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Andrews, ED, Antweiler RC, Neiman PJ, Ralph FM.  2004.  Influence of ENSO on flood frequency along the California coast. Journal of Climate. 17:337-348.   10.1175/1520-0442(2004)017<0337:Ioeoff>2.0.Co;2   AbstractWebsite

The influence of the El Nino-Southern Oscillation (ENSO) phenomenon on flooding in California coastal streams is investigated by analyzing the annual peak floods recorded at 38 gauging stations. The state of ENSO prior to and during flooding is characterized by the multivariate ENSO index (MEI), where MEI, 20.5 is defined as the La Nina phase and MEI>0.5 as the El Nino phase. Flood magnitude in all 20 streams located south of 35degreesN has a significant positive correlation (r=0.3 to 0.6), whereas in 3 of the 4 streams located north of 41degreesN flood magnitude has a significant negative correlation (r=-0.3 to 20.4), with MEI from -2.2 to +3.2. Correlations with MEI are uniformly weak and insignificant, however, when the floods are subdivided into El Nino and non-El Nino phases. A comparison of the geometric mean El Nino flood to the geometric mean non-El Nino flood determined that the means were statistically different at gauging stations south of 35degreesN and north of 41degreesN. For 20 streams located south of 35degreesN, the geometric mean of annual peak floods recorded at a stream gauge during El Nino phases is 2-14 times the geometric mean of annual peak floods recorded during non-El Nino phases. Thus, south of 35degreesN along the California coast, floods are significantly larger during an El Nino phase than a non-El Nino phase. For the three streams located north of 41degreesN, the geometric mean of annual peak floods during an El Nino phase was less than 70% of the geometric mean of annual peak floods during a non-El Nino phase. The relative strength of the El Nino phase, however, has, at most, a weak influence on flood magnitude. Flood exceedance probabilities for the El Nino and non-El Nino periods were calculated for all gauging stations using a three-parameter log gamma distribution. For exceedance probabilities from 0.50 to 0.02, the ratio of the El Nino to non-El Nino floods varies from greater than 10 near 32degreesN to less than 0.7 near 42degreesN. Latitude explains 76%-90% of the observed variation in the relative magnitude of El Nino versus non-El Nino floods over the range of exceedance probabilities.

Ault, AP, Williams CR, White AB, Neiman PJ, Creamean JM, Gaston CJ, Ralph FM, Prather KA.  2011.  Detection of Asian dust in California orographic precipitation. Journal of Geophysical Research-Atmospheres. 116   10.1029/2010jd015351   AbstractWebsite

Aerosols impact the microphysical properties of clouds by serving as cloud condensation nuclei (CCN) and ice nuclei (IN). By modifying cloud properties, aerosols have the potential to alter the location and intensity of precipitation, but determining the magnitude and reproducibility of aerosol-induced changes to precipitation remains a significant challenge to experimentalists and modelers. During the CalWater Early Start campaign (22 February to 11 March 2009), a uniquely comprehensive set of atmospheric chemistry, precipitation, and meteorological measurements were made during two extratropical cyclones. These two storms showed enhanced integrated water vapor concentrations and horizontal water vapor transports due to atmospheric river conditions and, together, produced 23% of the annual precipitation and 38% of the maximum snowpack at California's Central Sierra Snow Lab (CSSL). Precipitation measurements of insoluble residues showed very different chemistry occurring during the two storms with the first one showing mostly organic species from biomass burning, whereas the second storm showed a transition from biomass burning organics to the dominance of Asian dust. As shown herein, the dust was transported across the Pacific during the second storm and became incorporated into the colder high-altitude precipitating orographic clouds over the Sierra Nevada. The second storm produced 1.4 times as much precipitation and increased the snowpack by 1.6 times at CSSL relative to the first storm. As described in previous measurement and modeling studies, dust can effectively serve as ice nuclei, leading to increased riming rates and enhanced precipitation efficiency, which ultimately can contribute to differences in precipitation. Future modeling studies will help deconvolute the meteorological, microphysical, and aerosol factors leading to these differences and will use CalWater's meteorological and aerosol observations to constrain the model-based interpretations. The ultimate goal of such combined efforts is to use the results to improve aerosol-cloud impacts on precipitation in regional climate models.

