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  • Public Library of Science (PLoS)
  • American Meteorological Society
  • 2015-2019  (50)
  • 2000-2004  (28)
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  • 11
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    On the Discrepancies in Tropical Belt Expansion between Reanalyses and Climate Models and among Tropical Belt Width Metrics (2017)
    American Meteorological Society
    Details
    Publikationsdatum: 2017-02-01
    Beschreibung: The arid subtropics are situated at the edges of the tropical belt, where subsidence in the Hadley cells suppresses precipitation. Any meridional shift in these edge latitudes could have significant impacts on surface climate. Recent studies have investigated past and future changes in the tropical belt width and have found discrepancies in the rates of expansion estimated with different metrics and between climate models and reanalyses. Here, CMIP5 simulations and four modern reanalyses are analyzed using an ensemble of objective tropical belt width metrics to reexamine if such inconsistencies exist. The authors do not find sufficient evidence to demonstrate this discrepancy between models and reanalyses, as reanalysis trends in the tropical belt width fall within the range of model trends for any given metric. Furthermore, only metrics based on the Hadley cells are found to exhibit robust historical and future expansion. Metrics based on the subtropical jet and the tropopause show no robust response. This differentiation may be due to the strong correlation, on all time scales, between the Hadley cell edge latitudes and the latitudes of the eddy-driven jets, which consistently shift poleward in response to radiative forcings. In contrast, the subtropical jet and tropopause metrics appear to be decoupled from the Hadley cells and the eddy-driven jets and essentially measure a different tropical belt. The tropical belt width metrics are inconsistently correlated with surface climate indices based on precipitation and surface evaporation. This may make assessing the surface impacts of observed and future tropical expansion challenging.
    Print ISSN: 0894-8755
    Digitale ISSN: 1520-0442
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 12
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    Regional and Seasonal Characteristics of the Recent Expansion of the Tropics (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-06-07
    Print ISSN: 0894-8755
    Digitale ISSN: 1520-0442
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 13
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    Temporal Variability in Thermally Driven Cross-Shore Exchange: The Role of Semidiurnal Tides (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-07-01
    Beschreibung: We examine temporal variability of thermally driven baroclinic cross-shore exchange in the context of a tropical fringing reef system focusing on the role of tidally driven alongshore flow. Ensemble diurnal phase averaging of cross-shore flow at the Kilo Nalu Observatory (KNO) in Oahu, Hawaii, shows a robust diurnal signal associated with an unsteady buoyancy/diffusive dynamic balance, although significant variability is observed at subdiurnal time scales. In particular, persistent fortnightly variability in the cross-shore diurnal flow pattern is consistent with modulation by the semidiurnal alongshore tidal flow. The alongshore flow plays a direct role in the cross-shore exchange momentum balance via Coriolis acceleration but also affects the cross-shore circulation indirectly via its influence on vertical turbulent diffusion. An idealized linear theoretical model for thermally driven cross-shore flow is formulated using the long-term time-averaged diurnal dynamic balance at KNO as a baseline. The model is driven at leading order by the surface heat flux, with contributions from the alongshore flow and cross-shore wind appearing as linear perturbations. Superposition of the idealized solutions for Coriolis and time-varying eddy viscosity perturbations are able to reproduce key aspects of the fortnightly variability. Modifying the model to consider a more realistic alongshore flow and considering effects of nightly convection lead to further improvements in comparisons with KNO observations. The ability of the theoretical approach to reproduce the fortnightly patterns indicates that semidiurnal variations in the alongshore flow are effective in modulating the cross-shore flow via Coriolis and vertical turbulent transport mechanisms.
    Print ISSN: 0022-3670
    Digitale ISSN: 1520-0485
    Thema: Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 14
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    The Rapid Deployments to Wildfires Experiment (RaDFIRE): Observations from the Fire Zone (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-12-01
    Beschreibung: The Rapid Deployments to Wildfires Experiment (RaDFIRE) was a meteorological field campaign aimed at observing fire–atmosphere interactions during active wildfires. Using a rapidly deployable scanning Doppler lidar, airborne Doppler radar, and a suite of other instruments, the field campaign sampled 21 wildfires from 2013 to 2016 in the western United States. Observations include rotating convective plumes, plume interactions with stable layers and multilayered smoke detrainment, convective plume entrainment processes, smoke-induced density currents, and aircraft in situ observations of developing pyrocumulus. Collectively, these RaDFIRE observations highlight the range of meteorological phenomena associated with wildfires, especially plume dynamics, and will provide a valuable dataset for the modeling community.
