WRF simulation of downslope wind events in coastal Santa Barbara County

Abstract

The National Weather Service (NWS) considers frequent gusty downslope winds, accompanied by rapid warming and decreased relative humidity, among the most significant weather events affecting southern California coastal areas in the vicinity of Santa Barbara (SB). These extreme conditions, commonly known as "sundowners", have affected the evolution of all major wildfires that impacted SB in recent years. Sundowners greatly increase fire, aviation and maritime navigation hazards and are thus a priority for regional forecasting. Currently, the NWS employs the Weather Research Forecasting (WRF) model at 2 km resolution to complement forecasts at regional-to-local scales. However, no systematic study has been performed to evaluate the skill of WRF in simulating sundowners. This research presents a case study of an 11-day period in spring 2004 during which sundowner events were observed on multiple nights. We perform sensitivity experiments for WRF using available observations for validation and demonstrate that WRF is skillful in representing the general mesoscale structure of these events, though important shortcomings exist. Furthermore, we discuss the generation and evolution of sundowners during the case study using the best performing configuration, and compare these results to hindcasts for two major SB fires. Unique, but similar, profiles of wind and stability are observed over SB between case studies despite considerable differences in large-scale circulation, indicating that common conditions may exist across all events. These findings aid in understanding the evolution of sundowner events and are potentially valuable for event prediction.

Brief Communication: Synoptic-scale differences between Sundowner and Santa Ana wind regimes in the Santa Ynez Mountains, California

“…This thermally driven pressure gradient creates strong northeasterly winds and gravity waveforced downward momentum transfer that yields regional downslope warming and low relative humidity. Despite the high impact of fires in the Santa Ynez Mountains on urban communities (i.e., WUI; Martinuzzi et al, 2015) and agricultural operations, little research has focused on the smallerscale Sundowner winds and is limited to case studies (Blier, 1998;Cannon et al, 2017). These studies indicate that different atmospheric processes are involved in Sundowner events compared to classic SAW events at the synoptic scale (Blier, 1998;Cannon et al, 2017).…”

“…Anomalies of MSLP and 500 hPa heights were calculated as differences from the 1981-2010 long-term daily means. Although our primary goal is to explore synoptic-scale differences between wind regimes, Cannon et al (2017) pointed out the importance of northerly winds in Sundowners, which we would expect to be absent during SAW-only regimes. To do this, we examined vertical cross sections of northerly (v-component) winds from 32 to 36 • N at levels between 1000 and 300 hPa from NARR.…”

“…Despite the high impact of fires in the Santa Ynez Mountains on urban communities (i.e., WUI; Martinuzzi et al, 2015) and agricultural operations, little research has focused on the smallerscale Sundowner winds and is limited to case studies (Blier, 1998;Cannon et al, 2017). These studies indicate that different atmospheric processes are involved in Sundowner events compared to classic SAW events at the synoptic scale (Blier, 1998;Cannon et al, 2017). However, these few case stud-ies avoid generalizing their results in a climatological sense and to our knowledge no studies have yet attempted to relate Sundowner winds to SAWs.…”

“…In the Santa Ynez mountains, these winds are locally called "Sundowner" winds due to their characteristic onset during late afternoon or early evening (Blier, 1998;Fig 1b and c). In an effort to explain the dynamics of Sundowners, Cannon et al (2017) performed 2 km horizontal-resolution numerical simulations of several case studies. Their simulations demonstrated the importance of northerly winds over the Santa Ynez that formed gravity waves in the lee of the Santa Ynez.…”

A Surface Observation Based Climatology of Diablo-Like Winds in California’s Wine Country and Western Sierra Nevada

“…Despite the massive destruction wrought by the Diablo winds, associated with the Northern California firestorm of October 2017 and the Oakland Hills firestorm of 1991, very little is known quantitatively about them, especially relative to the Southern California downslope winds that favor wildfires, such as Santa Ana [1][2][3][4][5][6][7][8][9][10] and Sundowner [11][12][13][14][15] winds. A prescient preliminary analysis of 'Santa Ana-like' winds in the Oakland Hills was performed in 1973 [16], some 18 years prior to the Oakland Hills fire of 1991, and analyses [17][18][19] subsequent to that fire confirmed their localization at the eastern San Francisco Bay Area.…”