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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.

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.

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
Ralph, FM, Prather KA, Cayan D, Spackman JR, DeMott P, Dettinger M, Fairall C, Leung R, Rosenfeld D, Rutledge S, Waliser D, White AB, Cordeira J, Martin A, Helly J, Intrieri J.  2016.  CalWater field studies designed to quantify the roles of atmospheric rivers and aerosols in modulating US West Coast precipitation in a changing climate. Bulletin of the American Meteorological Society. 97:1209-1228.   10.1175/bams-d-14-00043.1   AbstractWebsite

The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions.To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemistry, cloud microphysics, and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on the remaining gaps, a vision was developed to extend these studies offshore over the eastern North Pacific and to enhance land-based measurements from 2014 to 2018 (CalWater-2). The dataset and selected results from CalWater-1 are summarized here. The goals of CalWater-2, and measurements to date, are then described.CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.

Ralph, FM, Cordeira JM, Neiman PJ, Hughes M.  2016.  Landfalling atmospheric rivers, the Sierra barrier jet, and extreme daily precipitation in Northern California's Upper Sacramento River watershed. Journal of Hydrometeorology. 17:1905-1914.   10.1175/jhm-d-15-0167.1   AbstractWebsite

The upper Sacramento River watershed is vital to California's water supply and is susceptible to major floods. Orographic precipitation in this complex terrain involves both atmospheric rivers (ARs) and the Sierra barrier jet (SBJ). The south-southeasterly SBJ induces orographic precipitation along south-facing slopes in the Mt. Shasta-Trinity Alps, whereas landfalling ARs ascend up and over the statically stable SBJ and induce orographic precipitation along west-facing slopes in the northern Sierra Nevada. This paper explores the occurrence of extreme daily precipitation (EDP) in this region in association with landfalling ARs and the SBJ. The 50 wettest days (i.e., days with EDP) for water years (WYs) 2002-11 based on the average of daily precipitation from eight rain gauges known as the Northern Sierra 8-Station Index (NS8I) are compared to dates from an SSM/I satellite-based landfalling AR-detection method and dates with SBJ events identified from nearby wind profiler data. These 50 days with EDP accounted for 20% of all precipitation during the 10-WY period, or 5 days with EDP per year on average account for one-fifth of WY precipitation. In summary, 46 of 50 (92%) days with EDP are associated with landfalling ARs on either the day before or the day of precipitation, whereas 45 of 50 (90%) days with EDP are associated with SBJ conditions on the day of EDP. Forty-one of 50 (82%) days with EDP are associated with both a landfalling AR and an SBJ. The top 10 days with EDP were all associated with both a landfalling AR and an SBJ.

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.

Neiman, PJ, Moore BJ, White AB, Wick GA, Aikins J, Jackson DL, Spackman JR, Ralph FM.  2016.  An airborne and ground-based study of a long-lived and intense atmospheric river with mesoscale frontal waves impacting California during CalWater-2014. Monthly Weather Review. 144:1115-1144.   10.1175/mwr-d-15-0319.1   AbstractWebsite

The wettest period during the CalWater-2014 winter field campaign occurred with a long-lived, intense atmospheric river (AR) that impacted California on 7-10 February. The AR was maintained in conjunction with the development and propagation of three successive mesoscale frontal waves. Based on Lagrangian trajectory analysis, moist air of tropical origin was tapped by the AR and was subsequently transported into California. Widespread heavy precipitation (200-400 mm) fell across the coastal mountain ranges northwest of San Francisco and across the northern Sierra Nevada, although only modest flooding ensued due to anomalously dry antecedent conditions. A NOAA G-IV aircraft flew through two of the frontal waves in the AR environment offshore during a ~24-h period. Parallel dropsonde curtains documented key three-dimensional thermodynamic and kinematic characteristics across the AR and the frontal waves prior to landfall. The AR characteristics varied, depending on the location of the cross section through the frontal waves. A newly implemented 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 documented prolonged AR conditions linked to the propagation of the three frontal waves and highlighted the orographic character of the coastal-mountain rainfall with the waves' landfall. A vertically pointing S-PROF radar in the coastal mountains provided detailed information on the bulk microphysical characteristics of the rainfall. 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.

