An Airborne and Ground-based Study of a Long-lived and Intense Atmospheric River Impacting California during the CalWater-2014 Early-Start Field Campaign

Emerging research has identified two phenomena that can significantly impact water supply and the occurrence of extreme precipitation across the western United States: (1) atmospheric rivers (ARs) and their role in delivering considerable precipitation associated with major winter storms affecting the region, and, secondarily, (2) aerosols – from local sources as well as those transported across the Pacific – and their role in modulating western U.S. precipitation and snowmelt. To advance our understanding of these important phenomena and their impacts across semiarid California – the nation's most populous state – a major field campaign utilizing four aircraft, the NOAA research ship Ron Brown, and a wide array of experimental land-based assets was carried out during the winter of 2015 called CalWater-2015. Prior to this, a smaller field campaign, without an aerosol emphasis, was implemented during the winter of 2014 using the NOAA G-IV aircraft. The purpose of this CalWater-2014 "Early-Start" project was to primarily demonstrate the viability of flying AR-focused missions off the coast of California.

By far, the wettest period during CalWater-2014 occurred with a long-lived AR event impacting California on 7-10 February. Based on Lagrangian trajectory analysis, the AR tapped into the tropical water-vapor reservoir, and the water vapor was subsequently advected toward the California coast. Widespread heavy precipitation (~200 to 400 mm) fell across the coastal mountain ranges northwest of San Francisco and across the northern Sierra Nevada inland from the coast, although only modest flooding ensued due to anomalously dry antecedent winter conditions. During the course of this event, the G-IV aircraft – which represents the cornerstone observing platform for this study – flew through two different mesoscale frontal waves in the AR environment offshore of the coast in a ~24-h period. Parallel dropsonde curtains documented key three-dimensional thermodynamic and kinematic structures across the AR and frontal waves prior to landfall. A tail-mounted Doppler radar on the G-IV simultaneously captured coherent precipitation features. Along the coast, a 449-MHz wind profiler and collocated global positioning system (GPS) receiver continuously monitored the tropospheric winds and water vapor during the AR landfall. These instruments also observed the transient frontal waves and highlighted the orographic character of the extreme rainfall occurring in the coastal mountains. A vertically pointing S-PROF radar in the coastal mountains provided detailed information on the bulk microphysical characteristics of the rainfall throughout the event. Farther inland, a pair of 915-MHz wind profilers and GPS receivers quantified the orographic precipitation forcing as the AR ascended the Sierra Nevada, and as the terrain-induced Sierra barrier jet ascended the northern terminus of California's Central Valley. We will share these analysis results during the presentation.