Opposite anomalous synoptic patterns for potential California large wildfire spread and extinguishing in 2018 cases

Weihong Qian,Yang Ai,Jin-Yi Yu,Jun Du

doi:10.1016/j.atmosres.2021.105804

Abstract

The consecutive occurrence of three large wildfires in 1 year is rarely seen in northern California including the deadliest case during 8–25 November 2018 in Butte County. They happened under favorable fuel and meteorological conditions including long-term warming trends, normal summer-autumn warm-dry climate, and extreme heated air masses with strong offshore wind. The former two meteorological conditions contribute a background for large wildfires but extreme heated air masses and strong offshore winds are critical potentials for a fire spread. The relationship between extreme heated/cooled air masses and the three large wildfires was carefully examined from their starts to extinguishing. An anomaly-based synoptic analysis method is used by separating atmospheric variables into climatology and anomalies. A 3-dimensional anomalous synoptic pattern of atmospheric variables is established for the three wildfire cases in 2018. The analysis showed that one anomalous warm-air mass in the mid-to-low troposphere dynamically associated with a positive center of geopotential height (GPH) anomalies at the upper troposphere (150–300 hPa) is an anomalous synoptic pattern indicating a potential large wildfire spread, and conversely by negative anomalies when the fires were extinguished. The ECMWF model seems to be capable of predicting such anomalous temperature-pressure patterns to indicate possible fire spread and extinguishing.

Highlights

Wildfires can threaten native ecosystems and human lives (Rager et al, 2021; Santos et al, 2021; Xiang et al, 2021)
In northern California, North Coast and the Sierra Nevada are largely covered by forests, where fuel condition is favorable for large wildfires under suitable meteoro logical conditions
To identify anomalous synoptic patterns related to California large wildfires, four datasets are used in this study

Summary

Introduction

Wildfires can threaten native ecosystems and human lives (Rager et al, 2021; Santos et al, 2021; Xiang et al, 2021). Atmospheric Research 262 (2021) 105804 in recent decades due to the increased frequency and size of large wildfires (Abatzoglou and Williams, 2016; Balch et al, 2018; Dennison et al, 2014) This long-term trend of increasing burned area has reached a top stage in 2017 and 2018 by extreme wildfire activity with substantial loss of life and property. Except for the long-term global-warming induced increase trend and the seasonal cycle of a high frequency of large wildfires occurred in summer and autumn, large wildfires could happen in an off-peak season when short-term individual weather extreme (such as heated air mass and high offshore winds) occurs. The ECMWF model products are used to demonstrate if current numerical weather prediction (NWP) models have the ability to predict the 3-dimensional anomalous atmospheric structure revealed by this study related to wildfire spread and extinguishing

Datasets

Anomaly-based analysis approach

Three large wildfires in 2018

General evolutions of vertical anomalous variables

Summary and discussions