Cloud‐to‐ground lightning with minimal rainfall (“dry” lightning) is a major wildfire ignition source in the western United States (WUS). Although dry lightning is commonly defined as occurring with ...<2.5 mm of daily‐accumulated precipitation, a rigorous quantification of precipitation amounts concurrent with lightning‐ignited wildfires (LIWs) is lacking. We combine wildfire, lightning and precipitation data sets to quantify these ignition precipitation amounts across ecoprovinces of the WUS. The median precipitation for all LIWs is 2.8 mm but varies with vegetation and fire characteristics. “Holdover” fires not detected until 2–5 days following ignition occur with significantly higher precipitation (5.1 mm) compared to fires detected promptly after ignition (2.5 mm), and with cooler and wetter environmental conditions. Further, there is substantial variation in precipitation associated with promptly‐detected (1.7–4.6 mm) and holdover (3.0–7.7 mm) fires across ecoprovinces. Consequently, the widely‐used 2.5 mm threshold does not fully capture lightning ignition risk and incorporating ecoprovince‐specific precipitation amounts would better inform WUS wildfire prediction and management.
Plain Language Summary
Cloud‐to‐ground lightning with minimal rainfall, also known as “dry lightning,” is a major wildfire ignition source in the western United States (WUS). Typically, daily‐accumulated precipitation of less than 2.5 mm is used to identify dry lightning occurrence. However, there is limited knowledge of (a) the true precipitation amounts that occur with lightning‐ignited wildfires (LIWs), and (b) how these amounts vary across different landscapes and vegetation types. We combine wildfire, lightning and precipitation data sets to quantify these ignition precipitation amounts across different regions of the WUS. Although we find a 2.8 mm median ignition precipitation for all LIWs, we show that “holdover” fires not detected until 2–5 days following ignition occur with significantly higher precipitation (5.1 mm) compared to fires detected promptly after ignition (2.5 mm). Holdover fires also occur with cooler and wetter environmental conditions. Further, ignition precipitation amounts associated with promptly‐detected and holdover fires vary substantially across ecoprovinces. Consequently, the widely‐used 2.5 mm threshold does not fully capture lightning ignition risk. WUS wildfire prediction and management could be improved through incorporating ecoprovince‐specific precipitation amounts and accounting for differing characteristics of holdover fires.
Key Points
The widely‐used 2.5 mm daily precipitation threshold to define “dry” lightning does not fully capture fire ignition risk across the WUS
Ignition precipitation amounts range from 1.7 to 7.7 mm depending on ecoprovince and whether the fire was promptly‐detected or a holdover
Holdover fires occur when conditions are cooler and wetter compared to promptly‐detected fires
Fire-Generated Tornadic Vortices Lareau, Neil P.; Nauslar, Nicholas J.; Bentley, Evan ...
Bulletin of the American Meteorological Society,
05/2022, Letnik:
103, Številka:
5
Journal Article
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Abstract Fire-generated tornadic vortices (FGTVs) linked to deep pyroconvection, including pyrocumulonimbi (pyroCbs), are a potentially deadly, yet poorly understood, wildfire hazard. In this study ...we use radar and satellite observations to examine three FGTV cases during high-impact wildfires during the 2020 fire season in California. We establish that these FGTVs each exhibit tornado-strength anticyclonic rotation, with rotational velocity as strong as 30 m s −1 (60 kt), vortex depths of up to 4.9 km AGL, and pyroCb plume tops as high as 16 km MSL. These data suggest similarities to EF2+ strength tornadoes. Volumetric renderings of vortex and plume morphology reveal two types of vortices: embedded vortices anchored to the fire and residing within high-reflectivity convective columns and shedding vortices that detach from the fire and move downstream. Time-averaged radar data further show that each case exhibits fire-generated mesoscale flow perturbations characterized by flow splitting around the fire’s updraft and pronounced flow reversal in the updraft’s lee. All the FGTVs occur during deep pyroconvection, including pyroCb, suggesting an important role of both fire and cloud processes. The commonalities in plume and vortex morphology provide the basis for a conceptual model describing when, where, and why these FGTVs form.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Global climatology of synoptically‐forced downslope winds Abatzoglou, John T.; Hatchett, Benjamin J.; Fox‐Hughes, Paul ...
