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Open Access Publications from the University of California

Scholarly Works (29 results)

  • Article
Research Reports (2023)

This project is to investigate how accessibility of city blocks is quantified through the transport systems and real traffic flow datafrom the Los Angeles Metropolitan Area. The authors investigate the reachability problem and provide a solution with a functional system that is capable of visualizing the reachability map (isochrone). Unlike other studies, this approach is data-driven and does not depend on mathematical graph-theory to compute the isochrone which requires intensive computation. Instead, it focuses on directly processing the large amount of traffic flow data that the Integrated Media Systems Center at USC has collected from the Regional Integration of Intelligent Transportation Systems (RIITS) for more than 10 years under the Center’s existing Archived Traffic Data Management System (ADMS) project. The reachability map construction is based on vehicle trajectories so the researchers devised the Data-Driven Trajectory Generator (DDTG), a data-driven, model-free, and parameter-less algorithm for generating realistic vehicle trajectory datasets from ADMS data. Since real world traffic is incomplete with lots of temporal and spatial missing data, the researchers studied imputation and interpolation methods to complete the dataset. Their experiments with real-world trajectory and traffic data show that the datasets generated by DDTG follow distributions that are very close to the distributions of a real trajectory dataset. Furthermore, to demonstrate the resultsfrom the proposed research, a web application was developed in which users can select a location, travel time, and the time of year to see the evaluated accessibility info in the form of an isochrone map. The outcomes of this project—synthetic vehicle trajectory dataset and reachability map construction—will be helpful in evaluating accessibility of city blocks for transport systems over a large area, essential for policymakers for effective city planning as well as to improve the well-being of citizens.

