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  • 1
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    Quantifying sensitivities of ice crystal number and sources of ice crystal number variability in CAM 5.1 using the adjoint of a physically-based cirrus formation parameterization (2015)
    Wiley
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    Publication Date: 2015-03-11
    Description: We present the adjoint of a cirrus formation parameterization that computes the sensitivity of ice crystal number concentration to updraft velocity, aerosol, and ice deposition coefficient. The adjoint is driven by simulations from the NCAR Community Atmosphere Model Version 5.1 to understand the sensitivity of formed ice crystal number concentration to 13 variables and quantify which contribute to its variability. Sensitivities of formed ice crystal number concentration to updraft velocity, sulfate number, and sulfate diameter are positive and largest over the coldest regions of the globe. Sulfate number sensitivity is also large over the tropics where updraft cooling is sufficient but sulfate number concentration is low, indicating a sulfate-limited regime in CAM 5.1. Outside of the tropics, competition between homogeneous and heterogeneous nucleation may shift annually-averaged sensitivities to higher magnitudes, when infrequent strong updrafts shift crystal production away from purely heterogeneous nucleation. Outside the tropics, updraft velocity is responsible for approximately 52.70% of the ice crystal number variability. In the tropics, sulfate number concentration and updraft jointly control variability in formed crystal number concentration. Insoluble aerosol species play a secondary, but still important, role in influencing the variability in crystal concentrations, with coarse mode dust being the largest contributor at nearly 50% in certain regions. On a global scale, more than 95% of the temporal variability in crystal number concentration can be described by temperature, updraft velocity, sulfate number, and coarse mode dust number concentration.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    A single column model ensemble approach applied to the twp-ice experiment (2013)
    Wiley
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    Publication Date: 2013-05-02
    Description: [1]  Single column models (SCM) are useful testbeds for investigating the parameterisation schemes of numerical weather prediction and climate models. The usefulness of SCM simulations are limited, however, by the accuracy of the best-estimate large-scale observations prescribed. Errors estimating the observations will result in uncertainty in modelled simulations. One method to address the modelled uncertainty is to simulate an ensemble where the ensemble members span observational uncertainty. This study first derives an ensemble of large-scale data for the Tropical Warm Pool International Cloud Experiment (TWP-ICE) based on an estimate of a possible source of error in the best-estimate product. This data is then used to carry out simulations with 11SCM and 2 cloud-resolving models (CRM). Best-estimate simulations are also performed. All models show that moisture-related variables are close to observations and there are limited differences between the best-estimate and ensemble mean values. The models, however, show different sensitivities to changes in the forcing particularly when weakly forced. The ensemble simulations highlight important differences in the surface evaporation term of the moisture budget between the SCM and CRM. Differences are also apparent between the models in the ensemble mean vertical structure of cloud variables whilst for each model cloud properties are relatively insensitive to forcing. The ensemble is further used to investigate cloud variables and precipitation and identifies differences between CRM and SCM particularly for relationships involving ice. This study highlights the additional analysis that can be performed using ensemble simulations and hence enables a more complete model investigationcompared to using the more traditional single best-estimate simulation only.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Effect of cloud-scale vertical velocity on the contribution of homogeneous nucleation to cirrus formation and radiative forcing (2016)
    Wiley
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    Publication Date: 2016-05-28
    Description: Ice nucleation is a critical process for the ice crystal formation in cirrus clouds. The relative contribution of homogeneous nucleation versus heterogeneous nucleation to cirrus formation differs between measurements and predictions from general circulation models (GCMs). Here we perform large-ensemble simulations of the ice nucleation process using a cloud parcel model driven by observed vertical motions, and find that homogeneous nucleation occurs rather infrequently, in agreement with recent measurement findings. When the effect of observed vertical velocity fluctuations on ice nucleation is considered in the Community Atmosphere Model version 5 (CAM5), the relative contribution of homogeneous nucleation to cirrus cloud occurrences decreases to only a few percent. However, homogeneous nucleation still has strong impacts on the cloud radiative forcing. Hence, the importance of homogeneous nucleation for cirrus cloud formation should not be dismissed on the global scale.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Changes in the fluorescence composition of multiple DOM sources over pH gradients assessed by combining parallel factor analysis and self-organizing maps (2014)
