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arXiv2014

Electrical charging of ash in Icelandic volcanic plumes

The existence of volcanic lightning and alteration of the atmospheric potential gradient in the vicinity of near-vent volcanic plumes provides strong evidence for the charging of volcanic ash. More subtle electrical effects are also visible in balloon soundings of distal volcanic plumes. Near the vent, some proposed charging mechanisms are fractoemission, triboelectrification, and the so-called "dirty thunderstorm" mechanism, which is where ash and convective clouds interact electrically to enhance charging. Distant from the vent, a self-charging mechanism, probably triboelectrification, has been suggested to explain the sustained low levels of charge observed on a distal plume. Recent research by Houghton et al. (2013) linked the self-charging of volcanic ash to the properties of the particle size distribution, observing that a highly polydisperse ash distribution would charge more effectively than a monodisperse one. Natural radioactivity in some volcanic ash could also contribute to self-charging of volcanic plumes. Here we present laboratory measurements of particle size distributions, triboelectrification and radioactivity in ash samples from the Grímsvötn and Eyjafjallajökull volcanic eruptions in 2011 and 2010 respectively, and discuss the implications of our findings.

physics.ao-ph↗
arXiv2014

Future climate trends from a first-difference atmospheric carbon dioxide regression model involving emissions scenarios for business as usual and for peak fossil fuel

This paper investigates the implications of the future continuation of the demonstrated past (1960-2012) strong correlation between first-difference atmospheric CO2 and global surface temperature. It does this, for the period from the present to 2050, for a comprehensive range of future global fossil fuel energy use scenarios. The results show that even for a business-as-usual (the mid-level IPCC) fossil fuel use estimate, global surface temperature will rise at a slower rate than for the recent period 1960-2000. Concerning peak fossil fuel, for the most common scenario the currently observed (1998-2013)temperature plateau will turn into a decrease. The observed trend to date for temperature is compared with that for global climate disasters: these peaked in 2005 and are notably decreasing. The temperature and disaster results taken together are consistent with either a reduced business-as-usual fossil fuel use scenario into the future, or a peak fossil fuel scenario, but not with the standard business-as-usual scenario. If the future follows a peak fossil fuel pathway, a markedly decreasing trend in global surface temperature should become apparent over the next few years. If entertained, these results are evidence that the climate problem may require less preventative action. If so, the same evidence is support for the case that the peak fossil fuel problem does require preventative action. This action is the same as it would have been for climate - the rapid transition to a predominantly renewable global energy system.

physics.ao-ph↗
arXiv2014

Phase-field theory of brine entrapment in sea ice: Short-time frozen microstructures

We analyze the early phase of brine entrapment in sea ice, using a phase field model. This model for a first-order phase transition couples non-conserved order parameter kinetics to salt diffusion. The evolution equations are derived from a Landau-Ginzburg order parameter gradient dynamics together with salinity conservation. The numerical solution of model equations by an exponential time differencing scheme describes the time evolution of phase separation between liquid water with high salinity and the ice phase with low salinity. The numerical solution in one and two dimensions indicates the formation of one dominant wavelength which sets the length scale of short-time frozen structures. A stability analysis provides the phase diagram in terms of two Landau parameters. It is distinguished an uniform ice phase, a homogeneous liquid saline water solution and a phase where solidification structures can be formed. The Landau parameters are extracted from the supercooling and superheating as well as the freezing point temperature of water. With the help of realistic parameters the distribution of brine inclusions is calculated and found in agreement with the measured samples. The size of the ice domains separating regions of concentrated seawater depends on salinity and temperature and corresponds to the size of sea ice platelets obtained from a morphological stability analysis for the solidification of salt water.

physics.ao-ph↗
arXiv2014

Deepening of the ocean mixed layer at the northern Patagonian continental shelf: a numerical study

