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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.

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.

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.

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.

Ralph, FM, Sukovich E, Reynolds D, Dettinger M, Weagle S, Clark W, Neiman PJ.  2010.  Assessment of extreme quantitative precipitation forecasts and development of regional extreme event thresholds using data from HMT-2006 and COOP observers. Journal of Hydrometeorology. 11:1286-1304.   10.1175/2010jhm1232.1   AbstractWebsite

Extreme precipitation events, and the quantitative precipitation forecasts (QPFs) associated with them, are examined. The study uses data from the Hydrometeorology Testbed (HMT), which conducted its first field study in California during the 2005/06 cool season. National Weather Service River Forecast Center (NWS RFC) gridded QPFs for 24-h periods at 24-h (day 1), 48-h (day 2), and 72-h (day 3) forecast lead times plus 24-h quantitative precipitation estimates (QPEs) from sites in California (CA) and Oregon-Washington (OR-WA) are used. During the 172-day period studied, some sites received more than 254 cm (100 in.) of precipitation. The winter season produced many extreme precipitation events, including 90 instances when a site received more than 7.6 cm (3.0 in.) of precipitation in 24 h (i.e., an "event'') and 17 events that exceeded 12.7 cm (24 h)(-1) [5.0 in. (24 h)(-1)]. For the 90 extreme events {> 7.6 cm (24 h)(-1) [3.0 in. (24 h)(-1)]}, almost 90% of all the 270 QPFs (days 1-3) were biased low, increasingly so with greater lead time. Of the 17 observed events exceeding 12.7 cm (24 h)(-1) [5.0 in. (24 h)(-1)], only 1 of those events was predicted to be that extreme. Almost all of the extreme events correlated with the presence of atmospheric river conditions. Total seasonal QPF biases for all events {i.e., >= 0.025 cm (24 h)(-1) [0.01 in. (24 h)(-1)]} were sensitive to local geography and were generally biased low in the California-Nevada River Forecast Center (CNRFC) region and high in the Northwest River Forecast Center(NWRFC) domain. The low bias in CA QPFs improved with shorter forecast lead time and worsened for extreme events. Differences were also noted between the CNRFC and NWRFC in terms of QPF and the frequency of extreme events. A key finding from this study is that there were more precipitation events > 7.6 cm (24 h)(-1) [3.0 in. (24 h)(-1)] in CA than in OR-WA. Examination of 422 Cooperative Observer Program (COOP) sites in the NWRFC domain and 400 in the CNRFC domain found that the thresholds for the top 1% and top 0.1% of precipitation events were 7.6 cm (24 h)(-1) [3.0 in. (24 h)(-1)] and 14.2 cm (24 h)(-1) [5.6 in. (24 h)(-1)] or greater for the CNRFC and only 5.1 cm (24 h)(-1) [2.0 in. (24 h)(-1)] and 9.4 cm (24 h)(-1) [3.7 in. (24 h)(-1)] for the NWRFC, respectively. Similar analyses for all NWS RFCs showed that the threshold for the top 1% of events varies from similar to 3.8 cm (24 h)(-1) [1.5 in. (24 h)(-1)] in the Colorado Basin River Forecast Center (CBRFC) to similar to 5.1 cm (24 h)(-1) [3.0 in. (24 h)(-1)] in the northern tier of RFCs and; 7.6 cm (24 h)(-1) [3.0 in. (24 h)(-1)] in both the southern tier and the CNRFC. It is recommended that NWS QPF performance in the future be assessed for extreme events using these thresholds.

Ma, ZZ, Kuo YH, Ralph FM, Neiman PJ, Wick GA, Sukovich E, Wang B.  2011.  Assimilation of GPS radio occultation data for an intense atmospheric river with the NCEP regional GSI system. Monthly Weather Review. 139:2170-2183.   10.1175/2010mwr3342.1   AbstractWebsite

