The Landfall and Inland Penetration of a Flood-Producing Atmospheric River in Arizona. Part I: Observed Synoptic-Scale, Orographic, and Hydrometeorological Characteristics

Abstract

Atmospheric rivers (ARs) are a dominant mechanism for generating intense wintertime precipitation along the U.S. West Coast. While studies over the past 10 years have explored the impact of ARs in, and west of, California’s Sierra Nevada and the Pacific Northwest’s Cascade Mountains, their influence on the weather across the intermountain west remains an open question. This study utilizes gridded atmospheric datasets, satellite imagery, rawinsonde soundings, a 449-MHz wind profiler and global positioning system (GPS) receiver, and operational hydrometeorological observing networks to explore the dynamics and inland impacts of a landfalling, flood-producing AR across Arizona in January 2010. Plan-view, cross-section, and back-trajectory analyses quantify the synoptic and mesoscale forcing that led to widespread precipitation across the state. The analyses show that a strong AR formed in the lower midlatitudes over the northeastern Pacific Ocean via frontogenetic processes and sea surface latent-heat fluxes but without tapping into the adjacent tropical water vapor reservoir to the south. The wind profiler, GPS, and rawinsonde observations document strong orographic forcing in a moist neutral environment within the AR that led to extreme, orographically enhanced precipitation. The AR was oriented nearly orthogonal to the Mogollon Rim, a major escarpment crossing much of central Arizona, and was positioned between the high mountain ranges of northern Mexico. High melting levels during the heaviest precipitation contributed to region-wide flooding, while the high-altitude snowpack increased substantially. The characteristics of the AR that impacted Arizona in January 2010, and the resulting heavy orographic precipitation, are comparable to those of landfalling ARs and their impacts along the west coasts of midlatitude continents.

The Regional Influence of an Intense Sierra Barrier Jet and Landfalling Atmospheric River on Orographic Precipitation in Northern California: A Case Study

“…Two atmospheric phenomena significantly modulate the distribution of precipitation, high-altitude snowpack, and runoff in the mountains surrounding California's northern CV: terrain-locked Sierra barrier jets (SBJs) and transient atmospheric rivers (ARs) (e.g., Dettinger 2004; Galewsky and Sobel 2005;Ralph et al 2006;Kim and Kang 2007;Reeves et al 2008;Guan et al 2010;Lundquist et al 2010;Smith et al 2010;Neiman et al 2008bNeiman et al , 2013aDettinger et al 2011;Kim et al 2012;Ralph et al 2013b). Landfalling extratropical cyclones along the U.S. West Coast are often accompanied by ARs, which are long (.2000 km), narrow (,1000 km) plumes of enhanced horizontal water vapor flux embedded within the broader region of generally poleward heat transport in the cyclone warm sector (Zhu and Newell 1998;Ralph et al 2004Ralph et al , 2006Ralph et al , 2011Neiman et al 2008aNeiman et al ,b, 2013bSmith et al 2010;Sodemann and Stohl 2013).…”

“…Landfalling extratropical cyclones along the U.S. West Coast are often accompanied by ARs, which are long (.2000 km), narrow (,1000 km) plumes of enhanced horizontal water vapor flux embedded within the broader region of generally poleward heat transport in the cyclone warm sector (Zhu and Newell 1998;Ralph et al 2004Ralph et al , 2006Ralph et al , 2011Neiman et al 2008aNeiman et al ,b, 2013bSmith et al 2010;Sodemann and Stohl 2013). SBJs, which were first documented during the Sierra Cooperative Pilot Project (SCPP; Reynolds and Dennis 1986) and have since been studied extensively (e.g., Parish 1982;Marwitz 1983Marwitz , 1987Smutz 1986;Hughes et al 2012;Kingsmill et al 2013;Neiman et al 2013a), form in response to the deceleration of stably stratified flow as it approaches the western Sierra foothills. This deceleration leads to a weakened Coriolis force, causing the flow to turn leftward toward the north end of the CV in response to the (no longer balanced) pressure gradient force.…”

“…The resulting blocked flow, which is maintained in conjunction with a statically stable pressure ridge dammed against the Sierra's windward slope, parallels the range's long axis on its west side below crest level [e.g., see conceptual Fig. 13 in Neiman et al (2013a)]. …”

“…An observationally based composite study by Neiman et al (2013a) revealed that 1) strong, moisture-laden, low-level southwesterly flow in ARs can enter a prominent gap in the coastal mountains east of San Francisco and get diverted northward by the Sierra into the SBJ within the CV and 2) the upper portion of the AR flow can override the SBJ. The composite study expanded on the observational case study of 14-16 February 2011 by Kingsmill et al (2013), which was the first to document key interactions between SBJs and ARs.…”

Snow Level Characteristics and Impacts of a Spring Typhoon-Originating Atmospheric River in the Sierra Nevada, USA

“…The orientation of the northwest-southeast trending Sierra Nevada ( Figure 1) perpendicular to the west-southwest flow during most cool season storms leads to orographic precipitation enhancement when ARs interact with the topography [9,10]. A meridional elevation gradient spans the range; the northern region is composed of lower characteristic elevations (median = 1.6 km above sea level) than the central (2.1 km) and southern (1.9 km) regions.…”