arXiv
(422.153 recursos)
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Mostrando recursos 1 - 17 de 17
1.
North Atlantic thermohaline circulation predictability in a coupled
ocean-atmosphere model - Griffies, Stephen M.; Bryan, