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2019
Reynolds, CA, Doyle JD, Ralph FM, Demirdjian R.  2019.  Adjoint sensitivity of North Pacific atmospheric river forecasts. Monthly Weather Review. 147:1871-1897.   10.1175/mwr-d-18-0347.1   AbstractWebsite

The initial-state sensitivity and optimal perturbation growth for 24- and 36-h forecasts of low-level kinetic energy and precipitation over California during a series of atmospheric river (AR) events that took place in early 2017 are explored using adjoint-based tools from the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS). This time period was part of the record-breaking winter of 2016-17 in which several high-impact ARs made landfall in California. The adjoint sensitivity indicates that both low-level winds and precipitation are most sensitive to mid- to lower-tropospheric perturbations in the initial state in and near the ARs. A case study indicates that the optimal moist perturbations occur most typically along the subsaturated edges of the ARs, in a warm conveyor belt region. The sensitivity to moisture is largest, followed by temperature and winds. A 1 g kg(-1) perturbation to moisture may elicit twice as large a response in kinetic energy and precipitation as a 1 m s(-1) perturbation to the zonal or meridional wind. In an average sense, the sensitivity and related optimal perturbations are very similar for the kinetic energy and precipitation response functions. However, on a case-by-case basis, differences in the sensitivity magnitude and optimal perturbation structures result in substantially different forecast perturbations, suggesting that optimal adaptive observing strategies should be metric dependent. While the nonlinear evolved perturbations are usually smaller (by about 20%, on average) than the expected linear perturbations, the optimal perturbations are still capable of producing rapid nonlinear perturbation growth. The positive correlation between sensitivity magnitude and wind speed forecast error or precipitation forecast differences supports the relevance of adjoint-based calculations for predictability studies.

Guirguis, K, Gershunov A, Shulgina T, Clemesha RES, Ralph FM.  2019.  Atmospheric rivers impacting Northern California and their modulation by a variable climate. Climate Dynamics. 52:6569-6583.   10.1007/s00382-018-4532-5   AbstractWebsite

Understanding the role of climate variability in modulating the behavior of land-falling atmospheric rivers (ARs) is important for seasonal and subseasonal predictability for water resource management and flood control. We examine daily activity of ARs targeting the Northern California coast over six decades using observations of synoptic-scale circulation, high-resolution precipitation, and a long-term AR detection catalog to quantify distinct types of land-falling ARs categorized by their circulation features. We demonstrate how dramatically different atmospheric states evolve into landfalling ARs along distinct pathways that are modulated by interannual (El Nino/Southern Oscillation (ENSO)and the Pacific Decadal Oscillation) and subseasonal (Arctic Oscillation, Pacific North American Pattern, Western Pacific Oscillation, and the Eastern Pacific Oscillation) modes of large-scale climate variability. Different configurations of climate variability modes are shown to favor ARs having different characteristics in terms of synoptic evolution, integrated vapor transport and landfall orientation resulting in different patterns of precipitation over the landscape. In particular, our results show that while ENSO plays an important role in modulating the synoptic evolution of ARs and their orientation at landfall, subseasonal regional climate modes, which also influence landfall orientation as well as the position of the storm track, appear to be more influential than ENSO in modulating precipitation variability in California. This could have implications for seasonal to subseasonal (S2S) forecasting. Finally, we examine AR activity over the most recent and highly anomalous winter 2016-2017 and show how the unprecedented wet conditions in Northern California were at least partly due to the persistence of ARs characterized by a southward storm track and southerly orientation, which represent the type of ARs associated with heavy rainfall in California, and which are associated with the negative phase of subseasonal regional teleconnection patterns.