A possible deepening of the ocean mixed layer was investigated at a selected point of the Patagonian continental shelf where a significant positive wind speed trend was estimated. Using a 1-dimensional vertical numerical model (S2P3) forced by atmospheric data from NCEP/NCAR I reanalysis and tidal constituents from TPXO 7.2 global model on a long term simulation (1979-2011), it was found that the mixed layer thickness presents a significant and positive trend of 10.1 +/- 1.4 cm/yr. Several numerical experiments were carried out in order to evaluate the impact of the different atmospheric variables (surface zonal and latitudinal wind components, air temperature, atmospheric pressure, specific humidity and cloud coverage) considered in this study. As a result it was found that an increase in the wind speed can be considered as the main responsible of the ocean mixed layer deepening at the selected location of the Patagonian continental shelf. A possible increasing in the mixed layer thickness could be directly impacting on the sea surface temperature. Preliminary results obtained in this paper show that a slight but significant cooling in the temperature of the sea upper layer (of the order of 1 C every 50 years) could be happening since some decades ago at the northern Patagonian continental shelf waters.

physics.ao-ph↗
arXiv2014

Sensitivity of Displaced-Beam Scintillometer Measurements of Area-Average Heat Fluxes to Uncertainties in Topographic Heights

Displaced-beam scintillometer measurements of the turbulence inner-scale length $l_o$ and refractive index structure function $C_n^2$ resolve area-average turbulent fluxes of heat and momentum through the Monin-Obukhov similarity equations. Sensitivity studies have been produced for the use of displaced-beam scintillometers over flat terrain. Many real field sites feature variable topography. We develop here an analysis of the sensitivity of displaced-beam scintillometer derived sensible heat fluxes to uncertainties in spacially distributed topographic measurements. Sensitivity is shown to be concentrated in areas near the center of the beam and where the underlying topography is closest to the beam height. Uncertainty may be decreased by taking precise topographic measurements in these areas.

physics.ao-ph↗
arXiv2014

Accurately Estimating the State of a Geophysical System with Sparse Observations: Predicting the Weather

Utilizing the information in observations of a complex system to make accurate predictions through a quantitative model when observations are completed at time $T$, requires an accurate estimate of the full state of the model at time $T$. When the number of measurements $L$ at each observation time within the observation window is larger than a sufficient minimum value $L_s$, the impediments in the estimation procedure are removed. As the number of available observations is typically such that $L \ll L_s$, additional information from the observations must be presented to the model. We show how, using the time delays of the measurements at each observation time, one can augment the information transferred from the data to the model, removing the impediments to accurate estimation and permitting dependable prediction. We do this in a core geophysical fluid dynamics model, the shallow water equations, at the heart of numerical weather prediction. The method is quite general, however, and can be utilized in the analysis of a broad spectrum of complex systems where measurements are sparse. When the model of the complex system has errors, the method still enables accurate estimation of the state of the model and thus evaluation of the model errors in a manner separated from uncertainties in the data assimilation procedure.

nlin.CD↗
arXiv2014

Logarithmic Wind Profile: A Stability Wind Shear Term

A stability wind shear term of logarithmic wind profile based on the terms of turbulent kinetic energy equation is proposed. The fraction influenced by thermal stratification is considered in the shear production term. This thermally affected shear is compared with buoyant term resulting in a stability wind shear term. It is also considered Reynolds stress as a sum of two components associated with wind shear from mechanical and thermal stratification process. The stability wind shear is responsible to Reynolds stress of thermal stratification term, and also to Reynolds stress of mechanical term at no neutral condition. The wind profile and its derivative are validated with data from Pedra do Sal experiment in a flat terrain and 300m from shoreline located in northeast coast of Brazil. It is close to the Equator line, so the meteorological condition are strongly influenced by trade winds and sea breeze. The site has one 100m tower with five instrumented levels, one 3D sonic anemometer, and a medium-range wind lidar profiler up 500m. The dataset are processed and filter from September to November of 2013 which results in about 550 hours of data available. The results show the derivative of wind profile with R^2 of 0.87 and RMSE of 0.08 m/s. The calculated wind profile performances well up to 400m at unstable condition and up to 280m at stable condition with R^2 better than 0.89. The proposed equation is valid for this specific site and is limited to a stead state condition with constant turbulent fluxes in the surface layer.