Bao, JW, Michelson SA, Neiman PJ, Ralph FM, Wilczak JM.  2006.  Interpretation of enhanced integrated water vapor bands associated with extratropical cyclones: Their formation and connection to tropical moisture. Monthly Weather Review. 134:1063-1080.   10.1175/Mwr3123.1   AbstractWebsite

Trajectory analysis using a weather prediction model is performed for five cases to interpret the formation of enhanced bands of vertically integrated water vapor (IWV) in the central and eastern Pacific that are frequently seen in satellite images from the Special Sensor Microwave Imager. The connection of these enhanced bands with poleward water vapor transport from the Tropics is also examined. It is shown that the leading end of the enhanced IWV bands (defined as the most eastward and poleward end) is the manifestation of moisture convergence in the warm conveyor bell associated with extratropical cyclones, while the bands away from the leading end result mainly from moisture convergence along the trailing cold fronts. There is evidence that some enhanced IWV bands may be associated with a direct poleward transport of tropical moisture along the IWV bands from the Tropics all the way to the extratropics. The trajectory analysis, together with the seasonal mean sea level pressure analysis, indicates that a favorable condition for the occurrence of a direct, along-IWV band transport of tropical (defined as south of 23.5 degrees N) moisture to the U.S. West Coast in the eastern Pacific is a weakened subtropical ridge in the central Pacific with an enhanced southwesterly low-level flow. The authors hypothesize that the direct poleward transport of tropical moisture within an enhanced IWV band in the eastern Pacific is most possible in the neutral El Nino-Southern Oscillation (ENSO) phase and is least possible in the El Nino phase.

Cannon, F, Ralph FM, Wilson AM, Lettenmaier DP.  2017.  GPM satellite radar measurements of precipitation and freezing level in atmospheric rivers: comparison with ground-based radars and reanalyses. Journal of Geophysical Research-Atmospheres. 122:12747-12764.   10.1002/2017jd027355   AbstractWebsite

Atmospheric rivers (ARs) account for more than 90% of the total meridional water vapor flux in midlatitudes, and 25-50% of the annual precipitation in the coastal western United States. In this study, reflectivity profiles from the Global Precipitation Measurement Dual-Frequency Precipitation Radar (GPM-DPR) are used to evaluate precipitation and temperature characteristics of ARs over the western coast of North America and the eastern North Pacific Ocean. Evaluation of GPM-DPR bright-band height using a network of ground-based vertically pointing radars along the West Coast demonstrated exceptional agreement, and comparison with freezing level height from reanalyses over the eastern North Pacific Ocean also consistently agreed, indicating that GPM-DPR can be used to independently validate freezing level in models. However, precipitation comparison with gridded observations across the western United States indicated deficiencies in GPM-DPR's ability to reproduce the spatial distribution of winter precipitation, likely related to sampling frequency. Over the geographically homogeneous oceanic portion of the domain, sampling frequency was not problematic, and significant differences in the frequency and intensity of precipitation between GPM-DPR and reanalyses highlighted biases in both satellite-observed and modeled AR precipitation. Reanalyses precipitation rates below the minimum sensitivity of GPM-DPR accounted for a 20% increase in total precipitation, and 25% of radar-derived precipitation rates were greater than the 99th percentile precipitation rate in reanalyses. Due to differences in the proportions of precipitation in convective, stratiform bright-band, and non-bright-band conditions, AR conditions contributed nearly 10% more to total precipitation in GPM-DPR than reanalyses.

Clark, TL, Hall WD, Kerr RM, Middleton D, Radke L, Ralph FM, Neiman PJ, Levinson D.  2000.  Origins of aircraft-damaging clear-air turbulence during the 9 December 1992 Colorado downslope windstorm: Numerical simulations and comparison with observations. Journal of the Atmospheric Sciences. 57:1105-1131.   10.1175/1520-0469(2000)057<1105:Ooadca>2.0.Co;2   AbstractWebsite

