Aircraft Observations and Numerical Simulations of the Developing Stage of a Southerly Surge near Southern California

摘要

Summertime low-level winds in the marine boundary layer off the California coast are predominantly from the north. This pattern is interrupted periodically by southerly winds and low stratus that can propagate for hundreds of kilometers northward along the coast. These events have been termed coastally trapped disturbances, coastally trapped wind reversals, and southerly surges; their forcing has been the subject of extensive study and debate. Southerly surges remain difficult to forecast, yet have a significant impact on coastal activities.The beginning stage of a southerly surge on 16 June 2012 was explored during the Precision Atmospheric Marine Boundary Layer Experiment. Measurements of the horizontal wind and pressure field in the marine layer offshore from Cape Arguello hours prior to the onset of the surge were made using the University of Wyoming King Air research aircraft. Airborne measurements show that a horizontal pressure field is established with higher pressure to the south just prior to the surge, supporting southerly ageostrophic winds and a south-to-north movement of marine stratus. Aircraft soundings and lidar returns confirm the existence of offshore flow of warm, continental air north of Point Arguello that alters the pressure field adjacent to the coast. The southerly surge originates near Point Arguello and propagates northward past San Francisco during the early morning hours on 17 June 2012. Results from the Weather Research and Forecasting Model are consistent with the King Air observations. Analyses and model output presented here confirm that the large-scale environment is critical to the initiation of these wind reversals.