The recent past and possible future decline of California coastal fog

by O'Brien, Travis Allen, Ph.D., UNIVERSITY OF CALIFORNIA, SANTA CRUZ, 2011, 193 pages; 3471817

Abstract:

The author of this dissertation has coupled the University of Washington boundary layer model to the International Centre for Theoretical Physics's regional climate model, RegCM, for the purposes of modeling the climatology of stratocumulus clouds and coastal fog. This dissertation compares results from this newly coupled model (RegCM-UW) wide a wide variety of climatological datasets to show that it improves the physical representation of coastal processes while maintaining its usability as a general-purpose regional climate model. The validation of RegCM-UW shows a number of areas where the UW model distinctly improves the climatology of RegCM over the Holtslag model, though there are a number of outstanding issues that warrant further development and research: the modeled near-surface temperatures tend to be cold relative to observed temperatures, the model has a positive precipitation bias in many areas, and the model has a low bias in stratocumulus liquid water path (especially near the coast). This work shows that the addition of stratocumulus physics to a regional climate model also allows for the simulation of coastal fog. The climatology of coastal fog in RegCM-UW has a similar magnitude, spatial pattern, and temporal variability (at multiple scales) to observed fog climatology. The diurnal cycle of fog over the coast-adjacent ocean opposes the observed and modeled diurnal cycle on land; the fog frequency increases during the day over the ocean while it decreases over land. Analysis of the model output gives evidence that this phenomenon may be real and not a model artifact. A long simulation of California climate, forced by the NOAA 20 th Century Reanalysis (version 2), exhibits a century-long decline in coastal fog that is statistically indistinguishable from the observed decline. Analysis of this simulation and a series of sensitivity tests provide evidence that coastal fog climatology is extremely sensitive to relative humidity, which essentially controls cloud base height. Results from this analysis suggest that changes in large-scale circulation have caused the decline in coastal fog. A set of three RegCM-UW simulations driven by output from a global climate model project a weak, but statistically significant decline in fog throughout the 21st century.

AdviserLisa C. Sloan
SchoolUNIVERSITY OF CALIFORNIA, SANTA CRUZ
Source TypeDissertation
SubjectsClimate change; Atmospheric sciences
Publication Number3471817

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