From Local- to Large-Scale, the Meteorology Associated with Rapid-Growth California Wildfires

Abstract

This dissertation serves to clarify the connection between short periods of meteorology (near surface weather conditions and upper air pressure patterns) and periods of rapid growth in California wildfires at a comprehensive scale not yet seen in the literature—across thousands of wildfires. In doing so, the dissertation also explores how the atmosphere produces periods of fire-relevant weather and considers the ability of a commonly used meteorological dataset to capture that meteorology. Our results indicate that periods of large growth in California wildfires occur when fuels are driest, trailing longer periods of atmospheric dryness. Particularly rapid growth—during which fires grow a great amount from a relatively smaller size in a short period of time—may occur in the presence of strong, dry (usually downslope) winds. The circulation patterns that produce these downslope winds are consistent from wind event to wind event in the lower-troposphere but vary substantially in the mid-troposphere. These circulation patterns can be generated as a result of numerous different antecedent atmospheric pathways (e.g., downslope wind events can follow the buildup of a high pressure ridge or the passage of an upper-level trough) but differences in antecedent drying from different evolutions are found to have little effect on ensuing fires. For describing fire-relevant meteorology, the ERA5—the most heavily used weather dataset in the world—is generally found to be a useable, self-consistent option. However, it struggles to reproduce the magnitude of strong winds observed by surface stations which may limit its ability to be used for other wildfire-related purposes (e.g., modeling fire spread).