Ralph, FM, Wilson AM, Shulgina T, Kawzenuk B, Sellars S, Rutz JJ, Lamjiri MA, Barnes EA, Gershunov A, Guan B, Nardi KM, Osborne T, Wick GA.  2019.  ARTMIP-early start comparison of atmospheric river detection tools: how many atmospheric rivers hit northern California's Russian River watershed? Climate Dynamics. 52:4973-4994.   10.1007/s00382-018-4427-5   AbstractWebsite

Many atmospheric river detection tools (ARDTs) have now been developed. However, their relative performance is not well documented. This paper compares a diverse set of ARDTs by applying them to a single location where a unique 12-year-long time-series from an atmospheric river observatory at Bodega Bay, California is available. The study quantifies the sensitivity of the diagnosed number, duration, and intensity of ARs at this location to the choice of ARDT, and to the choice of reanalysis data set. The ARDTs compared here represent a range of methods that vary in their use of different variables, fixed vs. percentile-based thresholds, geometric shape requirements, Eulerian vs. Lagrangian approaches, and reanalyses. The ARDTs were evaluated first using the datasets documented in their initial publication, which found an average annual count of 19 +/- 7. Applying the ARDTs to the same reanalysis dataset yields an average annual count of 19 +/- 4. Applying a single ARDT to three reanalyses of varying grid sizes (0.5 degrees, 1.0 degrees-2.5 degrees) showed little sensitivity to the choice of reanalysis. While the annual average AR event count varied by about a factor of two (10-25 per year) depending on the ARDT, average AR duration and maximum intensity varied by less than +/- 10%, i.e., 24 +/- 2h duration; 458 +/- 44kg m(-1) s(-1) maximum IVT. ARDTs that use a much higher threshold for integrated vapor transport were compared separately, and yielded just 1-2 ARs annually on average. Generally, ARDTs that include either more stringent geometric criteria or higher thresholds identified the fewest AR events.

Martin, AC, Cornwell G, Beall CM, Cannon F, Reilly S, Schaap B, Lucero D, Creamean J, Ralph FM, Mix HT, Prather K.  2019.  Contrasting local and long-range-transported warm ice-nucleating particles during an atmospheric river in coastal California, USA. Atmospheric Chemistry and Physics. 19:4193-4210.   10.5194/acp-19-4193-2019   AbstractWebsite

Ice-nucleating particles (INPs) have been found to influence the amount, phase and efficiency of precipitation from winter storms, including atmospheric rivers. Warm INPs, those that initiate freezing at temperatures warmer than -10 degrees C, are thought to be particularly impactful because they can create primary ice in mixed-phase clouds, enhancing precipitation efficiency. The dominant sources of warm INPs during atmospheric rivers, the role of meteorology in modulating transport and injection of warm INPs into atmospheric river clouds, and the impact of warm INPs on mixed-phase cloud properties are not well-understood. In this case study, time-resolved precipitation samples were collected during an atmospheric river in northern California, USA, during winter 2016. Precipitation samples were collected at two sites, one coastal and one inland, which are separated by about 35 km. The sites are sufficiently close that air mass sources during this storm were almost identical, but the inland site was exposed to terrestrial sources of warm INPs while the coastal site was not. Warm INPs were more numerous in precipitation at the inland site by an order of magnitude. Using FLEXPART (FLEXible PARTicle dispersion model) dispersion modeling and radar-derived cloud vertical structure, we detected influence from terrestrial INP sources at the inland site but did not find clear evidence of marine warm INPs at either site. We episodically detected warm INPs from long-range-transported sources at both sites. By extending the FLEXPART modeling using a meteorological reanalysis, we demonstrate that long-range-transported warm INPs were observed only when the upper tropospheric jet provided transport to cloud tops. Using radar-derived hydrometeor classifications, we demonstrate that hydrometeors over the terrestrially influenced inland site were more likely to be in the ice phase for cloud temperatures between 0 and -10 degrees C. We thus conclude that terrestrial and long-range-transported aerosol were important sources of warm INPs during this atmospheric river. Meteorological details such as transport mechanism and cloud structure were important in determining (i) warm INP source and injection temperature and (ii) ultimately the impact of warm INPs on mixed-phase cloud properties.

