7. Causes of change in Southwest coastal fog

Urbanization causes increased cloud base height and decresed fog in coastal Southern California - PDF

Subtropical marine stratus clouds regulate coastal and global climate, but future trends in theseclouds are uncertain. In coastal Southern California (CSCA), interannual variations in summer stratus cloud occurrence are spatially coherent across 24 airfields and dictated by positive relationships with stability above the marine boundary layer (MBL) and MBL height. Trends, however, have been spatially variable since records began in the mid-1900s due to differences in night time warming. Among CSCA airfields, differences in nighttime warming, but not daytime warming, are strongly and positively related to fraction of nearby urban cover, consistent with an urban heat island effect. Night time warming raises the near-surface dew point depression, which lifts the altitude of condensation and cloud base height, thereby reducing fog frequency. Continued urban warming, rising cloud base heights, and associated effects on energy and water balance would profoundly impact ecological and human systems in highly populated and ecologically diverse CSCA.

• Williams, A. P., R. E. Schwartz, S. Iacobellis, R. Seager, B. I. Cook, C. J. Still, G. Husak and J. Michaelsen, 2015: Urbanization causes increased cloud base height and decreased fog in coastal Southern California. Geophysical Research Letters, 42(5): 1527-1536, doi: 10.1002/2015GL063266. PDF