A Climatology of Narrow Cold Frontal Rainbands in Southern California

Abstract:

Narrow cold-frontal rainbands (NCFR) produce high-intensity, short-duration precipitation that has been associated with a number of flash flooding and debris flows events in southern California in recent years. Despite colloquial knowledge of NCFR frequency and impacts in southern California, and a deep body of literature evaluating their meteorology, predictability, and impacts in regional case studies, a climatological record of NCFR occurrences in southern California does not yet exist. Here, the first catalog of regional NCFR events during winter seasons from 1995-2020 is presented. A manual approach to identifying NCFRs in radar imagery that considers region-specific radar limitations and NCFR meteorology as well as leveraging rain gauges and reanalyses is described. In total, 94 NCFRs were observed within our southern California domain. General statistics regarding NCFR synoptic conditions and tendencies as well as characteristics in radar imagery are described herein. Importantly, the climatological record demonstrates consistent meteorological processes across events, including a deepening surface low pressure system off of the California coast and a cold front signature sweeping through southern California. However, individual NCFRs exist on a spectrum of large-scale conditions, precipitation intensity, and associated hazards. The extreme and impactful NCFRs described in recent case studies are endmembers on this spectrum. This catalog provides a record of local NCFR events that is useful for future efforts to develop an objective NCFR detection algorithm, evaluate regional rain gauge and radar network limitations in detecting hazard precipitation, as well as to establish skill metrics for NCFR forecast guidance products from operational numerical weather prediction. The methods established here are also transferrable to other regions of the western U.S. that experience short-duration, high-intensity meteorological hazards and have limited radar coverage.