DeFlorio, MJ, Waliser DE, Guan B, Ralph FM, Vitart F.  2019.  Global evaluation of atmospheric river subseasonal prediction skill. Climate Dynamics. 52:3039-3060.   10.1007/s00382-018-4309-x   AbstractWebsite

Subseasonal-to-Seasonal (S2S) forecasts of weather and climate extremes are being increasingly demanded by water resource managers, operational forecasters, and other users in the applications community. This study uses hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) S2S forecast system to evaluate global subseasonal prediction skill of atmospheric rivers (ARs), which are intense lower tropospheric plumes of moisture transport that often project strongly onto extreme precipitation. An aggregate quantity is introduced to assess AR subseasonal prediction skill, defined as the number of AR days occurring over a week-long period (AR1wk occurrence). The observed pattern of seasonal mean AR1wk occurrence strongly resembles the general pattern of daily AR frequency. The ECMWF S2S forecast system generally shows positive (negative) biases relative to reanalysis in the mid-latitude regions in summer (winter) of up to 0.5-1.0 AR days in AR1wk occurrence in regions of highest AR activity. ECMWF AR1wk occurrence forecast skill outperforms a reference forecast based on monthly climatology of AR1wk occurrence at week-3 (14-20days) lead over a number of subtropical to midlatitude regions, with slightly better skill evident in wintertime. The magnitude and subseasonal forecast skill of AR1wk occurrence are shown to vary interannually, and both quantities are modulated during certain phases of the El Nino-Southern Oscillation, Arctic Oscillation, Pacific-North America teleconnection pattern, and Madden-Julian Oscillation.

Zhang, ZH, Ralph FM, Zheng MH.  2019.  The relationship between extratropical cyclone strength and atmospheric river intensity and position. Geophysical Research Letters. 46:1814-1823.   10.1029/2018gl079071   AbstractWebsite

Extratropical cyclones (ECs) and atmospheric rivers (ARs) impact precipitation over the U.S. West Coast and other analogous regions globally. This study investigates the relationship between ECs and ARs by exploring the connections between EC strength and AR intensity and position using a new AR intensity scale. While 82% of ARs are associated with an EC, only 45% of ECs have a paired AR and the distance between the AR and EC varies greatly. Roughly 20% of ARs (defined by vertically integrated water vapor transport) occur without a nearby EC. These are usually close to a subtropical/tropical moisture source and include an anticyclone. AR intensity is only moderately proportional to EC strength. Neither the location nor intensity of an AR can be simply determined by an EC. Greater EC intensification occurs with stronger ARs, suggesting that ARs enhance EC deepening by providing more water vapor for latent heat release. Plain Language Summary Both extratropical cyclones and atmospheric rivers have impact on precipitation over the U.S. West Coast, and they are often mentioned together. However, the relationship between the two is not completely understood. In this study, we have examined the connections between extratropical cyclone strength and atmospheric river intensity and position. While 82% of atmospheric rivers are related to a cyclone, only 45% of cyclones have an accompanied atmospheric river. The distance between the two varies from about 300 km to over 2,000 km. Roughly 20% of atmospheric rivers occur without a nearby cyclone. These cases are close to the subtropical/tropical moisture source and are related to a high pressure. While cyclones can enhance atmospheric rivers with stronger wind, neither the location nor the intensity of an atmospheric river can be simply determined by a cyclone. On the other hand, atmospheric rivers with strong water vapor transport provide favorable conditions for cyclone intensification. Our results provide a comprehensive analysis of the relationship between atmospheric rivers and extratropical cyclones. This work improves the understanding of the dynamical mechanism for heavy precipitation over the U.S. West Coast and thus provides more reliable information on long-term flood control and water planning.

