Mountain Wave Interactions Between the Santa Ynez Mountains and the San Rafael Mountains During Eastern Sundowner Winds in Santa Barbara, CA

Abstract:

The residents of Santa Barbara County know Sundowners as the extreme northerly downslope windstorms that occur after sunset on the southern slopes of the Santa Ynez Mountains (SYM). Sundowners are notoriously known to dramatically and unpredictably influence wildfire spread. The southern Santa Barbara County coast exhibits unique topography with two east-west oriented mountains, the SYM and the San Rafael Mountains (SRM), separated by the Santa Ynez Valley (SYV). The SYM rises abruptly from the coast and to the north the SRM has peaks exceeding the highest elevations in the SYM. The large range of elevations in the local area causes the mixing layer to behave very differently in the mountain peaks, slopes, and foothills, and the valley, causing a wide range of spatial variability in stability and wind profiles during Sundowner winds. Previous research on Sundowners has indicated that the interaction between flows over the SRM and SYM, and the resulting mountain waves, play a role in the spatial and temporal variability of Sundowners. The goal of the present research is to investigate the interaction between the SYM and SRM wind regimes and their mountain waves during Eastern Sundowner events. This study analyzes the available 30-year climatology for the greater Santa Barbara area from Weather Research and Forecasting Model (WRF) simulations at 1km resolution. I found that peak downslope winds and mountain wave activity in the SRM lead the onset of Sundowners over eastern SYM. Additionally, I demonstrate that during the period of intensification of mountain wave activity in the SYM, the height of the SRM self-induced critical layer aligns with theoretical resonance height for wave amplification. With my continued investigation into the upstream influence of the SRM on the SYM, the SRM can act as a warning system to Sundowners and therefore increase the predictability of the fire dangerous downslope windstorms.