Urbanization reduces fogginess in coastal Southern California, possibly counteracting global-warming induced increases in foggines
Monday, 15 December 2014
Stratus clouds provide summer moisture and shade for ecological and human systems in coastal Southern California (CSCA). Subtropical marine clouds such as those in CSCA also regulate earth’s energy budget. Uncertainties in how these clouds respond to global warming contribute to uncertain projections of local and global climate. We use hourly cloud-height observations from 22 airfields in CSCA to develop records of summer stratus cloudiness (cloud base <1000 m) since 1948. We dissect these records by hour, altitude, and location to diagnose interannual variability and multi-decade trends. We find that low and high stratus clouds are promoted by distinct climate processes. Low stratus, including fog, is promoted by a cool surface and warmth aloft, enhancing stability above the marine layer. High stratus (cloud base: 600–1000 m) is promoted by synoptic frontal systems that reduce stability throughout the atmosphere. Additionally, fog (very low stratus clouds) frequency has declined during the past ~60 years by over 50% at night and in early morning at some sites, but has not changed at others. Fog reductions appear linked to urbanization. Among the 22 airfields, trends in fogginess correlate negatively (r = -0.84) with the percent of urban cover within 10 km. The mechanism linking urban cover to fog reduction is nighttime warming. Relatively urban sites experienced more rapid nighttime warming since 1948 (r = 0.90) and have lower relative humidity as a result (r = -0.71). Decreased relative humidity causes condensation level to lift, increasing cloud height and reducing fog frequency. Intriguingly, fog frequency increased by 50–75% on the nearby, non-urbanized Channel Islands. Increased fogginess is consistent with secular warming above the marine layer, which has thus far occurred without local sea-surface warming. We conclude that trends toward a potentially foggier climate have been counteracted, and overwhelmed, by artificial nighttime warming in much of CSCA.