    Wiley
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    Publication Date: 2014-08-24
    Description: Dissolved organic matter is a ubiquitous constituent of natural waters that plays key roles in several important processes. The fluorescence properties of DOM have been linked to its functionality, but these properties may vary with pH. In this study the Kohonen's self-organizing maps (SOM)were applied to EEMs of fresh dissolved organic matter (DOM) from three sources: senescent sugar-maple leaves and white spruce needles, and humified white spruce needles, over a pHrangeof ~4.5 – 12.5.SOM were applied to: raw EEMs, EEMs reduced in dimensionality by pre-processing using parallel factor analysis (PARAFAC), and PARAFAC loading proportions normalized to values at initial pH. Some separation of EEMs into source-based clusters was achieved in the SOM of raw EEMs, but commingling was apparent and evidence of changes over pH gradients was overshadowed. SOMs of PARAFAC component proportions demonstrated clear source-based clustering, and pH-based gradients were visible for DOM from senescent and humified spruce needles. Changes in optical properties were obvious over pH gradientsin the SOM of components normalized to starting condition.Component proportions decreased to values as low as 5 % of initial values for microbial humic-like peak M, and increased to as high as 278 % for a humic-like component. Tyrosine-like fluorescence increased to 112 % of initial over increasing pH in humified spruce leachates, but decreased to as low as 45 % in the other leachates.The combination of PARAFAC and SOMdrastically enhanced visualization and interpretability of pH-induced changesin DOM compared to either method alone.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Nightside Pi2 wave properties during an extended period with stable plasmapause location and variable geomagnetic activity (2017)
    Wiley
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    Publication Date: 2017-11-14
    Description: The frequencies and amplitudes of inner magnetosphere Pi2 waves are affected by the radial plasma density profile. Variable geomagnetic activity and external driving conditions can affect both wave properties and density profiles simultaneously. When interpreting observations, this can lead to ambiguity about whether changing wave properties are due to changing external conditions, density profiles, or a combination of factors. We present a case study using multi-point ground-based and in situ measurements to examine Pi2 wave properties during a period of variable geomagnetic activity. Multiple satellite passes demonstrate the density profile and plasmapause location is stable for at least two hours over a wide range of MLT. This stability allows us to examine how factors besides the radial density profile affect Pi2 wave properties. We find evidence for Pi2 waves with a broadband frequency spectrum as well as a discrete frequency plasmaspheric virtual resonance (PVR) that is observed at low, mid, and high-latitudes and both inside and outside the plasmapause. The PVR is excited in repeated bursts before, during and after (1) the development of a substorm, (2) several auroral intensifications, (3) the development of Sub-Auroral Polarization Stream (SAPS) flows/electric fields/conductivities, and (4) variable Interplanetary Magnetic Field (IMF) conditions. Through all these changes the PVR frequency remains remarkably stable (8.2 +/− 0.53 mHz, based on low latitude ground magnetometer observations), suggesting these variations have little effect on the frequency. This is consistent with PVR model predictions for a stationary plasmapause.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Mass wasting on Phobos triggered by an evolving tidal environment (2016)
    Wiley
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    Publication Date: 2016-12-10
    Description: The Martian moon Phobos is experiencing orbital decay, increasing tidal forces, and possible disruption in the near future. We suggest that this changing dynamic tidal environment has contributed to re-surfacing of the odd-shaped small moon. We investigated mass wasting features in craters using high-resolution images acquired by the Mars Express onboard High Resolution Stereo Camera. We model the evolving dynamical environment of Phobos by reconstructing dynamic slopes on its surface with Phobos at different distances to Mars. We note that the increase in slope by tidal effect correlates with most of the observed landslides, which is particularly obvious for craters near the sub-Mars and anti-Mars points. We suggest that the effect has triggered mass displacements that are responsible for observed asymmetries of some craters. The most recent landslides are likely to be geologically young and occurred within the past few hundred million years.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Sensitivity of homogeneous ice nucleation to aerosol perturbations and its implications for aerosol indirect effects through cirrus clouds (2018)