International journal of climatology,
January 2021, 2021-01-00, 20210101, Letnik:
41, Številka:
1
Journal Article
Recenzirano
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Downslope winds are mesoscale mountain meteorological phenomena that contribute to localized temperature extremes and contribute to numerous societal and environmental impacts. Whereas previous ...studies have examined local downslope winds, no known efforts have attempted to identify and characterize meso‐ to synoptic‐scale downslope winds globally using a common approach. We use a conceptual model for downslope winds that employs cross‐barrier wind speed, near‐mountain top static stability, and downward vertical velocity using thresholds guided by a chronology of local downslope winds and meta‐analysis of downslope wind case studies. This approach was applied to ERA‐5 reanalysis during 1979–2018 to develop a global atlas of downslope winds. Downslope winds adhered to distinct geographic and seasonal patterns, with peak occurrence in north–south oriented midlatitude mountains in the winter hemisphere associated with strong cross‐mountain winds and stability. However, we identify numerous locations from the tropics to the high‐latitudes where downslope winds occur at least 60 days a year as a byproduct of the general circulation and local‐scale circulation interacting with topography. The four‐decade‐long data set is also used to examine statistical relationships between the occurrence of downslope winds and El Niño‐Southern Oscillation as well as long‐term trends in downslope wind occurrence.
Downslope winds can result in numerous impacts to human and natural systems. Understanding where and when these winds occur is of importance in to understand synoptic drivers and to better inform hazards posed by such winds. This study provides a first‐known effort to develop a climatology downslope winds globally during 1979–2018.
The increasing complexity and impacts of fire seasons in the United States have prompted efforts to improve early warning systems for wildland fire management. Outlooks of potential fire activity at ...lead‐times of several weeks can help in wildland fire resource allocation as well as complement short‐term meteorological forecasts for ongoing fire events. Here, we describe an experimental system for developing downscaled ensemble‐based subseasonal forecasts for the contiguous US using NCEP's operational Climate Forecast System version 2 model. These forecasts are used to calculate forecasted fire danger indices from the United States (US) National Fire Danger Rating System in addition to forecasts of evaporative demand. We further illustrate the skill of subseasonal forecasts on weekly timescales using hindcasts from 2011 to 2021. Results show that while forecast skill degrades with time, statistically significant week 3 correlative skill was found for 76% and 30% of the contiguous US for Energy Release Component and evaporative demand, respectively. These results highlight the potential value of experimental subseasonal forecasts in complementing existing information streams in weekly‐to‐monthly fire business decision making for suppression‐based decisions and geographic reallocation of resources during the fire season, as well for proactive fire management actions outside of the core fire season.
Fire impacts in the western United States have garnered the need for improved intelligence to inform local‐to‐regional mobilization of fire‐suppression resources. We show widespread forecast skill into week 3 for certain fire danger metrics across the United States highlighting the potential for subseasonal forecasts to complement existing early warning systems for wildland fire management.
Santa Ana winds (SAW) are among the most notorious fire-weather conditions in the United States and are implicated in wildfire and wind hazards in Southern California. This study employs large-scale ...reanalysis data to diagnose SAW through synoptic-scale dynamic and thermodynamic factors using mean sea level pressure gradient and lower-tropospheric temperature advection, respectively. A two-parameter threshold model of these factors exhibits skill in identifying surface-based characteristics of SAW featuring strong offshore winds and extreme fire weather as viewed through the Fosberg fire weather index across Remote Automated Weather Stations in southwestern California. These results suggest that a strong northeastward gradient in mean sea level pressure aligned with strong cold-air advection in the lower troposphere provide a simple, yet effective, means of diagnosing SAW from synoptic-scale reanalysis. This objective method may be useful for medium- to extended-range forecasting when mesoscale model output may not be available, as well as being readily applied retrospectively to better understand connections between SAW and wildfires in Southern California.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Downslope Sundowner winds in southern California's
Santa Ynez Mountains favor wildfire growth. To explore differences between
Sundowners and Santa Ana winds (SAWs), we use surface observations ...from 1979 to 2014
to develop a climatology of extreme Sundowner days. The
climatology was compared to an existing SAW index from 1979 to 2012.
Sundowner (SAW) occurrence peaks in late spring (winter). SAWs demonstrate
amplified 500 hPa geopotential heights over western North America and
anomalous positive inland mean sea-level pressures. Sundowner-only
conditions display zonal 500 hPa flow and negative inland sea-level pressure
anomalies. A low-level northerly coastal jet is present during Sundowners
but not SAWs.
