Fire-weather interaction fed the 2020 western USA gigafire

Wildfires threaten human lives, destroy infrastructure, disrupt economic activity, and damage ecosystem services. A record-breaking gigafire event ravaged the western United States (USA) in mid-September 2020, burning 1.2 million acres (4,900 km2) in Oregon and California, and resulting in severe smoke pollution with daily fine particulate matter (PM2.5) concentrations over 300 µg/m3 for multiple days in many cities. Although previous studies have shown that regional warming escalates wildfire in the western USA, such an unprecedented fire cannot be explained by climate variability alone. Here we show that the synoptic-scale feedback between the wildfires and weather played an unexpectedly important role in accelerating the spread of this fire and also trapped pollutants in the shallow boundary layer over valley cities. Specifically, we find that aerosol-radiation interaction of the smoke plumes over the Cascade Mountains enhanced the downslope winds and weakened the moisture transport, thereby forming a positive feedback loop that amplified the fires and contributed to ~54% of estimated air-pollution related deaths. Our study underscores the complexity of the Earth system and the importance of understanding fundamental mechanisms to effectively mitigate disaster risks in a changing climate.

Episode with strong downslope wind in the Southern and Curvature Carpathians in the period 5-6 february 2020

The phenomenon that occurred during the blizzard from February 5-6 in the mountains and especially on the southern slopes of the Southern Carpathians, is known in the literature as "strong downslope winds". This phenomenon occurred in a typical blizzard configuration, in which the differentiated advection of temperature led to the formation of a very stable air layer, with thermal inversion approximately between the levels of 850 and 700 hPa; and it also contributed in this layer to the change of wind direction to vertical. Thus, the existence in the same air layer of two factors favorable to the formation of a critical level, created the ideal conditions for generating strong downslope winds.

Episode with strong downslope wind in the Southern and Curvature Carpathians in the period 5-6 February 2020

The phenomenon that occurred during the blizzard from February 5-6 in the mountains and especially on the southern slopes of the Southern Carpathians, is known in the literature as "strong downslope winds". This phenomenon occurred in a typical blizzard configuration, in which the differentiated advection of temperature led to the formation of a very stable air layer, with thermal inversion approximately between the levels of 850 and 700 hPa; and it also contributed in this layer to the change of wind direction to vertical. Thus, the existence in the same air layer of two factors favorable to the formation of a critical level, created the ideal conditions for generating strong downslope winds.

Air-traffic restrictions at the Madeira International Airport due to adverse winds: links to synoptic-scale patterns and mountain wave phenomena

<p>The Madeira International Airport (MIA) lies on the island south-eastern coast and it is known to be exposed to wind hazards. A link between these adverse winds at MIA and the synoptic-scale circulation is established using a weather type (WT) classification. From April to September (summer period), five WTs prevail, cumulatively representing nearly 70% of days. These WTs reflect the presence of well-established Azores high, with some variations on location and strength. Although with a low frequency of occurrence (<5%), this anticyclone occasionally strengthens and extends towards Iberia, inducing anomalously strong NNE/NE up to 3-5 km over Madeira. The most severe and longer-lasting wind conditions at the MIA, with a higher frequency of gusts above 35 kt, are driven by this synoptic-scale pattern and are more common in summer. An episode of adverse winds at the MIA is analyzed, illustrating the occurrence of upstream stagnation, flow splitting, and lee wake formation. The upstream conditions include a low-level inversion, strong NNE/NE winds near and above the inversion and a Froude number less than 1. AROME model predicted the occurrence of downslope winds, in association with a large-amplitude mountain wave. At this time, the strongest wind gusts were registered and a missed approach occurred. The wind regime in different places of the island suggests that these conditions are relatively frequent, mostly in summer. Lastly, this study provides an objective verification of the AROME wind forecasts, for a 3-year period and from June to August.</p>

Air-Traffic Restrictions at the Madeira International Airport Due to Adverse Winds: Links to Synoptic-Scale Patterns and Orographic Effects

The Madeira International Airport (MIA) lies on the island’s south-eastern coast and it is known to be exposed to wind hazards. A link between these adverse winds at MIA and the synoptic-scale circulation is established using a weather type (WT) classification. From April to September (summer period), five WTs prevail, cumulatively representing nearly 70% of days. These WTs reflect the presence of well-established Azores high, with some variations on location and strength. Although with a low frequency of occurrence (<5%), this anticyclone occasionally strengthens and extends towards Iberia, inducing anomalously strong NNE/NE up to 3–5 km over Madeira. The most severe and longer-lasting wind conditions at the MIA, with a higher frequency of gusts above 35 kt, are driven by this synoptic-scale pattern and are more common in summer. An episode of adverse winds at the MIA is analysed, illustrating the occurrence of upstream stagnation, flow splitting, and lee wake formation. The upstream conditions include a low-level inversion, strong NNE/NE winds near and above the inversion and a Froude number less than 1. The AROME (Application of Research to Operations at Mesoscale) model predicted the occurrence of downslope winds, in association with a large-amplitude mountain wave. At this time, the strongest wind gusts were registered and one aircraft executed a missed approach. The wind regime in different places of the island suggests that these conditions are relatively frequent, mostly in summer. Finally, objective verification of AROME wind forecast, for a three-year period and from June to August, is discussed.

