°C warmer and close to the observed at this time (not shown). The 100-m winds over the interior were similar to a few hours earlier, but the winds just east of New Jersey had become more southerly at 10–12 m s−1 as the coastal pressure gradient had increased. Southwesterly winds at 8–9 m s−1 still existed well offshore to the east of the New Jersey coast. The 500-m winds had also veered to more southwesterly over the coastal waters than a few hours earlier (Fig. 14b), but there was no well-defined wind maximum.

At 2200 UTC 2 June (Fig. 13c), the NYB jet winds at 100 m reached their peak of 15–16 m s−1 at 100 m MSL. The strongest winds were near the apex of New Jersey and Long Island around ALSN6. This was in response to an enhanced east–west pressure gradient across the central New Jersey coast. The 100-m jet winds were more ageostrophic (45° from geostrophic) than the nearly geostrophic winds over the coastal waters of southern New Jersey and farther offshore. Another 100-m wind maximum was located around the southern tip of New Jersey and Delaware Bay. Meanwhile, at 500 m (Fig. 14c), the winds near the coast (11–12 m s−1) were similar to another wind maximum developing offshore to the southeast.

By 0300 UTC 3 June (Fig. 13d), the pressure gradient across the New Jersey coast at 100 m was ~50% weaker than 5 h earlier. As a result, there was no well-defined jet along the coast, and the winds had become more geostrophic (south-southwesterly). The strongest (13–15 m s−1) winds had spread well offshore (100–200 km south of Long Island), where the pressure gradient was slightly greater than offshore areas to the south (Fig. 13d). The strongest winds at 500 m (13–15 m s−1) had also spread over the coastal and offshore region to the northeast and east of New Jersey (Fig. 14d).

A west–east cross section (AA′) across northern New Jersey to the offshore waters illustrates the structural evolution of the jet in the cross-shore direction (Fig. 15), while cross section BB′ illustrates the structure of the jet in the north–south direction along the coast (Fig. 16). At 1800 UTC 2 June (Fig. 15a), along section AA′ the isentropes and stable layer sloped downward from 400 m MSL at 100 km offshore to 200 m MSL near the coast given the deeper marine layer to the east and the mixed layer inland. The southerly wind enhancement (11–12 m s−1) was around 100 m MSL within 20 km of the coast at this time. Across section BB′ at this time (Fig. 16a), a southerly wind maximum (10–11 m s−1) was developing within a shallow inversion at ~100 m MSL. This inversion gradually deepens to the south at this time, with weak subsidence above the low-level stable layer. Near-surface flow convergence along the south shore of Long Island created a plume of upward motion and compensating subsidence, which has the characteristics of a small-amplitude gravity wave, as shown in Colle and Yuter (2007) for this area, as well as some thermal mixing.

By 2200 UTC (Figs. 15b and 16b), the slope of the marine layer along section AA′ increased within 30 km of the coast. Meanwhile, the jet had increased to 16–17 m s−1 and was centered near the coast at the top of the stable layer (~175 m MSL), with 12–15 m s−1 winds sloping upward to 225 m MSL at ~100 km offshore. The meridional winds near the coast steadily decreased to <10 m s−1 by 800 m MSL, and the e-folding offshore decay of the low-level wind enhancement extends about a Rossby radius (Lr = ND/f ~ 105 km, where N ~ 0.025 s−1, D ~ 400 m, and f = 9.48 × 10−5 s−1) off the coast. Along section BB′ at 2200 UTC (Fig. 16b), the southerly winds at 190 m MSL on the southern end of section BB′ accelerate from 12 to 16 m s−1 and lower slightly to 175 m MSL in the jet-core region. The isentropes descend slightly in this jet acceleration region, which is likely in response to the low-level divergence near the surface (Fig. 13a). The upward motion at the coast mixes some of the higher momentum (11 m s−1) upward to 700 m MSL.

