Improving the forecasting of barrier jets along coastal Alaska using remote sensing, modeling, and a SARJET field study

The Alaskan coastline bordering the Gulf of Alaska has steep coastal topography, as well as a variety of bays, straits, and sounds. The low-level flow accelerates down the pressure gradient along the coast and out of coastal gaps, which can be dangerous to the aviation and shipping industries. Because this region has little in situ observational data, this study used a combination of spaceborne synthetic aperture radar (SAR), high-resolution atmospheric modeling using the PSU-NCAR MM5, and a field study in order to improve our understanding and forecasting of these coastal wind events. First, this paper will show how high-resolution wind estimates from over 3000 SAR images between 1998-2003 were used to document the different types of barrier jet responses and their frequency along the coast. It was found that a majority of coastal barrier jets occur during the cool season and most jets are located along the coastline from Cross Sound northeast to Prince William Sound, with the coastline near Mount Fairweather and the Valdez-Cordova mountains experiencing the greatest number of barrier jets. Our analysis suggests that there is a type barrier jet (“hybrids”) in coastal Alaska, which has some gap flow characteristics, with cold and dry continental flow feeding into the jet through a coastal gap. During some of these events, the upstream boundary to barrier jet can develop sharp frontal-like characteristics in the temperature and wind field (shock-jet). Another subset of jet events in the SAR climatology have irregular wind maximum over the coastal waters (variable jet), suggesting localized downward transport of winds to the surface by turbulent eddies or convective downdrafts. Large-scale and sounding composites are constructed for all barrier jets objectively identified around Yakatat, AK (YAK) using the daily NCAR reanalysis and twice-daily soundings at YAK and Whitehorse, YT (YXY). It will be shown that each jet type (shock, variable, and other-type) has a significant large-scale and thermodynamic signal, which is useful for forecasting these events. Namely, during jet events there is an anomalously deep upper-level trough approaching the Gulf of Alaska and an anomalous ridge over western Canada and interior Alaska. The associated surface cyclone and surface ridging result in strong low-level southerlies over southeast Alaska and the advection of 850-mb warm anomalies northward from the subtropics to Alaska. The largest cool and dry anomalies are over the interior at YXY, especially for the shock events, which suggests that the interior cold and dry air source is important in the development of sharp front-like boundaries and hybrid characteristics to the barrier jet. In contrast, the variable jets have weaker low-level stability, which may favor the development shallow convection and subsequent mixing of higher momentum to the surface. Finally, some examples of these jet types will be shown using modeling, SAR, and in situ aircraft data from the Southern Alaska Regional Jets (SARJET) experiment, which occurred from 21 September – 22 October 2004 near Mount Fairweather northwest of Juneau, AK.