    Wiley
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    Publication Date: 2018-01-30
    Description: The magnitude and sign of anthropogenic aerosol impacts on cirrus clouds through ice nucleation are still very uncertain. In this study, aerosol sensitivity ( η α ), defined as the sensitivity of the number concentration ( N i ) of ice crystals formed from homogeneous ice nucleation to aerosol number concentration ( N a ), is examined based on simulations from a cloud parcel model. The model represents the fundamental process of ice crystal formation that results from homogeneous nucleation. We find that the geometric dispersion ( σ ) of the aerosol size distribution used in the model is a key factor for η α . For a monodisperse size distribution, η α is close to zero in vertical updrafts ( V 〈 50 cm s -1 ) typical of cirrus clouds. However, η α increases to 0.1-0.3 (i.e., N i increases by a factor of 1.3-2.0 for a 10-fold increase in N a ) if aerosol particles follow log-normal size distributions with a σ of 1.6-2.3 in the upper troposphere. By varying the input aerosol and environmental parameters, our model reproduces a large range of η α values derived from homogeneous ice nucleation parameterizations widely used in global climate models (GCMs). The differences in η α from these parameterizations can translate into a range of anthropogenic aerosol longwave indirect forcings through cirrus clouds from 0.05 to 0.36 W m -2 with a GCM. Our study suggests that a larger η α (0.1-0.3) is more plausible and the homogeneous nucleation parameterizations should include a realistic aerosol size distribution to accurately quantify anthropogenic aerosol indirect effects.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 8
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    Simultaneous Space and Ground-Based Observations of a Plasmaspheric Virtual Resonance (2017)
    Wiley
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    Publication Date: 2017-03-25
    Description: We present simultaneous space and ground-based observations of Pi2 pulsations which occurred during a substorm on 25 September 2014. The timeline for this event starts at ∼ 06:04 UT when the THEMIS probe D located inside the plasmasphere detected Pi2 pulsations in the electric and magnetic fields. Cross-spectral analysis shows the azimuthal electric field and compressional magnetic field oscillated nearly in quadrature, highly suggestive of a standing fast-mode wave. Simultaneous Pi2 observations from dayside and nightside ground magnetometers at low latitudes indicate a global wave mode. A latitudinal magnetometer chain on the nightside observed a phase reversal in the H component of the Pi2 pulsations when crossing the footprint of the plasmapause, estimated from THEMIS spacecraft measurements. Spectral analysis of data from ground magnetometers in this latitudinal chain showed fundamental and second harmonic spectral peaks in their H and D components. Similar pulsation signatures at comparable harmonic frequencies were observed by three mid-latitude SuperDARN HF radars, both poleward and equatorward of the plasmapause ionospheric footprint. Finally, the longitudinal polarization pattern and azimuthal phase propagation of mid-latitude Pi2 pulsations are consistent with previous observations of a plasmaspheric virtual resonance being excited by a longitudinally localized source near midnight.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Quantifying the effects of historical land cover conversion uncertainty on global carbon and climate estimates (2018)
    Wiley
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    Publication Date: 2018-01-04
    Description: Previous studies have examined land use change as a driver of global change, but the translation of land use change into land cover conversion has been largely unconstrained. Here, we quantify the effects of land cover conversion uncertainty on the global carbon and climate system using the integrated Earth System Model. Our experiments use identical land use change data and vary land cover conversions to quantify associated uncertainty in carbon and climate estimates. Land cover conversion uncertainty is large, constitutes a 5ppmv range in estimated atmospheric CO 2 in 2004, and generates carbon uncertainty that is equivalent to 80% of the net effects of CO 2 and climate and 124% of the effects of nitrogen deposition during 1850-2004. Additionally, land cover uncertainty generates differences in local surface temperature of over 1 °C. We conclude that future studies addressing land use, carbon, and climate need to constrain and reduce land cover conversion uncertainties.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 10
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    Soil Assessment Unit Scale Affects Quantifying CH4 Emissions from Rice Fields (2013)
    Wiley
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    Publication Date: 2013-02-25
    Print ISSN: 0361-5995
    Electronic ISSN: 1435-0661
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Published by Wiley
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