Results from numerical simulations of the Colorado Front Range downslope windstorm of 9 December 1992 are presented. Although this case was not characterized by severe surface winds, the event caused extreme clear-air turbulence (CAT) aloft, as indicated by the severe structural damage experienced by a DC-8 cargo jet at 9.7 km above mean sea level over the mountains. Derailed measurements from the National Oceanic and Atmospheric Administration/Environmental Research Laboratories/Environmental Technology Laboratory Doppler lidar and wind profilers operating on that day and from the Defense Meteorological Satellite Program satellite allow for a uniquely rich comparison between the simulations and observations.Four levels of grid refinement were used in the model. The outer domain used National Centers for Environmental Prediction data for initial and boundary conditions. The finest grid used 200 m in all three dimensions over a 48 km by 48 km section. The range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time-evolving windstorm forced during a frontal passage. This full range of resolution and model complexity was essential in this case. Many aspects of this windstorm are inherently three-dimensional and are not represented in idealized models using either 2D or so-called 2D-3D dynamics.Both the timing and location of wave breaking compared well with observations. The model also reproduced cross-stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar. Model results also show that the observed CAT derives from interactions between these wavelike jet stream disturbances and mountain-forced internal gravity waves. Due to the nearly east-west orientation of the jet stream. these two interacting wave modes were orthogonal to each other. Thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes (HVTs) aligned with the mean Row. These HVTs remained aloft while they propagated downstream at about the elevation of the aircraft incident, and evidence for such a vortex was seen by the lidar. The model and observations suggest that one of these intense vortices may have caused the aircraft incident.Reports of strong surface gusts were intermittent along the Front Range during the period of this study. The model showed that interactions between the gravity waves and how-aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations. The simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic "downburst of turbulence." In the present case, the perturbations of the jet stream provide a funnel-shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves. The physical picture for the upper levels is similar to the surface gusts described by Clark and Farley resulting from vortex tilting. The CAT feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream. Visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind.

Coplen, TB, Neiman PJ, White AB, Landwehr JM, Ralph FM, Dettinger MD.  2008.  Extreme changes in stable hydrogen isotopes and precipitation characteristics in a landfalling Pacific storm. Geophysical Research Letters. 35   10.1029/2008gl035481   AbstractWebsite

With a new automated precipitation collector we measured a remarkable decrease of 51 parts per thousand in the hydrogen isotope ratio (delta(2)H) of precipitation over a 60-minute period during the landfall of an extratropical cyclone along the California coast on 21 March 2005. The rapid drop in delta(2)H occurred as precipitation generation transitioned from a shallow to a much deeper cloud layer, in accord with synoptic-scale ascent and deep ''seeder-feeder'' precipitation. Such unexpected delta(2)H variations can substantially impact widely used isotope-hydrograph methods. From extreme delta(2)H values of -26 and -78 parts per thousand, we calculate precipitation temperatures of 9.7 and -4.2 degrees C using an adiabatic condensation isotope model, in good agreement with temperatures estimated from surface observations and radar data. This model indicates that 60 percent of the moisture was precipitated during ascent as temperature decreased from 15 degrees C at the ocean surface to -4 degrees C above the measurement site.

Cordeira, JM, Ralph FM, Moore BJ.  2013.  The development and evolution of two atmospheric rivers in proximity to western North Pacific tropical cyclones in October 2010. Monthly Weather Review. 141:4234-4255.   10.1175/Mwr-D-13-00019.1   AbstractWebsite

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.

Cordeira, JM, Ralph FM, Martin A, Gaggggini N, Spackman JR, Neiman PJ, Rutz JJ, Pierce R.  2017.  Forecasting atmospheric rivers during CalWater 2015. Bulletin of the American Meteorological Society. 98:449-459.   10.1175/bams-d-15-00245.1   AbstractWebsite
Creamean, JM, Lee C, Hill TC, Ault AP, DeMott PJ, White AB, Ralph FM, Prather KA.  2014.  Chemical properties of insoluble precipitation residue particles. Journal of Aerosol Science. 76:13-27.   10.1016/j.jaerosci.2014.05.005   AbstractWebsite

Precipitation chemistry can provide unique insights into the composition of aerosol particles involved in precipitation processes. Until recently, precipitation chemistry studies focused predominantly on soluble components. Analyzing the single particle insoluble components in addition to soluble ions in precipitation can provide detailed information on the individual particles originally in the cloud or removed by precipitation as well as the source of the aerosols. Herein, we present an in-depth analysis of resuspended residues from precipitation samples collected at a remote site in the Sierra Nevada Mountains in California during the 2009-2011 winter seasons. In addition, we present results from laboratory control experiments of dust, leaf litter, smoke, and sea salt samples that were conducted to better understand how insoluble and soluble residues are distributed during the atomization process and aid in the classification of the residue compositions in the precipitation samples. Further, immersion freezing ice nuclei (IN) measurements of insoluble residues from precipitation water enabled the determination of residue types that likely seeded clouds. Long-range transported dust mixed with biological material tended to be more IN active, while purely biological residues contained a variety of high and low temperature IN. Overall, results from this study can be used as a benchmark for classification of insoluble precipitation residues in future studies. Knowledge of the precipitation chemistry of insoluble residues coupled with meteorological and cloud microphysical measurements will ultimately improve our understanding of the link between aerosols, clouds, and precipitation. (C) 2014 Published by Elsevier Ltd.

