04 Jan 2024
 | 04 Jan 2024

Satellite Observations Reveal Northern California Wildfire Aerosols Reduce Cloud Cover in California and Nevada Through Semi-Direct Effects

James L. Gomez, Robert J. Allen, and King-Fai Li

Abstract. Wildfires in the southwestern United States, particularly in northern California (nCA), have grown in size and severity in the past decade. As they have grown larger, they have been associated with large emissions of absorbing aerosols in to the troposphere. Utilizing satellite observations from MODIS, CERES, AIRS, and CALIPSO, the meteorological effects of aerosols associated with fires during the wildfire season (June–October) were discerned over the nCA-NV (northern California and Nevada) region in the 2003–2022 time frame. As higher temperatures and low relative humidity RH dominate during high surface pressure ps atmospheric conditions, the effects of the aerosols on high (90th percentile) fire days compared to low fire (10th percentile) days were stratified based on whether ps was anomalously high or anomalously low (10th percentile). An increase in tropospheric temperatures was found to be concurrent with more absorbing aerosol aloft, which is associated with significant reductions in tropospheric RH during both 90th and 10th percentile ps conditions. Furthermore, high fire days under low ps conditions are associated with reduced cloud fraction CF, which is consistent with the traditionally-defined aerosol- cloud semi-direct effect. The reduced CF, in turn, is associated with reduced T OA SW radiative flux, a warmer surface, and less precipitation. These changes could create a positive feedback that could intensify fire weather, and therefore extend fire lifetime and impacts.

James L. Gomez, Robert J. Allen, and King-Fai Li

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2827', Anonymous Referee #1, 31 Jan 2024
  • RC2: 'Comment on egusphere-2023-2827', Anonymous Referee #2, 12 Feb 2024
James L. Gomez, Robert J. Allen, and King-Fai Li
James L. Gomez, Robert J. Allen, and King-Fai Li


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Short summary
Wildfires in the southwestern United States have grown very large throughout the last 20 years. These fires emit sunlight absorbing aerosols. Our study finds that aerosols emitted from fires in northern California spread throughout California and Nevada, warming the atmosphere. The warmer temperatures remove clouds from the atmosphere. This may lead to surface warming, as well as decreases in rain in the region affected. This could create conditions that prolong the fires and their consequences.