Ralph, FM, Rutz JJ, Cordeira JM, Dettinger M, Anderson M, Reynolds D, Schick LI, Smallcomb C.  2019.  A scale to characterize the strength and impacts of atmospheric rivers. Bulletin of the American Meteorological Society. 100:269-290.   10.1175/bams-d-18-0023.1   AbstractWebsite

Atmospheric rivers (ARs) play vital roles in the western United States and related regions globally, not only producing heavy precipitation and flooding, but also providing beneficial water supply. This paper introduces a scale for the intensity and impacts of ARs. Its utility may be greatest where ARs are the most impactful storm type and hurricanes, nor'easters, and tornadoes are nearly nonexistent. Two parameters dominate the hydrologic outcomes and impacts of ARs: vertically integrated water vapor transport (IVT) and AR duration [i.e., the duration of at least minimal AR conditions (IVT >= 250 kg m(-1) s(-1))]. The scale uses an observed or predicted time series of IVT at a given geographic location and is based on the maximum IVT and AR duration at that point during an AR event. AR categories 1-5 are defined by thresholds for maximum IVT (3-h average) of 250, 500, 750, 1,000, and 1,250 kg m(-1) s(-1), and by IVT exceeding 250 kg m(-1) s(-1) continuously for 24-48 h. If the AR event duration is less than 24 h, it is downgraded by one category. If it is longer than 48 h, it is upgraded one category. The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snowpack, while stronger ARs can become mostly hazardous, for example, if they strike an area with antecedent conditions that enhance vulnerability, such as burn scars or wet conditions. Extended durations can enhance impacts. Short durations can mitigate impacts.

2018
Cannon, F, Hecht CW, Cordeira JM, Ralph FM.  2018.  Synoptic and mesoscale forcing of Southern California extreme precipitation. Journal of Geophysical Research-Atmospheres. 123:13714-13730.   10.1029/2018jd029045   AbstractWebsite

Southern California water resources are heavily dependent on a small number of extreme precipitation events each winter season, which dictate the region's highly variable interannual accumulations. In the Santa Ana River Watershed, on average, three extreme events per year contribute half of annual precipitation, yet there are relatively few studies of the synoptic to mesoscale processes that drive precipitation during these events. This study uses an ingredient-based approach in identifying the contributions of orographic forcing, dynamical forcing, and convective instability to extreme precipitation in the watershed in 107 storms that produced roughly 50% of all precipitation from 1981 to 2017. The influence of dynamical forcing and convective instability on event precipitation distributions is investigated relative to the dominant influence of orographic forcing that is typically found in landfalling atmospheric rivers. Case studies of two high-impact events from the 2017 winter season demonstrate differences in the roles of synoptic ascent and mesoscale convective features in modifying precipitation location, rate, and accumulation over the watershed. The 17 and 18 February 2017 case study included a narrow cold-frontal rainband that produced high-intensity short-duration precipitation over low elevations of the watershed. In the 107 extreme event records, similar modification of the precipitation distribution toward non-orographic rainfall was related to significant changes in the synoptic-scale circulation that favored enhanced dynamics and upstream ascent associated with frontogenesis. Variability in precipitation mechanisms is of primary interest to weather forecasters and water managers as it modifies event impacts and predictability.

Oakley, NS, Cannon F, Munroe R, Lancaster JT, Gomberg D, Ralph FM.  2018.  Brief communication: Meteorological and climatological conditions associated with the 9 January 2018 post-fire debris flows in Montecito and Carpinteria, California, USA. Natural Hazards and Earth System Sciences. 18:3037-3043.   10.5194/nhess-18-3037-2018   AbstractWebsite

The Thomas Fire burned 114078 ha in Santa Barbara and Ventura counties, southern California, during December 2017-January 2018. On 9 January 2018, high-intensity rainfall occurred over the Thomas Fire burn area in the mountains above the communities of Montecito and Carpinteria, initiating multiple devastating debris flows. The highest rainfall intensities occurred with the passage of a narrow rainband along a cold front oriented north to south. Orographic enhancement associated with moist southerly flow immediately ahead of the cold front also played a role. We provide an explanation of the meteorological characteristics of the event and place it in historic context.