The North American monsoon (NAM) is an annual climate system phenomenon that develops over the Sierra Madre Occidental in western Mexico and spreads northwards into the southwest United States from ...June through September bringing large quantities of rainfall and lightning, which can vary greatly on intra‐ and inter‐annual timescales. The timing of the NAM onset can lengthen or shorten the wildfire season in the southwest United States. Here we determine NAM onset thresholds and subsequent dates for the Southwest Area (SWA; Arizona, New Mexico, west Texas, and Oklahoma panhandle) and each SWA Predictive Services Area (PSA) April through September from 1995–2013. Various wildfire activity thresholds were defined to capture days or events associated with increased wildfire activity that are considered “busy” by wildland fire management in the context of an impact on firefighting resources. These defined thresholds allow for a unique examination of the relationship between the NAM and wildfire. Self‐organizing maps (SOMs), utilizing 500‐hPa geopotential heights and precipitable water, were implemented to identify atmospheric patterns contributing to the NAM onset and “busy” days for the SWA and each PSA. Map types from the SOMs analysis showed the transition to, during, and from the NAM. Northwards and eastwards displacements of the subtropical ridge over the SWA were associated with NAM onset. Restructuring of the subtropical ridge in time (i.e., amplification or breakdown) as inferred from map types over the SWA was directly associated with increased wildfire activity. By identifying atmospheric patterns pertinent to busy days of wildfire activity and a wildfire‐based perspective of the NAM in the SWA, fire weather forecasters can proactively identify potential periods that may be particularly impactful on wildfire suppression resources.
Wildfire activity in the southwest United States is inextricably linked with the timing and strength of the North American monsoon (NAM). Using synoptic classification techniques, certain atmospheric patterns were associated with significant wildfire episodes and the NAM onset in the southwest United States. Identifying atmospheric patterns pertinent to significant wildfire episodes in the SWA would aid operational fire meteorologists in better prediction of significant wildfire episodes and provide lead time to fire managers more efficiently allocate firefighting resources.
Analysis was performed to determine whether a lightning flash could be associated with every reported lightning-initiated wildfire that grew to at least 4 km(exp 2). In total, 905 lightning-initiated ...wildfires within the Continental United States (CONUS) between 2012 and 2015 were analyzed. Fixed and fire radius search methods showed that 81–88% of wildfires had a corresponding lightning flash within a 14 day period prior to the report date. The two methods showed that 52–60% of lightning-initiated wildfires were reported on the same day as the closest lightning flash. The fire radius method indicated the most promising spatial results, where the median distance between the closest lightning and the wildfire start location was 0.83 km, followed by a 75th percentile of 1.6 km and a 95th percentile of 5.86 km. Ninety percent of the closest lightning flashes to wildfires were negative polarity. Maximum flash densities were less than 0.41 flashes km(exp 2) for the 24 h period at the fire start location. The majority of lightning-initiated holdover events were observed in the Western CONUS, with a peak density in north-central Idaho. A twelve day holdover event in New Mexico was also discussed, outlining the opportunities and limitations of using lightning data to characterize wildfires.
We implemented a fire risk assessment framework that combines spatially-explicit burn probabilities, post-fire mortality models and public auction timber prices, to estimate expected economic losses ...from wildfires in 155 black pine stands covering about 450ha in the Juslapeña Valley of central Navarra, northern Spain. A logit fire occurrence model was generated from observed historic fires to provide required fire ignition input data. Wildfire likelihood and intensity were estimated by modeling 50,000 fires with the minimum travel time algorithm (MTT) at 30m resolution under 97th percentile fire weather conditions. Post-fire tree mortality due to burning fire intensity at different successional stages ranged from 0.67% in the latest stages to 9.22% in the earliest. Stands showed a wide range of potential economic losses, and intermediate successional stage stands presented the highest values, with about 124€ha−1 on average. A fire risk map of the target areas was provided for forest management and risk mitigation purposes at the individual stand level. The approach proposed in this work has a wide potential for decision support, policy making and risk mitigation in southern European commercial conifer forests where large wildfires are the main natural hazard.
•We quantify economic losses from wildfires in black pine afforestation.•A fire modeling approach accounting for historic ignition probability and extreme fire weather was used to assess wildfire exposure at fine resolution.•We used successional stage specific response functions and timber prices.•Wildfire exposure and potential losses showed stand specific complex spatial patterns.•Results can help policy making and managing wildfire risk at individual stand level.
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