Creamean, JM, Ault AP, White AB, Neiman PJ, Ralph FM, Minnis P, Prather KA.  2015.  Impact of interannual variations in sources of insoluble aerosol species on orographic precipitation over California's central Sierra Nevada. Atmospheric Chemistry and Physics. 15:6535-6548.   10.5194/acp-15-6535-2015   AbstractWebsite

Aerosols that serve as cloud condensation nuclei (CCN) and ice nuclei (IN) have the potential to profoundly influence precipitation processes. Furthermore, changes in orographic precipitation have broad implications for reservoir storage and flood risks. As part of the CalWater field campaign (2009-2011), the variability and associated impacts of different aerosol sources on precipitation were investigated in the California Sierra Nevada using an aerosol time-of-flight mass spectrometer for precipitation chemistry, S-band profiling radar for precipitation classification, remote sensing measurements of cloud properties, and surface meteorological measurements. The composition of insoluble residues in precipitation samples collected at a surface site contained mostly local biomass burning and longrange- transported dust and biological particles (2009), local sources of biomass burning and pollution (2010), and longrange transport (2011). Although differences in the sources of insoluble residues were observed from year to year, the most consistent source of dust and biological residues were associated with storms consisting of deep convective cloud systems with significant quantities of precipitation initiated in the ice phase. Further, biological residues were dominant (up to 40 %) during storms with relatively warm cloud temperatures (up to -15 degrees C), supporting the important role bioparticles can play as ice nucleating particles. On the other hand, lower percentages of residues from local biomass burning and pollution were observed over the three winter seasons (on average 31 and 9 %, respectively). When precipitation quantities were relatively low, these insoluble residues most likely served as CCN, forming smaller more numerous cloud droplets at the base of shallow cloud systems, and resulting in less efficient riming processes. Ultimately, the goal is to use such observations to improve the mechanistic linkages between aerosol sources and precipitation processes to produce more accurate predictive weather forecast models and improve water resource management.

Creamean, JM, Suski KJ, Rosenfeld D, Cazorla A, DeMott PJ, Sullivan RC, White AB, Ralph FM, Minnis P, Comstock JM, Tomlinson JM, Prather KA.  2013.  Dust and biological aerosols from the Sahara and Asia influence precipitation in the Western U.S.. Science. 339:1572-1578.   10.1126/science.1227279   AbstractWebsite

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 orographic precipitation processes over the western United States.

Crochet, M, Cuq F, Ralph FM, Venkateswaran SV.  1990.  Clear-Air Radar Observations of the Great October Storm of 1987. Dynamics of Atmospheres and Oceans. 14:443-461. AbstractWebsite
Dabberdt, WF, Schlatter TW, Carr FH, Friday EWJ, Jorgensen D, Koch S, Pirone M, Ralph FM, Sun JZ, Welsh P, Wilson JW, Zou XL.  2005.  Multifunctional mesoscale observing networks. Bulletin of the American Meteorological Society. 86:961-+.   10.1175/Bams-86-7-961   AbstractWebsite

More than 120 scientists, engineers, administrators, and users met on 8-10 December 2003 in a workshop format to discuss the needs for enhanced three-dimensional mesoscale observing networks. Improved networks are seen as being critical to advancing numerical and empirical modeling for a variety of mesoscale applications, including severe weather warnings and forecasts, hydrology, air-quality forecasting, chemical emergency response, transportation safety, energy management, and others. The participants shared a clear and common vision for the observing requirements: existing two-dimensional mesoscale measurement networks do not provide observations of the type, frequency, and density that are required to optimize mesoscale prediction and nowcasts. To be viable, mesoscale observing networks must serve multiple applications, and the public, private, and academic sectors must all actively participate in their design and implementation, as well as in the creation and delivery of value-added products. The mesoscale measurement challenge can best be met by an integrated approach that considers all elements of an end-to-end solution-identifying end users and their needs, designing an optimal mix of observations, defining the balance between static and dynamic (targeted or adaptive) sampling strategies, establishing long-term test beds, and developing effective implementation strategies. Detailed recommendations are provided pertaining to nowcasting, numerical prediction and data assimilation, test beds, and implementation strategies.