As the jet weakens at 0300 UTC 3 June, cross section AA′ indicates less west–east tilt of the isentropes (Fig. 15c). With no more solar heating over land, the marine air is advected westward over New Jersey, which helped to reduce the cross-shore temperature gradient. The jet (14 m s−1) was ~2 m s−1 weaker and more elevated at 250 m than at 2200 UTC. Meanwhile, relatively strong winds (13–14 m s−1) had spread eastward and were elevated as the stratification associated with the marine layer deepened offshore. The jet also weakened and became more elevated along section BB′ by this time (Fig. 16c), with gradually downward-sloping isentropes toward the north.

d. Physical processes

1) Trajectories and momentum budget

To illustrate the origin of the air within the NYB jet, backward trajectories were calculated using the 1.33-km WRF output along a west to east line across the point of maximum southerly winds at 100 m MSL to the east of the New Jersey coast (Fig. 17). A trajectory time step of 5 min was used with the spatial and temporal interpolations of the 15-min model data, and the time of trajectory release was 2200 UTC 2 June 2007. There is some diffluence in the trajectories in the NYB region, with trajectories 1–6 curving more to the north, while trajectories 7 and 8 traveled from southwest to northeast. Trajectories 1–6 exhibit some weak subsidence, as they descend gradually from 200 m MSL to the east of the southern New Jersey coast to 100 m south of Long Island. The largest subsidence occurs along the coast from 500 to 100 m MSL (trajectory 1); since the local diffluence between trajectories 1 and 2 is largest in this region, as trajectory 1 descends, it also accelerates as it enters the jet.

A momentum budget was calculated in order to diagnose the mechanisms for the wind variations near the coast. The following momentum equation was separated into its components:
i1520-0493-138-6-2385-e1
where d/dt is the total derivative with respect to time and V is the horizontal velocity vector. The first two terms on the right-hand side represent the pressure gradient and Coriolis acceleration, respectively. The last term (F) shows the tendencies output from the WRF PBL scheme, which include friction and turbulent mixing. Using this equation, the zonal and meridional momentum terms were output at all grid points within the 1.33-km WRF for a specified period. The total acceleration was calculated as a net forcing of all terms on the right-hand side of Eq. (1).

Figure 18 shows the momentum terms plotted spatially near the height of the maximum winds of the jet, which is at model sigma level 34 or ~150 m MSL. At 1900 UTC 2 June (Fig. 18a), air parcels moving north-northeastward along the central New Jersey coast experience an increasing pressure gradient orientated to the northwest. The unbalanced friction and Coriolis force with this pressure gradient results in a northerly acceleration along the central and northern New Jersey coast. Meanwhile, those points 150–200 km to the southeast experience little acceleration, and the flow is nearly geostrophic, with some cross-isobar flow induced by the frictional term.

Later in the jet event at 2300 UTC 2 June (Fig. 18b), the pressure gradient to the east of the central New Jersey coast is only slightly weaker than 1900 UTC 2 June (Fig. 18a), but there is little total acceleration given the increase in the Coriolis and frictional forces balancing much of this pressure gradient. These more balanced winds are 2–5 m s−1 stronger than at 1900 UTC, and the Coriolis force has veered the jet winds south-southwesterly. The stronger winds have also expanded offshore given the pressure gradient increase over this region.

Figure 19 shows the temporal evolution (every 20 min) of the wind components and the momentum terms at sigma level 34 (~150 m MSL) from 1200 UTC 2 June to 0600 UTC 3 June for a 20 × 20 (26.7 km × 26.7 km) box around ALSN6 in Fig. 18b. During the morning (1200–1400 UTC), the west-southwesterly flow is nearly geostrophic (Fig. 19a). From 1500 to 2000 UTC, the flow is unbalanced as the meridional wind accelerates northward as a result of the increasing north–south pressure gradient (Fig. 19b). Meanwhile, in the west–east direction (Fig. 19c), there is an eastward acceleration around 1500–1600 UTC as a result of a rapidly increasing onshore- (westward-) directed pressure gradient given the late morning inland heating. The west–east pressure gradient remains nearly steady from 1600 to 2100 UTC and the zonal acceleration returns to near zero as the increasing Coriolis balances more of the pressure gradient. The north–south pressure gradient peaks near 2030 UTC and then it slowly decreases, while the increasing Coriolis and frictional forces gradually increase and reduce the northerly accelerations through a geostrophic adjustment process. As a result, the peak meridional wind is established near 2130 UTC, and the total wind speed (16.3 m s−1) is nearly geostrophic (18.0 m s−1). After 2200 UTC, with the exception of a small pressure enhancement in the north–south pressure gradient around 0300 UTC, the pressure gradient gradually weakens and so do the geostrophic and total winds.

