scholarly journals The Sensitivity of the Isentropic Slope in a Primitive Equation Dry Model

2008 ◽  
Vol 65 (1) ◽  
pp. 43-65 ◽  
Author(s):  
Pablo Zurita-Gotor
Keyword(s):  
Surface Temperature ◽  
Heating Rate ◽  
Mass Flux ◽  
Time Scale ◽  
Diabatic Heating ◽  
Return Flow ◽  
Mean Flow ◽  
Primitive Equation ◽  
The Mean ◽  
Flow Changes

Abstract This paper discusses the sensitivity of the isentropic slope in a primitive equation dry model forced with Newtonian cooling when the heating is varied. This is done in two different ways, changing either the radiative equilibrium baroclinicity or the diabatic time scale for the zonal-mean flow. When the radiative equilibrium baroclinicity is changed, the isentropic slope remains insensitive against changes in the forcing, in agreement with previous results. However, the isentropic slope steepens when the diabatic heating rate is accelerated for the zonal-mean flow. Changes in the ratio between the interior and the boundary diffusivities as the diabatic heating rate is varied appear to be responsible for the violation of the constant criticality constraint in this model. Theoretical arguments are used to relate the sensitivity of the isentropic slope to that of the isentropic mass flux, which also remains constant when the radiative-equilibrium baroclinicity is changed. The sensitivity of the isentropic mass flux on the heating depends on how the gross stability changes. Bulk stabilities calculated from isobaric averages and gross stabilities estimated from isentropic diagnostics are not necessarily equivalent because a significant part of the return flow occurs at potential temperatures colder than the mean surface temperature.

scholarly journals Optimal Surface Excitation of the Thermohaline Circulation

2008 ◽  
Vol 38 (8) ◽  
pp. 1820-1830 ◽  
Author(s):  
Laure Zanna ◽  
Eli Tziperman
Keyword(s):  
Surface Temperature ◽  
Time Scale ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Initial Conditions ◽  
Singular Problem ◽  
Mean Flow ◽  
L2 Norm ◽  
Basin Scale ◽  
The Mean

Abstract The amplification of thermohaline circulation (THC) anomalies resulting from heat and freshwater forcing at the ocean surface is investigated in a zonally averaged coupled ocean–atmosphere model. Optimal initial conditions of surface temperature and salinity leading to the largest THC growth are computed, and so are the structures of stochastic surface temperature and salinity forcing that excite maximum THC variance (stochastic optimals). When the THC amplitude is defined as its sum of squares (equivalent to using the standard L2 norm), the nonnormal linearized dynamics lead to an amplification with a time scale on the order of 100 yr. The optimal initial conditions have a vanishing THC anomaly, and the complex amplification mechanism involves the advection of both temperature and salinity anomalies by the mean flow and of the mean temperature and salinity by the anomaly flow. The L2 characterization of THC anomalies leads to physically interesting results, yet to a mathematically singular problem. A novel alternative characterizing the THC amplitude by its maximum value, as often done in general circulation model studies, is therefore introduced. This complementary method is shown to be equivalent to using the L-infinity norm, and the needed mathematical approach is developed and applied to the THC problem. Under this norm, an amplification occurs within 10 yr explained by the classic salinity advective feedback mechanism. The analysis of the stochastic optimals shows that the character of the THC variability may be very sensitive to the spatial pattern of the surface forcing. In particular, a maximum THC variance and long-time-scale variability are excited by a basin-scale surface forcing pattern, while a significantly higher frequency and to some extent a weaker variability are induced by a smooth and large-scale, yet mostly concentrated in polar areas, surface forcing pattern. Overall, the results suggest that a large THC variability can be efficiently excited by atmospheric surface forcing, and the simple model used here makes several predictions that would be interesting to test using more complex models.

DNS of the thermal effects of laser energy deposition in isotropic turbulence

2010 ◽  
Vol 654 ◽  
pp. 387-416
Author(s):  
SHANKAR GHOSH ◽  
KRISHNAN MAHESH
Keyword(s):  
Shock Wave ◽  
Time Scale ◽  
Energy Deposition ◽  
Isotropic Turbulence ◽  
Radial Distance ◽  
Working Fluid ◽  
Shock Propagation ◽  
Mean Flow ◽  
Turbulent Reynolds Number ◽  
The Mean

