Meteorological Characteristics and Overland Precipitation Impacts of Atmospheric Rivers Affecting the West Coast of North America Based on Eight Years of SSM/I Satellite Observations

Abstract

The pre-cold-frontal low-level jet within oceanic extratropical cyclones represents the lower-tropospheric component of a deeper corridor of concentrated water vapor transport in the cyclone warm sector. These corridors are referred to as atmospheric rivers (ARs) because they are narrow relative to their length scale and are responsible for most of the poleward water vapor transport at midlatitudes. This paper investigates landfalling ARs along adjacent north- and south-coast regions of western North America. Special Sensor Microwave Imager (SSM/I) satellite observations of long, narrow plumes of enhanced integrated water vapor (IWV) were used to detect ARs just offshore over the eastern Pacific from 1997 to 2005. The north coast experienced 301 AR days, while the south coast had only 115. Most ARs occurred during the warm season in the north and cool season in the south, despite the fact that the cool season is climatologically wettest for both regions. Composite SSM/I IWV analyses showed landfalling wintertime ARs extending northeastward from the tropical eastern Pacific, whereas the summertime composites were zonally oriented and, thus, did not originate from this region of the tropics. Companion SSM/I composites of daily rainfall showed significant orographic enhancement during the landfall of winter (but not summer) ARs. The NCEP–NCAR global reanalysis dataset and regional precipitation networks were used to assess composite synoptic characteristics and overland impacts of landfalling ARs. The ARs possess strong vertically integrated horizontal water vapor fluxes that, on average, impinge on the West Coast in the pre-cold-frontal environment in winter and post-cold-frontal environment in summer. Even though the IWV in the ARs is greater in summer, the vapor flux is stronger in winter due to much stronger flows associated with more intense storms. The landfall of ARs in winter and north-coast summer coincides with anomalous warmth, a trough offshore, and ridging over the Intermountain West, whereas the south-coast summer ARs coincide with relatively cold conditions and a near-coast trough. ARs have a much more profound impact on near-coast precipitation in winter than summer, because the terrain-normal vapor flux is stronger and the air more nearly saturated in winter. During winter, ARs produce roughly twice as much precipitation as all storms. In addition, wintertime ARs with the largest SSM/I IWV are tied to more intense storms with stronger flows and vapor fluxes, and more precipitation. ARs generally increase snow water equivalent (SWE) in autumn/winter and decrease SWE in spring. On average, wintertime SWE exhibits normal gains during north-coast AR storms and above-normal gains during the south-coast AR storms. The north-coast sites are mostly lower in altitude, where warmer-than-normal conditions more frequently yield rain. During those events when heavy rain from a warm AR storm falls on a preexisting snowpack, flooding is more likely to occur.

Long-term macroinvertebrate responses to climate change: implications for biological assessment in mediterranean-climate streams

“…Not surprisingly, the number of storms (using the 1005 mb threshold index) is positively correlated with the number of heavy precipitation events and also with the annual total precipitation, although these correlations are only modest (about 0.3 level). The continued occurrence of significant storms within the model simulations would suggest that future decades would continue to be occasionally affected by floods in the California region (Neiman et al 2008). …”

Storms, floods, and the science of atmospheric rivers

“…For example, a recent numerical model study [Smith et al, 2010] estimated that roughly 20-40% of the water vapor transported ashore by an AR crossing over northern California was rained out there. This rainout happens because when ARs make landfall on the West Coast of North America (as well as on other continents [e.g., Stohl et al, 2008]), they are forced up and over coastal mountains, where they cool and condense large parts of their heavy burden of vapor [e.g., Neiman et al, 2008;Leung and Qian, 2009].…”

Evaluation of cloud microphysics schemes in simulations of a winter storm using radar and radiometer measurements

“…This system was characterized by a strong prefrontal low-level jet (LLJ) that transported abundant moisture onshore in the cyclone's warm sector. The moisture-laden LLJ is often referred to as an "Atmospheric River (AR)" and is readily observed by water vapor channels in microwave radiometers [e.g., Neiman et al, 2008;Ralph et al, 2011]. Smith et al, [2010] documented the synoptic environment and moisture fluxes for the atmospheric river discussed in the current study.…”