4,591 research outputs found
Recommended from our members
How tropical Pacific surface cooling contributed to accelerated sea ice melt from 2007 to 2012 as ice is thinned by anthropogenic forcing
Over the past 40 years the Arctic sea ice minimum in September has declined. The period between 2007 and 2012 showed accelerated melt contributed to the record minima of 2007 and 2012. Here, observational and model evidence shows that the changes in summer sea ice since the 2000s reflects a continuous anthropogenically forced melting masked by interdecadal variability of Arctic atmospheric circulation. This variation is partially driven by teleconnections originating from sea surface temperature (SST) changes in the east-central tropical Pacific via a Rossby wave train propagating into the Arctic (hereafter referred to as the “Pacific-Arctic teleconnection (PARC)”), which represents the leading internal mode connecting the pole to lower latitudes. This mode has contributed to accelerated warming and Arctic sea ice loss from 2007 to 2012, followed by slower declines in recent years, resulting in the appearance of a slowdown over the past 11 years. A pacemaker model simulation, in which we specify observed SST in the tropical eastern Pacific, demonstrates a physically plausible mechanism for the PARC mode. However, the model-based PARC mechanism is considerably weaker and only partially accounts for the observed acceleration of sea ice loss from 2007 to 2012. We also explore features of large-scale circulation patterns associated with extreme melting periods in a long (1800-yr) CESM preindustrial simulation. These results further support the role of remote SST forcing originating from the tropical Pacific in exciting significant warm episodes in the Arctic. However, further research is needed to identify the reasons for model limitations in reproducing the observed PARC mode featuring a Cold Pacific - Warm Arctic connection
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments
with Dynamics (ROCKE-3D) is a 3-Dimensional General Circulation Model (GCM)
developed at the NASA Goddard Institute for Space Studies for the modeling of
atmospheres of Solar System and exoplanetary terrestrial planets. Its parent
model, known as ModelE2 (Schmidt et al. 2014), is used to simulate modern and
21st Century Earth and near-term paleo-Earth climates. ROCKE-3D is an ongoing
effort to expand the capabilities of ModelE2 to handle a broader range of
atmospheric conditions including higher and lower atmospheric pressures, more
diverse chemistries and compositions, larger and smaller planet radii and
gravity, different rotation rates (slowly rotating to more rapidly rotating
than modern Earth, including synchronous rotation), diverse ocean and land
distributions and topographies, and potential basic biosphere functions. The
first aim of ROCKE-3D is to model planetary atmospheres on terrestrial worlds
within the Solar System such as paleo-Earth, modern and paleo-Mars,
paleo-Venus, and Saturn's moon Titan. By validating the model for a broad range
of temperatures, pressures, and atmospheric constituents we can then expand its
capabilities further to those exoplanetary rocky worlds that have been
discovered in the past and those to be discovered in the future. We discuss the
current and near-future capabilities of ROCKE-3D as a community model for
studying planetary and exoplanetary atmospheres.Comment: Revisions since previous draft. Now submitted to Astrophysical
Journal Supplement Serie
Experimental investigations on the characteristics of snow accretion using the EMU-320 model train
This paper presents a snow accretion test conducted in a climate wind tunnel
to investigate the icing process on a model train. The model used within this
experiment was the cleaned-up and 2/3-scaled version of EMU-320, which is a
high-speed train in Korea. The model was designed without an electronic power
source or heat source so that the wheels did not rotate and snow accretion on
the model did not occur due to heat sources. To investigate snow accretion,
four cases with different ambient temperatures were considered in the climate
wind tunnel on Rail Tec Arsenal. Before analyzing the snow accretion on the
train, the snow flux and liquid water content of snow were measured so that
they could be used as the input conditions for the simulation and to ensure the
analysis of the icing process was based on the characteristics of the snow.
Both qualitative and quantitative data were obtained, whereby photographs was
used for qualitative analysis, and the density of the snow sample and the
thickness of snow accreted on the model were used for quantitative analysis.
Based on the visual observations, it was deduced that as the ambient
temperature increased, the range of the snow accreted was broader. The
thickness of snow accreted on the model nose was the largest on the upper and
lower part at -3 oC, and on the middle part at -5 oC. Additionally, the cross
section of snow accreted was observed to be trench-like. Similar icing
processes were observed to occur on the slope of nose. Snow accreted on all
components of the bogie, and for all cases, the thickness of snow at wheel was
the largest at an arc angle of 40 to 70 o. These detailed data of experimental
conditions can be applied as an input to simulations to improve simulations of
ice conditions. Thus, they can facilitate the development of appropriate
anti-icing designs for trainsComment: 31 pages, 23 Figures, 8 Table
Recommended from our members
Un-mixing the Ocean: Double Diffusion and Turbulence in Polar Oceans
This thesis concerns theory, numerical simulations and observations of double-diffusion in polar settings. Double diffusion refers to processes occurring due to the difference in molecular diffusivities between two components that both contribute to the density. Specifically, these processes occur in the ocean due to the much slower diffusion of salinity compared to temperature. Within polar regions, thermohaline staircases have been frequently observed. These are layered structures in both temperature and salinity that can form due to double-diffusive processes, that give a characteristic `staircase' shape to profiles of temperature and salinity. Thermohaline staircases provide observational evidence of the importance of double diffusion to small scale ocean mixing, and so motivate our discussion of double-diffusive convection in polar environments.
