Evidence of Post-Frontal Mountain Wave Enhanced Wind Shear in Juneau Alaska

On 30 January 1993, an aircraft departing the Juneau International airport encountered severe wind shear resulting in a temporarily loss of control and a near accident. While on a curved, right turn departure, off runway 10 at a 30 degree bank, the crew reported extreme crosswinds when the aircraft was opposite the Fish Creek valley on Douglas Island. The timing of the incident closely corresponded with the passage of a strong cold front, which was supported by airport surface observations of rapidly rising pressure and an abrupt wind shift. A high resolution WRF case study of this event using North American Regional Reanalysis (NARR) data suggests that wind shear in the affected area was enhanced by a topographically induced gravity wave. This short-lived mountain wave developed in the low-level stable layer immediately behind the front, accelerating lee-side winds with a downward component out from the west side of the Fish Creek valley and across the departure path of the aircraft.

As a result of the January 1993 event, as well as other wind shear and turbulence problems, development of an alert system called Juneau Airport Wind System (JAWS) was initiated by the Federal Aviation Administration (FAA) and National Center for Atmospheric Research (NCAR) in 1995. The sensor network for this system includes three boundary layer wind profilers and an array of anemometers at both sea level and mountain top locations. One of the JAWS wind profilers, near the northwestern mouth of the Fish Creek valley, is in an ideal location for detecting other occurences of post-frontal topographically enhanced wind shear, and an examination of historical data has found evidence of similar events. An example is presented.

Based on the WRF case study and historical observational evidence, a set of criteria has been identified that would favor the development of this type of wave enhanced wind shear: (1) A deep low making landfall in the northeast Gulf of Alaska; (2) pressure rising rapidly at the Juneau airport following passage of a strong front; (3) low level winds shifting to south or southwest after frontal passage; (4) low level cooling behind the front. The forecast of these criteria should be an important consideration when evaluating wind shear potential in the vicinity of the Juneau airport.