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Dive into the research topics where Armando Howard is active.

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Featured researches published by Armando Howard.


Journal of Physical Oceanography | 2001

Ocean Turbulence. Part I: One-Point Closure Model—Momentum and Heat Vertical Diffusivities

V. M. Canuto; Armando Howard; Y. Cheng; M. S. Dubovikov

Since the early forties, one-point turbulence closure models have been the canonical tools used to describe turbulent flows in many fields. In geophysics, Mellor and Yamada applied such models using the 1980 state-of-the art. Since then, no improvements were introduced to alleviate two major difficulties: 1) closure of the pressure correlations, which affects the correct determination of the critical Richardson number Ri(sub cr) above which turbulent mixing is no longer possible and 2) the need to express the non-local third-order moments (TOM) in terms of lower order moments rather than via the down-gradient approximation as done thus far, since the latter seriously underestimates the TOMs. Since 1) and 2) are still being dealt with adjustable parameters which weaken the credibility of the models, alternative models, not based on turbulence modeling, have been suggested. The aim of this paper is to show that new information, partly derived from the newest 2-point closure model discussed, can be used to solve these shortcomings. The new one-point closure model, which in its simplest form is algebraic and thus simple to implement, is first shown to reproduce a variety of data. Then, it is used in a Ocean-General Circulation Model (O-GCM) where it reproduces well a large variety of ocean data. While phenomenological models are specifically tuned to ocean turbulence, the present model is not. It is first tested against laboratory data on stably stratified flows and then used in an O-GCM. It is more general, more predictive and more resilient, e.g., it can incorporate phenomena like wave-breaking at the surface, salinity diffusivity, non-locality, etc. One important feature that naturally comes out of the new model is that the predicted Richardson critical value Ri(sub cr) is Ri (sub cr approx. = 1) in agreement with both Large Eddy Simulations (LES) and empirical evidence while all previous models predicted Ri (sub cr approx. = 0.2) which led to a considerable underestimate of the extent of turbulent mixing and thus to an incorrect mixed layer depth. The predicted temperature and salinity profiles (vs. depth) are presented and compared with those of the Kolmogorov-Petruvsky-Piskunuv (KPP) model and Levitus data.


Journal of Physical Oceanography | 2002

Ocean turbulence. Part II: Vertical diffusivities of momentum, heat, salt, mass, and passive scalars

V. M. Canuto; Armando Howard; Y. Cheng; M. S. Dubovikov

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 Munks suggestion RiT ≈ c. Thus, below the ML, the Kα become functions of Rρ, N, and ϵ. The dissipation ϵ representing the physical ...


The Astrophysical Journal | 1997

The Evolution of a primordial galactic magnetic field

Armando Howard; Russell M. Kulsrud

We consider the hypothesis that galactic magnetic fields are primordial. We also discuss the various objections to this hypothesis. To test this hypothesis properly we assume that there was a magnetic field present in the galactic plasma before the galaxy formed and explore how such a field would evolve assuming a specific model for the interstellar medium in the galactic disk. 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 by the differential rotation of the galactic disk, amplifying the toroidal component of the field and increasing 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 that, 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 rate of horizontal stretching and finally a reduction in the magnetic field strength. After a sufficient time, the magnetic field at all points goes through this reduction and the field strength approaches a universal function of time. This function is slowly decreasing and only depends on the ambipolar properties of the interstellar medium. At any given time the magnetic field is toroidal and has the same strength everywhere. On the other hand, it turns out that its direction varies rapidly with radius, changing sign every 100 parsecs or so. However, if the initial cosmic magnetic field is not uniform, the areas of one sign of the toroidal field dominate over the other. The resulting field has a net Faraday rotation. If such a field were observed with low resolution in an external galaxy, then the field would appear toroidal in between the spiral arms. The spiral density wave would turn it so that the lines appear to trace out the spiral arm, although the apparent lines really are the sum of pieces of magnetic lines as they cross the disk. They do not necessarily extend very far along the arms. We contend that this model of the magnetic field, which arises naturally from a primordial origin, can fit the observations as well as other models for the magnetic field, such as those arising from the mean field dynamo theory. Finally, because the field lines are topologically threaded through the disk, they cannot be expelled from the disk. This counters the objection against the primordial origin, namely that such a field could not survive very long in the galaxy.


Journal of Physical Oceanography | 2018

Parameterization of Mixed Layer and Deep-Ocean Mesoscales including Nonlinearity

V. M. Canuto; Y. Cheng; M. S. Dubovikov; Armando Howard; Anthony Leboissetier

AbstractIn 2011, Chelton et al. carried out a comprehensive census of mesoscales using altimetry data and reached the following conclusions: “essentially all of the observed mesoscale features are nonlinear” and “mesoscales do not move with the mean velocity but with their own drift velocity,” which is “the most germane of all the nonlinear metrics.” Accounting for these results in a mesoscale parameterization presents conceptual and practical challenges since linear analysis is no longer usable and one needs a model of nonlinearity. A mesoscale parameterization is presented that has the following features: 1) it is based on the solutions of the nonlinear mesoscale dynamical equations, 2) it describes arbitrary tracers, 3) it includes adiabatic (A) and diabatic (D) regimes, 4) the eddy-induced velocity is the sum of a Gent and McWilliams (GM) term plus a new term representing the difference between drift and mean velocities, 5) the new term lowers the transfer of mean potential energy to mesoscales, 6) th...