physics.ao-ph↗
arXiv2014

An accurate and efficient numerical framework for adaptive numerical weather prediction

We present an accurate and efficient discretization approach for the adaptive discretization of typical model equations employed in numerical weather prediction. A semi-Lagrangian approach is combined with the TR-BDF2 semi-implicit time discretization method and with a spatial discretization based on adaptive discontinuous finite elements. The resulting method has full second order accuracy in time and can employ polynomial bases of arbitrarily high degree in space, is unconditionally stable and can effectively adapt the number of degrees of freedom employed in each element, in order to balance accuracy and computational cost. The p-adaptivity approach employed does not require remeshing, therefore it is especially suitable for applications, such as numerical weather prediction, in which a large number of physical quantities are associated with a given mesh. Furthermore, although the proposed method can be implemented on arbitrary unstructured and nonconforming meshes, even its application on simple Cartesian meshes in spherical coordinates can cure effectively the pole problem by reducing the polynomial degree used in the polar elements. Numerical simulations of classical benchmarks for the shallow water and for the fully compressible Euler equations validate the method and demonstrate its capability to achieve accurate results also at large Courant numbers, with time steps up to 100 times larger than those of typical explicit discretizations of the same problems, while reducing the computational cost thanks to the adaptivity algorithm.

math.NA↗
arXiv2014

Tropical cyclone boundary layer shocks

This paper presents numerical solutions and idealized analytical solutions of axisymmetric, $f$-plane models of the tropical cyclone boundary layer. In the numerical model, the boundary layer radial and tangential flow is forced by a specified pressure field, which can also be interpreted as a specified gradient balanced tangential wind field $v_{\rm gr}(r)$ or vorticity field $ζ_{\rm gr}(r)$. When the specified $ζ_{\rm gr}(r)$ field is changed from one that is radially concentrated in the inner core to one that is radially spread, the quasi-steady-state boundary layer flow transitions from a single eyewall shock-like structure to a double eyewall shock-like structure. To better understand these structures, analytical solutions are presented for two simplified versions of the model. In the simplified analytical models, which do not include horizontal diffusion, the $u(\partial u/\partial r)$ term in the radial equation of motion and the $u[f+(\partial v/\partial r)+(v/r)]$ term in the tangential equation of motion produce discontinuities in the radial and tangential wind, with associated singularities in the boundary layer pumping and the boundary layer vorticity. In the numerical model, which does include horizontal diffusion, the radial and tangential wind structures are not true discontinuities, but are shock-like, with wind changes of 20 or 30 m s$^{-1}$ over a radial distance of a few kilometers. When double shocks form, the outer shock can control the strength of the inner shock, an effect that likely plays an important role in concentric eyewall cycles.

physics.ao-ph↗
arXiv2014

Deep-water gravity waves: nonlinear theory of wave groups

Nonlinear initial-boundary value problem on deep-water gravity waves of finite amplitude is solved approximately (up to small terms of higher order) assuming that the waves are generated by an initial disturbance to the water and the horizontal dimensions of the initially disturbed body of the water are much larger than the magnitude of the free surface displacement. A numerable set of specific free surface waves is obtained in closed form and it is shown that free surface waves produced by an arbitrary initial disturbance to the water is a combination (not superposition: the waves are nonlinear) of the specific waves. A set of dispersive wave packets is found with one-to-one correspondence between the packets and positive integers, say, packet numbers, such that any initial free surface displacement gradually disintegrates into a number (limited or unlimited, depending on initial conditions) of the wave packets. The greater the packet number, the shorter the wavelength of the packet's carrier wave component, the slower the packet travels, the slower the packet disperses; evolution of any of the packets is not influenced by evolution of any other one. It is found that in case of infinitesimal wave amplitude the present theory is in agreement with linear wave theory. On the other hand, the behaviour of wave packets of large numbers and asymptotic behaviour of solutions of Schrödinger equation for weakly nonlinear waves are found to be similar, except for dispersion The theory is tested against experiments performed in a water tank.