    Print ISSN: 0003-0007
    Digitale ISSN: 1520-0477
    Thema: Geographie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 15
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    Coral Reef Drag Coefficients—Surface Gravity Wave Enhancement (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-07-01
    Beschreibung: A primary challenge in modeling flow over shallow coral reefs is accurately characterizing the bottom drag. Previous studies over continental shelves and sandy beaches suggest surface gravity waves should enhance the drag on the circulation over coral reefs. The influence of surface gravity waves on drag over four platform reefs in the Red Sea is examined using observations from 6-month deployments of current and pressure sensors burst sampling at 1 Hz for 4–5 min. Depth-average current fluctuations U′ within each burst are dominated by wave orbital velocities uw that account for 80%–90% of the burst variance and have a magnitude of order 10 cm s−1, similar to the lower-frequency depth-average current Uavg. Previous studies have shown that the cross-reef bottom stress balances the pressure gradient over these reefs. A bottom stress estimate that neglects the waves (ρCdaUavg|Uavg|, where ρ is water density and Cda is a drag coefficient) balances the observed pressure gradient when uw is smaller than Uavg but underestimates the pressure gradient when uw is larger than Uavg (by a factor of 3–5 when uw = 2Uavg), indicating the neglected waves enhance the bottom stress. In contrast, a bottom stress estimate that includes the waves [ρCda(Uavg + U′)|Uavg + U′|)] balances the observed pressure gradient independent of the relative size of uw and Uavg, indicating that this estimate accounts for the wave enhancement of the bottom stress. A parameterization proposed by Wright and Thompson provides a reasonable estimate of the total bottom stress (including the waves) given the burst-averaged current and the wave orbital velocity.
    Print ISSN: 0022-3670
    Digitale ISSN: 1520-0485
    Thema: Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 16
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    Effects of Low-Level Flow Orientation and Vertical Shear on the Structure and Intensity of Tropical Cyclones (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-07-17
    Beschreibung: This article explores the simultaneous effect of vertical wind shear (VWS) and low-level mean flow (LMF) on tropical cyclone (TC) structure evolution. The structural evolution of 180 western North Pacific TCs from 2002 to 2014 was measured by a new parameter, the RV ratio, which is defined as the ratio of a TC’s radius of 34-kt (17.5 m s−1) wind to its maximum wind speed at the ending point of the intensification period. Whereas TCs with RV ratios in the lowest quartile of all 180 samples favored intensification over expansion, and 82% of these TCs experienced rapid intensification, TCs with RV ratios in the topmost quartile favored size expansion over intensification. A novel result of this study is that TC RV ratios were found to correlate with the LMF orientation relative to the deep-layer VWS vector. Specifically, whereas an LMF directed toward the left-of-shear orientation favors TC intensification, a right-of-shear LMF favors TC size expansion. This study further analyzed the TC rainfall asymmetry and asymmetric surface flow using satellite observations. Results show that for a TC affected by an LMF with right-of-shear orientation, the positive surface flux anomaly in the upshear outer region promotes convection in the downshear rainband region. On the other hand, a left-of-shear LMF induces a positive surface flux anomaly in the downshear outer region, thus promoting convection in the upshear inner core. Enhancement of the symmetric inner-core convection favors intensification, whereas enhancement of the downshear rainband favors expansion.
    Print ISSN: 0027-0644
    Digitale ISSN: 1520-0493
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 17
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    Diurnal Evolution and Structure of Long-Lived Mesoscale Convective Vortices along the Mei-Yu Front over the East China Plains (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-03-01
    Beschreibung: The structure and diurnal evolution of long-lived, eastward-propagating mesoscale convective vortices (MCVs) along typical summertime mei-yu fronts over the east China plains are investigated through composite analysis of a 30-day semi-idealized simulation. The simulation uses lateral boundary conditions that vary only diurnally in time using analyses of recurring MCV events during 1–10 July 2007. Hence, the behavior of convection and vorticity follows a closely repeating diurnal cycle for each day during the simulation. Assisted by the eastward extension of enhanced vorticity anomalies from the Sichuan basin, the incipient MCV forms in the morning hours over the immediate lee (east) of the central China mountain ranges (stage 1). From local afternoon to early evening, as the MCV moves over the plains, convection weakens in the daytime downward branch of the mountain–plains solenoid. This allows the upper-level and lower-level portions of the vortex to partially decouple, and for convection to shift to the east-southeast side of the surface vortex (stage 2). Immediately after sunset, convection reinvigorates above the low-level MCV center as a result of moistening and destabilization from a combination of radiative forcing and an intensified low-level jet. This intensifies the MCV to maturity (stage 3). The mature MCV eventually evolves into an occluding subsynoptic cyclone with strong convection across all sectors of the low-level vorticity center during the subsequent day’s morning hours along the east China coastal plains before it moves offshore (stage 4).
