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

Cloud shading limits surface radiation, thus reducing vegetation water stress and, presumably, flammability. Since the early 1970s, cloud observations from airfields in coastal Southern California (CSCA) indicate reductions of similar to 25-50% in warm-season frequency of daytime stratus clouds at many sites, including fire-prone wildland-urban interface zones. We use 10years of meteorological, surface radiation, and cloud observations to statistically model the effects of clouds on warm-season surface energy fluxes in CSCA. Forcing our model with cloud observations, we estimate 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 and northern San Diego. Correlation with burned area and live fuel moisture implicates stratus cloud shading as an important driver of warm-season wildfire activity in CSCA. Large reductions in cloud shading have likely enhanced warm-season wildfire potential in many CSCA areas when and where fuels are not limiting. Plain Language Summary 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. These 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. Clouds appear to be important regulators of summer wildfire activity in this region, as the shade they provide slows loss of moisture from vegetation. On the vegetated mountainsides that ring coastal Southern California's large cities, increases in summer sunlight and evaporative demand have likely enhanced summer wildfire potential over the past several decades. This effect is expected to continue due to continued urban expansion and positive feedbacks, where warming due to cloud loss promotes further warming and cloud loss.