Appendix B. Southern California Santa Ana foehn wind characteristics and their relationship to fires in the region.
These foehn winds are a synoptic weather condition developing from a high pressure cell in the Great Basin coupled with a low pressure trough off the Pacific Coast. They are an annual occurrence in spring and autumn, but the latter season follows an ~ 6 month drought and results in the most extreme fire weather of any region in the country (Schroeder et al. 1964). They are often more severe in the Santa Ana Mountains than in other parts of southern California (Appendix B-1), with wind speeds over 160 kph (44 m/s) recorded there during some events (Froke 1993, p. 19).
Commonly Santa Ana wind driven fires are pushed by an intense off-shore flow for only one to several days, however, these fires typically continue burning under the milder on-shore flow for a week or more. It has been observed that this on-shore flow following Santa Ana wind events is significantly stronger than the normal onshore breezes (Mike Rohde, personal communication, Nov 2007). The reasons for stronger onshore flow are not known but may be tied to the substantial drop in ocean surface temperature offshore during Santa Ana wind events (Hu and Liu 2003, Trasviña et al. 2003). Alternatively, the perception of this more intense onshore flow could be due to the fact that it is pushing the very dry Santa Ana air masses back on shore (Dave Sapsis, Cal Fire, personal communication 10 April 2008).
FIG. B1. Streamline map for the most frequently observed wind at each station in the southern California region, Averaged across 149 Santa Ana wind days between 1956–1962, wind speed in mph (Edinger et al. 1964). Values are in mph. |
FIG. B2. Percent frequency of wind direction for 0900 hr; these patterns are typical for most of the morning, late afternoon and night; 2% per scale division, based on average of 149 days (Edinger et al. 1964). |
FIG. B3. Percent frequency of wind direction for 1300 hr, illustrating mid-day onshore effect; 2% per scale division patterns are typical for most of the morning, late afternoon and night; based on average of 149 days (Edinger et al. 1964). |
LITERATURE CITED
Edinger, J. G., R. A. Helvey, and D. Baumhefner. 1964. Surface wind patterns in the Los Angeles Basin during " Santa Ana " conditions. Part I of Final Report on Research Project No. 2606. Supplement No. 49, USFS-UC Contract No. A5fs-16563.
Froke, J. B. 1993. Evolution of a settled landscape: A biogeographical study of Rancho Mission Viejo, California. Ph.D. dissertation. University of California, Los Angeles, California, USA.
Hu, H., and W. T. Liu. 2003. Oceanic thermal and biological responses to Santa Ana winds. Geophysical Research Letters 30:50.51 - 50.54; doi:10.1029/2003GL017208.
Schroeder, M. J., M. Glovinski, and V. F. Hendricks. 1964. Synoptic weather types associated with critical fire weather. U.S. Department of Commerce, National Bureau of Standards, Institute for Applied Technology, AD 449-630, Washington, D.C., USA.
Trasviña, A., M. Ortiz-Figueroa, H. Herrera, M. a. Cosio, and E. Gonzalez. 2003. 'Santa Ana' winds and upwelling filaments off northern Baja California. Dynamics of Atmospheres and Oceans 37:113–129.