Lavers, DA, Waliser DE, Ralph FM, Dettinger MD.  2016.  Predictability of horizontal water vapor transport relative to precipitation: Enhancing situational awareness for forecasting western US extreme precipitation and flooding. Geophysical Research Letters. 43:2275-2282.   10.1002/2016gl067765   AbstractWebsite

The western United States is vulnerable to socioeconomic disruption due to extreme winter precipitation and floods. Traditionally, forecasts of precipitation and river discharge provide the basis for preparations. Herein we show that earlier event awareness may be possible through use of horizontal water vapor transport (integrated vapor transport (IVT)) forecasts. Applying the potential predictability concept to the National Centers for Environmental Prediction global ensemble reforecasts, across 31 winters, IVT is found to be more predictable than precipitation. IVT ensemble forecasts with the smallest spreads (least forecast uncertainty) are associated with initiation states with anomalously high geopotential heights south of Alaska, a setup conducive for anticyclonic conditions and weak IVT into the western United States. IVT ensemble forecasts with the greatest spreads (most forecast uncertainty) have initiation states with anomalously low geopotential heights south of Alaska and correspond to atmospheric rivers. The greater IVT predictability could provide warnings of impending storminess with additional lead times for hydrometeorological applications.

Lavers, DA, Ralph FM, Waliser DE, Gershunov A, Dettinger MD.  2015.  Climate change intensification of horizontal water vapor transport in CMIP5. Geophysical Research Letters. 42:5617-5625.   10.1002/2015gl064672   AbstractWebsite

Global warming of the Earth's atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most previous research has used precipitation. This study, however, investigates projected changes to global atmospheric water vapor transport (integrated vapor transport (IVT)), the key link between water source and sink regions. Using 22 global circulation models from the Climate Model Intercomparison Project Phase 5, we evaluate, globally, the mean, standard deviation, and the 95th percentiles of IVT from the historical simulations (1979-2005) and two emissions scenarios (2073-2099). Considering the more extreme emissions, multimodel mean IVT increases by 30-40% in the North Pacific and North Atlantic storm tracks and in the equatorial Pacific Ocean trade winds. An acceleration of the high-latitude IVT is also shown. Analysis of low-altitude moisture and winds suggests that these changes are mainly due to higher atmospheric water vapor content.

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.

White, AB, Neiman PJ, Creamean JM, Coleman T, Ralph FM, Prather KA.  2015.  The impacts of California's San Francisco Bay Area gap on precipitation observed in the Sierra Nevada during HMT and CalWater. Journal of Hydrometeorology. 16:1048-1069.   10.1175/jhm-d-14-0160.1   AbstractWebsite

Atmospheric rivers (ARs) are narrow regions of enhanced water vapor transport, usually found on the warm-sector side of the polar cold front in many midlatitude storms formed primarily over the oceans. Nonbrightband (NBB) rain is a shallow orographic rainfall process driven by collision and coalescence that has been observed in some of these storms. NBB rain accounts for about one-third, on average, of the total winter season rainfall occurring at a coastal mountain site in Northern California. During the California Energy Commission's CalWater project, nearly the same fraction of NBB rain was observed at a northern Sierra Nevada foothills site as compared to the coastal mountains, whereas less than half of the fractional amount of NBB rain was observed at a southern Sierra Nevada foothills site. Both Sierra Nevada sites often experience terrain-induced blocked flow, that is, Sierra barrier jet (SBJ) during landfalling winter storms. However, the northern Sierra Nevada site often is oriented geographically downwind of a gap in the coastal terrain near San Francisco during AR landfall. This gap allows maritime air in the AR to arrive at the northern site and enhance the collision-coalescence process in orographic feeder clouds as compared with the southern site. As a result, a greater amount and intensity of NBB rain and overall precipitation was produced at the northern site. This study uses a variety of observations collected in the coastal and Sierra Nevada ranges from the Hydrometeorology Testbed and CalWater field campaigns to document this behavior. A detailed case study provides additional context on the interaction between AR flow, the SBJ, and precipitation processes.

