Stratospheric intrusions, the Santa Ana winds, and wildland fires in Southern California

Abstract

The Santa Ana winds of Southern California have long been associated with wildland fires that can adversely affect air quality and lead to loss of life and property. These katabatic winds are driven primarily by thermal gradients but can be exacerbated by northerly flow associated with upper level troughs passing through the western U.S. In this paper, we show that the fire danger associated with the passage of upper level troughs can be further increased by the formation of deep tropopause folds that transport extremely dry ozone‐rich air from the upper troposphere and lower stratosphere to the surface. Stratospheric intrusions can thus increase surface ozone both directly through transport and indirectly through their influence on wildland fires. We illustrate this situation with the example of the Springs Fire, which burned nearly 25,000 acres in Ventura County during May 2013.

Detection of deep stratospheric intrusions by cosmogenic 35 S

“…A recent study suggested that the coupling between Santa Ana winds and stratospheric intrusions might pose serious O 3 pollution threats across the coast of southern California (22). In this study, groundlevel O 3 mixing ratios during the Santa Ana event increased significantly (Fig.…”

“…Such deep stratospheric intrusion events compromise high-altitude regions of the western United States in attaining US National Ambient Air Quality Standard (NAAQS) for O 3 (6). Additional field-based observation studies indicated that this O 3 -rich stratospheric air can even be transported to lowaltitude regions, such as Los Angeles at the southern California coast (8,22,23). Consequently, this region is a natural laboratory for studying the potential of 35 S as a tracer for stratospheric intrusions.…”

“…3B). These abnormal meteorological conditions are typical signatures of Santa Ana winds, which are highly dry, hot, and strong winds that descend from inland desert regions to the Pacific coastal region in southern California (18,22,(30)(31)(32). These foehn-like katabatic winds result from a strong pressure gradient between a high pressure over the Great Basin and an offshore low pressure.…”

“…Santa Ana wind events are often behind a cold front associated with an upper-level trough (31), which cannot only exacerbate the katabatic winds but also, can lead to the formation of deep tropopause folds and stratospheric intrusions (22). A recent study suggested that the coupling between Santa Ana winds and stratospheric intrusions might pose serious O 3 pollution threats across the coast of southern California (22).…”

Tropospheric ozone seasonal and long-term variability as seen by lidar and surface measurements at the JPL-Table Mountain Facility, California

“…In the case of deep stratospheric intrusions, ozone-rich stratospheric air masses embedded in the lower half of the fold can reach lower altitudes, and occasionally the planetary boundary layer mixing down to the surface (Chung and Dann, 1985;Langford et al, 2012Langford et al, , 2015Lefohn et al, 2012;Lin et al, 2012a), leading to an ozone increase in the lower troposphere (Fig. 14b).…”