Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, but knowledge of their ...influence over the interior is limited. Here, the authors use Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) observations to describe the characteristics of cool-season (November-April) ARs over the western United States. It is shown that AR frequency and duration exhibit a maximum along the Oregon-Washington coast, a strong transition zone upwind (west) of and over the Cascade-Sierra ranges, and a broad minimum that extends from the "high" Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade-Sierra ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade-Sierra ranges. Farther east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.
Extratropical cyclones (ECs) and atmospheric rivers (ARs) impact precipitation over the U.S. West Coast and other analogous regions globally. This study investigates the relationship between ECs and ...ARs by exploring the connections between EC strength and AR intensity and position using a new AR intensity scale. While 82% of ARs are associated with an EC, only 45% of ECs have a paired AR and the distance between the AR and EC varies greatly. Roughly 20% of ARs (defined by vertically integrated water vapor transport) occur without a nearby EC. These are usually close to a subtropical/tropical moisture source and include an anticyclone. AR intensity is only moderately proportional to EC strength. Neither the location nor intensity of an AR can be simply determined by an EC. Greater EC intensification occurs with stronger ARs, suggesting that ARs enhance EC deepening by providing more water vapor for latent heat release.
Plain Language Summary
Both extratropical cyclones and atmospheric rivers have impact on precipitation over the U.S. West Coast, and they are often mentioned together. However, the relationship between the two is not completely understood. In this study, we have examined the connections between extratropical cyclone strength and atmospheric river intensity and position. While 82% of atmospheric rivers are related to a cyclone, only 45% of cyclones have an accompanied atmospheric river. The distance between the two varies from about 300 km to over 2,000 km. Roughly 20% of atmospheric rivers occur without a nearby cyclone. These cases are close to the subtropical/tropical moisture source and are related to a high pressure. While cyclones can enhance atmospheric rivers with stronger wind, neither the location nor the intensity of an atmospheric river can be simply determined by a cyclone. On the other hand, atmospheric rivers with strong water vapor transport provide favorable conditions for cyclone intensification. Our results provide a comprehensive analysis of the relationship between atmospheric rivers and extratropical cyclones. This work improves the understanding of the dynamical mechanism for heavy precipitation over the U.S. West Coast and thus provides more reliable information on long‐term flood control and water planning.
Key Points
Eighty‐two percent of atmospheric rivers are associated with an extratropical cyclone, while 45% of extratropical cyclones have an atmospheric river
An extratropical cyclone often intensifies the atmospheric river with stronger wind‐driven meridional water vapor transport
Atmospheric rivers can enhance the precipitation and latent heat release, which contributes to extratropical cyclone intensification
A recent study presented nearly two decades of airborne atmospheric river (AR) observations and concluded that, on average, an individual AR transports ∼5 × 10⁸ kg s−1 of water vapor. The study here ...compares those cases to ARs independently identified in reanalyses based on a refined algorithm that can detect less well-structured ARs, with the dual-purpose of validating reanalysis ARs against observations and evaluating dropsonde representativeness relative to reanalyses. The first comparison is based on 21 dropsonde-observed ARs in the northeastern Pacific and those closely matched, but not required to be exactly collocated, in ERA-Interim (MERRA-2), which indicates a mean error of −22% (−8%) in AR width and +3% (−1%) in total integrated water vapor transport (TIVT) and supports the effectiveness of the AR detection algorithm applied to the reanalyses. The second comparison is between the 21 dropsonde ARs and ∼6000 ARs detected in ERA-Interim (MERRA-2) over the same domain, which indicates a mean difference of 5% (20%) in AR width and 5% (14%) in TIVT and suggests the limited number of dropsonde observations is a highly (reasonably) representative sampling of ARs in the northeastern Pacific. Sensitivities of the comparison to seasonal and geographical variations in AR width/TIVT are also examined. The results provide a case where dedicated observational efforts in specific regions corroborate with global reanalyses in better characterizing the geometry and strength of ARs regionally and globally. The results also illustrate that the reanalysis depiction of ARs can help inform the selection of locations for future observational and modeling efforts.