The interaction of a laser-induced plasma with isotropic turbulence is studied using numerical simulations. The simulations use air as the working fluid and assume local thermodynamic equilibrium. The numerical method is fully spectral and uses a shock-capturing scheme in a corrector step. A model problem involving the effect of energy deposition on an isolated vortex is studied as a first step towards plasma/turbulence interaction. Turbulent Reynolds number Reλ = 30 and fluctuation Mach numbers Mt = 0.001 and 0.3 are considered. A tear-drop-shaped shock wave is observed to propagate into the background, and progressively become spherical in time. The turbulence experiences strong compression due to the shock wave and strong expansion in the core. This behaviour is spatially inhomogeneous and non-stationary in time. Statistics are computed as functions of radial distance from the plasma axis and angular distance across the surface of the shock wave. For Mt = 0.001, the shock wave propagates on a much faster time scale compared to the turbulence evolution. At Mt of 0.3, the time scale of the shock wave is comparable to that of the background. For both cases the mean flow is classified into shock formation, shock propagation and subsequent collapse of the plasma core, and the effect of turbulence on each of these phases is studied in detail. The effect of mean vorticity production on the turbulent vorticity field is also discussed. Turbulent kinetic energy budgets are presented to explain the mechanism underlying the transfer of energy between the mean flow and background turbulence.

scholarly journals Transport and instability in driven two-dimensional magnetohydrodynamic flows

2016 ◽  
Vol 799 ◽  
pp. 541-578 ◽  
Author(s):  
Sam Durston ◽  
Andrew D. Gilbert
Keyword(s):  
Magnetic Field ◽  
Time Scale ◽  
Large Scale ◽  
Body Force ◽  
Passive Scalar ◽  
Fast Time ◽  
Two Dimensional ◽  
Mean Flow ◽  
The Mean ◽  
Background Shear

This paper concerns the generation of large-scale flows in forced two-dimensional systems. A Kolmogorov flow with a sinusoidal profile in one direction (driven by a body force) is known to become unstable to a large-scale flow in the perpendicular direction at a critical Reynolds number. This can occur in the presence of a ${\it\beta}$-effect and has important implications for flows observed in geophysical and astrophysical systems. It has recently been termed ‘zonostrophic instability’ and studied in a variety of settings, both numerically and analytically. The goal of the present paper is to determine the effect of magnetic field on such instabilities using the quasi-linear approximation, in which the full fluid system is decoupled into a mean flow and waves of one scale. The waves are driven externally by a given random body force and move on a fast time scale, while their stress on the mean flow causes this to evolve on a slow time scale. Spatial scale separation between waves and mean flow is also assumed, to allow analytical progress. The paper first discusses purely hydrodynamic transport of vorticity including zonostrophic instability, the effect of uniform background shear and calculation of equilibrium profiles in which the effective viscosity varies spatially, through the mean flow. After brief consideration of passive scalar transport or equivalently kinematic magnetic field evolution, the paper then proceeds to study the full magnetohydrodynamic system and to determine effective diffusivities and other transport coefficients using a mixture of analytical and numerical methods. This leads to results on the effect of magnetic field, background shear and ${\it\beta}$-effect on zonostrophic instability and magnetically driven instabilities.

scholarly journals A Simple Model of a Balanced Boundary Layer Coupled to a Large-Scale Convective Circulation

2019 ◽  
Vol 76 (3) ◽  
pp. 837-849
Author(s):  
Robert J. Beare ◽  
Michael J. P. Cullen
Keyword(s):  
Boundary Layer ◽  
Simple Model ◽  
Surface Temperature ◽  
Mass Flux ◽  
Time Scale ◽  
Large Scale ◽  
Climate Models ◽  
Convective Boundary ◽  
Convective Circulation ◽  
Convective Mass Flux

Abstract Many simple models of large-scale tropical circulations do not include a frictional boundary layer. A simple model is presented where the convective circulation is coupled to the boundary layer convergence. In the free troposphere, convection and boundary layer heating try to relax to a moist adiabat from the local sea surface temperature with a time scale τc, but other processes act to maintain a weak temperature gradient. There is a mass balance between radiatively driven subsidence and the large-scale convective mass flux. For a prescribed Gaussian surface temperature, the model predicts a mass flux that varies as and a convective width proportional to its reciprocal. In the boundary layer, there can be significant horizontal temperature gradients and a balance between the pressure gradient and drag is assumed. Coupling between the two layers is mediated by the vertical velocity at the top of the boundary layer. The boundary layer constrains the circulation in three ways. First, it may lengthen the relaxation time scale compared to deep convection. Second, the evaporation in the nonconvecting region constrains the horizontal moisture advection. Third, it maintains a convective boundary layer where there is a convective mass flux; this condition cannot be satisfied if τc is too small or if the drag is too large, thus showing that such values are physically impossible. These results provide testable hypotheses concerning the physics and large-scale dynamics in weather and climate models.

scholarly journals Litter Surface Temperature: A Driving Factor Affecting Foraging Activity in Dinoponera lucida (Hymenoptera: Formicidae)

