The Relevance of Mean-State Critical Levels for the Intensification of Downslope Winds

Downslope windstorms occur when atmospheric flow interacts with sufficiently large mountain barriers. Under influence of these windstorms, wildfires may grow substantially as shown by recent major wildfires in the western US. Besides sufficient atmospheric flow, ingredients necessary for downslope windstorm to develop include self-induced wave breaking from a large-amplitude wave, strong stratification near ridge level below a layer of lower stability, or wave breaking from a mean state critical level (MSCL), defined here when the wind component perpendicular to the mountain range reaches zero. Sundowner winds are downslope windstorms that frequently occur on the slopes of the west-east oriented Santa Ynez Mountains (SYM) in coastal Santa Barbara, California. While recent efforts have shown that each of these processes occur during Sundowner winds, the relevance of MSCL is yet unknown. Here, we use data obtained during the Sundowner Winds EXperiment (SWEX) and 32-year hourly 1-km grid spacing Weather Research & Forecasting (WRF-1km) simulations to investigate the importance of the MSCL for Sundowner development. SWEX collected data from April 1 to May 15 2022 from a suite of ground-based instruments including 18 flux towers, 6 lidars, and 3 microwave wind profilers. During 10 Sundowner events (IOPs), additional 3-hourly radiosondes were collected at four sites, along with airborne and ground-based mobile lidars and dropsondes. The 32-yrs of WRF-1km hourly climatology indicated that during spring (peak of Sundowner frequency) about 25% of these events occurred in association with a MSCL, while in summer more than 50% occurred under the influence of a MSCL. The results from the WRF-1km climatology suggest that during conditions when a MSCL is present, weaker tropospheric forcing can cause similar strength of windstorms than when MSCL is absent. The influence of the MSCL was further demonstrated during SWEX IOP 9, when less tropospheric forcing attained similar lee slope wind strength comparatively to other IOPs that occurred without MSCL. Our findings indicate that the MSCL is a critical factor that should be incorporated in operational forecasts of mountain windstorms in coastal ranges, with significant impact on fire weather forecast and red-flag warnings.