Using synthetic aperture radar (SAR), modeling, and a SARJET field study to understand the terrain-enhanced flows along the southeast Alaskan coast

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 and WSR-88D radars are blocked by terrain for many locations over the coastal waters, 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.

Several simulations of coastal wind events have been simulated using the MM5 down to 1.33-km grid spacing. Since obtaining a good estimate of the winds over water using SAR requires knowing the wind direction at the surface, this paper will show the positive impact a high- resolution wind field from the MM5 can have on the observed SAR wind field. Finally, the Southern Alaska Regional Jets (SARJET) experiment was conducted near Mount Fairweather northwest of Juneau, AK in late September to October 2004 using the Wyoming King Air aircraft. A spectrum of jet types were sampled, and this paper will present some brief comparisons of a more classical jet (26 September 2004) versus a hybrid event (10 October 2004). The model will be quantitatively compared with flight-level observations, and reasons for the mesoscale differences near the coast between the two cases will be discussed.