Downslope Windstorms in Coastal Mountains: Observations and Modeling during the Sundowner Wind Experiment (SWEX)

Coastal Santa Barbara is among the most wildfire-prone communities in Southern California. Downslope, dry, and gusty windstorms frequently occur along the south-facing slopes of the east–west-oriented Santa Ynez Mountains (SYM), which separate the Pacific Ocean from the Santa Ynez Valley. These winds, known as Sundowner Winds, typically peak after sunset and often persist overnight. They represent the most critical fire-weather phenomenon in the region.

The Sundowner Winds Experiment (SWEX), conducted from 1 April to 15 May 2022, integrated airborne and ground-based observations to examine interactions between continental and marine atmospheric boundary layers (ABLs), assess mountain waves and hydraulic jumps and their influence on surface winds and dew point, and evaluate forecasting challenges in mesoscale models.

This study analyzes two Sundowner events—IOP-2 (April 5–6) and IOP-10 (May 12–13)—affecting the eastern SYM. IOP-2 occurred during a heat wave, with temperatures reaching the 95th percentile, whereas IOP-10 reflected typical spring conditions.

During IOP-2, observations revealed sharp elevated inversions near the SYM, with mountain waves propagating across these layers. The free atmosphere was extremely dry, and strong horizontal winds were confined near inversion height. On the lee side, 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 the eastern foothills, while nighttime marine boundary layer (MBL) intrusion—capped by a strong inversion—played a key role in the Sundowner cycle. Descending wave structures and rotor circulations produced reversed flows and enhanced surface winds. A nocturnal mid-channel eddy over the Santa Barbara Channel further stratified the MBL and decoupled it from the downslope jet. WRF simulations at 1-km resolution underestimated ridgetop and lee slope winds and overestimated coastal winds, with biases linked to misrepresentation of ABL height, inversion strength, and delayed MBL advection.

IOP-10 was investigated using ground-based instruments and radiosondes. It featured the second-largest observed mean sea level pressure difference between Santa Barbara and Bakersfield during SWEX. However, winds exceeding 20 m/s occurred on eastern slopes hours before peak pressure differences. LiDAR detected vertical motions near 6 m/s, associated with lifting of the lee-slope jet and weakening of surface winds—evidence of mountain wave activity influencing wind intermittency. Similar to IOP-2, the nocturnal mid-channel eddy contributed to lifting the lee jet and terminating Sundowners near the surface.

These findings emphasize the need for accurate representation of inversion structure and height, as well as marine–continental ABL interactions, in mesoscale models. Realistic simulation of complex flow dynamics—such as mountain waves and hydraulic jumps—is essential to improve forecasts of downslope winds in coastal environments. The SWEX campaign provided unique measurements to evaluate these features.