Rutz, JJ, Steenburgh WJ, Ralph FM.  2015.  The inland penetration of atmospheric rivers over Western North America: A Lagrangian analysis. Monthly Weather Review. 143:1924-1944.   10.1175/mwr-d-14-00288.1   AbstractWebsite

Although atmospheric rivers (ARs) typically weaken following landfall, those that penetrate inland can contribute to heavy precipitation and high-impact weather within the interior of western North America. In this paper, the authors examine the evolution of ARs over western North America using trajectories released at 950 and 700 hPa within cool-season ARs along the Pacific coast. These trajectories are classified as coastal decaying, inland penetrating, or interior penetrating based on whether they remain within an AR upon reaching selected transects over western North America. Interior-penetrating AR trajectories most frequently make landfall along the Oregon coast, but the greatest fraction of landfalling AR trajectories that eventually penetrate into the interior within an AR is found along the Baja Peninsula. In contrast, interior-penetrating AR trajectories rarely traverse the southern "high'' Sierra. At landfall, interior-penetrating AR trajectories are associated with a more amplified flow pattern, more southwesterly (vs westerly) flow along the Pacific coast, and larger water vapor transport (qv). The larger initial qv of interior-penetrating AR trajectories is due primarily to larger initial water vapor q and wind speed v for those initiated at 950 and 700 hPa, respectively. Inland- and interior-penetrating AR trajectories maintain large qv over the interior partially due to increases in v that offset decreases in q, particularly in the vicinity of topographical barriers. Therefore, synoptic conditions and trajectory pathways favoring larger initial qv at the coast, limited water vapor depletion by orographic precipitation, and increases in v over the interior are keys to differentiating interior-penetrating from coastal-decaying ARs.

Mahoney, K, Ralph FM, Wolter K, Doesken N, Dettinger M, Gottas D, Coleman T, White A.  2015.  Climatology of extreme daily precipitation in Colorado and its diverse spatial and seasonal variability. Journal of Hydrometeorology. 16:781-792.   10.1175/jhm-d-14-0112.1   AbstractWebsite

The climatology of Colorado's historical extreme precipitation events shows a remarkable degree of seasonal and regional variability. Analysis of the largest historical daily precipitation totals at COOP stations across Colorado by season indicates that the largest recorded daily precipitation totals have ranged from less than 60 mm day(-1) in some areas to more than 250 mm day(-1) in others. East of the Continental Divide, winter events are rarely among the top 10 events at a given site, but spring events dominate in and near the foothills; summer events are most common across the lower-elevation eastern plains, while fall events are most typical for the lower elevations west of the Divide. The seasonal signal in Colorado's central mountains is complex; high-elevation intense precipitation events have occurred in all months of the year, including summer, when precipitation is more likely to be liquid (as opposed to snow), which poses more of an instantaneous flood risk. Notably, the historic Colorado Front Range daily rainfall totals that contributed to the damaging floods in September 2013 occurred outside of that region's typical season for most extreme precipitation (spring-summer). That event and many others highlight the fact that extreme precipitation in Colorado has occurred historically during all seasons and at all elevations, emphasizing a year-round statewide risk.

Neiman, PJ, Gottas DJ, White AB, Schick LJ, Ralph FM.  2014.  The use of snow-level observations derived from vertically profiling radars to assess hydrometeorological characteristics and forecasts over Washington's Green River Basin. Journal of Hydrometeorology. 15:2522-2541.   10.1175/jhm-d-14-0019.1   AbstractWebsite