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.

Nardi, KM, Barnes EA, Ralph FM.  2018.  Assessment of numerical weather prediction model reforecasts of the occurrence, intensity, and location of atmospheric rivers along the west coast of North America. Monthly Weather Review. 146:3343-3362.   10.1175/mwr-d-18-0060.1   AbstractWebsite

Atmospheric rivers (ARs)-narrow corridors of high atmospheric water vapor transport-occur globally and are associated with flooding and maintenance of the water supply. Therefore, it is important to improve forecasts of AR occurrence and characteristics. Although prior work has examined the skill of numerical weather prediction (NWP) models in forecasting atmospheric rivers, these studies only cover several years of reforecasts from a handful of models. Here, we expand this previous work and assess the performance of 10-30 years of wintertime (November-February) AR landfall reforecasts from the control runs of nine operational weather models, obtained from the International Subseasonal to Seasonal (S2S) Project database. Model errors along the west coast of North America at leads of 1-14 days are examined in terms of AR occurrence, intensity, and landfall location. Occurrence-based skill approaches that of climatology at 14 days, while models are, on average, more skillful at shorter leads in California, Oregon, and Washington compared to British Columbia and Alaska. We also find that the average magnitude of landfall integrated water vapor transport (IVT) error stays fairly constant across lead times, although overprediction of IVT is common at later lead times. Finally, we show that northward landfall location errors are favored in California, Oregon, and Washington, although southward errors occur more often than expected from climatology. These results highlight the need for model improvements, while helping to identify factors that cause model errors.

Viale, M, Valenzuela R, Garreaud RD, Ralph FM.  2018.  Impacts of atmospheric rivers on precipitation in southern South America. Journal of Hydrometeorology. 19:1671-1687.   10.1175/jhm-d-18-0006.1   AbstractWebsite

This study quantifies the impact of atmospheric rivers (ARs) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from 6-hourly CFSR reanalysis data over a 16-yr period (2001-16). AR landfalls were linked to precipitation using a comprehensive observing network that spanned large variations in terrain along and across the Andes from 27 degrees to 55 degrees S, including some sites with hourly data. Along the Pacific (west) coast, AR landfalls are most frequent between 38 degrees and 50 degrees S, averaging 35-40 days yr(-1). This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of similar to 43 degrees S. ARs contribute 45%-60% of the annual precipitation in subtropical Chile (37 degrees-32 degrees S) and 40%-55% along the midlatitude west coast (37 degrees-47 degrees S). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions, roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs is 2-3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather, and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events, and support local nowcasting and forecasting.

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.

Lavers, DA, Rodwell MJ, Richardson DS, Ralph FM, Doyle JD, Reynolds CA, Tallapragada V, Pappenberger F.  2018.  The gauging and modeling of rivers in the sky. Geophysical Research Letters. 45:7828-7834.   10.1029/2018gl079019   AbstractWebsite

Atmospheric rivers (ARs) are responsible for most of the horizontal water vapor flux outside of the tropics and can cause extreme precipitation and affect the atmospheric dynamics and predictability. For their impacts to be skillfully predicted, it is essential for weather forecasting systems to accurately represent AR characteristics. Using the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System and dropsonde observations from the 2018 AR Reconnaissance field campaign over the Northeast Pacific Ocean, it is shown that the AR structure is modeled well but that short-range water vapor flux forecasts have a root-mean-square error of 60.0 kgm(-1) s(-1) (21.9% of mean observed flux). These errors are most related to uncertainties in the winds near the top of the planetary boundary layer. The findings identify a potential barrier in the prediction of high-impact weather and suggest an area where research should be focused to improve atmospheric forecast systems. Plain Language Summary Atmospheric rivers (ARs) are responsible for most of the horizontal transport of water vapor outside of the tropics and can cause extreme precipitation and affect the atmospheric circulation. In this study, we evaluate the ability of a state-of-the-science weather forecasting system to model ARs by using unique atmospheric observations from the 2018 AR Reconnaissance field campaign. Results show that while the AR structure is modeled well, there can be large errors in the water vapor transport which are most related to uncertainties in the low-level winds. These findings identify a potential barrier in the prediction of high-impact weather.

