UCLA

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 ~25–50% in warm-season frequency of daytime stratus clouds at many sites, including fire-prone wildland-urban interface zones. We use 10 years 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.