Atmospheric rivers (ARs) play vital roles in the western United States and related regions globally, not only producing heavy precipitation and flooding, but also providing beneficial water supply. ...This paper introduces a scale for the intensity and impacts of ARs. Its utility may be greatest where ARs are the most impactful storm type and hurricanes, nor’easters, and tornadoes are nearly nonexistent. Two parameters dominate the hydrologic outcomes and impacts of ARs: vertically integrated water vapor transport (IVT) and AR duration i.e., the duration of at least minimal AR conditions (IVT ≥ 250 kg m−1 s−1). The scale uses an observed or predicted time series of IVT at a given geographic location and is based on the maximum IVT and AR duration at that point during an AR event. AR categories 1–5 are defined by thresholds for maximum IVT (3-h average) of 250, 500, 750, 1,000, and 1,250 kg m−1 s−1, and by IVT exceeding 250 kg m−1 s−1 continuously for 24–48 h. If the AR event duration is less than 24 h, it is downgraded by one category. If it is longer than 48 h, it is upgraded one category. The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snowpack, while stronger ARs can become mostly hazardous, for example, if they strike an area with antecedent conditions that enhance vulnerability, such as burn scars or wet conditions. Extended durations can enhance impacts. Short durations can mitigate impacts.
A uniform, global approach is used to quantify how atmospheric rivers (ARs) change between Coupled Model Intercomparison Project Phase 5 historical simulations and future projections under the ...Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios. The projections indicate that while there will be ~10% fewer ARs in the future, the ARs will be ~25% longer, ~25% wider, and exhibit stronger integrated water vapor transports (IVTs) under RCP8.5. These changes result in pronounced increases in the frequency (IVT strength) of AR conditions under RCP8.5: ~50% (25%) globally, ~50% (20%) in the northern midlatitudes, and ~60% (20%) in the southern midlatitudes. The models exhibit systematic low biases across the midlatitudes in replicating historical AR frequency (~10%), zonal IVT (~15%), and meridional IVT (~25%), with sizable intermodel differences. A more detailed examination of six regions strongly impacted by ARs suggests that the western United States, northwestern Europe, and southwestern South America exhibit considerable intermodel differences in projected changes in ARs.
Plain Language Summary
Atmospheric rivers (ARs) are elongated strands of horizontal water vapor transport, accounting for over 90% of the poleward water vapor transport across midlatitudes. These “rivers in the sky” have important implications for extreme precipitation when they make landfall, particularly along the west coasts of many midlatitude continents (e.g., North America, South America, and West Europe) due to orographic lifting. ARs are important contributors to extreme weather and precipitation events, and while their presence can contribute to beneficial rainfall and snowfall, which can mitigate droughts, they can also lead to flooding and extreme winds. This study takes a uniform, global approach that is used to quantify how ARs change between Coupled Model Intercomparison Project Phase 5 historical simulations and future projections under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios globally. The projections indicate that while there will be ~10% fewer ARs in the future, the ARs will be ~25% longer, ~25% wider, and exhibit stronger integrated water vapor transports under RCP8.5. These changes result in pronounced increases in the frequency (integrated water vapor transport strength) of AR conditions under RCP8.5: ~50% (25%) globally, ~50% (20%) in the northern midlatitudes, and ~60% (20%) in the southern midlatitudes.
Key Points
Globally, atmospheric rivers (ARs) are ~10% fewer, ~25% longer, ~25% wider, and with stronger moisture transport under the RCP8.5 scenario
In the midlatitudes where ARs are most frequent, AR conditions are ~50–60% more frequent and AR transport is ~20% stronger in the future
Systematic low biases exist in the midlatitudes in historical AR frequency (~10%), zonal (~15%), and meridional (~25%) moisture transport
Daily precipitation in California has been projected to become less frequent even as precipitation extremes intensify, leading to uncertainty in the overall response to climate warming. Precipitation ...extremes are historically associated with Atmospheric Rivers (ARs). Sixteen global climate models are evaluated for realism in modeled historical AR behavior and contribution of the resulting daily precipitation to annual total precipitation over Western North America. The five most realistic models display consistent changes in future AR behavior, constraining the spread of the full ensemble. They, moreover, project increasing year-to-year variability of total annual precipitation, particularly over California, where change in total annual precipitation is not projected with confidence. Focusing on three representative river basins along the West Coast, we show that, while the decrease in precipitation frequency is mostly due to non-AR events, the increase in heavy and extreme precipitation is almost entirely due to ARs. This research demonstrates that examining meteorological causes of precipitation regime change can lead to better and more nuanced understanding of climate projections. It highlights the critical role of future changes in ARs to Western water resources, especially over California.
