May 24, 2018

Effect of Reduced Summer Cloud Shading on Evaporative Demand and Wildfire in Coastal Southern California

A. Park Williams, Pierre Gentine, Max A. Moritz, Dar A. Roberts, John T. Abatzoglou
Geophysical Research Letters
  • States that in much of coastal southern California, the frequency of summer clouds has declined rapidly in recent decades due to warming from urbanization and greenhouse gases
  • Finds that since the early 1970s, cloud observations from airfields in coastal southern California (CSCA) indicate reductions of ~25–50% in warm‐season frequency of daytime stratus clouds at many sites, including fire‐prone wildland‐urban interface (WUI) zones
  • Estimates that reduced warm‐season cloud shading since the 1970s significantly enhanced daytime solar radiation and evaporative demand throughout much of CSCA, particularly in greater Los Angeles
  • In other words, cloud reductions have significantly reduced cloud shading and increased evaporative demand, particularly in greater Los Angeles and northern San Diego, such that a relatively cloudy summer today is similar to a relatively clear summer in the 1970s
  • Correlation with burned area and live fuel moisture implicates stratus cloud shading as an important driver of warm‐season wildfire activity in CSCA