Mountain Waves, Downslope Jets, and Boundary Layer Interactions During the Sundowner Winds Experiment (Swex) – IOP2

Authors

• Daisuke Seto, University of California, Santa Barbara
• Leila M.V. Carvalho, University of California, Santa Barbara
• Gert Jan Duine, University of California, Santa Barbara
• Stephan De Wekker, University of Virginia
• Zhien Wang, Stony Brook University
• Charles Jones, University of California, Santa Barbara
• David Emmitt, Simpson Weather Associates
• Steve Greco, Simpson Weather Associates
• Harindra J.S. Fernando, University of Notre Dame
• Griffin Modjeski, University of Notre Dame
• Craig B. Clements, San Jose State UniversityFollow
• Marian de Orla-Barile, University of California, Santa Barbara
• Callum Thompson, University of California, Santa Barbara
• Zhixing Liu, University of California, Santa Barbara
• Anthony Bucholtz, Naval Postgraduate School
• William Brown, National Center for Atmospheric Research
• Steven Oncley, National Center for Atmospheric Research
• Holger Vӧmel, National Center for Atmospheric Research

Publication Date

7-1-2026

Document Type

Article

Publication Title

Atmospheric Research

Volume

337

DOI

10.1016/j.atmosres.2026.108920

Abstract

Sundowner winds are downslope windstorms occurring along the lee of the Santa Ynez Mountains (SYM) in Santa Barbara County, California, and are recognized as the region's most critical fire-weather condition. The Sundowner Winds Experiment (SWEX) provided a rare opportunity to integrate airborne and ground-based observations to examine interactions between continental and marine atmospheric boundary layers (ABLs), and their modulation by mountain waves. This study focuses on an eastern Sundowner event (IOP2) observed on the evening of 5–6 April 2022, driven by strong mean sea-level pressure gradients. Multi-platform observations revealed sharp elevated temperature inversions near the SYM and San Rafael Mountains (SRM), with mountain waves propagating across these layers. The free atmosphere was extremely dry, and strong horizontal winds were confined near the inversion height. On the lee side of the SYM, a large-amplitude lee wave evolved into a hydraulic jump, followed by wave breaking and a downslope jet. Despite strong offshore forcing, a shallow sea breeze developed over eastern SYM foothills, while nighttime marine boundary layer (MBL) intrusion—capped by a strong inversion—played a key role in the Sundowner cycle. Mountain waves strongly influenced the timing and intensity of the Sundowner event, with descending waves and rotor circulations producing reversed flows and enhanced surface winds. A cyclonic atmospheric eddy that formed in the eastern Santa Barbara Channel at night further stratified the MBL, decoupling it from the downslope jet. WRF simulations at 1-km grid spacing underestimated ridgetop and lee slope winds and overestimated winds at hoothill and coastal sites.

Funding Number

LFR-20652467

Funding Sponsor

National Center for Atmospheric Research

Keywords

Aircraft observations, Boundary layer, Coastal meteorology, Downslope winds, Lidar observations, Mountain waves

Department

Meteorology and Climate Science

Recommended Citation

Daisuke Seto, Leila M.V. Carvalho, Gert Jan Duine, Stephan De Wekker, Zhien Wang, Charles Jones, David Emmitt, Steve Greco, Harindra J.S. Fernando, Griffin Modjeski, Craig B. Clements, Marian de Orla-Barile, Callum Thompson, Zhixing Liu, Anthony Bucholtz, William Brown, Steven Oncley, and Holger Vӧmel. "Mountain Waves, Downslope Jets, and Boundary Layer Interactions During the Sundowner Winds Experiment (Swex) – IOP2" Atmospheric Research (2026). https://doi.org/10.1016/j.atmosres.2026.108920