Mesoscale Controls on the Shasta County Convergence Zone

In this presentation we examine the Shasta County Convergence Zone (SCCZ), a poorly understood terrain-forced convergence zone that develops at the northern terminus of California’s Sacramento Valley during cool season trough passages. The SCCZ is characterized by long-lived quasi-stationary bands of convection that contribute to localized flash flooding and mudslides. 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. For example, in some cases the convective initiation occurs over the terrain, whereas in others the initiation is displaced upstream over the valley floor. In addition, some events are transient whereas others persist for many hours, leading to significant societal impacts.

To improve our understanding of these events, Level-II NEXRAD radar data are used examine the onset, duration, orientation, geographical extent, and precipitation rate of 20 SCCZ events during the cool season (Oct.-Apr.) 2016-2017. The analyses reveal distinct event morphologies including (1) isolated convective bands over the topography (Fig. 1a), (2) isolated convective bands displaced off of the topography (Fig. 1b), and (3) linear convective bands embedded within broader regional precipitation (Fig. 1c). Event duration ranges from 1 h to more than 9 h, and single event estimated rainfall is as high as 100 mm in some locations. Additionally, RAP model analyses and in situ observations are used to investigate stability parameters and vertical wind 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. The non-dimensional mountain height is also examined as a control parameter, showing some utility in distinguishing between convective initiation locations.