气候变化领域集成服务门户

In California's Central Valley, tule fog frequency increased 85% from 1930 to 1970, then declined 76% in the last 36 winters. Throughout these changes, fog frequency exhibited a consistent north-south trend, with maxima in southern latitudes. We analyzed seven decades of meteorological data and five decades of air pollution data to determine the most likely drivers changing fog, including temperature, dew point depression, precipitation, wind speed, and NOx (oxides of nitrogen) concentration. Climate variables, most critically dew point depression, strongly influence the short-term (annual) variability in fog frequency; however, the frequency of optimal conditions for fog formation show no observable trend from 1980 to 2016. NOx concentration, which has a decreasing north-south concentration gradient, declined continuously over this period, consistent with the long-term temporal and spatial trends in fog. As development in the Central Valley increased direct particle and other pollutant emissions from 1930 to 1970, fog frequency increased. Following the Clean Air Act, particle emissions quickly declined, and NOx emissions declined steadily, reducing the cloud condensation nuclei (CCN) available for fog formation. As a precursor of ammonium nitrate aerosols, which are efficient CCN, we used NOx measurements and emission trends as a proxy for the CCN trend. We conclude that while the short-term fog variability is dominantly driven by climate fluctuations, the longer-term temporal and spatial changes in fog have been driven by changes in air pollution. For conditions close to the dew point, a decrease in fog of 5days/year per 10 parts per billion NOx decrease occurred across the Central Valley.

Plain Language Summary California's Central Valley is known for dense wintertime ground fog, called tule fog, which dramatically reduces visibility, causing weather-related accidents, while also helping provide the winter chill required to improve productivity of fruit and nut trees. Fog frequency changed dramatically over the past century, increasing 85% from 1930 to 1970 then decreasing by similar to 75% since 1980. We found that climate variables, most importantly dew point depression (a measure of how low the temperature must drop for water droplets to form) is the main controller of short-term (year-to-year) changes in fog frequency. However, changing air pollution is the main driver of the long-term trends in fog frequency because it provides water-attracting particles on which fog droplets form. Air pollution increased in the Central Valley of California prior to 1970 due to development with lack of emissions regulation, then decreased since 1980 due to effective air pollution control, matching the observed long-term trend in fog frequency.