2) Sensitivity experiments

Additional WRF simulations were completed for this event to determine the role of the diurnal heating and the coastal geometry. First, a simulation down to 1.33-km grid spacing was run in which the diurnal cycle was removed by turning off the radiational processes within WRF (NORAD run). The lack of heating and cooling completely removes the NYB jet at 2200 UTC 2 June (Fig. 20a). Rather than 15–16 m s−1 southerlies at 100 m MSL along the northern New Jersey coast for the control run at this time (Fig. 13c), there are only ~10 m s−1 southwesterlies in this region in the NORAD run.

The 12-km NORAD run was also compared with the control in order to look at the larger-scale wind response along the coast later at 0000 UTC 3 June (Figs. 21a and 21b). The control run with its heating over the interior Northeast produces 2–3 m s−1 stronger flow than the NORAD run over a relatively large region from the mid-Atlantic to coastal Massachusetts. This result is consistent with the NARR composite of the coastal winds for these NYB jet events (Fig. 8f). Thus, the NYB jet is part of a larger-scale diurnal wind enhancement along the coast. The 12-km WRF winds near the jet are also within 1–2 m s−1 of the observed at all times (not shown), so high resolution is not needed to simulate most of the coastal wind enhancement.

It was hypothesized that the bend in the coastal geometry of central New Jersey was important in modifying the low-level winds. An experiment was completed in which the coast of central New Jersey was extended southward to Cape Hatteras, North Carolina, in all WRF domains (not shown). This was done by replacing the water with a flat land surface using the same land-use and soil properties as a 50-km average of the properties to the west of the original coast. In this NSCOAST run at 2200 UTC 2 June (Fig. 20b), the jet is uniform along the New Jersey coast. Thus, the concave coastline of New Jersey is important in limiting the jet to the northern locations over the NYB. This can be understood by looking at the orientation between the isobars relative to the coast. In the control run, where the isobars are parallel to the coast of southern New Jersey, the flow is nearly geostrophic (Fig. 18a). In contrast, to the north where the isobars intersect with the coast at a ~45° angle, as the land warms, the surface pressure decreases relative to the water to the east; thus, the isobars are orientated more north–south along the coast, which increases the pressure gradient and accelerations along the coast. This NSCOAST run also illustrates that the jet wind speeds reach a nearly constant magnitude (~15 m s−1), which is similar in magnitude to the control run (cf. Figs. 20b and 13c). Thus, further accelerations would likely not occur for the NYB jet if Long Island and coastal Connecticut were not there. In fact, a separate simulation without Long Island revealed that the jet did not increase in magnitude to the north of ALSN6 (not shown).

Since this jet is maximized just south of NYC, an additional WRF experiment was completed to determine whether urban heating was a factor in the near-coast accelerations. For this run the WRF land use for New York was converted from urban to deciduous forest (NONYC run). However, this simulation had very little (<0.5 m s−1) impact on the NYB winds around ALSN6 (not shown). An additional run was also completed in which the interior terrain was replaced by flat land over the eastern United States, and this also only weakened the NYB jet winds by <10% (not shown). Overall, the sensitivity experiments illustrate that the most important factor in NYB jet occurrence is the larger-scale differential heating between the land and ocean.

4. Discussion

a. Comparison with the U.S. west coast northerly jets

There are similarities between the NYB jet and the northerly jets along the U.S. west coast (Burk and Thompson 1996). The coastal jet is situated within a sloping marine inversion and represents an enhancement of the background synoptic flow around the anticyclone (northerly on West Coast and southerly on East Coast). For both jets the maximum coastal baroclinicity occurs in the midafternoon, while the LLJ maximum occurs in the late afternoon and evening. On the West Coast the jets are enhanced by the flow around coastal bends and topography. There is no topographic enhancement on the East Coast, but the change in orientation of the coast can change the structure of the jet.