DeFlorio, MJ, Waliser DE, Guan B, Lavers DA, Ralph FM, Vitart F.  2018.  Global assessment of atmospheric river prediction skill. Journal of Hydrometeorology. 19:409-426.   10.1175/jhm-d-17-0135.1   AbstractWebsite

Atmospheric rivers (ARs) are global phenomena that transport water vapor horizontally and are associated with hydrological extremes. In this study, the Atmospheric River Skill (ATRISK) algorithm is introduced, which quantifies AR prediction skill in an object-based framework using Subseasonal to Seasonal (S2S) Project global hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The dependence of AR forecast skill is globally characterized by season, lead time, and distance between observed and forecasted ARs. Mean values of daily AR prediction skill saturate around 7-10 days, and seasonal variations are highest over the Northern Hemispheric ocean basins, where AR prediction skill increases by 15%-20% at a 7-day lead during boreal winter relative to boreal summer. AR hit and false alarm rates are explicitly considered using relative operating characteristic (ROC) curves. This analysis reveals that AR forecast utility increases at 10-day lead over the North Pacific/western U.S. region during positive El Nino-Southern Oscillation (ENSO) conditions and at 7-and 10-day leads over the North Atlantic/U.K. region during negative Arctic Oscillation (AO) conditions and decreases at a 10-day lead over the North Pacific/western U.S. region during negative Pacific-North America (PNA) teleconnection conditions. Exceptionally large increases in AR forecast utility are found over the North Pacific/western United States at a 10-day lead during El Nino + positive PNA conditions and over the North Atlantic/United Kingdom at a 7-day lead during La Nina + negative PNA conditions. These results represent the first global assessment of AR prediction skill and highlight climate variability conditions that modulate regional AR forecast skill.

Dettinger, MD, Ralph FM, Das T, Neiman PJ, Cayan DR.  2011.  Atmospheric rivers, floods and the water resources of California. Water. 3:445-478.   10.3390/W3020445   AbstractWebsite

California's highly variable climate and growing water demands combine to pose both water-supply and flood-hazard challenges to resource managers. Recently important efforts to more fully integrate the management of floods and water resources have begun, with the aim of benefitting both sectors. California is shown here to experience unusually large variations in annual precipitation and streamflow totals relative to the rest of the US, variations which mostly reflect the unusually small average number of wet days per year needed to accumulate most of its annual precipitation totals (ranging from 5 to 15 days in California). Thus whether just a few large storms arrive or fail to arrive in California can be the difference between a banner year and a drought. Furthermore California receives some of the largest 3-day storm totals in the country, rivaling in this regard the hurricane belt of the southeastern US. California's largest storms are generally fueled by landfalling atmospheric rivers (ARs). The fractions of precipitation and streamflow totals at stations across the US that are associated with ARs are documented here and, in California, contribute 20-50% of the state's precipitation and streamflow. Prospects for long-lead forecasts of these fractions are presented. From a meteorological perspective, California's water resources and floods are shown to derive from the same storms to an extent that makes integrated flood and water resources management all the more important.

Dettinger, MD, Ralph FM, Rutz JJ.  2018.  Empirical return periods of the most intense vapor transports during historical atmospheric river landfalls on the US West Coast. Journal of Hydrometeorology. 19:1363-1377.   10.1175/jhm-d-17-0247.1   AbstractWebsite

Atmospheric rivers (ARs) come in all intensities, and clear communication of risks posed by individual storms in observations and forecasts can be a challenge. Modest ARs can be characterized by the percentile rank of their integrated water vapor transport (IVT) rates compared to past ARs. Stronger ARs can be categorized more clearly in terms of return periods or, equivalently, historical probabilities that at least one AR will exceed a given IVT threshold in any given year. Based on a 1980-2016 chronology of AR landfalls on the U.S. West Coast from NASA's Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), datasets, the largest instantaneous IVTsgreater than 1700 kg m(-1) s(-1)have occurred in ARs making landfall between 41 degrees and 46 degrees N with return periods longer than 20 years. IVT values with similar return periods are smaller to the north and, especially, to the south (declining to similar to 750 kg m(-1) s(-1)). The largest storm-sequence IVT totals have been centered near 42.5 degrees N, with scatter among the top few events, and these large storm-sequence totals depend more on sequence duration than on the instantaneous IVT that went into them. Maximum instantaneous IVTs are largest in the Pacific Northwest in autumn, with largest IVT values arriving farther south as winter and spring unfold, until maximum IVTs reach Northern California in spring.

