Spatial Distribution of Wildfires Ignited under Katabatic versus Non-Katabatic Winds in Mediterranean Southern California USA

Abstract

Wildfires are a major hazard to humans in the southern California Mediterranean ecosystem and improving our understanding and delineation of different fire regimes is critical to mitigating wildfire-related hazards. Recent research has demonstrated that there are two distinct fire regimes in this region based on the presence or absence of katabatic winds (primarily Santa Ana winds) concurrent with the fire. Here, we expand the katabatic wind category to include Sundowner winds along the Santa Barbara front range and analyze the spatial relationships and difference in ignition sources between fires associated with katabatic and non-katabatic wind events from 1948-2017. We found distinct spatial extents for katabatic versus non-katabatic fires, with areas of the higher number of repeat fires generally associated with one fire type or the other. These spatial delineations were consistent with prior analyses of katabatic wind patterns and were also related to the climatology of marine influences across the region. Finally, we contextualize the burn perimeter of the 2017 Thomas Fire, the largest fire in modern California history, relative to spatial patterns of katabatic and non-katabatic fires. The 2017 Thomas Fire began during the longest Santa Ana event in the last 70 years in an area that has been burned repeatedly by Santa Ana fires. However, the Thomas Fire ultimately burned into a region where there were no prior Santa Ana fires. The spatial delineation of two relatively distinct fire regimes is critical to making management decisions, such as where to locate suppression resources at critical times and where fuel treatments might be most effective. However, the anomalous pattern of the Thomas Fire also points to the potential for changes in anthropogenic and environmental factors to disrupt historical spatial patterns and suggests that spatial patterns of fire regimes are themselves prospective metrics of global change.

Winds and Gusts during the Thomas Fire

“…A Santa Ana wind event of exceptional temporal extent, unmatched in 70 years [12], started on 4 December 2017, quickly leading to the Thomas fire. [13] has discussed this episode's synoptic setup and the extreme fire conditions present during this event.…”

Climate Change Suppresses Santa Ana Winds of Southern California and Sharpens Their Seasonality

“…The Santa Ana winds (SAWs; Guzman‐Morales et al, ; Hughes & Hall, ; Raphael, ), rooted in cold air masses over the elevated Great Basin, are notorious for fanning California's largest wildfires. SAW‐fanned wildfires typically rage in the sloping coastal backcountry, with its encroaching wildland‐urban‐interface (WUI), where the gusty downslope SAWs are strongest (Guzman‐Morales et al, ; Moritz et al, ) and ignitions are nearly always human caused (Kolden & Abatzoglou, ; Syphard & Keeley, ). While the local impacts of these wildfires are devastating (e.g., Medina, ), smoke blowing toward the densely populated coastal zone exposes much larger and diverse populations to remote respiratory health impacts (Delfino et al, ).…”

Characterizing peak exposure of secondhand smoke using a real‐time PM 2.5 monitor

“…Risks from tobacco smoke are so significant that cigarette equivalence has long been used to illustrate the hazards of other sources of air pollutants. 15,57,58 Compared with predicting PM 2.5 exposure of active smokers, predicting the level of PM 2.5 obtained from SHS exposure for non-smokers is more challenging due to the variable dilution due to distance between smokers and non-smokers, housing type, and ventilation rates. 46 Questionnaire-based approaches are also challenged by different perceptions of SHS exposure.…”