physics.ao-ph↗
arXiv2014

Hydrodynamic model of Fukushima-Daiichi NPP Industrial site flooding

While the Fukushima-Daiichi was designed and constructed the maximal tsunami height estimate was about 3 m based on analysis of statistical data including Chile earthquake in 1960. The NPP project industrial site height was 10 m. The further deterministic estimates TPCO-JSCE confirmed the impossibility of the industrial site flooding by a tsunami and therefore confirmed ecological safety of the NPP. However, as a result of beyond design earthquake of 11 March 2011 the tsunami height at the shore near the Fukushima-Daiichi NPP reached 15 m. This led to flooding and severe emergencies having catastrophic environmental consequences. This paper proposes hydrodynamic model of tsunami emerging and traveling based on conservative assumptions. The possibility of a tsunami wave reaching 15 m height at the Fukushima-Daiichi NPP shore was confirmed for deduced hydrodynamic resistance coefficient of 1.8. According to the model developed a possibility of flooding is determined not only by the industrial site height, magnitude and distance to the earthquake epicenter, but also by duration of seismic impacts, power dissipation conditions, epicenter size and other conditions.

physics.flu-dyn↗
arXiv2014

The Shifts Hypothesis - an alternative view of global climate change

In this study we used HadCRUT4 temperature anomalies for 1950-2013 years in order to investigate properties of recent warming. Our aim was to separate changes produced by short-term ENSO variations and to look on temporal and spatial dynamics of residual temperature anomalies. For this we subtract linear influence of ENSO index from each HadCRUT4 grid box. We found that residual global temperature dynamics looks like staircase function: linear trends for three quasi-stable periods 1950-1987, 1988-1997 and 1998-2013 are near zero and near all warming occurred during two shifts of 1987/1988 and 1997/1998 years. All these allows us to formulate a new hypothesis about recent warming - the Shifts Hypothesis. It explains the structure of recent warming as follows: during shifts of 1987/1988 and 1997/1998 the mean value of global temperature quickly rose, over which natural variability associated with ENSO, PDO and many other local factors occurs. According to IPCC forcing-response paradigm warming process (WP) cleared from all factors should be continuous warming (may be near linear on the considered time interval). This idea is inconsistent to the obtained staircase function. This staircase function may be explained by current IPCC forcing-response paradigm only if it happened so that the combination of compensating factors is near the same as anthropogenic greenhouse gases (GHGs) forcing for each quasi-stable period - 1950-1987, 1988-1997 and 1998-2013. This seems very unlikely. An alternative and more simple explanation is the existence of some regulation mechanism (e.g. global thermostat) not presented in IPCC climate models. This mechanism should maintain global temperature near stable in 1950-1987, 1988-1997 and 1998-2013 periods nevertheless all the time growing forcing due to anthropogenic GHGs.

physics.ao-ph↗
arXiv2014

Identify the diapycnical eddy diffusivities by salt fingers and turbulence with vertical microstructure measurements

Diapycnical eddy diffusivities are formulated from physical relations according to a simple fact that the different formulas are identical for the same parameter. It is found that the dispassion ratio Γis a crucial parameter. When it is above a critical value (about 0.2), the flow is salt finger type; otherwise, it is turbulence. All the density ratio $R_ρ=1/(1-Γ^2)$, eddy flux ratio $γ=1/(1+Γ)$, and eddy diffusivity ratio $R_k=1-Γ$ are simply dependent on the dispassion ratio gamma Γ. We apply these relations to the measurement data in the western tropical Atlantic Ocean. The effective diapycnal diffusivity is $0.96*10^{-4} m^2/s$ that agrees quite well with the observation from 0.8 to $0.9*10^{-4} m^2/s$ by tracer.

