Climatology of Barrier Jets along the Alaskan Coast. Part I: Spatial and Temporal Distributions

Abstract

This paper investigates the temporal and spatial climatology of coastal barrier jets in the Gulf of Alaska. The jets are divided into two categories based upon the origin of the air involved: "classic" barrier jets fed primarily by onshore flow and "hybrid" jets fed primarily by gap flow from the continental interior. The analyses are compiled from five years (1998)(1999)(2000)(2001)(2002)(2003) of synthetic aperture radar images from the Radarsat-1 satellite totaling 3000 images. Thermodynamic and kinematic data from the NCEP reanalysis is used in the interpretation of the results.The majority of coastal barrier jets occur during the cool season, with the coastline near Mount Fairweather and the Valdez-Cordova mountains experiencing the greatest number of barrier jets. Hybrid jets are even more strongly restricted to the cool season and are commonly found to the west of Cross Sound, Yakutat Bay, and Icy Bay. Some interannual variability in the total number of jets is observed.Coastal barrier jet formation is associated with onshore wind directions and maximum terrain heights exceeding 2 km within 100 km of the coast, features that support low-level flow blocking. Hybrid jet formation requires the additional condition of an abnormally large offshore-directed pressure gradient force.Half of the barrier and hybrid jets exhibit surface wind speeds in excess of 20 m s Ϫ1 (strong gale), although their widths are typically less than 100 km. The maximum speed of both types of jet tends to be 2-3 times that of the ambient synoptic flow. A small percentage of the jets detach from the coastline, with the typical detachment distance being 10 km.

Using SAR Remote Sensing, Field Observations, and Models to Better Understand Coastal Flows in the Gulf of Alaska

“…The along-coast variations in both terrain height and width of the coastal plain contribute to the along-coast variation in the frequency of coastal barrier jets. Loescher et al (2006) also found that barrier jets tended to exhibit a 2-3 times enhancement of the winds over the ambient synoptic flow. Approximately 20% of the barrier jet cases were quite strong, with surface winds in excess of 25 m s _1 .…”

“…This archive contains a plethora of snapshots of the near-surface wind fields associated with a wide variety of synoptic-scale and mesoscale atmospheric phenomena. Some examples of the phenomena imaged within the archive include polar lows (Sikora et al 2000), synoptic-scale fronts (Young et al 2005), boundary layer convection (Babin et al 2003), gap flows (Liu et al 2006), vortex streets and atmospheric gravity waves (Li 2004), and barrier jets (Loescher et al 2006;Colle et al 2006).…”

“…The dynamics of these various jet categories are discussed below. In addition to developing a seasonal climatology of each subclass, Loescher et al (2006) examined the width and strength of each jet type.…”

“…Using NOGAPS at 1°-1° grid spacing has proved to be acceptable in determining the SAR wind structures near the coast for wind climatologies (Loescher et al 2006). However, as illustrated by a region of high winds along the Gulf of Alaska coast offshore of Cross Sound (labeled as "Cross Sound Gap" in Fig.…”

Simulating wind channelling over Frobisher Bay and its interaction with downslope winds during the 7–8 November 2006 wind event

“…A cross-strait pressure-gradient force induced by lee troughing caused by a flow descending over the strait from southern Vancouver Island forced the gap flow to bend, impeding an antitriptic balance along the strait. Loescher et al (2006) differentiate between classic barrier jets and 'hybrid' jets along the coast of the Gulf of Alaska. The former occur when the onshore low-level flow is blocked by the coastal topographic barrier, whereas the latter develop when offshore-directed flows exiting gaps in the same barrier are deflected to the right by the Coriolis force and then merge with a coastal barrier jet.…”