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Colloquium

Department of Atmospheric Science

Mon., Aug. 27, 2:00 pm, EN6060

Understanding Downslope Windstorms in the Santa Ynez Mountains in Santa Barbara, CA and the Sundowner Winds Experiment (SWEX) First Campaign

Dr. Leila Carvalho

University of California, Santa Barbara

Abstract

Sundowner winds (Sundowners for short) are the northerly gusty winds frequently observed leeward of the Santa Ynez Mountains (SYM) in coastal Santa Barbara (SB), CA. These winds typically peak from early evening through early morning with gale force gusts exhibiting features of downslope windstorms. The SYM rise abruptly from coastal SB separating the Pacific Ocean on its south face from the Santa Ynez Valley on its north face. Sundowners are considered the most relevant fire weather condition in all seasons and represent a major hazard for aviation, particularly small crafts. All major wildfires affecting the region exhibited significant fire spread rates toward the SB wildland-urban interface during Sundowners. The Sundowner Wind Experiment-first intensive campaign (hereafter, SWEX-I) was the first to evaluate vertical profiles of winds, temperature, humidity and other thermodynamic variables from the boundary layer to mid-high troposphere and leeward of the SYM during episodes of Sundowners (wind gusts exceeding 13 m/s or 30 mph). This was accomplished by launching 3-hourly radiosondes during the Sundowner events on April 28-29, 2018. SWEX-I demonstrated that cross-mountain winds in the lee of the SYM exhibit complex spatial and temporal patterns. Profiles of wind speed and direction, temperature and humidity near ground level showed a transition from humid onshore winds from morning-to-mid afternoon hours to very pronounced offshore winds during the evening. These offshore winds accompanied a northerly nocturnal low-level jet leeward of the SYM at very low elevation with variable behavior with respect to intensity and elevation. The experiment showed evidence of mountain waves and critical layers associated with the intensification of winds at sunset. Variations in the jet were accompanied by changes in stability profiles and the Richardson number. Additionally, we examined the skill of the Weather and Forecast Model (WRF) (at 1km grid spacing) in forecasting this event, with focus on profiles of winds and stability. These results advanced our understanding of Sundowners and indicated that a comprehensive field campaign is critical to properly characterize the main mechanisms driving these winds and to advance studies on predictability of these events.


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