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 18
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    Spray Underwater Glider Operations (2016)
    American Meteorological Society
    Details
    Publikationsdatum: 2016-05-23
    Beschreibung: Operational statistics for the Spray underwater glider are presented to demonstrate capabilities for sustained observations. An underwater glider is an autonomous device that profiles vertically by changing buoyancy and flies horizontally on wings. The focus has been on sustained observations of boundary currents to take advantage of the glider’s small size, which allows it to be deployed and recovered from small vessels close to land, and the fine horizontal resolution delivered by the glider, which is scientifically desirable in boundary regions. Since 2004, Spray underwater gliders have been deployed for over 28 000 days, traveling over 560 000 km, and delivering over 190 000 profiles. More than 10 gliders, on average, have been in the water since 2012. Statistics are given in the form of histograms for 297 completed glider missions of longer than 5 days. The statistics include mission duration, number of dives, distance over ground, and horizontal and vertical distance through water. A discussion of problems, losses, and short missions includes a survival analysis. The most extensive work was conducted in the California Current system, where observations on three across-shorelines have been sustained, with 97% coverage since 2009. While the authors have certain advantages as developers and builders of the Spray underwater glider and Spray may have design and construction advantages, they believe these statistics are a sound basis for optimism about the widespread future of gliders in oceanographic observing.
    Print ISSN: 0739-0572
    Digitale ISSN: 1520-0426
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 19
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    Competing Effects of Elevated Vertical Mixing and Increased Freshwater Input on the Stratification and Sea Ice Cover in a Changing Arctic Ocean (2016)
    American Meteorological Society
    Details
    Publikationsdatum: 2016-04-29
    Beschreibung: The Arctic Ocean is undergoing a period of rapid transition. Freshwater input is projected to increase, and the decline in Arctic sea ice is likely to drive periodic increases in vertical mixing during ice-free periods. Here, a 1D model of the Arctic Ocean is used to explore how these competing processes will affect the stratification, the stability of the cold halocline, and the sea ice cover at the surface. Initially, stronger shear leads to elevated vertical mixing that causes the mixed layer to warm. The change in temperature, however, is too small to affect the sea ice cover. Most importantly, in the Eurasian Basin, the elevated shear also deepens the halocline and strengthens the stratification over the Atlantic Water thermocline, reducing the vertical heat flux. After about a decade this effect dominates, and the mixed layer begins to cool. The sea ice cover can only be significantly affected if the elevated mixing is sufficient to erode the stratification barrier associated with the cold halocline. While freshwater generally dominates in the Canadian Basin (further isolating the mixed layer from the Atlantic Water layer), in the Eurasian Basin elevated shear reduces the strength of the stratification barrier, potentially allowing Atlantic Water heat to be directly entrained into the mixed layer during episodic mixing events. Therefore, although most sea ice retreat to date has occurred in the Canadian Basin, the results here suggest that, in future decades, elevated vertical mixing may play a more significant role in sea ice melt in the Eurasian Basin.
    Print ISSN: 0022-3670
    Digitale ISSN: 1520-0485
    Thema: Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 20
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    An Ensemble Approach to Investigate Tropical Cyclone Intensification in Sheared Environments. Part I: Katia (2011) (2015)
    American Meteorological Society
    Details
    Publikationsdatum: 2015-12-11
    Beschreibung: The mechanisms responsible for tropical cyclone (TC) intensification in the presence of moderate vertical shear magnitudes are not well understood. To investigate how TCs intensify in spite of moderate shear, this study employed a 96-member ensemble generated with the Advanced Hurricane Weather Research and Forecasting (AHW) Model. In this first part, AHW ensemble forecasts for TC Katia (2011) were evaluated when Katia was a weak tropical storm in an environment of 12 m s−1 easterly shear. The 5-day AHW forecasts for Katia were characterized by large variability in the intensity, presenting an opportunity to compare the underlying mechanisms between two subsets of members that predicted different intensity scenarios: intensification and weakening. The key difference between these two subsets was found in the lower-tropospheric moisture north of Katia (i.e., right-of-shear quadrant). With more water vapor in the lower troposphere, buoyant updrafts helped to moisten the midtroposphere and enhanced the likelihood of deep and organized convection in the subset that predicted intensification. This finding was validated with a vorticity budget, which showed that deep cyclonic vortex stretching and tilting contributed to spinning up the circulation after the midtroposphere had moistened. Sensitivity experiments, in which the initial conditions were perturbed, also demonstrated the importance of lower-tropospheric moisture, which suggests that moisture observations may help reduce uncertainty in forecasts of weak, sheared tropical storms.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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