Sociobiology ◽  
2021 ◽  
Vol 68 (1) ◽  
Author(s):  
Flávio Curbani ◽  
Cássio Zocca ◽  
Rodrigo B. Ferreira ◽  
Cecilia Waichert ◽  
Tathiana Guerra Sobrinho ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Activity Patterns ◽  
Red List ◽  
Foraging Activity ◽  
Mean Flow ◽  
Lowland Forest ◽  
Habitat Temperature ◽  
Eastern Brazil ◽  
Forest Remnant ◽  
The Mean

Dinoponera lucida is a poneromorph ant endemic to the Atlantic Forest of Brazil. The species is classified as endangered in Brazil’s Red List due to its peculiar reproductive biology and high habitat fragmentation. Herein, we characterize D. lucida foraging activity and response to litter surface temperature in a lowland forest remnant in south-eastern Brazil. The mean flow of workers at nest openings was 3.8 ± 0.6 per hour, mean foraging trip was 14.2 ± 2.2 min, and mean foraging distance was 3.8 ± 0.4 m. The time spent per foraging trip and litter surface temperature were positively correlated. Flow of workers at nest openings was higher with mean temperature of litter surface between 21.0 and 27.0 °C. Our results show that D. lucida has a diurnal foraging activity related to habitat temperature. Our data contribute to the knowledge about the ecology of D. lucida and support the hypothesis of optimal food foraging regulated by habitat temperature. In addition, the better understanding of D. lucida activity patterns can assist on conservation planning of this endangered and endemic ant.

On the Mean Flow in the Western Gulf of Mexico and a Reappraisal of Errors in Ship-Drift Data

2020 ◽  
Vol 50 (7) ◽  
pp. 1983-1988
Author(s):  
Wilton Sturges
Keyword(s):  
Gulf Of Mexico ◽  
Surface Flow ◽  
Return Flow ◽  
Loop Current ◽  
Mean Flow ◽  
The West ◽  
Near Surface ◽  
Southern Half ◽  
The Mean ◽  
Layer Flow

AbstractShip-drift data in the Gulf of Mexico have led to a perplexing result, that the near-surface flow in the west has a north–south mean, of the east–west flow, ~5–10 cm s−1 into a closed basin. Ship-drift data have been used in the past hundred years under the assumption that they are reasonably accurate; the present study examines that assumption carefully, finding that the standard deviation of individual observations is typically ~20 cm s−1. In a monthly mean composed of order 400 observations or more, as examined here, the standard error of the mean will be reduced accordingly. In the southern part of the western Gulf of Mexico, the observed upper-layer flow is clearly to the west and is consistent with our expectations. In the northern part, however, the apparent flow as reported by ship drift in deep water is not significantly different from zero. Thus, the puzzling result remains: three different datasets in the southern half of the basin clearly show flow to the west, with speeds of 10 cm s−1 or more, yet there is no clear evidence of a near-surface return flow back to the east. The convergent wind stress forces downwelling of the upper layer; its return flow could be at some intermediate depth. The transport to the west from Loop Current rings is possibly returned in a deep boundary flow driven by the rectification of deep topographic Rossby waves.

scholarly journals Optimal Excitation of Interannual Atlantic Meridional Overturning Circulation Variability

2011 ◽  
Vol 24 (2) ◽  
pp. 413-427 ◽  
Author(s):  
Laure Zanna ◽  
Patrick Heimbach ◽  
Andrew M. Moore ◽  
Eli Tziperman
Keyword(s):  
High Latitude ◽  
Time Scale ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Mean Flow ◽  
Overturning Circulation ◽  
Optimal Excitation ◽  
Meridional Overturning ◽  
The Mean

Abstract The optimal excitation of Atlantic meridional overturning circulation (MOC) anomalies is investigated in an ocean general circulation model with an idealized configuration. The optimal three-dimensional spatial structure of temperature and salinity perturbations, defined as the leading singular vector and generating the maximum amplification of MOC anomalies, is evaluated by solving a generalized eigenvalue problem using tangent linear and adjoint models. Despite the stable linearized dynamics, a large amplification of MOC anomalies, mostly due to the interference of nonnormal modes, is initiated by the optimal perturbations. The largest amplification of MOC anomalies, found to be excited by high-latitude deep density perturbations in the northern part of the basin, is achieved after about 7.5 years. The anomalies grow as a result of a conversion of mean available potential energy into potential and kinetic energy of the perturbations, reminiscent of baroclinic instability. The time scale of growth of MOC anomalies can be understood by examining the time evolution of deep zonal density gradients, which are related to the MOC via the thermal wind relation. The velocity of propagation of the density anomalies, found to depend on the horizontal component of the mean flow velocity and the mean density gradient, determines the growth time scale of the MOC anomalies and therefore provides an upper bound on the MOC predictability time. The results suggest that the nonnormal linearized ocean dynamics can give rise to enhanced MOC variability if, for instance, overflows, eddies, and/or deep convection can excite high-latitude density anomalies in the ocean interior with a structure resembling that of the optimal perturbations found in this study. The findings also indicate that errors in ocean initial conditions or in model parameterizations or processes, particularly at depth, may significantly reduce the Atlantic MOC predictability time to less than a decade.