Oakley, NS, Lancaster JT, Hatchett BJ, Stock J, Ralph FM, Roj S, Lukashov S.  2018.  A 22-year climatology of cool season hourly precipitation thresholds conducive to shallow landslides in California. Earth Interactions. 22:1-35.   10.1175/ei-d-17-0029.1   AbstractWebsite

California's winter storms produce intense rainfall capable of triggering shallow landslides, threatening lives and infrastructure. This study explores where hourly rainfall in the state meets or exceeds published values thought to trigger landslides after crossing a seasonal antecedent precipitation threshold. We answer the following questions: 1) Where in California are overthreshold events most common? 2) How are events distributed within the cool season (October-May) and interannually? 3) Are these events related to atmospheric rivers? To do this, we compile and quality control hourly precipitation data over a 22-yr period for 147 Remote Automated Weather Stations (RAWS). Stations in the Transverse and Coast Ranges and portions of the northwestern Sierra Nevada have the greatest number of rainfall events exceeding thresholds. Atmospheric rivers coincide with 60%-90% of these events. Overthreshold events tend to occur in the climatological wettest month of the year, and they commonly occur multiple times within a storm. These state-wide maps depict where to expect intense rainfalls that have historically triggered shallow landslides. They predict that some areas of California are less susceptible to storm-driven landslides solely because high-intensity rainfall is unlikely.

Martin, A, Ralph FM, Demirdjian R, DeHaan L, Weihs R, Helly J, Reynolds D, Iacobellis S.  2018.  Evaluation of atmospheric river predictions by the WRF model using aircraft and regional mesonet observations of orographic precipitation and its forcing. Journal of Hydrometeorology. 19:1097-1113.   10.1175/jhm-d-17-0098.1   AbstractWebsite

Accurate forecasts of precipitation during landfalling atmospheric rivers (ARs) are critical because ARs play a large role in water supply and flooding for many regions. In this study, we have used hundreds of observations to verify global and regional model forecasts of atmospheric rivers making landfall in Northern California and offshore in the midlatitude northeast Pacific Ocean. We have characterized forecast error and the predictability limit in AR water vapor transport, static stability, onshore precipitation, and standard atmospheric fields. Analysis is also presented that apportions the role of orographic forcing and precipitation response in driving errors in forecast precipitation after AR landfall. It is found that the global model and the higher-resolution regional model reach their predictability limit in forecasting the atmospheric state during ARs at similar lead times, and both present similar and important errors in low-level water vapor flux, moist-static stability, and precipitation. However, the relative contribution of forcing and response to the incurred precipitation error is very different in the two models. It can be demonstrated using the analysis presented herein that improving water vapor transport accuracy can significantly reduce regional model precipitation errors during ARs, while the same cannot be demonstrated for the global model.

Nash, D, Waliser D, Guan B, Ye HC, Ralph FM.  2018.  The role of atmospheric rivers in extratropical and polar hydroclimate. Journal of Geophysical Research-Atmospheres. 123:6804-6821.   10.1029/2017jd028130   AbstractWebsite