This paper uses a case study to explore the potential of Constellation Observing System for Meteorology. Ionosphere, and Climate (COSMIC) and Challenging Minisatellite Payload (CHAMP) global positioning system (GPS) radio occultation (RO) satellite data over the eastern Pacific Ocean to improve analyses and mesoscale forecasts of landfalling atmospheric rivers (ARs) along the U.S. West Coast. The case study is from early November 2006 and was a very high-impact event in the Pacific Northwest where it created torrential rainfall and severe flooding. Recent studies have shown that the COSMIC data offshore have the ability to better define the vertical and horizontal structure of the strong AR. This paper extends the earlier work by assessing the impact of assimilating the COSMIC data into the Advanced Research Weather Research and Forecasting (ARW-WRF) mesoscale numerical model (using a nested mode with 36-, 12-, and 4-km grid sizes) on a key 24-h forecast.The data are assimilated using NCEP's Gridpoint Statistical Interpolation (GSI), and impacts are evaluated using Special Sensor Microwave Imager (SSM/I) satellite observations over the ocean and precipitation observations over land. The assimilation of GPS RO soundings made use of a local refractivity observation operator as well as an advanced nonlocal excess phase observation operator that considers the effects of atmospheric horizontal gradients. The results show that the assimilation of G PS RO soundings improved the moisture analysis for this AR event. This result supports conclusions from earlier observing systems simulation experiment (OSSE) studies, but in a real event. The use of a nonlocal excess phase observation operator can produce larger and more robust analysis increments. Although this is a single case study, the results are likely representative of the potential impacts of assimilating COSMIC data in other extreme AR and precipitation events and in other regions affected by landfalling ARs, for example, western Europe, western South America, and New Zealand.

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.

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

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

White, AB, Gottas DJ, Strem ET, Ralph FM, Neiman PJ.  2002.  An automated brightband height detection algorithm for use with Doppler radar spectral moments. Journal of Atmospheric and Oceanic Technology. 19:687-697.   10.1175/1520-0426(2002)019<0687:Aabhda>2.0.Co;2   AbstractWebsite

Because knowledge of the melting level is critical to river forecasters and other users, an objective algorithm to detect the brightband height from profiles of radar reflectivity and Doppler vertical velocity collected with a Doppler wind profiling radar is presented. The algorithm uses vertical profiles to detect the bottom portion of the bright band, where vertical gradients of radar reflectivity and Doppler vertical velocity are negatively correlated. A search is then performed to find the peak radar reflectivity above this feature, and the brightband height is assigned to the altitude of the peak. Reflectivity profiles from the off-vertical beams produced when the radar is in the Doppler beam swinging mode provide additional brightband measurements. A consensus test is applied to subhourly values to produce a quality-controlled, hourly averaged brightband height. A comparison of radar-deduced brightband heights with melting levels derived from temperature profiles measured with rawinsondes launched from the same radar site shows that the brightband height is, on average, 192 m lower than the melting level. A method for implementing the algorithm and making the results available to the public in near-real time via the Internet is described. The importance of melting level information in hydrological prediction is illustrated using the NWS operational river forecast model applied to mountainous watersheds in California. It is shown that a 2000-ft increase in the melting level can triple run off during a modest 24-h rainfall event. The ability to monitor the brightband height is likely to aid in melting-level forecasting and verification.

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.

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.

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.

Crochet, M, Cuq F, Ralph FM, Venkateswaran SV.  1990.  Clear-Air Radar Observations of the Great October Storm of 1987. Dynamics of Atmospheres and Oceans. 14:443-461. AbstractWebsite
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.

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.

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.

White, AB, Neiman PJ, Ralph FM, Kingsmill DE, Persson POG.  2003.  Coastal orographic rainfall processes observed by radar during the California land-falling jets experiment. Journal of Hydrometeorology. 4:264-282.   10.1175/1525-7541(2003)4<264:Corpob>2.0.Co;2   AbstractWebsite

Radar and rain gauge observations collected in coastal mountains during the California Land-Falling Jets Experiment (CALJET) are used to diagnose the bulk physical properties of rainfall during a wet winter season ( January-March 1998). Three rainfall types were clearly distinguishable by differences in their vertical profiles of radar reflectivity and Doppler vertical velocity: nonbright band, bright band, and hybrid ( seeder-feeder). The contribution of each rainfall type to the total rainfall observed at the radar site ( 1841 mm) was determined by a new, objective algorithm. While hybrid rain occurred most often, nonbrightband rain (NBB rain) contributed significantly (28%) to the total. This paper focuses on characterizing NBB rain because of the need to document this key physical process and because of its impact on Weather Surveillance Radar-1988 Doppler (WSR-88D) precipitation surveillance capabilities.NBB rain is a quasi-steady, shallow rain process that does not exhibit a radar bright band, that occurs largely beneath the melting level, and that can produce rain rates exceeding 20 mm h(-1). Composite vertical profiles were produced for NBB rain using 1417 samples and brightband rain using 5061 samples. Although the mean rain rate for each composite was 3.95 mm h(-1), at all altitudes NBB rain had systematically weaker equivalent radar reflectivity (e.g., 20.5 dBZ(e) vs 28.5 dBZ(e) at 263 m above ground level) and much smaller Doppler vertical fall velocities (e.g., 2.25 m s(-1) vs 6.25 m s(-1) at 263 m) than did brightband rain. The reflectivity-rain-rate (Z-R) relationship for NBB rain (Z = 1.2R(1.8)) differs significantly from that of brightband/ hybrid rain (Z = 207R(1.1)).The meteorological context in which NBB rain occurred is described through case studies and seasonal statistics. NBB rain occurred in a wide variety of positions relative to frontal zones within land-falling storms, but three-quarters of it fell when the layer-mean, profiler-observed wind direction at 1250 m MSL ( the altitude of the composite low-level jet) was between 190degrees and 220degrees. The importance of orographic forcing during NBB rain, relative to all rain events, was indicated by a stronger correlation between upslope wind speed and coastal rain rates at 1250 m MSL (r = 0.74 vs r = 0.54), stronger low-level wind speeds, and wind directions more orthogonal to the mean terrain orientation.

