A case study of high winds induced by upper-level frontogenesis and tropopause folding

Abstract

High surface winds over California and the bordering Pacific Ocean resulted in the death of one man and the loss of power to approximately 50,000 residences across the state. These damaging winds are hypothesized to result from an upper level front and associated tropopause folding that rapidly intensify as they move south across the region, causing high momentum air to be transported to the lower troposphere. Once the high momentum air reaches the top of the planetary boundary layer, the combined effects of destabilization of the planetary boundary layer by cold air advection aloft and shear induced turbulence at the top of the layer provide the initial mechanism by which the high momentum air is entrained into the layer and mixed to the surface. After sunrise, convectively driven turbulence provides an additional source of mixing in the planetary boundary layer. The high surface winds have a strong cross isobaric component in the direction of the upper level winds, and the upper level frontal movement to the south over central California is synchronous with the increase of surface winds over the same region. The winds decrease as the upper level front moves into the base of the upper level trough and the high momentum source in the lower troposphere disappears.