Analysis of Sundowner Winds during the April 4-5, 2022 Event from the Sundowner Winds EXperiment (SWEX)

The Sundowner Winds EXperiment (SWEX) was a six-week field campaign from April 1st to May 15th, 2022, in Santa Barbara County, California. The campaign utilized ground and air operations through collaborative efforts between several universities and organizations including

the National Weather Service at Oxnard to study the dynamics of Sundowner winds. On April 4-

5, 2022, the second Intensive Operation Period (IOP2), is analyzed using a ground-based LiDAR, four on-site upper-air soundings, 1 dropsonde from the NOAA Twin Otter aircraft and six

Integrated Surface Flux System (ISFS) stations. The onset of the Sundowner event was due to a weakening trough at 500 hPa that moved east aligning northwest, cross-mountain flow at 850 hPa that transitioned to northeast. Mean Sea-Level Pressure (MSLP) differences were notably strong for this case with a maximum value of -8.1 hPa between Santa Barbara Airport (KSBA) and Bakersfield Airport (KBFL); therefore, this event is categorized as an eastern regime Sundowner. Mountain wave activity was indicated by strong downward momentum transfer captured by vertical wind profiles, strongest at 1930 PDT as well as by vertical velocity profiles that observed strong downward motion that quickly transitioned to upward motion suggesting mountain wave breaking in the foothills. The intensification of downslope winds during the onset of the Sundowner was observed at Station 12, along the ridgeline. Soon after, a hydraulic jump was observed at Station 1, in the foothills, suggesting a transport of warm air from the ridgeline. At the end of the analysis period, observed by several ISFS stations and by the LiDAR, there was a rapid onset of weak, southwest winds and increased dewpoint in coordination with the onset of the marine layer thus signifying the end of IOP2. Future work will include the analysis of WRF model forecast outputs compared to observations to analyze the intensity and extent of downslope winds and the influence of the Marine Boundary Layer (MBL).