Short Warm Distribution Tails Accelerate the Increase of Humid-Heat Extremes Under Global Warming

Humid-heat extremes threaten human health and are increasing in frequency with global warming, so elucidating factors affecting their rate of change is critical. We investigate the role of wet-bulb temperature (TW) frequency distribution tail shape on the rate of increase in extreme TW threshold exceedances under 2°C global warming. Results indicate that non-Gaussian TW distribution tails are common worldwide across extensive, spatially coherent regions. More rapid increases in the number of days exceeding the historical 95th percentile are projected in locations with shorter-than-Gaussian warm side tails. Asymmetry in the specific humidity distribution, one component of TW, is more closely correlated with TW tail shape than temperature, suggesting that humidity climatology strongly influences the rate of future changes in TW extremes. Short non-Gaussian TW warm tails have notable implications for dangerous humid-heat in regions where current-climate TW extremes approach human safety limits

Lightning-Ignited Wildfires in the Western United States: Ignition Precipitation and Associated Environmental Conditions

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 \u3c2.5 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

Short Warm-Side Wet-Bulb Temperature Distribution Tails Lead to Accelerated Increases in Extreme Threshold Exceedances Under Global Warming

Humid-heat extremes threaten human health and are increasing in frequency with global warming, so elucidating factors affecting their rate of change is critical. This thesis examines the role of historical (1985-2014) wet-bulb temperature distribution tail shape on the probability of wet-bulb temperature extreme threshold exceedances under 2°Celsius global warming. Analysis of global climate models and reanalysis reveals that non-Gaussian wet-bulb temperature distribution tails are common worldwide across extensive, spatially coherent regions. More rapid increases in the number of days exceeding the historical 95th percentile are projected in locations with shorter-than-Gaussian warm-side tails. Of the two primary components of wet-bulb temperature, specific humidity and temperature, specific humidity tail shape is much more closely correlated with wet-bulb temperature tail shape and future exceedances. This suggests that humidity tail shape is more influential on the rate of future changes in wet-bulb temperature extreme exceedances than temperature tail shape. Short non-Gaussian wet-bulb temperature warm tails have notable implications for dangerous humid-heat stress in regions where current-climate wet-bulb temperature extremes approach human safety limits