Sundowners and Mountain Wave Activity: The 28-29 April, 2022 Event during the Sundowner Winds Experiment (SWEX)

Wildfires in complex terrain have demonstrated tragic loss and the vulnerability of communities within the interface over the past several years. The added intensity created by downslope windstorms and the lack of high temporal and spatial resolution of ground observations motivates the ongoing need to understand and increase predictability. Sundowners, downslope wind storms that are typically intensified around sunset, are a common and significant weather phenomenon observed along the Santa Barbara coastline and are historically known to increase challenges during wildfire events. The Sundowner Winds EXperiment (SWEX) was a six-week field campaign from April 1st to May 15th, 2022 held in Santa Barbara County, California. The campaign utilized ground and air operations to study the dynamics of sundowner winds by increasing spatial and temporal measurements of the local climate and several vertical profiles by acquiring a suite of thermodynamic meteorological variables. The goal of the experiment is to understand the mechanisms behind sundowner winds, mesoscale simulations, and increase the forecast lead time and predictability of an event.

On April 28-29, 2022, the sixth Intensive Operation Period (IOP), a strong trough at 500 mb lead the alignment of northwest winds over the Santa Ynez Mountains and thus a western regime sundowner event in the coastal Santa Barbara foothills. This event is analyzed using a suite of instruments, ground-based and airborne, including a doppler LiDAR, radiosondes, dropsondes, and surface micrometeorological stations. Strong mountain wave activity aloft was observed by the LiDAR as well as strong northwest surface winds observed by ISFS station’s. Radiosonde profiles demonstrate there was a pocket of warm, dry air (750 m AGL) situated above cool, moist air at 0200 UTC (0700 PDT) that corresponded to a mountain wave structure also observed in the LiDAR velocity profiles. At two notable times there was an increase in temperature and decrease in relative humidity associated with increased surface wind speeds and near surface mountain wave activity. At the end of the period, captured at several ISFS stations and the LiDAR, there was a rapid onset of weak, southeast winds and increased relative humidity in association with the onset of the marine layer thus ending the event. Further analysis will focus on the impact of mountain waves at the surface and the influence of the marine boundary layer (MBL).