Atmospheric rivers (ARs) are narrow, long, transient, water vapor-rich corridors of the atmosphere that are responsible for over 90% of the poleward water vapor transport in and across midlatitudes. However, the role of ARs in modulating extratropical and polar hydroclimate features (e.g., water vapor content and precipitation) has not been fully studied, even though moistening of the polar atmosphere is both a key result and amplifier of Arctic warming and sea ice melt, and precipitation is key to the surface mass balance of polar sea ice and ice sheets. This study uses the Modern-Era Retrospective analysis for Research and Applications, Version 2 reanalysis to characterize the roles of AR water vapor transport on the column-integrated atmospheric water vapor budget in the extratropical and polar regions of both hemispheres. Meridional water vapor transport by ARs across a given latitude (examined for 40 degrees, 50 degrees, 60 degrees, and 70 degrees) is strongly related to variations in area-averaged (i.e., over the cap poleward of the given latitude) total water vapor storage and precipitation poleward of that latitude. For the climatological annual cycle, both AR transport (i.e., nonlocal sources) and total evaporation (i.e., local sources) are most correlated with total precipitation, although with slightly different phases. However, for monthly anomalies, the water budget at higher latitudes is largely dominated by the relationship between AR transport and precipitation. For pentad and daily anomalies, AR transport is related to both precipitation and water vapor storage variations. These results demonstrate the important role of episodic, extreme water vapor transports by ARs in modulating extratropical and polar hydroclimate. Plain Language Summary The term atmospheric river (AR) was coined by scientists Zhu and Newell in the early 1990s with the main result highlighting the importance of relatively infrequent, long conduits of strong moisture transport being responsible for most of the poleward transport of moisture across the midlatitudes and into the polar regions. While it is generally understood that this moisture is critical to the water and energy budgets of high latitudes, there have been no studies that have ever quantified the relationship between AR poleward moisture transports and the hydroclimate features of high latitudes. After a long hiatus in the consideration of the role of ARs on global climate since those of Zhu and Newell, this study quantifies the connections between water vapor transport by ARs across specific latitudes (e.g., 40 degrees) and the hydroclimate poleward of this latitude. The findings show there are strong, time scale-dependent (e.g., daily and monthly) connections between ARs and high-latitude hydroclimate features. For example, the findings show a strong relationship between AR water vapor transport at a given latitude and the area-averaged total precipitation of the region poleward. This and other results in this study indicate the importance of ARs in shaping our global weather and climate.

Shields, CA, Rutz JJ, Leung LY, Ralph FM, Wehner M, Kawzenuk B, Lora JM, McClenny E, Osborne T, Payne AE, Ullrich P, Gershunov A, Goldenson N, Guan B, Qian Y, Ramos AM, Sarangi C, Sellars S, Gorodetskaya I, Kashinath K, Kurlin V, Mahoney K, Muszynski G, Pierce R, Subramanian AC, Tome R, Waliser D, Walton D, Wick G, Wilson A, Lavers D, Prabhat, Collow A, Krishnan H, Magnusdottir G, Nguyen P.  2018.  Atmospheric River Tracking Method Intercomparison Project (ARTMIP): project goals and experimental design. Geoscientific Model Development. 11:2455-2474.   10.5194/gmd-11-2455-2018   AbstractWebsite

The Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is an international collaborative effort to understand and quantify the uncertainties in atmospheric river (AR) science based on detection algorithm alone. Currently, there are many AR identification and tracking algorithms in the literature with a wide range of techniques and conclusions. ARTMIP strives to provide the community with information on different methodologies and provide guidance on the most appropriate algorithm for a given science question or region of interest. All ARTMIP participants will implement their detection algorithms on a specified common dataset for a defined period of time. The project is divided into two phases: Tier 1 will utilize the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalysis from January 1980 to June 2017 and will be used as a baseline for all subsequent comparisons. Participation in Tier 1 is required. Tier 2 will be optional and include sensitivity studies designed around specific science questions, such as reanalysis uncertainty and climate change. High-resolution reanalysis and/or model output will be used wherever possible. Proposed metrics include AR frequency, duration, intensity, and precipitation attributable to ARs. Here, we present the ARTMIP experimental design, timeline, project requirements, and a brief description of the variety of methodologies in the current literature. We also present results from our 1-month "proof-of-concept" trial run designed to illustrate the utility and feasibility of the ARTMIP project.

