Publication Date

Summer 2019

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Meteorology and Climate Science

Advisor

Craig C. Clements

Keywords

California, Convergence Zones, Mesoscale Processes, Mountain Meteorology, Orographic Convection, Precipitation

Subject Areas

Meteorology

Abstract

The Shasta County Convergence Zone (SCCZ) is a poorly understood terrain-forced convergence zone that develops at the northern terminus of California’s Sacramento Valley during cool season (Oct.-Apr.) trough passages. While terrain-flow interactions clearly factor into the SCCZ development, the nature of these interactions is unknown as are the mesoscale processes affecting event duration and orientation. Level-II NEXRAD radar data are used to examine the onset, duration, orientation, geographical extent, and precipitation rate of 23 SCCZ events during the 2016-2017 cool season. The analyses reveal distinct event morphologies including (1) isolated convective bands over the topography, (2) isolated convective bands displaced off of the topography over the Sacramento Valley, and (3) linear convective bands embedded within broader regional precipitation. Event duration ranges from 1 hour to more than 9 hours, and single event estimated rainfall is as high as 100 mm in some locations. Additionally, RAP model analyses, WRF simulations and in situ observations are used to investigate stability parameters and vertical wind and moisture profiles between different cases. These data indicate that location of convective bands is dependent on wind speed and direction in the low and middle levels of the atmosphere. Linearity and orientation of convective lines appear to be linked to the low level shear profile and mean winds. The non-dimensional mountain height is also examined as a control parameter, showing some utility in distinguishing between convective initiation locations.

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