Two vertically pointing S-band radars (coastal and inland) were operated in western Washington during two winters to monitor brightband snow-level altitudes. Similar snow-level characteristics existed at both sites, although the inland site exhibited lower snow levels by similar to 70 m because of proximity to cold continental air, and snow-level altitude changes were delayed there by several hours owing to onshore translation of weather systems. The largest precipitation accumulations and rates occurred when the snow level was largely higher than the adjacent terrain. A comparison of these observations with long-term operational radiosonde data reveals that the radar snow levels mirrored climatological conditions. The inland radar data were used to assess the performance of nearby operational freezing-level forecasts. The forecasts possessed a lower-than-observed bias of 100-250 m because of a combination of forecast error and imperfect representativeness between the forecast and observing points. These forecast discrepancies increased in magnitude with higher observed freezing levels, thus representing the hydrologically impactful situations where a greater fraction of mountain basins receive rain rather than snow and generate more runoff than anticipated. Vertical directional wind shear calculations derived from wind-profiler data, and concurrent surface temperature data, reveal that most snow-level forecast discrepancies occurred with warm advection aloft and low-level cold advection through the Stampede Gap. With warm advection, forecasts were too high (low) for observed snow levels below (above) 1.25 km MSL. An analysis of sea level pressure differences across the Cascades indicated that mean forecasts were too high (low) for observed snow levels below (above) 1.25 km MSL when higher pressure was west (east) of the range.

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.

Neiman, PJ, Wick GA, Moore BJ, Ralph FM, Spackman JR, Ward B.  2014.  An airborne study of an atmospheric river over the subtropical Pacific during WISPAR: Dropsonde budget-box diagnostics and precipitation impacts in Hawaii. Monthly Weather Review. 142:3199-3223.   10.1175/mwr-d-13-00383.1   AbstractWebsite

The Winter Storms and Pacific Atmospheric Rivers (WISPAR) experiment was carried out in January-March 2011 from the National Aeronautics and Space Administration (NASA) Dryden Flight Research Center as a demonstration for utilizing unmanned aerial systems in meteorological research and operations over data-sparse oceans. One of the campaign's three missions was coordinated with a manned National Oceanic and Atmospheric Administration Gulfstream-IV (G-IV) flight out of Honolulu, Hawaii, on 3-4 March 2011. The G-IV, which flew through a developing atmospheric river (AR) west of Hawaii, represents the cornerstone observing platform for this study and provided the southernmost dropsonde observations of an AR published to date in the subtropical Northern Hemisphere. The AR exhibited characteristics comparable to those observed in previous studies farther north in the subtropics and midlatitudes, save for larger integrated water vapor and weaker winds in the AR core and stronger equatorward vapor fluxes in the shallow post-cold-frontal northeasterly flow. Eight dropsondes released in a similar to 200-km-wide box formation provided a novel kinematic assessment of tropospheric vorticity, divergence (mass, water vapor, sensible heat), and vertical velocity in the AR region, as well as sea surface fluxes. The budget-box diagnostics were physically consistent with global-gridded reanalysis datasets, while also providing useful additional kinematic and thermodynamic information on the mesoscale. Meteorological impacts of the AR were assessed on Hawaii's island of Kauai, where the state's heaviest rainfall was observed for this case. Rainfall on Kauai was modulated significantly by its steep orography, including on the normally dry side of the island where heavy rains fell.

Neiman, PJ, Ralph FM, Moore BJ, Zamora RJ.  2014.  The regional influence of an intense Sierra Barrier jet and landfalling atmospheric river on orographic precipitation in Northern California: A case study. Journal of Hydrometeorology. 15:1419-1439.   10.1175/jhm-d-13-0183.1   AbstractWebsite

A 915-MHz wind profiler, a GPS receiver, and surface meteorological sites in and near California's northern Central Valley (CV) provide the observational anchor for a case study on 23-25 October 2010. The study highlights key orographic influences on precipitation distributions and intensities across northern California during a landfalling atmospheric river (AR) and an associated Sierra barrier jet (SBJ). A detailed wind profiler/GPS analysis documents an intense AR overriding a shallow SBJ at similar to 750 m MSL, resulting in record early season precipitation. The SBJ diverts shallow, pre-cold-frontal, incoming water vapor within the AR poleward from the San Francisco Bay gap to the northern CV. The SBJ ultimately decays following the passage of the AR and trailing polar cold front aloft. A statistical analysis of orographic forcing reveals that both the AR and SBJ are crucial factors in determining the amount and spatial distribution of precipitation in the northern Sierra Nevada and in the Shasta-Trinity region at the northern terminus of the CV. As the AR and SBJ flow ascends the steep and tall terrain of the northern Sierra and Shasta-Trinity region, respectively, the precipitation becomes enhanced. Vertical profiles of the linear correlation coefficient quantify the orographic linkage between hourly upslope water vapor flux profiles and hourly rain rate. The altitude of maximum correlation (i.e., orographic controlling layer) is lower for the shallow SBJ than for the deeper AR (i.e., 0.90 versus 1.15 km MSL, respectively). This case study expands the understanding of orographic precipitation enhancement from coastal California to its interior. It also quantifies the connection between dry antecedent soils and reduced flood potential.