View the NCST Project Webpage

    Cover page: Evaluating Accessibility of Los Angeles Metropolitan Area Using Data-Driven Time-Dependent Reachability Analysis
    • Article
    • Peer Reviewed
    Faculty Publications (2011)
    This study examines the linkages between leading patterns of interannual sea level pressure (SLP) variability over the extratropical Pacific (20°–60°N) and the eastern Pacific (EP) and central Pacific (CP) types of El Niño–Southern Oscillation (ENSO). The first empirical orthogonal function (EOF) mode of the extratropical SLP anomalies represents variations of the Aleutian low, and the second EOF mode represents the North Pacific Oscillation (NPO) and is characterized by a meridional SLP anomaly dipole with a nodal point near 50°N. It is shown that a fraction of the first SLP mode can be excited by both the EP and CP types of ENSO. The SLP response to the EP type is stronger and more immediate. The tropical–extratropical teleconnection appears to act more slowly for the CP ENSO. During the decay phase of EP events, the associated extratropical SLP anomalies shift from the first SLP mode to the second SLP mode. As the second SLP mode grows, subtropical SST anomalies are induced beneath via surface heat flux anomalies. The SST anomalies persist after the peak in strength of the second SLP mode, likely because of the seasonal footprinting mechanism, and lead to the development of the CP type of ENSO. This study shows that the CP ENSO is an extratropically excited mode of tropical Pacific variability and also suggests that the decay of an EP type of ENSO can lead to the onset of a CP type of ENSO with the aid of the NPO. This extratropical linking mechanism appears to be at work during the 1972, 1982, and 1997 strong El Niño events, which were all EP events and were all followed by strong CP La Niña events after the NPO was excited in the extratropics. This study concludes that extratropical SLP variations play an important role in exciting the CP type of ENSO and in linking the transitions from the EP to CP events.
      Cover page: Relationships between Extratropical Sea Level Pressure Variations and the Central Pacific and Eastern Pacific Types of ENSOCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2011)
      This study examines preindustrial simulations from Coupled Model Intercomparison Project, phase 3 (CMIP3), models to show that a tendency exists for El Niño sea surface temperature anomalies to be located farther eastward than La Niña anomalies during strong El Niño-Southern Oscillation (ENSO) events but farther westward than La Niña anomalies during weak ENSO events. Such reversed spatial asymmetries are shown to force a slow change in the tropical Pacific Ocean mean state that in return modulates ENSO amplitude. CMIP3 models that produce strong reversed asymmetries experience cyclic modulations of ENSO intensity, in which strong and weak events occur during opposite phases of a decadal variability mode associated with the residual effects of the reversed asymmetries. It is concluded that the reversed spatial asymmetries enable an ENSO-tropical Pacific mean state interaction mechanism that gives rise to a decadal modulation of ENSO intensity and that at least three CMIP3 models realistically simulate this interaction mechanism. © 2011 American Meteorological Society.
        Cover page: Reversed Spatial Asymmetries between El Niño and La Niña and Their Linkage to Decadal ENSO Modulation in CMIP3 ModelsCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        Faculty Publications (2012)
        [1] In this study, we evaluate the intensity of the Central-Pacific (CP) and Eastern-Pacific (EP) types of El Niño-Southern Oscillation (ENSO) simulated in the pre-industrial, historical, and the Representative Concentration Pathways (RCP) 4.5 experiments of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Compared to the CMIP3 models, the pre-industrial simulations of the CMIP5 models are found to (1) better simulate the observed spatial patterns of the two types of ENSO and (2) have a significantly smaller inter-model diversity in ENSO intensities. The decrease in the CMIP5 model discrepancies is particularly obvious in the simulation of the EP ENSO intensity, although it is still more difficult for the models to reproduce the observed EP ENSO intensity than the observed CP ENSO intensity. Ensemble means of the CMIP5 models indicate that the intensity of the CP ENSO increases steadily from the pre-industrial to the historical and the RCP4.5 simulations, but the intensity of the EP ENSO increases from the pre-industrial to the historical simulations and then decreases in the RCP4.5 projections. The CP-to-EP ENSO intensity ratio, as a result, is almost the same in the pre-industrial and historical simulations but increases in the RCP4.5 simulation.
          Cover page: The two types of ENSO in CMIP5 modelsCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          Faculty Publications (2011)
          This study examines preindustrial simulations from Coupled Model Intercomparison Project, phase 3 (CMIP3), models to show that a tendency exists for El Niño sea surface temperature anomalies to be located farther eastward than La Niña anomalies during strong El Niño–Southern Oscillation (ENSO) events but farther westward than La Niña anomalies during weak ENSO events. Such reversed spatial asymmetries are shown to force a slow change in the tropical Pacific Ocean mean state that in return modulates ENSO amplitude. CMIP3 models that produce strong reversed asymmetries experience cyclic modulations of ENSO intensity, in which strong and weak events occur during opposite phases of a decadal variability mode associated with the residual effects of the reversed asymmetries. It is concluded that the reversed spatial asymmetries enable an ENSO–tropical Pacific mean state interaction mechanism that gives rise to a decadal modulation of ENSO intensity and that at least three CMIP3 models realistically simulate this interaction mechanism.
            Cover page: Reversed Spatial Asymmetries between El Niño and La Niña and Their Linkage to Decadal ENSO Modulation in CMIP3 ModelsCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            Faculty Publications (2012)