Fire and Rain: The Legacy of Hurricane Lane in Hawaiʻi

Hurricane Lane (2018) was an impactful event for the Hawaiian Islands and provided a textbook example of the compounding hazards that can be produced from a single storm. Over a 4-day period, the island of Hawaiʻi received an island-wide average of 424 mm (17 in.) of rainfall, with a 4-day single-station maximum of 1,444 mm (57 in.), making Hurricane Lane the wettest tropical cyclone ever recorded in Hawaiʻi (based on all available quantitative records). Simultaneously, fires on the islands of nearby Maui and Oʻahu burned 1,043 ha (2,577 ac) and 162 ha (400 ac), respectively. Land-use characteristics and antecedent moisture conditions exacerbated fire hazard, and both fire and rain severity were influenced by the storm environment and local topographical features. Broadscale subsidence around the storm periphery and downslope winds resulted in dry and windy conditions conducive to fire, while in a different region of the same storm, preexisting convection, incredibly moist atmospheric conditions, and upslope flow brought intense, long-duration rainfall. The simultaneous occurrence of rain-driven flooding and landslides, high-intensity winds, and multiple fires complicated emergency response. The compounding nature of the hazards produced during the Hurricane Lane event highlights the need to improve anticipation of complex feedback mechanisms among climate- and weather-related phenomena.

Mountain Waves Produced by a Stratified Boundary Layer Flow. Part I: Hydrostatic Case

A hydrostatic theory for mountain waves with a boundary layer of constant eddy viscosity is presented. It predicts that dissipation impacts the dynamics over an inner layer whose depth is controlled by the inner-layer scale δ of viscous critical-level theory. The theory applies when the mountain height is smaller or near δ and is validated with a fully nonlinear model. In this case the pressure drag and the wave Reynolds stress can be predicted by inviscid theory, if one takes for the incident wind its value around the inner-layer scale. In contrast with the inviscid theory and for small mountains the wave drag is compensated by an acceleration of the flow in the inner layer rather than of the solid earth. Still for small mountains and when stability increases, the emitted waves have smaller vertical scale and are more dissipated when traveling through the inner layer: a fraction of the wave drag is deposited around the top of the inner layer before reaching the outer regions. When the mountain height becomes comparable to the inner-layer scale, nonseparated upstream blocking and downslope winds develop. Theory and the model show that (i) the downslope winds penetrate well into the inner layer and (ii) upstream blocking and downslope winds are favored when the static stability is strong and (iii) are not associated with upper-level wave breaking.

Uncertainties in forecasting orographic flows

&lt;p&gt;The accuracy of a large set of high-resolution wind speed forecasts with different lead times is assessed for different parts of orographic flows, including upstream blockings, gap winds, corner winds, wakes and downslope winds.&amp;#160; The by far largest errors are in areas of downslope windstorms, but there are also considerable errors in the other parts of orographic disturbances to the flow and they are greater than in non-orographic flows in the same region.&amp;#160; The errors are discussed in view of the different dynamics and kinematics of the flows.&amp;#160; They are partly related to intermittency of i.e. gravity waves as well as strong spatial gradients in the wind field.&lt;/p&gt;

Observed Near-Storm Environment Variations across the Southern Cumberland Plateau System in Northeastern Alabama

The Sand Mountain and Lookout Mountain Plateaus in northeastern Alabama have been established as a regional relative maximum in tornadogenesis reports within the southeastern United States. Investigation of long-term surface datasets has revealed (i) stronger and more backed winds atop Sand Mountain than over the Tennessee Valley, and (ii) measured cloud-base heights are lower to the surface atop Sand Mountain than over the Tennessee Valley. These observations suggest that low-level wind shear and lifting condensation level (LCL) height changes may lead to conditions more favorable for tornadogenesis atop the plateaus than over the Tennessee Valley. However, prior to fall 2016, no intensive observations had been made to further investigate low-level flow or thermodynamic changes in the topography of northeastern Alabama. This paper provides detailed analysis of observations gathered during VORTEX-SE field campaign cases from fall 2016 through spring 2019. These observations indicate that downslope winds form along the northwest edge of Sand Mountain in at least some severe storm environments in northeastern Alabama. Wind profiles gathered across northeastern Alabama indicate that low-level helicity changes can be substantial over small distances across different areas of the topographic system. LCL height changes often scale to changes in land elevation, which can be on the order of 200–300 m across northeastern Alabama.