Burk and Thompson (1996) explained the delay in the jet maximum in the early evening using a modification of the technique used by Haurwitz (1947), in which an oscillatory pressure forcing term in the momentum equation was added in the west–east direction to represent the diurnal heating, which is a midafternoon maximum. In this explanation, the northerly wind perturbation (acceleration) is initially directed onshore during maximum heating; however, as the Coriolis force acts on this perturbed flow, it traces a portion of an inertial circle to the right over 4–5 h. Although the average wind evolution for the NYB jets suggests a clockwise inertial response (Figs. 6 and 11), the pressure evolutions near the coast and WRF momentum budgets suggest that the winds peak and rotate to more south-southwesterly through a geostrophic adjustment process. Namely, the accelerations to the north gradually weaken 1–3 h after the pressure gradient peaks during the midafternoon as the Coriolis and friction nearly balance the pressure gradient.

b. NYB jet impacts

The NYB jet has several potential impacts. The importance of diurnal southerly winds on coastal ocean mixing and upwelling in the NYB region has been previously documented (e.g., Hunter et al. 2007), and it is likely that the NYB jet enhances such oceanic processes. The NYB jet may also impact wind power (Kempton et al. 2010). For example, during many NYB jet events, such as the 2 June 2007 event, there are relatively warm (~30°C) temperatures inland and thus increased electrical demand. The potential for wind power may be limited inland, where the winds are light; however, the NYB jet may provide a source of regional wind energy on these days. Unfortunately, these southerly winds may also favor rip currents, which are favored when there are persistent moderate to strong onshore winds over a relatively large (>500 km) fetch (Short and Hogan 1994). The enhanced NYB jet winds along the New Jersey coast can push large amounts of water to the south-facing beaches of Long Island, which may increase the rip current threat in this region.

When ozone and other pollutants are transported over the coastal waters of southern New Jersey during southwesterly low-level flow, they can be advected northward within the NYB jet to the NYC–Long Island area. On the other hand, when the large-scale flow is more southerly or south-southwesterly, such as during the 2 June 2007 event, the NYB jet may advect relatively clean (marine) air to the coastal NYC–Long Island area. The stable and shallow marine layer associated with the NYB jet also prevents vertical mixing and may increase the pollutant concentrations within the boundary layer. High ozone concentrations have been found along the New England coast because of a lack of mixing and shallow sea-breeze circulations (Angevine et al. 2004; Mao and Talbot 2004).

During the spring and early summer, the NYB jet can transport cooler marine air into southern New England. This southerly (onshore) flow pattern has been shown to decrease convection during this region (Murray 2009). As the water warms later in the summer, there is an early evening and nocturnal maximum of convection over the Atlantic coastal waters (Parker and Ahijevych 2007; Murray 2009), and the low-level shear associated with the NYB jet may help organize convection during these evening periods. More work is needed to better understand the regional implications of the NYB jet.

5. Summary and conclusions

High-resolution observations and model simulations using the Weather Research and Forecasting (WRF) model were used to understand the structural evolution, dynamics, and climatology of a low-level jet over the coastal waters of the New York Bight (NYB) region. A 1997–2006 climatology of the jet using hourly data from a tower (ALSN6) and a buoy (44025) in the NYB shows that the jet is most common during the warm season (June–July peak), with a skew in the monthly distribution toward spring, since the jet is driven by the differential heating between land and water. About 28% of the events have winds greater than 13 m s−1 (25 kt), which meets or exceeds the small craft advisory wind conditions for the National Weather Service. The wind directions for the jet trace out an elliptical orbit for the 24-h period around the jet maximum, which is at 2300 UTC (1800 LST) on average. In addition to the inertial forces, there is also an increasing west-northwest–east-southeast-directed pressure gradient that peaks 1–3 h before the time of maximum southerly wind. This 1–3-h delay is qualitatively consistent with geostrophic adjustment. Spatial composites reveal that the NYB jet occurs when there is large-scale southwesterly flow around a Bermuda high and a short-wave ridge along the East Coast. The composites also illustrate that the jet is part of a larger-scale offshore wind enhancement along the mid-Atlantic and to the northeast U.S. coasts during the early evening hours.

