1,720,979 research outputs found
Twentieth century temperature trends in CMIP3, CMIP5, and CESM-LE climate simulations – spatial-temporal uncertainties, differences and their potential sources
No full textThe twentieth century climate simulations from the Coupled Model Intercomparison Project Phase 3 (CMIP3) and Phase 5 (CMIP5) are compared to assess the models' ability to capture observed near-surface air temperature trends at global, continental, and regional scales. We computed trends by using a nonparametric method and considering long-term persistence in the time series. The role of internal variability is examined by using large ensemble climate simulations from the National Center for Atmospheric Research model Community Earth System Model (CESM). We computed temperature trends for three periods: (1) the twentieth century, (2) the second half of the twentieth century, and (3) the recent hiatus period to contrast the roles of external forcing and internal variability at various spatial and temporal scales. Both CMIP ensembles show statistically significant warming at global and continental scales during the twentieth century. We found a small but statistically significant difference between CMIP3 (0.57?±?0.07?°C/century) and CMIP5 (0.47?±?0.06?°C/century) twentieth century temperature trends, with the CMIP3 estimate being closer to the observations. The spatial structure of long-term temperature trends, and top-of-the atmosphere net radiation trends, suggests that differences in model parameterizations and feedback processes that lead to a smaller net radiative forcing are likely contributing to the differences between CMIP3 and CMIP5. The estimate of internal variability based on the CESM large ensemble spans 24% of the uncertainty in CMIP5 for the twentieth century temperature trends, and 76% for the recent hiatus period, both at global scales, and 43% and almost 100% during the corresponding time periods at regional scales
El Niño and the Southern Oscillation in Parameterized and Super-Parameterized Coupled General Circulation Models
No full textThe explicit treatment of cloud-scale processes in a super-parameterized coupled general circulation model (SP-CGCM) is known to produce improved total heating and low-level wind variability on interannual time scales relative to a model with conventional parameterization of convection. In this study, a novel linear statistical adjustment method has been explored to introduce a state-dependent adjustment such that the dominant modes of variability in the model with the explicit treatment of clouds are captured in the conventional model. This method was applied to isolate and quantify the impact of the horizontal gradients and time evolution of the surface stress forcing of the ocean due to the interannual surface stress variability of the SP-CGCM. The usefulness of this approach in isolating the effects of equatorial and off-equatorial oceanic wave response to the interannual surface stress forcing is demonstrated.This work is embargoed by the author and will not be available until June 2014
Multi-Decadal Trends of Precipitation and Flooding in the James River Watershed
Full text linkThe trends in precipitation and flooding for the James River watershed are examined across the timeframe of 1951-2022. Precipitation is analyzed using spatial linear and quantile regressions across the watershed, which includes individually examining the trends across the warm months (April-September) and cold months (October-March). Floods are catalogued at selected stream sites to determine recent changes in frequency of events, as well as correlated with precipitation trends in terms of annual days above a threshold value. The James River watershed shows statistically significant positive trends for daily and extreme precipitation in the nearly the entire basin, especially during the warm months. Minor flood events at Richmond increased by 46% in the cold months and 131% in the warm months during 1979-2022 compared to 1935-1978. Precipitation accumulations preceding floods at Richmond were much higher during the warm months than the cold months. Richmond also displays a highly significant positive relationship between days with extreme precipitation and days above action or minor flood stage. The results indicate a strong potential for applying the analysis to other watersheds on a regional scale, which would provide implications of precipitation and flooding trends for a much larger area
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Seasonality of the Tropical Intraseasonal Oscillations: Sensitivity to Mean Background State