Neff, W, Compo GP, Ralph FM, Shupe MD.  2014.  Continental heat anomalies and the extreme melting of the Greenland ice surface in 2012 and 1889. Journal of Geophysical Research-Atmospheres. 119:6520-6536.   10.1002/2014jd021470   AbstractWebsite

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 3212m 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.

Rutz, JJ, Steenburgh WJ, Ralph FM.  2014.  Climatological characteristics of atmospheric rivers and their inland penetration over the Western United States. Monthly Weather Review. 142:905-921.   10.1175/mwr-d-13-00168.1   AbstractWebsite

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.

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.

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.

Wick, GA, Neiman PJ, Ralph FM, Hamill TM.  2013.  Evaluation of forecasts of the water vapor signature of atmospheric rivers in operational numerical weather prediction models. Weather and Forecasting. 28:1337-1352.   10.1175/Waf-D-13-00025.1   AbstractWebsite

The ability of five operational ensemble forecast systems to accurately represent and predict atmospheric rivers (ARs) is evaluated as a function of lead time out to 10 days over the northeastern Pacific Ocean and west coast of North America. The study employs the recently developed Atmospheric River Detection Tool to compare the distinctive signature of ARs in integrated water vapor (IWV) fields from model forecasts and corresponding satellite-derived observations. The model forecast characteristics evaluated include the prediction of occurrence of ARs, the width of the IWV signature of ARs, their core strength as represented by the IWV content along the AR axis, and the occurrence and location of AR landfall. Analysis of three cool seasons shows that while the overall occurrence of ARs is well forecast out to a 10-day lead, forecasts of landfall occurrence are poorer, and skill degrades with increasing lead time. Average errors in the position of landfall are significant, increasing to over 800 km at 10-day lead time. Also, there is a 1 degrees-2 degrees southward position bias at 7-day lead time. The forecast IWV content along the AR axis possesses a slight moist bias averaged over the entire AR but little bias near landfall. The IWV biases are nearly independent of forecast lead time. Model spatial resolution is a factor in forecast skill and model differences are greatest for forecasts of AR width. This width error is greatest for coarser-resolution models that have positive width biases that increase with forecast lead time.

Neiman, PJ, Hughes M, Moore BJ, Ralph FM, Sukovich EM.  2013.  Sierra barrier jets, atmospheric rivers, and precipitation characteristics in Northern California: A composite perspective based on a network of wind profilers. Monthly Weather Review. 141:4211-4233.   10.1175/Mwr-D-13-00112.1   AbstractWebsite

Five 915-MHz wind profilers and GPS receivers across California's northern Central Valley (CV) and adjacent Sierra foothills and coastal zone, in tandem with a 6-km-resolution gridded reanalysis dataset generated from the Weather Research and Forecasting Model, document key spatiotemporal characteristics of Sierra barrier jets (SBJs), landfalling atmospheric rivers (ARs), and their interactions. Composite kinematic and thermodynamic analyses are based on the 13 strongest SBJ cases observed by the Sloughhouse profiler between 2009 and 2011. The analyses show shallow, cool, south-southeasterly (i.e., Sierra parallel) flow and associated water vapor transport strengthening with time early in the 24-h compositing period, culminating in an SBJ core at <1 km above ground over the eastern CV. The SBJ core increases in altitude up the Sierra's windward slope and poleward toward the north end of the CV, but it does not reach the westernmost CV. Above the developing SBJ, strengthening southwesterly flow descends temporally in response to the landfalling AR. The moistening SBJ reaches maximum intensity during the strongest AR flow aloft, at which time the core of the AR-parallel vapor transport slopes over the SBJ. The inland penetration of the AR through the San Francisco Bay gap in the coastal mountains contributes to SBJ moistening and deepening. The SBJ subsequently weakens with the initial cold-frontal period aloft, during which the shallow flow shifts to southwesterly and the heaviest precipitation falls in the Sierra foothills. An orographic precipitation analysis quantitatively links the Sierra-perpendicular (nearly AR parallel) vapor fluxes to enhanced precipitation along the Sierra's windward slope and the SBJ-parallel fluxes to heavy precipitation at the north end of the CV.