physics.ao-ph↗
arXiv2014

Predictability Horizon of Oceanic Rogue Waves

Prediction is a central goal and a yet-unresolved challenge in the investigation of oceanic rogue waves. Here we define a horizon of predictability for oceanic rogue waves and derive, via extensive computational experiments, the first statistically-converged predictability time-scale for these structures. We show that this time-scale is a function of the sea state as well as the strength (i.e. overall height) of the expected rogue wave. The presented predictability time-scale establishes a quantitative metric on the combined temporal effect of the variety of mechanisms that together lead to the formation of a rogue wave, and is crucial for the assessment of validity of rogue waves predictions, as well as for the critical evaluation of results from the widely-used model equations. The methodology and presented results can have similar implications in other systems admitting rogue waves, e.g. nonlinear optics and plasma physics.

physics.flu-dyn↗
arXiv2014

Downscaling near-surface atmospheric fields with multi-objective Genetic Programming

The coupling of models for the different components of the Soil-Vegetation-Atmosphere-System is required to investigate component interactions and feedback processes. However, the component models for atmosphere, land-surface and subsurface are usually operated at different resolutions in space and time owing to the dominant processes. The computationally often more expensive atmospheric models, for instance, are typically employed at a coarser resolution than land-surface and subsurface models. Thus up- and downscaling procedures are required at the interface between the atmospheric model and the land-surface/subsurface models. We apply multi-objective Genetic Programming (GP) to a training data set of high-resolution atmospheric model runs to learn equations or short programs that reconstruct the fine-scale fields (e.g., 400 m resolution) of the near-surface atmospheric state variables from the coarse atmospheric model output (e.g., 2.8 km resolution). Like artificial neural networks, GP can flexibly incorporate multivariate and nonlinear relations, but offers the advantage that the solutions are human readable and thus can be checked for physical consistency. Using the Strength Pareto Approach for multi-objective fitness assignment allows us to consider multiple characteristics of the fine-scale fields during the learning procedure.

physics.ao-ph↗
arXiv2014

An amplitude equation for long, nonlinear internal waves in a weak shear flow over topography

A forced, variable coefficients Kor\-te\-weg-de Vries equation for amplitudes of long, nonlinear internal waves in a stratified shear flow over topography is derived when the magnitude of the basic flow is small. The derivation is done by incorporating the basic flow in the perturbation series of the Taniuti and Wei's reductive perturbation method. Istead of the long-wave Tailor-Goldstein spectral problem, only the usual long-wave spectral problem for rested stratified media and a boundary value problem have to be solved for calculating the coefficients of obtained equation. Explicit expressions for these coefficients are presented.

physics.ao-ph↗
arXiv2014

The Role of Variability in Transport for Large-Scale Flow Dynamics

We develop a framework to study the role of variability in transport across a streamline of a reference flow. Two complementary schemes are presented: a graphical approach for individual cases, and an analytical approach for general properties. The spatially nonlinear interaction of dynamic variability and the reference flow results in flux variability. The characteristic time-scale of the dynamic variability and the length-scale of the flux variability in a unit of flight-time govern the spatio-temporal interaction that leads to transport. The non-dimensional ratio of the two characteristic scales is shown to be a a critical parameter. The pseudo-lobe sequence along the reference streamline describes spatial coherency and temporal evolution of transport. For finite-time transport from an initial time up to the present, the characteristic length-scale of the flux variability regulates the width of the pseudo-lobes. The phase speed of pseudo-lobe propagation averages the reference flow and the flux variability. In contrast, for definite transport over a fixed time interval and spatial segment, the characteristic time-scale of the dynamic variability regulates the width of the pseudo-lobes. Generation of the pseudo-lobe sequence appears to be synchronous with the dynamic variability, although it propagates with the reference flow. In either case, the critical characteristic ratio is found to be one, corresponding to a resonance of the flux variability with the reference flow. Using a kinematic model, we demonstrate the framework for two types of transport in a blocked flow of the mid-latitude atmosphere: across the meandering jet axis and between the jet and recirculating cell.

physics.flu-dyn↗