Meteorological Environments Associated With California Wildfires and Their Potential Roles in Wildfire Changes During 1984–2017

Abstract

California has experienced more wildfires in recent years, resulting in huge economic losses and threatening human health. Clarifying the meteorological environments of wildfires is foundational to improving the understanding and prediction of wildfires and their impacts. Here, 1,535 California wildfires during 1984–2017 are systematically investigated. Based on two key meteorological factors—temperature and moisture anomalies—all wildfires are classified into four groups: hot‐dry, hot‐wet, cold‐dry, cold‐wet. Most (∼60%) wildfires occurred on hot‐dry days. Compositing the meteorological environments of the four groups shows that persistent high pressure and strong northeasterly wind descending from inland favor hot‐dry conditions for wildfires. Self‐organizing map analysis lends confidence in the large‐scale meteorological pattern dominating hot‐dry wildfires in California. Meteorological anomalies also influence wildfire size through their magnitudes, with moisture anomaly explaining the largest fraction (∼69%) of variability in wildfire sizes. In addition, 12.2% of wildfires occurred on hot‐wet days, which may be related with lightning flashes. More lightning tends to trigger wildfires, but the wet condition helps to suppress the wildfire sizes. Total burned area by wildfires has significantly increased by ∼3.6% per year, indicating a doubling of burned area in 2017 relative to 1984, mainly dominated by hot‐dry wildfires in summer. Drying and warming in conjunction with strengthening of the high pressure in summer and fall have the potential to support more frequent and larger hot‐dry wildfires in California during the past several decades.

The Character and Changing Frequency of Extreme California Fire Weather

“…A summary of this analysis and the selected variable combinations that resulted in optimal skill in each region is shown in Figure 3. The variables that result in the highest predictive skill are mostly related to low‐level moisture and temperature patterns, which agrees well with previous studies that identified hot‐dry conditions as most favorable for wildfires in California (Dong et al., 2021). Additionally, 500 hPa geopotential height is also a good predictor in many regions.…”

Identifying Key Drivers of Wildfires in the Contiguous US Using Machine Learning and Game Theory Interpretation

“…For the NCA region, SVD1 features low RH, high temperature, high pressure, and northeasterly winds over the NCA region while SVD2 is characterized by low RH and northwesterly winds inland (Figures S1 and S2). The two SVD modes capture a semi‐persistent synoptic pattern conductive to wildfires in NCA, consistent with prior studies (Dong et al., 2021; Zhong et al., 2020). Time series of monthly standard deviation of the two SVDs and the monthly burned area over NCA show moderate negative correlations, indicating that large burned area is associated with the more stagnant or less stormy synoptic weather identified by the SVD analysis.…”

Increasing co-occurrence of fine particulate matter and ground-level ozone extremes in the western United States

“…4) suggest an increased likelihood of the type of atmospheric conditions that contributed to the August 2020 co-occurrence episode, if these trends continue. While recent studies have shown an intensification of western US summer ridging since the 1980s using atmospheric reanalysis (58) and tree-ring records (59), identification of trends in ridging frequency and persistence over the western US before the present analysis had been restricted to other seasons (60)(61)(62). Our findings of changing late-summer atmospheric patterns agree with recent studies that have highlighted the role of increasingly warmer and drier summer seasons, which are strongly favored by atmospheric ridging, across the western US in driving increased wildfire burned area extent and severity (38,39).…”