Dettinger, MD, Ralph FM, Hughes M, Das T, Neiman P, Cox D, Estes G, Reynolds D, Hartman R, Cayan D, Jones L.  2012.  Design and quantification of an extreme winter storm scenario for emergency preparedness and planning exercises in California. Natural Hazards. 60:1085-1111.   10.1007/s11069-011-9894-5   AbstractWebsite

The USGS Multihazards Project is working with numerous agencies to evaluate and plan for hazards and damages that could be caused by extreme winter storms impacting California. Atmospheric and hydrological aspects of a hypothetical storm scenario have been quantified as a basis for estimation of human, infrastructure, economic, and environmental impacts for emergency-preparedness and flood-planning exercises. In order to ensure scientific defensibility and necessary levels of detail in the scenario description, selected historical storm episodes were concatentated to describe a rapid arrival of several major storms over the state, yielding precipitation totals and runoff rates beyond those occurring during the individual historical storms. This concatenation allowed the scenario designers to avoid arbitrary scalings and is based on historical occasions from the 19th and 20th Centuries when storms have stalled over the state and when extreme storms have arrived in rapid succession. Dynamically consistent, hourly precipitation, temperatures, barometric pressures (for consideration of storm surges and coastal erosion), and winds over California were developed for the so-called ARkStorm scenario by downscaling the concatenated global records of the historical storm sequences onto 6- and 2-km grids using a regional weather model of January 1969 and February 1986 storm conditions. The weather model outputs were then used to force a hydrologic model to simulate ARkStorm runoff, to better understand resulting flooding risks. Methods used to build this scenario can be applied to other emergency, nonemergency and non-California applications.

Espinoza, V, Waliser DE, Guan B, Lavers DA, Ralph FM.  2018.  Global analysis of climate change projection effects on atmospheric rivers. Geophysical Research Letters. 45:4299-4308.   10.1029/2017gl076968   AbstractWebsite

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 similar to 10% fewer ARs in the future, the ARs will be similar to 25% longer, similar to 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: similar to 50% (25%) globally, similar to 50% (20%) in the northern midlatitudes, and similar to 60% (20%) in the southern midlatitudes. The models exhibit systematic low biases across the midlatitudes in replicating historical AR frequency (similar to 10%), zonal IVT (similar to 15%), and meridional IVT (similar to 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 similar to 10% fewer ARs in the future, the ARs will be similar to 25% longer, similar to 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: similar to 50% (25%) globally, similar to 50% (20%) in the northern midlatitudes, and similar to 60% (20%) in the southern midlatitudes.

Gershunov, A, Shulgina T, Ralph MF, Lavers DA, Rutz JJ.  2017.  Assessing the climate-scale variability of atmospheric rivers affecting western North America. Geophysical Research Letters.   10.1002/2017GL074175   Abstract

A new method for automatic detection of atmospheric rivers (ARs) is developed and applied to an atmospheric reanalysis, yielding an extensive catalog of ARs land-falling along the west coast of North America during 1948–2017. This catalog provides a large array of variables that can be used to examine AR cases and their climate-scale variability in exceptional detail. The new record of AR activity, as presented, validated and examined here, provides a perspective on the seasonal cycle and the interannual-interdecadal variability of AR activity affecting the hydroclimate of western North America. Importantly, AR intensity does not exactly follow the climatological pattern of AR frequency. Strong links to hydroclimate are demonstrated using a high-resolution precipitation data set. We describe the seasonal progression of AR activity and diagnose linkages with climate variability expressed in Pacific sea surface temperatures, revealing links to Pacific decadal variability, recent regional anomalies, as well as a generally rising trend in land-falling AR activity. The latter trend is consistent with a long-term increase in vapor transport from the warming North Pacific onto the North American continent. The new catalog provides unprecedented opportunities to study the climate-scale behavior and predictability of ARs affecting western North America.