Full text linkThis study investigates the seasonality of tropical intraseasonal oscillations (TISO) in Earths current climate and its relationship with the inter-hemispherical migration of the climatological mean maximum sea surface temperature (SST) and the tropical core of the low-level westerly wind. TISO is identified with anomalies of atmospheric convection with large spatial scale (10^5 km^2) that characteristically exist on the intra-seasonal time scale (20- 100 days period). A new method for tracking the large spatial scale features of convective anomalies, measured by outgoing long-wave radiation (OLR), is developed, based on a two-stage Kalman filter predictor-corrector method. Two dominant components of TISO (eastward-propagating and northward-propagating) are classified, and it is found that TISO remains active throughout the year, with eastward propagation of TISO events occurring from November to April and northward propagating events occurring from May to October. The eastward events have a phase speed of 4 m/s, while the northward events propagate at 2 m/s in both the Indian and Pacific Ocean basins. A composite analysis of the mean background states (zonal wind, SST and low-level moisture) reveals that the co-occurrence of warm climatological SST and mean westerly wind plays an important role in the direction of propagation and geographical location of TISO. It is hypothesized that the geographical location of TISO occurrences is coupled with SST, moisture and lower tropospheric circulation. The seasonal migration of the mean background state is a potential determinant of the seasonal changes in the characteristics of TISO. A Lagrangian composite analysis with respect to the center of mass of the each convective cloud system was done separately for eastward-propagating TISO events, northward propagating TISO events over the Indian Ocean and northward-propagating TISO events over the west Pacific Ocean. The analysis suggests that the average size of eastward propagating events is 10^6 km^2 and the OLR anomaly at the center of convection is -50 W/m2, and size of northward propagating events is 106 km2 and the OLR anomaly at the center of convection is -45 W/m2. The spatial asymmetry in the mean background state composite moisture, moist static energy, moisture convergence, and vertical velocity all suggest that the development phase of convection lies east of the convection center. A slight shift in moisture anomalies ahead of the convection center and moistening (drying) ahead of (behind) the convection is found in both eastward and northward propagating TISO events
THE ROLE OF SOIL MOISTURE STATE ON THE INCIDENCE OF SUMMER MESOSCALE CONVECTIVE SYSTEMS IN THE U.S. GREAT PLAINS
Full text linkMesoscale convective systems (MCS) are known to develop in strongly-forced synoptic environments, but recent work has shown that summer MCS events in the U.S. Great Plains can occur in weakly-forced synoptic environments, and convection in this case may be triggered by anomalous soil moisture (SM) conditions. Using an MCS database covering the contiguous U.S. east of the Rocky Mountains in boreal summers 2004–17, this dissertation aims to identify the SM conditions associated with summertime MCS initiations in the U.S. Great Plains, and to identify a mechanism of interaction between the SM state and MCS initiation through the lens of local land-atmosphere (L-A) coupling. MCS events that occur in weakly-forced synoptic environments are isolated using the Convective Triggering Potential and Low-Level Humidity Index (CTP-HI_low), and their SM conditions are evaluated centered on the mean initiation position of a 5° X 5° domain. At larger scales [O(100) km], soil moisture heterogeneity, where anomalously dry soils (anomalously wet soils) are located southwest (northeast) of the initiation point, is associated with MCS initiation. These dry and wet soils set up a surface temperature gradient, which is hypothesized to produce a surface pressure gradient that drives a near-surface wind perturbation with convergence and upward motion to stimulate convection. To test this hypothetical mechanism, numerical experiments are performed with the Weather Research and Forecasting (WRF) model by imposing idealized, exaggerated SM perturbations (PB) having varying organizations and alignments to the near-surface wind. The MCS initiation locations are used as the center point for a 5° X 5° domain where SM is changed, with five total SM PB. A storm-centered analysis of SM showed that PB-1, where SM is uniformly set to a near-wilting point in the experiment domain, was the only experiment that consisted of organized heterogeneity [O(100)km], structured as significantly drier soils in the southwest transitioning to wetter soils in the northeast. Further analyses showed the PB-1 experiment demonstrated the largest thermodynamic and dynamic response, where the mechanism of interaction was driven by SM values in the transitional regime promoting strong local L-A coupling (these SM values are associated with smaller evaporative fraction and larger sensible heat flux values that influenced the growth of the PBLH), and the spatial SM gradient aligned with the near-surface wind produced additional uplift to generate convection. While the PB-1 experiment was intended to evaluate the response to homogeneously dry SM, an artifact of imposition of the SM perturbation in the model domain set up a large dry-to-wet SM gradient along the eastern and northern edges of the PB area, and MCS initiations were subsequently preferred over those transition zones. These cumulative findings are similar to the previous results in the Sahel that suggest SM heterogeneity can drive MCS initiation, with meso-β circulations related to near-surface variable fluctuations as the main mechanism of interaction
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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