Nuss, WA, Bane JM, Thompson WT, Holt T, Dorman CE, Ralph FM, Rotunno R, Klemp JB, Skamarock WC, Samelson RM, Rogerson AM, Reason C, Jackson P.  2000.  Coastally trapped wind reversals: Progress toward understanding. Bulletin of the American Meteorological Society. 81:719-743.   10.1175/1520-0477(2000)081<0719:Ctwrpt>2.3.Co;2   AbstractWebsite

Coastally trapped wind reversals along the U.S. west coast, which are often accompanied by a northward surge of fog or stratus, are an important warm-season forecast problem due to their impact on coastal maritime activities and airport operations. Previous studies identified several possible dynamic mechanisms that could be responsible for producing these events, yet observational and modeling limitations at the time left these competing interpretations open for debate. In an effort to improve our physical understanding, and ultimately the prediction, of these events, the Office of Naval Research sponsored an Accelerated Research Initiative in Coastal Meteorology during the years 1993-98 to study these and other related coastal meteorological phenomena. This effort included two field programs to study coastally trapped disturbances as well as numerous modeling studies to explore key dynamic mechanisms. This paper describes the various efforts that occurred under this program to provide an advancement in our understanding of these disturbances. While not all issues have been solved, the synoptic and mesoscale aspects of these events are considerably better understood.

Wilczak, JM, Strauch RG, Ralph FM, Weber BL, Merritt DA, Jordan JR, Wolfe DE, Lewis LK, Wuertz DB, Gaynor JE, Mclaughlin SA, Rogers RR, Riddle AC, Dye TS.  1995.  Contamination of Wind Profiler Data by Migrating Birds - Characteristics of Corrupted Data and Potential Solutions. Journal of Atmospheric and Oceanic Technology. 12:449-467.   10.1175/1520-0426(1995)012<0449:Cowpdb>2.0.Co;2   AbstractWebsite

Winds measured with 915- and 404-MHz wind profilers are frequently found to have nonrandom errors as large as 15 m s(-1) when compared to simultaneously measured rawinsonde winds. Detailed studies of these errors, which occur only at night below about 4 km in altitude and have a pronounced seasonal pattern, indicate that they are due to the wind profilers' detection of migrating songbirds (passerines). Characteristics of contaminated data at various stages of data processing are described, including raw time series, individual spectra, averaged spectra, 30- or 60-s moments, 3- or 6-min winds, and hourly averaged winds. An automated technique for the rejection of contaminated data in historical datasets, based on thresholding high values of moment-level reflectivity and spectral width, is shown to be effective. Techniques designed for future wind profiler data acquisition systems are described that show promise for rejecting bird echoes, with the additional capability of being able to retrieve the true wind velocity in many instances. Finally, characteristics of bird migration revealed by wind profilers are described, including statistics of the spring (March-May) 1993 migration season determined from the 404-MHz Wind Profiler Demonstration Network (WPDN). During that time, contamination of moment data occurred on 43% of the nights monitored.

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.

Persson, POG, Neiman PJ, Walter B, Bao JW, Ralph FM.  2005.  Contributions from California coastal-zone surface fluxes to heavy coastal precipitation: A CALJET case study during the strong El Nino of 1998. Monthly Weather Review. 133:1175-1198.   10.1175/Mwr2910.1   AbstractWebsite

