Climatology of diablo winds in Northern California and their relationships with large-scale climate variabilities

Abstract

The most destructive wildfires recently in Northern California have been linked to the occurrences of Diablo Winds (DWs). This study investigates the climatology of DWs during September–December 1979–2018, and their relationships with various climate modes using observations and two high-resolution reanalysis datasets. Our finding shows that DWs do not have a long-term trend in terms of the annual total number, total duration, and associated maximum wind speeds of DWs over the past 4 decades. However, their associated minimum relative humidity (RH) has decreased significantly, especially in October, which suggests that the dryness during DWs has become more severe with time, possibly leading to an increased chance of fires, and their destructive potential. We also find that the annual total number and duration of DWs have exhibited an quasi-periodic variation, with intervals ranging from 2 to 4 years. The periodic variability of DWs might be attributed to the teleconnections between DWs and climate oscillations, specifically the El Niño–Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO), through their modulation of pressure systems near California and the location of the Pacific jet stream. It is suggested that when La Niña and the QBO westerly phases co-occur in the spring, DWs in the following fall and winter tend to occur more frequently, and are associated with more intense high winds and dryness as compared to other springtime QBO and ENSO phases. This relationship may be used to predict the seasonal outlook of DWs.

Increasing Wind-Driven Wildfire Risk Across California's Sierra Nevada Mountains

“…Hazardous Wind Events were defined as strong, dry, northeasterly winds lasting at least 6 h, where 'strong' is defined as a wind speed above its grid point 75th percentile wind speed and 'dry' is defined as a relative humidity below its grid point 25th percentile relative humidity. HWE trends were largely negligible over northern California during both JJA and SON (Figures 3a,b), consistent with Liu et al (2020), who investigated Diablo wind trends in the ERA5 under similar criteria. However, significant increasing trends were found on the western slopes of the Sierra Nevada Mountains and have occurred more frequently since the early 2000s (Figure 3c), with the autumn of 2018 standing out as a particularly above-average season which also saw the Camp Fire (2018).…”

“…Using percentilebased criteria for winds and humidity permits 'strong' winds to be defined relative to what the reanalysis can represent and dry conditions to be defined relative to the local climate. Similar criteria have been used to characterize Diablo and Santa Ana winds (Guzman-Morales & Gershunov, 2019;Liu et al, 2020) and, for the criteria used here, monthly averages of the number of HWE within California over 1979-2019 produced the expected seasonal cycle with a HWE peak over autumn and winter (Figure S1). Additionally, time series for regions showing significant HWE trends were calculated by averaging the number of HWE within each region for a given season during 1979-2019.…”

“…Despite the urgency to better understand changes in the wind landscape and winddriven fire events, however, such studies are relatively sparse and a state-wide picture is yet to emerge. In one study, Liu et al (2020) found no discernible trend in either Diablo maximum wind speeds or frequency in the ERA5 reanalysis over the San Francisco Bay. For Southern California, Guzman-Morales et al ( 2016) described a modest increase in Santa Ana wind intensity during 1948-2012, while Rolinski et al (2019) reported a marked increase in Santa Ana wind frequency after 2007 in a downscaling of the North American Regional Reanalysis (NARR).…”

Prescribed burns as a tool to mitigate future wildfire smoke exposure: Lessons for states and rural environmental justice communities

“…This large disparity in potential smoke exposure is again driven by the greater stagnation occurring later in the fire season. In October and November, average horizontal windspeeds in NorCal are lower than in previous months (Figure S1), which may be due to weakening of the "Diablo Winds" in the CVCal region (Liu et al, 2021). The mean West adjoint sensitivities in the 2020 fire season peak in October (3517 mg m -3 /g m -2 s -1 ) rather than September (3339 mg m -3 /g m -2 s -1 ) or the other months (July: 2489, August: 2894, November: 3348 mg m -3 /g m -2 s -1 ).…”

Toward a Better Understanding of Wildfire Behavior in the Wildland-Urban Interface: A Case Study of the 2021 Marshall Fire

“…One commonality between these wildfires is that they were considered winddriven fires, allowing them to expand rapidly and wreak havoc on communities in their paths. In California, so-called "Santa Ana" (e.g., Randles et al, 2003, and references therein) or "Diablo" (e.g., Liu et al, 2021) wind events are often associated with destructive winddriven wildfires (e.g., Nauslar et al, 2018;Smith et al, 2018). Wind-driven WUI fires are also a concern in regions outside of the U.S., including Australia (e.g., Cruz et al, 2012), France (e.g., Ganteaume, 2020), and Greece (e.g., Efthimiou et al, 2020).…”