The 2–3 June 2007 NYB jet event was simulated down to 1.33-km grid spacing using the WRF. The model was able to realistically simulate the jet event, although its magnitude was slightly too weak and it occurred 1–2 h too early. The jet is shallow (lowest 300 m), since it resides within the sloping marine layer from offshore (east) to the coast. Trajectories illustrate the low-level flow diffluence near the coast and the resulting subsidence of the air parcels in that region. A momentum budget shows the enhanced pressure gradient and westward accelerations helping to force this low-level diffluence. The peak jet winds are established 1–2 h after the maximum pressure gradient is established during the midafternoon through a geostrophic adjustment, as Coriolis force limits the accelerations and veers the flow to more south-southwesterly.

A WRF sensitivity run without diurnal (radiational) heating results in no NYB jet development. The run also shows that the differential heating at the coast enhances the low-level winds 200–300 km offshore and along a large portion of the coast from the mid-Atlantic to southern New England. Another WRF run with a straight north–south coast shows how the westward (concave) bend in the New Jersey coast favors limited accelerations there, since the isobars are nearly parallel to the coast and the flow is more geostrophic.

Future work will need to more closely investigate the mechanisms for the large-scale wind response along the East Coast to separate out the role of the marine layer depth and synoptic-scale control. The impacts this NYB jet has on convection also need to be investigated, since convection tends to be maximized in the coastal waters in the early evening during the warm season (Parker and Ahijevych 2007).

Acknowledgments

We thank the forecast staff of the New York City Weather Forecast Office for initially raising our awareness of this phenomenon. Use of the WRF was made possible by the Microscale and Mesoscale Meteorological (MMM) Division of the National Center for Atmospheric Research (NCAR), which is supported by the National Science Foundation. This study was supported in part by the National Science Foundation under Grant ATM-0705036 (Colle).

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Fig. 1.
Fig. 1.

(a) Topographic map of the region surrounding NYC, with geographic features labeled and elevation gray shaded (m). The NYB jet area of interest is outlined by a box. (b) Surface map at 0000 UTC 1 Apr 2006 for the box in (a) showing station model data including temperature (°C), dewpoint (°C), wind (1 full barb = 5 m s−1), sea level pressure, and cloud cover.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 2.
Fig. 2.

The number of NYB jet events per month from January 1997 to December 2006.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 3.
Fig. 3.

The number of NYB jet events for various wind speed ranges (m s−1) from January 1997 to December 2006.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 4.
Fig. 4.

(a) Wind speed (m s−1) and (b) wind direction evolution at ALSN (at ~30 m ASL) for the 15–19 Jul 1999 period (in UTC and LST). The 4 days of wind directions are contoured using the line shades given by the inset box.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 5.
Fig. 5.

The number of NYB jet events with a maximum wind at a particular hour (UTC and LST) from 1800 to 0300 UTC.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 6.
Fig. 6.

Hodograph of the 30-m winds (m s−1) at ALSN6 plotted every hour from 12 h before the time of the maximum winds (gray dots) at 2300 UTC and 12 h after the maximum winds (black dots).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 7.
Fig. 7.

Sea level pressure difference in hPa between (a) ALSN6 and Central Park, NYC (ALSN6-NYC), and (b) buoy 44025 and EWR (44025-EWR) for the time period in hours relative to the maximum winds at ALSN6.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 8.
Fig. 8.

NARR composite at 1500 UTC for the NYB jet days showing (a) 500-hPa height (every 30 m), (b) sea level pressure (every 1 hPa), and (c) 1000-hPa wind speed (contoured every 1 m s−1, with vectors showing direction). (d)–(f) As in (a)–(c), but for the time closest to the time of the NYB jet wind maximum.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 9.
Fig. 9.

(a) The 500-hPa geopotential heights (dashed every 30 m) and sea level pressure (solid every 4 hPa) from the 36-km WRF at 1200 UTC 2 Jun 2007 (forecast hour 12). (b) As in (a), but for 0000 UTC 3 Jun 2007. The boxes in (a) show the locations of the 12-, 4-, and 1.33-km domains, respectively.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 10.
Fig. 10.