Analysis of the case of 3 February 1998, using an extensive observational system in the California Bight during an El Nino winter, has revealed that surface sensible and latent heat fluxes within 150 km of the shore contributed substantially to the destabilization of air that subsequently produced strong convection and flooding along the coast. Aircraft, dropsonde, and satellite observations gathered offshore documented the sea surface temperatures (SSTs), surface fluxes, stratification, and frontal structures. These were used to extrapolate the effects of the fluxes on the warm-sector, boundary layer air ahead of a secondary cold front as this air moved toward the coast. The extrapolated structure was then validated in detail with nearshore aircraft, wind profiler, sounding, and buoy observations of the frontal convection along the coast, and the trajectory transformations were confirmed with a model simulation. The results show that the surface fluxes increased CAPE by about 26% such that the nearshore boundary layer values of 491 J kg(-1) were near the upper end of those observed for cool-season California thunderstorms.The increased CAPE due to upward sensible and latent heat fluxes was a result of the anomalously warm coastal SSTs (+1 degrees-3 degrees C) typical of strong El Nino events. Applications of the extrapolation method using a surface flux parameterization scheme and different SSTs suggested that convective destabilization due to nearshore surface fluxes may only occur during El Nino years when positive coastal SST anomalies are present. The fluxes may have no effect or a stabilizing effect during non-El Nino years, characterized by zero or negative coastal SST anomalies. In short, during strong El Ninos, it appears that the associated coastal SST anomalies serve to further intensify the already anomalously strong storms in southern California, thus contributing to the increased flooding. This modulating effect by El Nino-Southern Oscillation (ENSO) of a mesoscale process has not been considered before in attempts at assessing the impacts of ENSO on U.S. west coast precipitation.

Ralph, FM, Neiman PJ, Keller TL.  1999.  Deep-tropospheric gravity waves created by leeside cold fronts. Journal of the Atmospheric Sciences. 56:2986-3009.   10.1175/1520-0469(1999)056<2986:Dtgwcb>2.0.Co;2   AbstractWebsite

The water vapor channel of the Geostationary Operational Environmental Satellite GOES-8 reveals narrow (30 km wide), elongated (500-1000 km) bands that propagate southward on the eastern side of the Rocky Mountains from Colorado to Texas. Two events in which surface and wind profiler observations show that these bands are associated with leeside cold fronts are documented in detail, and several other cases are summarized. The wind profilers observe vertical motions exceeding 1 m s(-1) in narrow plumes at the leading edge of the fronts, in broader zones in the upper troposphere, and in the lower stratosphere. These cause vertical displacements of up to 1 km and are responsible for the signature in water vapor images.The bands occur when the Rocky Mountains block either arctic leeside cold fronts coining from the north or northeast or Pacific cold fronts coming from the northwest. The blocking changes the frontal orientation and disrupts geostrophic thermal wind balance near the terrain-modified fronts. This imbalance is manifested as strong (20 m s(-1)) prefrontal, front-relative, cross-front flow V-r. Observations and numerical simulations are presented showing that deep-tropospheric gravity waves are produced in this region by the obstacle effect of the surface leeside cold front. Farther east, V-r is near zero, and the waves are weak or absent.Along the western portion of the front the waves propagate with the front and resemble trapped lee waves: however, farther east the waves appear ahead of the surface front by up to 100 km. These prefrontal gravity waves occur when the wave forcing decays along the eastern portion of the Dent and the crapped waves that had developed there become decoupled from the front and propagate away. Numerical simulations of a well-observed event confirm that trapped waves would have developed, and profiler data confirm the trapped nature of the observed gravity wave's vertical structure. Such waves could create convection, including prefrontal squall lines, and can be seen in real-time satellite imagery before the convection is triggered.

Wick, GA, Neiman PJ, Ralph FM.  2013.  Description and validation of an automated objective technique for identification and characterization of the integrated water vapor signature of atmospheric rivers. Ieee Transactions on Geoscience and Remote Sensing. 51:2166-2176.   10.1109/Tgrs.2012.2211024   AbstractWebsite

An automated, objective tool for identifying and characterizing the integrated water vapor (IWV) signature of atmospheric rivers (ARs) based on satellite-observed or model-derived IWV fields has been developed, demonstrated, and validated. ARs are narrow plumes of intense water vapor transport that have been found to be an important contributor to major flooding events in the western U. S. and to seasonal water supply. Previous results demonstrated that the associated IWV signature is an effective proxy for the ARs themselves and that signature is used in this work to characterize the features. The technique employs basic objective criteria for the length (> 2000 km), width (< 1000 km), and IWV content (> 2 cm) of the plumes and standard image processing techniques including thresholding and skeletonization to identify the ARs. Extracted characteristics for the identified plumes include their position, width, core IWV content, orientation, lifetime, and propagation speed. The performance of the AR detection tool (ARDT) was validated over five cool seasons by comparing the AR IWV signature identified by the tool with visually identified events from an existing landfalling AR climatology. The ARDT performed extremely well with a critical success index of 92.4% and a 98.5% probability of detection. Differences were largely the result of subjective decisions in visual classification and tradeoffs in the tool sensitivity between missing actual ARs and inclusion of non-AR features. Future improvements include refined computations of the length and width of AR features and extension of the technique to apply directly to measurements of the water vapor transport. Overall, the ARDT appears well suited for the development of extended AR climatologies and the comparison and verification of forecasts of ARs.

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

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