This study investigates the evolution of two zonally elongated atmospheric rivers (ARs) that produced >200 mm of rainfall over mountainous regions of Northern California in late October 2010. ...Synoptic-scale analysis and air parcel trajectory analysis indicate that the ARs developed within high-CAPE environments characterized by troposphere-deep ascent as water vapor was transported directly from western North Pacific tropical cyclones (TCs) toward the equatorward entrance region of an intensifying North Pacific jet stream (NPJ). The same ARs were subsequently maintained as water vapor was transported from extratropical and subtropical regions over the central and eastern North Pacific in an environment characterized by quasigeostrophic forcing for ascent and strong frontogenesis along the anticyclonic shear side of an intense and zonally extended NPJ. Although the ARs developed in conjunction with water vapor transported from regions near TCs and in the presence of troposphere-deep ascent, an atmospheric water vapor budget illustrates that decreases in integrated water vapor (IWV) via precipitation are largely offset by the horizontal aggregation of water vapor along the AR corridors via IWV flux convergence in the presence of frontogenesis. The frameworks used for investigations of predecessor rain events ahead of TCs and of interactions between recurving TCs and the NPJ are also utilized to illustrate many dynamically similar processes related to AR development and evolution. Similarities include the following: water vapor transport directly from a TC, troposphere-deep ascent in a high-CAPE environment beneath the equatorward entrance region of an intensifying upper-tropospheric jet streak, interactions between diabatic outflow and an upper-tropospheric jet streak, and strong frontogenesis.
This study quantifies the impact of atmospheric rivers (ARs) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from ...6-hourly CFSR reanalysis data over a 16-yr period (2001–16). AR landfalls were linked to precipitation using a comprehensive observing network that spanned large variations in terrain along and across the Andes from 27° to 55°S, including some sites with hourly data. Along the Pacific (west) coast, AR landfalls are most frequent between 38° and 50°S, averaging 35–40 days yr−1. This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of ~43°S. ARs contribute 45%–60% of the annual precipitation in subtropical Chile (37°–32°S) and 40%–55% along the midlatitude west coast (37°–47°S). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions, roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs is 2–3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather, and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events, and support local nowcasting and forecasting.
Winter storms in California's Sierra Nevada increase seasonal snowpack and provide critical water resources and hydropower for the state. Thus, the mechanisms influencing precipitation in this region ...have been the subject of research for decades. Previous studies suggest Asian dust enhances cloud ice and precipitation, whereas few studies consider biological aerosols as an important global source of ice nuclei (IN). Here, we show that dust and biological aerosols transported from as far as the Sahara were present in glaciated high-altitude clouds coincident with elevated IN concentrations and ice-induced precipitation. This study presents the first direct cloud and precipitation measurements showing that Saharan and Asian dust and biological aerosols probably serve as IN and play an important role in orographie precipitation processes over the western United States.
Recent decades have seen increased melting of the Greenland ice sheet. On 11 July 2012, nearly the entire surface of the ice sheet melted; such rare events last occurred in 1889 and, prior to that, ...during the Medieval Climate Anomaly. Studies of the 2012 event associated the presence of a thin, warm elevated liquid cloud layer with surface temperatures rising above the melting point at Summit Station, some 3212 m above sea level. Here we explore other potential factors in July 2012 associated with this unusual melting. These include (1) warm air originating from a record North American heat wave, (2) transitions in the Arctic Oscillation, (3) transport of water vapor via an Atmospheric River over the Atlantic to Greenland, and (4) the presence of warm ocean waters south of Greenland. For the 1889 episode, the Twentieth Century Reanalysis and historical records showed similar factors at work. However, markers of biomass burning were evident in ice cores from 1889 which may reflect another possible factor in these rare events. We suggest that extreme Greenland summer melt episodes, such as those recorded recently and in the late Holocene, could have involved a similar combination of slow climate processes, including prolonged North American droughts/heat waves and North Atlantic warm oceanic temperature anomalies, together with fast processes, such as excursions of the Arctic Oscillation, and transport of warm, humid air in Atmospheric Rivers to Greenland. It is the fast processes that underlie the rarity of such events and influence their predictability.
Key Points
The North American heat wave was a key factor in the 2012 Greenland melting
Transport of warm air and water vapor to Greenland was via an Atmospheric River
Many factors in 1889 the last melting episode were very similar to those in 2012
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