Multidecadal simulation of coastal fog with a regional climate model
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In order to model stratocumulus clouds and coastal fog, we have coupled the University of Washington boundary layer model to the regional climate model, RegCM (RegCM-UW). By comparing fog occurrences observed at various coastal airports in the western United States, we show that RegCM-UW has success at modeling the spatial and temporal (diurnal, seasonal, and interannual) climatology of northern California coastal fog. The quality of the modeled fog estimate depends on whether coast-adjacent ocean or land grid cells are used; for the model runs shown here, the oceanic grid cells seem to be most appropriate. The interannual variability of oceanic northern California summertime fog, from a multi-decadal simulation, has a high and statistically significant correlation with the observed interannual variability (r = 0.72), which indicates that RegCM-UW is capable of investigating the response of fog to long-term climatological forcing. While RegCM-UW has a number of aspects that would benefit from further investigation and development, RegCM-UW is a new tool for investigating the climatology of coastal fog and the physical processes that govern it. We expect that with appropriate physical parameterizations and moderate horizontal resolution, other climate models should be capable of simulating coastal fog. The source code for RegCM-UW is publicly available, under the GNU license, through the International Centre for Theoretical Physics.
KeywordsRegional climate model Marine stratocumulus Planetary boundary layer Cloud topped boundary layer Turbulence closure Coastal climate Coastal fog Sequoia sempervirens Redwood
The authors would like to extend their deep gratitude to Professor Christopher Bretherton of the University of Washington for kindly sharing his turbulence closure model code with us. We would also like to thank two anonymous reviewers, whose thorough comments and suggestions improved the focus and clarity of this manuscript, and Professor Filippo Giorgi, who has kindly provided travel support for the lead author to work with the RegCM developers at ICTP on two occasions. This material is based upon work supported by the National Science Foundation under Grant No. ATM-0533482-001 and ATM-0736046-001. This work was partially supported by a grant from the California Energy Commission and a grant from the UC Santa Cruz STEPS Institute. This research was partially supported by the Director, Office of Science, Office of Biological and Environmental Research of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 as part of the Regional and Global Climate Modeling Program (RGCM). Figures were prepared using the NCAR Command Language (NCL 2012).
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