On the behaviour of impinging zero-net-mass-flux jets

2016 ◽  
Vol 810 ◽  
pp. 25-59 ◽  
Author(s):  
Carlo Salvatore Greco ◽  
Gennaro Cardone ◽  
Julio Soria
Keyword(s):  
Flow Field ◽  
Vortex Ring ◽  
Strouhal Number ◽  
Mass Flux ◽  
Mean Flow ◽  
Turbulent Statistics ◽  
Image Velocimetry ◽  
The Mean ◽  
Zero Net Mass Flux ◽  
Plate Distance

This paper reports on an experimental study of the influence of the Strouhal number (0.011, 0.022 and 0.044) and orifice-to-plate distances (2, 4 and 6 orifice diameters) on the flow field of an impinging zero-net-mass-flux jet at a Reynolds number equal to 35 000. These jets are generated by a reciprocating piston that oscillates in a cavity behind a circular orifice. Instantaneous two-dimensional in-plane velocity fields are measured in a plane containing the orifice axis using multigrid/multipass cross-correlation digital particle image velocimetry. These measurements have been used to investigate the mean flow quantities and turbulent statistics of the impinging zero-net-mass-flux jets. In addition, the vortex ring behaviour is analysed via its trajectory and azimuthal vorticity as well as the saddle point excursion, the flow rate and entrainment. The behaviour of all these quantities depends on the Strouhal number and the orifice-to-plate distance because the former governs the presence and the relative importance of the vortex ring and the trailing jet on the flow field and the latter delimits the downstream evolution of these structures.

scholarly journals The Linear Response Function of an Idealized Atmosphere. Part I: Construction Using Green’s Functions and Applications

2016 ◽  
Vol 73 (9) ◽  
pp. 3423-3439 ◽  
Author(s):  
Pedram Hassanzadeh ◽  
Zhiming Kuang
Keyword(s):  
Response Function ◽  
Linear Response ◽  
Mean Flow ◽  
Turbulent Eddies ◽  
Fluctuation Dissipation ◽  
Linear Response Function ◽  
Potential Applications ◽  
Neutral Vector ◽  
The Mean ◽  
Flow Changes

Abstract A linear response function (LRF) determines the mean response of a nonlinear climate system to weak imposed forcings, and an eddy flux matrix (EFM) determines the eddy momentum and heat flux responses to mean-flow changes. Neither LRF nor EFM can be calculated from first principles owing to the lack of a complete theory for turbulent eddies. Here the LRF and EFM for an idealized dry atmosphere are computed by applying numerous localized weak forcings, one at a time, to a GCM with Held–Suarez physics and calculating the mean responses. The LRF and EFM for zonally averaged responses are then constructed using these forcings and responses through matrix inversion. Tests demonstrate that LRF and EFM are fairly accurate. Spectral analysis of the LRF shows that the most excitable dynamical mode, the neutral vector, strongly resembles the model’s annular mode. The framework described here can be employed to compute the LRF and EFM for zonally asymmetric responses and more complex GCMs. The potential applications of the LRF and EFM constructed here are (i) forcing a specified mean flow for hypothesis testing, (ii) isolating/quantifying the eddy feedbacks in complex eddy–mean flow interaction problems, and (iii) evaluating/improving more generally applicable methods currently used to construct LRFs or diagnose eddy feedbacks in comprehensive GCMs or observations. As an example for (iii), in Part II, the LRF is also computed using the fluctuation–dissipation theorem (FDT), and the previously calculated LRF is exploited to investigate why FDT performs poorly in some cases. It is shown that dimension reduction using leading EOFs, which is commonly used to construct LRFs from the FDT, can significantly degrade the accuracy owing to the nonnormality of the operator.

scholarly journals The Mean Upper-Layer Flow in the Central Gulf of Mexico by a New Method

2016 ◽  
Vol 46 (10) ◽  
pp. 2915-2924 ◽  
Author(s):  
Wilton Sturges
Keyword(s):  
Gulf Of Mexico ◽  
Numerical Models ◽  
Independent Set ◽  
Sea Surface ◽  
Return Flow ◽  
Mean Flow ◽  
The Mean ◽  
Layer Flow ◽  
A New Technique ◽  
Height Data

AbstractPrevious studies have found a puzzling disagreement between two large datasets and the results of numerical models in the central Gulf of Mexico. The observations suggest an upper-layer mean flow to the west of order 10 cm s−1, while the numerical models find no such mean flow. A new technique is used here, using 23 yr of satellite-derived sea surface height data, to estimate the mean flow. This third, independent set of data yields the same westward flow found in previous studies. These findings require that there be sinking in the western Gulf. The details of the return flow remain an intriguing problem.

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