In coastal California, cloud shading is critical to drought-limited and flammable ecological systems because it reduces atmospheric water demand during the rainless warm season. Over the past 40-70 years, cloud observations from many coastal Californian airfields indicate major declines in warm-season (May–September) daytime cloud frequency, which appear to be linked lifting of the altitude of condensation due to the urban heat island effect. At Burbank, Santa Monica, and Santa Ana Airports, which are positioned adjacent to flammable coastal mountains that ring Los Angeles, warm-season daytime cloud frequency has declined by approximately half since the early 1970s. Reductions in daytime cloud frequency have caused increases incoming solar radiation, evaporative demand, and, presumably, fire danger, but these effects have not been quantified. We used 10 years of sub-hourly meteorological, surface radiation, and cloud observations (N > 101,000) at a coastal site in southern California to statistically model the effects of changes in cloud cover on warm-season evaporative demand.
Results/Conclusions
Forcing our model with cloud observations from 31 coastal airfields throughout California, including several near wildlands regularly at fire risk, we estimate that reduced cloud frequency and increased cloud-base height since the early 1970s significantly enhanced warm-season daytime solar radiation and atmospheric water demand at the majority of sites, especially in the greater Los Angeles and San Diego areas. At Burbank, Santa Monica, and Santa Ana Airports, the increases in warm-season daytime solar radiation and atmospheric moisture demand (as represented by Penman-Monteith reference evapotranspiration) were 17–31 W m-2 and 3–6%, respectively. Within the coastal grass and chaparral ecoregions of central and southern CA, annual wildfire area was significantly and positively correlated with summer atmospheric water demand during 1984-2017 and an increase in summer atmospheric water demand of just 5% corresponded to a tripling in wildfire area. Reduced warm-season cloud cover in and around coastal California’s large cities has very likely had important consequences for ecological systems and fire danger over the past half century that have been under-recognized and deserve further study.