Ocean Modelling | 2014

North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states

Gokhan Danabasoglu; Steve G. Yeager; David A. Bailey; Erik Behrens; Mats Bentsen; Daohua Bi; Arne Biastoch; Claus W. Böning; Alexandra Bozec; V. M. Canuto; Christophe Cassou; Eric P. Chassignet; Andrew C. Coward; Sergey Danilov; Nikolay Diansky; Helge Drange; Riccardo Farneti; Elodie Fernandez; Pier Giuseppe Fogli; Gael Forget; Yosuke Fujii; Stephen M. Griffies; A. A. Gusev; Patrick Heimbach; Armando Howard; Thomas Jung; Maxwell Kelley; William G. Large; Anthony Leboissetier; Jianhua Lu


Ocean Modelling | 2015

An assessment of Antarctic Circumpolar Current and Southern Ocean meridional overturning circulation during 1958–2007 in a suite of interannual CORE-II simulations

Riccardo Farneti; Stephanie M. Downes; Stephen M. Griffies; Simon J. Marsland; Erik Behrens; Mats Bentsen; Daohua Bi; Arne Biastoch; Claus W. Böning; Alexandra Bozec; V. M. Canuto; Eric P. Chassignet; Gokhan Danabasoglu; Sergey Danilov; Nikolay Diansky; Helge Drange; Pier Giuseppe Fogli; A. A. Gusev; Robert Hallberg; Armando Howard; Mehmet Ilicak; Thomas Jung; Maxwell Kelley; William G. Large; Anthony Leboissetier; Matthew C. Long; Jianhua Lu; Simona Masina; Akhilesh Mishra; Antonio Navarra


Ocean Modelling | 2016

North Atlantic Simulations in Coordinated Ocean-Ice Reference Experiments Phase II (CORE-II) . Part II; Inter-Annual to Decadal Variability

Gokhan Danabasoglu; Steve G. Yeager; Who M. Kim; Erik Behrens; Mats Bentsen; Daohua Bi; Arne Biastoch; Rainer Bleck; Claus W. Böning; Alexandra Bozec; V. M. Canuto; Christophe Cassou; Eric P. Chassignet; Andrew C. Coward; Sergey Danilov; Nikolay Diansky; Helge Drange; Riccardo Farneti; Elodie Fernandez; Pier Giuseppe Fogli; Gael Forget; Yosuke Fujii; Stephen M. Griffies; A. A. Gusev; Patrick Heimbach; Armando Howard; Mehmet Ilicak; Thomas Jung; Alicia Karspeck; Maxwell Kelley


Ocean Modelling | 2015

An assessment of Southern Ocean water masses and sea ice during 1988–2007 in a suite of interannual CORE-II simulations

Stephanie M. Downes; Riccardo Farneti; Petteri Uotila; Stephen M. Griffies; Simon J. Marsland; David A. Bailey; Erik Behrens; Mats Bentsen; Daohua Bi; Arne Biastoch; Claus W. Böning; Alexandra Bozec; V. M. Canuto; Eric P. Chassignet; Gokhan Danabasoglu; Sergey Danilov; Nikolay Diansky; Helge Drange; Pier Giuseppe Fogli; A. A. Gusev; Armando Howard; Mehmet Ilicak; Thomas Jung; Maxwell Kelley; William G. Large; Anthony Leboissetier; Matthew C. Long; Jianhua Lu; Simona Masina; Akhilesh Mishra


Ocean Modelling | 2010

Ocean turbulence, III : new GISS vertical mixing scheme

V. M. Canuto; Armando Howard; Y. Cheng; C. J. Muller; A. Leboissetier; Steven R. Jayne


Ocean Modelling | 2011

Vertical diffusivities of active and passive tracers

V. M. Canuto; Y. Cheng; Armando Howard

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V. M. Canuto

Goddard Institute for Space Studies

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Y. Cheng

Goddard Institute for Space Studies

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Anthony Leboissetier

Goddard Institute for Space Studies

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Gokhan Danabasoglu

National Center for Atmospheric Research

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Maxwell Kelley

Goddard Institute for Space Studies

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Stephen M. Griffies

Geophysical Fluid Dynamics Laboratory

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Mats Bentsen

Bjerknes Centre for Climate Research

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Daohua Bi

Commonwealth Scientific and Industrial Research Organisation

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