Gorodetskaya, IV, Tsukernik M, Claes K, Ralph MF, Neff WD, Van Lipzig NPM.  2014.  The role of atmospheric rivers in anomalous snow accumulation in East Antarctica. Geophysical Research Letters. 41:6199–6206.   10.1002/2014GL060881   AbstractWebsite

Recent, heavy snow accumulation events over Dronning Maud Land (DML), East Antarctica, contributed significantly to the Antarctic ice sheet surface mass balance (SMB). Here we combine in situ accumulation measurements and radar-derived snowfall rates from Princess Elisabeth station (PE), located in the DML escarpment zone, along with the European Centre for Medium-range Weather Forecasts Interim reanalysis to investigate moisture transport patterns responsible for these events. In particular, two high-accumulation events in May 2009 and February 2011 showed an atmospheric river (AR) signature with enhanced integrated water vapor (IWV), concentrated in narrow long bands stretching from subtropical latitudes to the East Antarctic coast. Adapting IWV-based AR threshold criteria for Antarctica (by accounting for the much colder and drier environment), we find that it was four and five ARs reaching the coastal DML that contributed 74–80% of the outstanding SMB during 2009 and 2011 at PE. Therefore, accounting for ARs is crucial for understanding East Antarctic SMB.

Guan, B, Waliser DE, Ralph MF, Fetzer EJ, Neiman PJ.  2016.  Hydrometeorological characteristics of rain-on-snow events associated with atmospheric rivers. 43:2973.   10.1002/2016GL067978   Abstract

Atmospheric rivers (ARs) are narrow, elongated, synoptic corridors of enhanced water vapor transport that play an important role in regional weather/hydrology. Rain-on-snow (ROS) events during ARs present enhanced flood risks due to the combined effects of rainfall and snowmelt. Focusing on California's Sierra Nevada, the study identifies ROS occurrences and their connection with ARs during the 1998–2014 winters. AR conditions, which occur during 17% of all precipitation events, are associated with 50% of ROS events (25 of 50). Composite analysis shows that compared to ARs without ROS, ARs with ROS are on average warmer by ~2 K, with snow water equivalent loss of ~0.7 cm/d (providing 20% of the combined water available for runoff) and ~50% larger streamflow/precipitation ratios. Atmospheric Infrared Sounder retrievals reveal distinct offshore characteristics of the two types of ARs. The results highlight the potential value of observing these events for snow, rain, and flood prediction.

Guan, B, Waliser DE, Ralph FM.  2018.  An intercomparison between reanalysis and dropsonde observations of the total water vapor transport in individual atmospheric rivers. Journal of Hydrometeorology. 19:321-337.   10.1175/jhm-d-17-0114.1   AbstractWebsite

A recent study presented nearly two decades of airborne atmospheric river (AR) observations and concluded that, on average, an individual AR transports similar to 5 x 10(8) 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 -2% (-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 similar to 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.

Guirguis, K, Gershunov A, Clemesha RES, Shulgina T, Subramanian AC, Ralph FM.  2018.  Circulation drivers of atmospheric rivers at the North American West Coast. Geophysical Research Letters. 45:12576-12584.   10.1029/2018gl079249   AbstractWebsite

Atmospheric rivers (ARs) are mechanisms of strong moisture transport capable of bringing heavy precipitation to the West Coast of North America, which drives water resources and can lead to large-scale flooding. Understanding links between climate variability and landfalling ARs is critical for improving forecasts on timescales needed for water resource management. We examined 69years of landfalling ARs along western North America using reanalysis and a long-term AR catalog to identify circulation drivers of AR landfalls. This analysis reveals that AR activity along the West Coast is largely associated with a handful of influential modes of atmospheric variability. Interaction between these modes creates favorable or unfavorable atmospheric states for landfalling ARs at different locations, effectively steering moisture plumes up and down the coast from Mexico to British Columbia. Seasonal persistence of certain modes helps explain interannual variability of landfalling ARs, including recent California drought years and the wet winter of 2016/2017. Plain Language Summary Understanding links between large-scale climate variability and landfalling ARs is important for improving subseasonal-to-seasonal (S2S) predictability of water resources in the western United States. We have analyzed a seven-decade-long catalog of ARs impacting western North America to quantify synoptic influence on AR activity. Our results identify dominant circulation patterns associated with landfalling ARs and show how seasonal variation in the prevalence of certain circulation features modulates the frequency of AR landfalls at different latitudes in a given year. AR variability played an important role in the recent California drought as well as the wet winter of 2016/2017, and we show how this variability was associated with the relative frequency of favorable versus unfavorable atmospheric states. Our findings also reveal that the bulk of AR landfalls along the West Coast is associated with only a handful of influential circulation features, which has implications for S2S predictability.