Doppler radial velocities (shaded in kt using the inset scale) from the KJFK TDWR and station model data including temperature (°F), dewpoint (°F), wind (1 full barb = 5 m s−1), and sea level pressure at (a) 1400, (b) 1800, and (c) 2300 UTC 2 Jun 2007.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 11.
Fig. 11.

(a) ACARS temperature and wind profile (full barb = 5 m s−1) on a skew T chart from JFK at 1541 UTC 2 Jun 2007. (b) As in (a), but for the 1.33-km WRF at 1600 UTC, and dewpoint temperature is also shown (in gray). (c) As in (a), but at 2206 UTC 2 Jun 2007. (d) As in (b), but at 2200 UTC 2 Jun 2007.

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Fig. 12.
Fig. 12.

Hodograph of the 30-m winds (m s−1) at ALSN6 plotted every hour for the observations (black) and 1.33-km WRF (gray) from 1200 UTC 2 Jun to 0600 UTC 3 Jun 2007 for WRF and to 1000 UTC 3 June 2007 for the observations.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 13.
Fig. 13.

The 1.33-km WRF pressure (solid every 1 hPa), wind barbs (full barb = 10 kt), wind speed (shaded in m s−1), and temperature (every 3°C) at 100 m ASL at (a) 1400 UTC 2 Jun, (b) 1800 UTC 2 Jun, (c) 2200 UTC 2 Jun, and (d) 0300 UTC 3 Jun 2007. The locations for the cross sections in Figs. 15 and 16 are shown in (a).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 14.
Fig. 14.

As in Fig. 13, but at 500 m ASL.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 15.
Fig. 15.

Cross section AA′ from the 1.33-km WRF showing potential temperature (solid every 1 K), and meridional wind speed into the section (shaded in m s−1) at (a) 1800 UTC 2 Jun, (b) 2200 UTC 2 Jun, and (c) 0300 UTC 3 Jun 2007. The location of the cross section is shown in Fig. 13a.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 16.
Fig. 16.

As in Fig. 15, but for along BB′ in Fig. 13a.

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Fig. 17.
Fig. 17.

Backward trajectories 1–8 starting at 30 m ASL at 2200 UTC 2 Jun. The height of the trajectory is given by the width of the ribbon (inset scale). The trajectory is divided into 1-h segments. The 30-m wind speed (shaded in m s−1) at 2200 UTC is also shown.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 18.
Fig. 18.

Momentum terms at model σ = 34 (~150 m above surface) at (a) 1900 UTC 2 Jun and (b) 2300 UTC 2 Jun showing the pressure (every 0.5 hPa) and the total acceleration (thick black; A), pressure gradient force (thick gray, P), Coriolis force (thin black to the right of the wind barb; C), and boundary layer drag and mixing (thin black; F). The terms A, P, C, and F are shown for two representative locations for reference. The box in (b) shows the location of the average momentum budget traces in Fig. 19.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 19.
Fig. 19.

(a) Time series of the total (black) and geostrophic (gray) wind speed (in m s−1) averaged for the black box in Fig. 18b within the 1.33-km domain for hours 12–30 (1200–0600 UTC). (b) As in (a), but for the υ momentum budget (m s−2) showing the total acceleration (vaccel; gray solid), frictional and mixing processes (vpbl; dotted), pressure gradient (pgy; short dashed), Coriolis (cory; long dashed), and the meridional wind (vtotal: thick black in m s−1). (c) As in (b), but for the u momentum budget.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 20.
Fig. 20.

The 1.33-km WRF pressure (solid every 1 hPa), winds barbs (full barb = 10 kt), wind speed (shaded in m s−1), and temperature (every 3°C) at 100 m ASL for the (a) NORAD and (b) and NSCOAST runs at 2200 UTC 2 Jun.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

Fig. 21.
Fig. 21.

The 12-km WRF pressure (solid every 2 hPa), winds barbs (full barb = 10 kt), wind speed (shaded in m s−1), and temperature (every 3°C) at 100 m ASL for the (a) control and (b) NORAD runs at 0000 UTC 3 Jun.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3231.1

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