Ralph, FM, Intrieri J, Andra D, Atlas R, Boukabara S, Bright D, Davidson P, Entwistle B, Gaynor J, Goodman S, Jiing JG, Harless A, Huang J, Jedlovec G, Kain J, Koch S, Kuo B, Levit J, Murillo S, Riishojgaard LP, Schneider T, Schneider R, Smith T, Weiss S.  2013.  The emergence of weather-related test beds linking research and forecasting operations. Bulletin of the American Meteorological Society. 94:1187-1211.   10.1175/Bams-D-12-00080.1   AbstractWebsite

Test beds have emerged as a critical mechanism linking weather research with forecasting operations. The U.S. Weather Research Program (USWRP) was formed in the 1990s to help identify key gaps in research related to major weather prediction problems and the role of observations and numerical models. This planning effort ultimately revealed the need for greater capacity and new approaches to improve the connectivity between the research and forecasting enterprise. Out of this developed the seeds for what is now termed test beds. While many individual projects, and even more broadly the NOAA/National Weather Service (NWS) Modernization, were successful in advancing weather prediction services, it was recognized that specific forecast problems warranted a more focused and elevated level of effort. The USWRP helped develop these concepts with science teams and provided seed funding for several of the test beds described. Based on the varying NOAA mission requirements for forecasting, differences in the organizational structure and methods used to provide those services, and differences in the state of the science related to those forecast challenges, test beds have taken on differing characteristics, strategies, and priorities. Current test bed efforts described have all emerged between 2000 and 2011 and focus on hurricanes (Joint Hurricane Testbed), precipitation (Hydrometeorology Testbed), satellite data assimilation (Joint Center for Satellite Data Assimilation), severe weather (Hazardous Weather Testbed), satellite data support for severe weather prediction (Short-Term Prediction Research and Transition Center), mesoscale modeling (Developmental Testbed Center), climate forecast products (Climate Testbed), testing and evaluation of satellite capabilities [Geostationary Operational Environmental Satellite-R Series (GOES-R) Proving Ground], aviation applications (Aviation Weather Testbed), and observing system experiments (OSSE Testbed).

White, AB, Anderson ML, Dettinger MD, Ralph FM, Hinojosa A, Cayan DR, Hartman RK, Reynolds DW, Johnson LE, Schneider TL, Cifelli R, Toth Z, Gutman SI, King CW, Gehrke F, Johnston PE, Walls C, Mann D, Gottas DJ, Coleman T.  2013.  A twenty-first-century California observing network for monitoring extreme weather events. Journal of Atmospheric and Oceanic Technology. 30:1585-1603.   10.1175/jtech-d-12-00217.1   AbstractWebsite

During Northern Hemisphere winters, the West Coast of North America is battered by extratropical storms. The impact of these storms is of paramount concern to California, where aging water supply and flood protection infrastructures are challenged by increased standards for urban flood protection, an unusually variable weather regime, and projections of climate change. Additionally, there are inherent conflicts between releasing water to provide flood protection and storing water to meet requirements for the water supply, water quality, hydropower generation, water temperature and flow for at-risk species, and recreation. To improve reservoir management and meet the increasing demands on water, improved forecasts of precipitation, especially during extreme events, are required. Here, the authors describe how California is addressing their most important and costliest environmental issue-water management-in part, by installing a state-of-the-art observing system to better track the area's most severe wintertime storms.