            This study develops a pattern correlation method to determine the type of major El Niño events since 1870 from a reconstructed sea surface temperature dataset. Different from other identification methods, this method allows an El Niño event to be of the Central-Pacific (CP) type, the Eastern-Pacific (EP) type, or the Mixed type (i.e. the both types coexist). Application of this method to the 39 major El Niño events identified by the Ocean Niño Index during the period 1870–2010 results in 8 events that are categorized to be of the EP type, 16 of the CP type, and 15 of Mixed type. Before the 1910s, the El Niño events are mostly of the EP type, but are mostly the CP type after 2000, while in between both types occurred. The consistencies and inconsistencies between the El Niño types identified by this method and other three methods, which have been proposed recently for El Niño-type classification, are examined and discussed. All four methods consistently identify the El Niño events occurring in the following years to be of the EP types: 1876–1877, 1881, 1884–1885, 1895–1896, 1896–1897, 1918–1919, 1982–1983, and 1997–1998; and the events occurring in the following years to be of the CP type: 1968–1969, 1977–1978, 1994–1995, 2004–2005, and 2009–2010. It is evident that the characteristics of the EP type of El Niño are more robust in the 19th century and the early part of the 20th century, whereas the characteristics of the CP type of El Niño is more robust in the late 20th century and the early 21st century. The list of the El Niño types produced by this study can be used for selecting El Niño events to further study the dynamics and climate impacts of the EP, CP, and Mixed types of El Niño.
              Cover page: Identifying the types of major El Niño events since 1870Creative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              Faculty Publications (2010)
              [1] Much understanding of the El Niño-Southern Oscillation (ENSO) has been obtained from the analyses of the climate simulations produced for World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3). However, most of these analyses do not consider the existence of the Eastern-Pacific (EP) and Central-Pacific (CP) types of ENSO events, which have been increasingly recognized as two distinct types of interannual sea surface temperature (SST) variation in the tropical Pacific. This study uses a regression-Empirical Orthogonal Function method to identify how well these two ENSO types are captured in the pre-industrial simulations of nineteen CMIP3 models. It is concluded that most CMIP3 models (13 out of 19) can produce realistically strong CP ENSOs, but only a few of them (9 out of 19) can produce realistically strong EP ENSOs. Six models that realistically simulate both the EP and CP ENSOs and their intensity ratio are identified. By separating the SST variability into these two types, it is further revealed that the leading periodicity of the simulated EP ENSO is linearly related to the latitudinal width of SST variability and varies from 1 to 5 years. As for the simulated CP ENSO, its leading periodicity is either 2 or 4 years depending on whether its SST variability is located to the east of the dateline or in the western-Pacific warm pool, respectively. The identification produced in this study offers useful information to further understand the two types of ENSO using the CMIP3 models.
                Cover page: Identification of Central-Pacific and Eastern-Pacific types of ENSO in CMIP3 ModelsCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2011)
                This study examines the linkages between leading patterns of interannual sea level pressure (SLP) variability over the extratropical Pacific (20°-60°N) and the eastern Pacific (EP) and central Pacific (CP) types of El Niño-Southern Oscillation (ENSO). The first empirical orthogonal function (EOF) mode of the extratropical SLP anomalies represents variations of the Aleutian low, and the second EOF mode represents the North Pacific Oscillation (NPO) and is characterized by a meridional SLP anomaly dipole with a nodal point near 50°N. It is shown that a fraction of the first SLP mode can be excited by both the EP and CP types of ENSO. The SLP response to the EP type is stronger and more immediate. The tropical-extratropical teleconnection appears to act more slowly for the CP ENSO. During the decay phase of EP events, the associated extratropical SLP anomalies shift from the first SLP mode to the second SLP mode. As the second SLP mode grows, subtropical SST anomalies are induced beneath via surface heat flux anomalies. The SST anomalies persist after the peak in strength of the second SLP mode, likely because of the seasonal footprinting mechanism, and lead to the development of the CP type of ENSO. This study shows that the CP ENSO is an extratropically excited mode of tropical Pacific variability and also suggests that the decay of an EP type of ENSO can lead to the onset of a CP type of ENSO with the aid of the NPO. This extratropical linking mechanism appears to be at work during the 1972, 1982, and 1997 strong El Niño events, which were all EP events and were all followed by strong CP La Niña events after the NPO was excited in the extratropics. This study concludes that extratropical SLP variations play an important role in exciting the CP type of ENSO and in linking the transitions from the EP to CP events. © 2011 American Meteorological Society.
                  Cover page: Relationships between Extratropical Sea Level Pressure Variations and the Central Pacific and Eastern Pacific Types of ENSOCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  Faculty Publications (2010)
                  [1] Three evolution patterns are identified for the Central-Pacific (CP) type of El Niño based on events that occurred during 1958–2007: (1) a symmetric-decaying pattern whose sea surface temperature anomalies grow and decay symmetrically with respect to a peak phase; (2) a prolonged-decaying pattern that decays slowly and is followed by a warm event in the eastern Pacific (EP); and (3) an abrupt-decaying pattern that terminates rapidly after the peak and is followed by a cold event in the EP. The depth of the equatorial thermocline is found to determine which evolution pattern occurs. If the CP El Niño occurs in a recharged thermocline state (i.e., deeper-than-normal depth), an EP warming may appear in the decaying phase of the CP event to slow down the decay, giving rise to the prolonged-decaying pattern. If the thermocline is in a discharged state (i.e., shallower-than-normal depth), an EP cooling may occur to abruptly terminate the CP El Niño. If the thermocline is in a neutral state (i.e., normal depth), the CP event may have a symmetric pattern of growth and decay. Although a few exceptions exist, these results indicate that the equatorial thermocline state at the peak phase of a CP El Niño event can be a potential predictor of the way the event may decay.
                    Cover page: Three Evolution Patterns of Central-Pacific El NiñoCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Irvine Previously Published Works (2012)
                      Cover page: The changing impact of El Niño on US winter temperatures
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