Synoptic and Mesoscale Forcing of Southern California Extreme Precipitation

Abstract

Southern California water resources are heavily dependent on a small number of extreme precipitation events each winter season, which dictate the region's highly variable interannual accumulations. In the Santa Ana River Watershed, on average, three extreme events per year contribute half of annual precipitation, yet there are relatively few studies of the synoptic to mesoscale processes that drive precipitation during these events. This study uses an ingredient‐based approach in identifying the contributions of orographic forcing, dynamical forcing, and convective instability to extreme precipitation in the watershed in 107 storms that produced roughly 50% of all precipitation from 1981 to 2017. The influence of dynamical forcing and convective instability on event precipitation distributions is investigated relative to the dominant influence of orographic forcing that is typically found in landfalling atmospheric rivers. Case studies of two high‐impact events from the 2017 winter season demonstrate differences in the roles of synoptic ascent and mesoscale convective features in modifying precipitation location, rate, and accumulation over the watershed. The 17 and 18 February 2017 case study included a narrow cold‐frontal rainband that produced high‐intensity short‐duration precipitation over low elevations of the watershed. In the 107 extreme event records, similar modification of the precipitation distribution toward non‐orographic rainfall was related to significant changes in the synoptic‐scale circulation that favored enhanced dynamics and upstream ascent associated with frontogenesis. Variability in precipitation mechanisms is of primary interest to weather forecasters and water managers as it modifies event impacts and predictability.

Simulating and Evaluating Atmospheric River‐Induced Precipitation Extremes Along the U.S. Pacific Coast: Case Studies From 1980–2017

“…Additionally, the finer resolution simulations at 9 and 3 km yield more detailed dynamical features than are present in the 27 km domain or in MERRA, including occasional double‐IVT maxima and well‐organized synoptic‐scale cyclones (see Video S1 and Figure S3 for each individual storm's maximum IVT snapshot for WRF9/3 km and reanalysis). Realistic representation of both large‐scale and mesoscale meteorological features provides initial confidence that a regional modeling framework similar to the one used in the present study is appropriate in further study of the associated precipitation extremes (Cannon et al, ).…”

Composite Synoptic-Scale Environments Conducive to North American Polar–Subtropical Jet Superposition Events

“…11c). The aspect ratio of this corridor of anomalous precipitable water strongly resembles the character of landfalling western U.S. atmospheric rivers (e.g., Newell et al 1992;Zhu and Newell 1998;Ralph et al 2004Ralph et al , 2018Ralph et al , 2019Cannon et al 2018) and is collocated with broad regions of 300-hPa geostrophic warm-air advection (Fig. 11a) and negative OLR anomalies along the jet axis (Fig.…”

“…Tropical anticyclonic PV anomalies result from the poleward transport of tropical, low-PV upper-tropospheric air via low-latitude troughs and tropical plumes (e.g., Iskenderian 1995;Roundy et al 2010;Fröhlich et al 2013;Winters and Martin 2016) and/or tropical cyclones (e.g., McTaggart-Cowan et al 2007;Archambault et al 2013Archambault et al , 2015. Tropical anticyclonic PV anomalies at midlatitudes typify a thermodynamic environment characterized by low upper-tropospheric static stability and can contribute to the development of an atmospheric river (e.g., Newell et al 1992;Zhu and Newell 1998;Ralph et al 2004Ralph et al , 2018Ralph et al , 2019 within the polewarddirected branch of the tropospheric-deep, nondivergent circulation induced by the anticyclonic PV anomaly.…”

Competing droughts affect dust delivery to Sierra Nevada

“…This research aimed to test the sensitivity of annual changes in the regional drought upon local dust generation and transport. California's fiveyear-drought ceased following an anomalously wet 2016-2017 water year in California (California Department of Water Resources, 2018) which included extreme precipitation events (described as atmospheric rivers) during early 2017 (Cannon et al, 2018).…”