Publication Date

Summer 2017

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Meteorology

Advisor

Craig B. Clements

Keywords

dry air, fire weather, inversion, nocturnal drying, overnight drying, subsidence

Subject Areas

Meteorology

Abstract

The second largest fire shelter deployment in U. S. history occurred during the Devil Fire in a remote and rugged region of the San Francisco Bay Area when relative humidity values abruptly dropped in the middle of the night, causing rapid fire growth. Nocturnal drying in the higher elevations along California’s central coast is a unique phenomenon that poses a great risk to firefighters. Single digit relative humidity with dew points below -25°C is not uncommon during summer nights in this region. In order to provide the firefighting community with knowledge of these hazardous conditions, an event criterion was established to develop a climatology of nocturnal drying and investigate the synoptic patterns surrounding such episodes. Furthermore, a detailed case study of the Devil Fire incident, including a high resolution numerical simulation, was used to understand the interaction between complex topography, the boundary layer, and a subsidence inversion. A lower tropospheric source region of dry air was found over the northeastern Pacific, corresponding to an area of maximum low-level divergence and associated subsidence. This dry air forms above a marine inversion and advects inland with the marine layer through gaps in the coastal barrier overnight. As the boundary layer over higher terrain collapses at night, the inversion lowers and immerses upper slopes with warm and dry air. An average of 15-20 nocturnal drying events per year occur in elevations greater than 700 m in the San Francisco Bay Area and their characteristics are highly variable, making them a challenge to forecast.

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