Hu, HC, Dominguez F, Wang Z, Lavers DA, Zhang G, Ralph FM.  2017.  Linking atmospheric river hydrological impacts on the US West Coast to Rossby wave breaking. Journal of Climate. 30:3381-3399.   10.1175/jcli-d-16-0386.1   AbstractWebsite

Atmospheric rivers (ARs) have significant hydrometeorological impacts on the U.S. West Coast. This study presents the connection between the characteristics of large-scale Rossby wave breaking (RWB) over the eastern North Pacific and the regional-scale hydrological impacts associated with landfalling ARs on the U.S. West Coast (36 degrees-49 degrees N). ARs associated with RWB account for two-thirds of the landfalling AR events and >70% of total AR-precipitation in the winter season. The two regimes of RWB-anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB)-are associated with different directions of the vertically integrated water vapor transport (IVT). AWB-ARs impinge in a more westerly direction on the coast whereas CWB-ARs impinge in a more southwesterly direction. Most of the landfalling ARs along the northwestern coast of the United States (states of Washington and Oregon) are AWB-ARs. Because of their westerly impinging angles when compared to CWB-ARs, AWBARs arrive more orthogonally to the western Cascades and more efficiently transform water vapor into precipitation through orographic lift than CWB-ARs. Consequently, AWB-ARs are associated with the most extreme streamflows in the region. Along the southwest coast of the United States (California), the southwesterly impinging angles of CWBARs are more orthogonal to the local topography. Furthermore, the southwest coast CWB-ARs have more intense IVT. Consequently, CWB-ARs are associated with the most intense precipitation. As a result, most of the extreme streamflows in southwest coastal basins are associated with CWB-ARs. In summary, depending on the associated RWB type, ARs impinge on the local topography at a different angle and have a different spatial signature of precipitation and streamflow.

Jin, Y, Koch SE, Lin YL, Ralph FM, Chen C.  1996.  Numerical simulations of an observed gravity current and gravity waves in an environment characterized by complex stratification and shear. Journal of the Atmospheric Sciences. 53:3570-3588.   10.1175/1520-0469(1996)053<3570:Nsoaog>2.0.Co;2   AbstractWebsite

Numerical simulations of a gravity current in an environment characterized by complex stratification and vertical wind shear have been performed using a nonhydrostatic, two-dimensional, dry, primitive-equation model. Data from one of the most complete documentations to date of gravity waves associated with a gravity current, presented in an earlier study, are used both to prescribe the gravity current's environment and for validation of the simulated gravity current and its associated gravity waves. These comparisons indicate that the gravity current observed by a Doppler wind profiler and sodars was well simulated in terms of depth, density contrast, and propagation speed and that the model produced a variety of gravity waves similar in many ways to those observed.Because uncertainties remained concerning the gravity wave generation mechanisms derived from the observations (e.g., wavelengths were not observed), the validated simulations are used to test these tentative hypotheses. The simulations confirm that trapped lee-type gravity waves formed in response to flow over the head of the gravity current and that Kelvin-Helmholtz (KH) waves were created because of shear atop the cold air within the gravity current. The 2.8-km wavelength of the simulated KH waves agrees with the 2- to 3-km wavelength inferred from the observations. However, the 6.4-km wavelength of the simulated lee-type waves is significantly shorter than the 12.5-km wavelength inferred from the observational data, even though wave periods (20-23 minutes) are nearly identical. Sensitivity tests indicate that the curvature in the wind profile associated with the low-level opposing inflow and an elevated isothermal layer worked together to support the development of the trapped lee-type waves. The model produces a deep vertically propagating wave above the gravity current head that was not present in the observations. As deduced in the earlier study, sensitivity tests indicate that the prefrontal, near-surface stable layer was too shallow to support the generation of a bore; that is, conditions were supercritical. Synthesis of detailed observations and numerical simulations of these mesoscale phenomena thus offers the broadest examination possible of the complex physical processes.