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.

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.

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.

2017
Lamjiri, MA, Dettinger MD, Ralph FM, Guan B.  2017.  Hourly storm characteristics along the US West Coast: Role of atmospheric rivers in extreme precipitation. Geophysical Research Letters. 44:7020-7028.   10.1002/2017gl074193   AbstractWebsite

Gridded hourly precipitation observations over the conterminous U.S., from 1948 to 2002, are analyzed to determine climatological characteristics of storm precipitation totals. Despite generally lower hourly intensities, precipitation totals along the U.S. West Coast (USWC) are comparable to those in southeast U.S. (SEUS). Storm durations, more so than hourly intensities, strongly modulate precipitation- total variability over the USWC, where the correlation coefficients between storm durations and storm totals range from 0.7 to 0.9. Atmospheric rivers (ARs) contribute 30-50% of annual precipitation on the USWC and make such large contributions to extreme storms that 60-100% of the most extreme storms, i.e., storms with precipitation- total return intervals longer than 2 years, are associated with ARs. These extreme storm totals are more strongly tied to storm durations than to storm hourly or average intensities, emphasizing the importance of AR persistence to extreme storms on the USWC.

Sellars, SL, Kawzenuk B, Nguyen P, Ralph FM, Sorooshian S.  2017.  Genesis, pathways, and terminations of intense global water vapor transport in association with large-scale climate patterns. Geophysical Research Letters. 44:12465-12475.   10.1002/2017gl075495   AbstractWebsite

The CONNected objECT (CONNECT) algorithm is applied to global Integrated Water Vapor Transport data from the NASA's Modern-Era Retrospective Analysis for Research and Applications - Version 2 reanalysis product for the period of 1980 to 2016. The algorithm generates life-cycle records in time and space evolving strong vapor transport events. We show five regions, located in the midlatitudes, where events typically exist (off the coast of the southeast United States, eastern China, eastern South America, off the southern tip of South Africa, and in the southeastern Pacific Ocean). Global statistics show distinct genesis and termination regions and global seasonal peak frequency during Northern Hemisphere late fall/winter and Southern Hemisphere winter. In addition, the event frequency and geographical location are shown to be modulated by the Arctic Oscillation, Pacific North American Pattern, and the quasi-biennial oscillation. Moreover, a positive linear trend in the annual number of objects is reported, increasing by 3.58 objects year-over-year. Plain Language Summary A computational science approach to tracking global atmospheric water vapor plumes is applied to a NASA data set from 1980 to 2016. Results show regions of the globe where intense water vapor transport often exists, including their genesis and termination locations. Winter time months tend to have more water vapor plumes in both the Southern and Northern Hemispheres. In addition, climate phenomena also have an impact on the frequency and location of these water vapor plumes.

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.

Ralph, FM, Galarneau TJ.  2017.  The Chiricahua Gap and the Role of Easterly Water Vapor Transport in Southeastern Arizona Monsoon Precipitation. Journal of Hydrometeorology. 18:2511-2520.   10.1175/jhm-d-17-0031.1   AbstractWebsite

Between North America's Sierra Madre and Rocky Mountains exists a little-recognized terrain "gap.'' This study defines the gap, introduces the term "Chiricahua Gap,'' and documents the role of easterly transport of water vapor through the gap in modulating summer monsoon precipitation in southeastern Arizona. The gap is near the Arizona-New Mexico border north of Mexico and is approximately 250 km wide by 1 km deep. It is the lowest section along a 3000-km length of the Continental Divide from 168 to 45 degrees N and represents 80% of the total cross-sectional area below 2.5 km MSL open to horizontal water vapor transport in that region. This study uses reanalyses and unique upper-air observations in a case study and a 15-yr climatology to show that 72% (76%) of the top-quartile (decile) monsoon precipitation days in southeast Arizona during 2002-16 occurred in conditions with easterly water vapor transport through the Chiricahua Gap on the previous day.