An Unusually Strong Gravity Wave over Western Washington

Abstract

A mesoscale wave disturbance, observed over western Washington State, is studied through local observations and recognized by comparisons with theory as a large-amplitude gravity wave. It is shown that this gravity wave propagated over 200 km in an atmosphere conductive to wave trapping. Abrupt wind shifts, strong gusty winds to 17 ms-1, moderate-to-severe turbulence, and a sea level pressure fall/rise couplet of 3 to 6 mb were observed as the wave propagated at an average speed of 13 ms-1 over the Puget Sound. Subsidence at the top of the marine boundary layer, associated with the trough passage, is estimated at 1100 m over a period of 30 min. This dramatic sinking motion generated rapid, short-lived changes in cloud heights and ceilings, which proved difficult to forecast and potentially dangerous to aviation trafffic. A convective downdraft appears to have provided the necessary push to initiate this gravity wave. However, geostrophic adjustment and wind shear cannot be ruled out as possible generation mechanisms.