Synoptic-Dynamics of Extreme Cold Air Outbreaks Over the California Central Valley

Abstract:

Cold air outbreaks (CAOs) have created multi-billion dollar losses in the state of California. Especially hard hit have been agricultural operations in the California Central Valley. Criteria based on duration and extreme values of daily minimum surface temperature at 17 stations over California Central Valley and 700hPa temperature at the Oakland radiosonde station are used to identify CAOs during the period of 1950-2013. 32 strong CAO events in total are obtained with our criterion. 10 stronger CAOs are selected for detailed study. Composite analyses and bootstrap statistical tests are applied to these 10 strong CAOs and find a similar large scale meteorological pattern (LSMP) in each event. This LSMP has a ridge-trough-ridge pattern in the mass field extending from Alaska across North America to the Southeastern part of the US as the LSMP in Grotjahn & Faure (2008). A challenging problem arises in the analyses caused by the different phase speeds of waves prior to different CAO events. We use dynamical analysis methods, such as wave activity flux, three-dimensional trajectories, and temperature tendency equation terms, to reveal the synoptic-dynamical mechanisms of how the LSMP and cold air formation/migration lead to these CAOs.