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Papers by Armando Howard

We consider the hypothesis that galactic magnetic fields are primordial. We also discuss the vari... more We consider the hypothesis that galactic magnetic fields are primordial. We also discuss the various objections to this hypothesis. We assume that there was a magnetic field present in the galactic plasma before the galaxy formed. After the galactic disk formed, the lines of force thread through it and remain connected to the external cosmic medium. They enter through one side of the disk, proceed horizontally a distance l in the disk, and then leave through the other side. We find that the lines of force are stretched rotation of the galactic disk, which amplifies the toroidal component of the field and increases l. When the magnetic field is strong enough, it produces ambipolar velocities that try to lift the line out of the galactic disk but in opposite directions on different parts of the line. The result is, instead of the line being expelled from the disk, its horizontal length l is shortened, both in the radial, and in the toroidal direction. This leads to a reduction of the ...

Journal of Physical Oceanography, 2001

Ocean mixing processes have traditionally been formulated using one-point turbulence closure mode... more Ocean mixing processes have traditionally been formulated using one-point turbulence closure models, specifically the Mellor and Yamada (MY) models, which were pioneered in geophysics using 1980 state-of-the-art turbulence modeling. These models have been widely applied over the years, but the underlying core physical assumptions have hardly improved since the 1980s; yet, in the meantime, turbulence modeling has made sufficient progress to allow four improvements to be made. 1) The value of Ri cr. MY-type models yield a low value for the critical Richardson number, Ri cr ϭ 0.2 (the result of linear stability is Ri cr ϭ 1/4). On the other hand, nonlinear stability analysis, laboratory measurements, direct numerical simulation, large eddy simulation, and mixed layer studies indicate that Ri cr ϳ 1. The authors show that by improving the closure for the pressure correlations, the result Ri cr ϳ 1 naturally follows. 2) Nonlocal, third-order moments (TOMs). The downgradient approximation used in all models thus far seriously underestimates the TOMs. A new expression that includes both stratification and shear is presented here for the first time. It is obtained by solving the dynamic equations for the third-order moments. 3) Rotation. The MY-type models with rotation assume that the latter does not affect turbulence, specifically, neither the pressure correlations nor the rate of dissipation of turbulent kinetic energy. Recent studies show that both quantities are affected. 4) Mixing below the mixed layer. Thus far, the momentum and heat diffusivities below the mixed layer have been treated as adjustable parameters. A new model that allows use of the same turbulence model throughout the ocean depth is proposed. A new model is presented that includes 1), 2), and 4). Rotation will be dealt with in a subsequent paper. The new model is fully algebraic and easy to use in an ocean code. The new model is used in an OGCM, and the predicted global temperature and salinity profiles are compared with those of the KPP model and Levitus data.

Journal of Physical Oceanography, 2002

Abstract A Reynolds stress–based model is used to derive algebraic expressions for the vertical d... more Abstract A Reynolds stress–based model is used to derive algebraic expressions for the vertical diffusivities Kα(α = m, h, s) for momentum, heat, and salt. The diffusivities are expressed as Kα(Rρ, N, RiT, ϵ)in terms of the density ratio Rρ = αs∂S/∂z(αT∂T/∂z)−1, the Brunt–Vaisala frequency N2 = −gρ−10∂ρ/∂z, the Richardson number RiT = N2/Σ2 (Σ is the shear), and the dissipation rate of kinetic energy ϵ. The model is valid both in the mixed layer (ML) and below it. Here Rρ and N are computed everywhere using the large-scale fields from an ocean general circulation model while RiT is contributed by resolved and unresolved shear. In the ML, the wind-generated large-scale shear dominates and can be computed within an OGCM. Below the ML, the wind is no longer felt and small-scale shear dominates. In this region, the model provides a new relation RiT = cf(Rρ) with c ≈ 1 in lieu of Munk's suggestion RiT ≈ c. Thus, below the ML, the Kα become functions of Rρ, N, and ϵ. The dissipation ϵ representing the physical ...

Journal of Physical …, 2001

The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark... more The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark matter and texture are discussed and compared with recent observational results of the COBE satellite. A couple of important statistical parameters are determined. The fluctuations are slightly non gaussian. The quadrupole anisotropy is 1.5±1.2×10 −5 and the fluctuations on a angular scale of 10 degrees are (3.8±2.6)×10 −5. The COBE are within about one standard deviation of the typical texture + CDM model discussed in this paper. Furthermore, we calculate fluctuations on intermediate scales (about 2 degrees) with the result ∆T/T(θ ∼ 2 o) = 3.9 ± 0.8) × 10 −5. Collapsing textures are modeled by spherically symmetric field configurations. This Global texture[1] is the latest of a set of models based on a simple physical idea: the universe begins in a hot, homogeneous state and then, as it cools, undergoes a symmetry breaking phase transition that leads to the formation of topological def...