After an introduction to the topic, the first results chapter discusses the energetics of double diffusion, developing a new model for the flow of energy within double-diffusive fluids. The second results chapter is motivated by observations of thermohaline staircases beneath George VI Ice Shelf, Antarctica. We conducted Large-Eddy-Simulations to explore the interaction of double diffusive convection with turbulence forced at a prescribed rate. Utilising the theory developed in chapter 1, the transition between double diffusive convection and stratified turbulence is identified and a criterion is developed for that transition in terms of profiles in temperature, salinity, and turbulence rate.
The third results chapter considers observational turbulence data collected in the Chukchi Sea in the marginal seas of the Arctic Ocean. This data shows an oceanographic section of a warm core intrahalocline eddy, where thermohaline layering was observed. We develop a criterion to predict the observed turbulent dissipation rates using fine-scale temperature and salinity data, assuming double-diffusive convection is active. This criterion is based on the energetic model from the first results chapter and assumes a lateral stirring of `spice’ variance (compensated thermohaline variance) along isopycnals is the driver of turbulence. The final results chapter consists of an analysis of mooring data from beneath George VI Ice Shelf, at the same location as thermohaline staircases were observed. We find that shear-driven turbulence cannot explain the observed dissipation rates. Utilising the method from the third results chapter, we show that lateral variations in spice can explain the observed turbulent mixing, suggesting it exerts control over the ice shelf basal melt rate
Modelling hydrodynamics and ice formation in a pump-storage system between two Norwegian reservoirs
publishedVersio
Phase change of molten-salt flows in energy systems
The possibility of molten salt freezing in pipe flow systems is a key concern for the
solar-energy industry and a safety issue in the Generation-IV molten-salt reactors,
worthy of careful consideration. The overriding aim of this thesis is to address this
issue by providing an approach to quantify the solidification of molten salts in piping
systems (in terms of mass build-up, effect on the flow and heat transfer, etc.).
In light of this aim, several aspects needed to be investigated which affect how
molten salt solidification can be predicted. Specifically, the work described in this
thesis is hereby described: 1) An experimental method was developed to measure
the thermal conductivity of molten salts, whose uncertainties significantly affect
further modelling efforts. The method can be applied to measure the thermal
conductivity of molten salts up to temperatures around 760 K, with an overall
error better than 4%. 2) The thermal conductivities of NaNO3 -NaNO2 -KNO3
eutectic (HTS) and LiCl -KCl eutectic were measured up to temperatures of 700 K
and 760 K respectively. In addition, data in the literature were re-evaluated by
taking into account the thermal losses present in a particular experimental apparatus;
the revised results were found to be in good agreement with other studies.
These re-evaluated data and the measurements conducted in the present study
were used to critically review and suggest the values of the thermal conductivities
of common salts, including FLiNaK. 3) A 1-dimensional thermo-hydraulic model
was developed under the steady state assumption and validated to predict transient
freezing in internal pipe flows. The model can be incorporated in standard
thermo-hydraulic codes and can be used to predict the solidi cation process in
complex piping system where CFD is computationally expensive. 4) An experimental
apparatus equipped with laser-based diagnostic measurement techniques
was built to measure the growing thickness of an ice layer in contact with a cold
surface and liquid water flow. The developed freezing model was validated against
these experimental data and the discrepancies were considered. 5) The freezing
model was then applied to study the behaviour of the Direct Reactor Auxiliary
Cooling System (DRACS) under Loss of Forced Circulation (LOFC) with blackout.
DRACS was found to be prone to failure due to freezing in the molten salt/air
heat exchanger; its transient response was characterised and discussed.Open Acces
The effects of climate change on hailstorms
Hailstorms are dangerous and costly phenomena that are expected to change in response to a warming climate. In this Review, we summarize current knowledge of climate change effects on hailstorms. As a result of anthropogenic warming, it is generally anticipated that low-level moisture and convective instability will increase, raising hailstorm likelihood and enabling the formation of larger hailstones; the melting height will rise, enhancing hail melt and increasing the average size of surviving hailstones; and vertical wind shear will decrease overall, with limited influence on the overall hailstorm activity, owing to a predominance of other factors. Given geographic differences and offsetting interactions in these projected environmental changes, there is spatial heterogeneity in hailstorm responses. Observations and modelling lead to the general expectation that hailstorm frequency will increase in Australia and Europe, but decrease in East Asia and North America, while hail severity will increase in most regions. However, these projected changes show marked spatial and temporal variability. Owing to a dearth of long-term observations, as well as incomplete process understanding and limited convection-permitting modelling studies, current and future climate change effects on hailstorms remain highly uncertain. Future studies should focus on detailed processes and account for non-stationarities in proxy relationships
- …