Ocean Modelling, Oct 1, 2015

We characterize the representation of the Southern Ocean water mass structure and sea ice within ... more We characterize the representation of the Southern Ocean water mass structure and sea ice within a suite of 15 global ocean-ice models run with the Coordinated Ocean-ice Reference Experiment Phase II (CORE-II) protocol. The main focus is the representation of the present (1988-2007) mode and intermediate waters, thus framing an analysis of winter and summer mixed layer depths; temperature, salinity, and potential vorticity structure; and temporal variability of sea ice distributions. We also consider the inter-annual variability over the same 20 year period. Comparisons are made between models as well as to observation-based analyses where available.

Physical review D: Particles and fields

The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark... more The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark matter and texture are discussed and compared with recent observational results of the COBE satellite. A couple of important statistical parameters are determined. The fluctuations are slightly non gaussian. The quadrupole anisotropy is 1.5±1.2×10 −5 and the fluctuations on a angular scale of 10 degrees are (3.8±2.6)×10 −5. The COBE are within about one standard deviation of the typical texture + CDM model discussed in this paper. Furthermore, we calculate fluctuations on intermediate scales (about 2 degrees) with the result ∆T /T (θ ∼ 2 o) = 3.9 ± 0.8) × 10 −5. Collapsing textures are modeled by spherically symmetric field configurations. This leads to uncertainties of about a factor of 2.

We present a derivation of the equation for the turbulence dissipation rate ε for a shear driven ... more We present a derivation of the equation for the turbulence dissipation rate ε for a shear driven flow. In 1961, Davydov used a one-point closure model to derive the ε-equation from first principles but the final result contained undetermined terms and thus lacked predictive power. In 1987, Schiestel and in 2001, Rubinstein and Zhou attempted to derive the ε-equation also from first principles using a two-point closure but their method relied on a phenomenological assumption. The standard practice has thus been to employ a heuristic form of the ε-equation that contains three empirical ingredients: two constants c1, c2 and a diffusion term Dε. We have employed a two-point closure and obtained the following results: 1) the empirical constants get replaced by new c1, c2 that are now functions of K and ε, 2) c1 and c2 are not independent since we derive a general relation between the two valid for any K, ε, 3) homogenous flows: c1, c2 become constant with values close to the empirical va...

Ocean Modelling, 2015

ABSTRACT In the framework of the second phase of the Coordinated Ocean-ice Reference Experiments ... more ABSTRACT In the framework of the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II), we present an analysis of the representation of the Antarctic Circumpolar Current (ACC) and Southern Ocean Meridional Overturning Circulation (MOC) in a suite of seventeen global ocean-sea ice models. We focus on the mean, variability and trends of both the ACC and MOC over the 1958--2007 period, and discuss their relationship with the surface forcing. We aim to quantify the degree of eddy saturation and eddy compensation in the models participating in CORE-II, and compare our results with available observations, previous fine-resolution numerical studies and theoretical constraints. Most models show weak ACC transport sensitivity to changes in forcing during the past five decades, and they can be considered to be in an eddy saturated regime. Larger contrasts arise when considering MOC trends, with a majority of models exhibiting significant strengthening of the MOC during the late 20th and early 21st century. Only a few models show a relatively small sensitivity to forcing changes, responding with an intensified eddy-induced circulation that provides some degree of eddy compensation, while still showing considerable decadal trends. Both ACC and MOC interannual variability are largely controlled by the Southern Annular Mode (SAM). Based on these results, models are clustered into two groups. Models with constant or two-dimensional (horizontal) specification of the eddy-induced advection coefficient k show larger ocean interior decadal trends, larger ACC transport decadal trends and no eddy compensation in the MOC. Eddy-permitting models or models with a three-dimensional time varying k show smaller changes in isopycnal slopes and associated ACC trends, and partial eddy compensation. As previously argued, a constant in time or space k is responsible for a poor representation of mesoscale eddy effects and cannot properly simulate the sensitivity of the ACC and MOC to changing surface forcing. Evidence is given for a larger sensitivity of the MOC as compared to the ACC transport, even when approaching eddy saturation. Future process studies designed for disentangling the role of momentum and buoyancy forcing in driving the ACC and MOC are proposed.

Ocean Modelling, 2014

Simulation characteristics from eighteen global ocean-sea-ice coupled models are presented with a... more Simulation characteristics from eighteen global ocean-sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort.

The Astrophysical Journal, 1997

Journal of Physical Oceanography, 2001

Journal of Physical Oceanography, 2002

Journal of Physical …, 2001

Ocean Modelling, Oct 1, 2015

Physical review D: Particles and fields

The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark... more The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark matter and texture are discussed and compared with recent observational results of the COBE satellite. A couple of important statistical parameters are determined. The fluctuations are slightly non gaussian. The quadrupole anisotropy is 1.5±1.2×10 −5 and the fluctuations on a angular scale of 10 degrees are (3.8±2.6)×10 −5. The COBE are within about one standard deviation of the typical texture + CDM model discussed in this paper. Furthermore, we calculate fluctuations on intermediate scales (about 2 degrees) with the result ∆T /T (θ ∼ 2 o) = 3.9 ± 0.8) × 10 −5. Collapsing textures are modeled by spherically symmetric field configurations. This leads to uncertainties of about a factor of 2.

Ocean Modelling, 2015

Ocean Modelling, 2014

The Astrophysical Journal, 1997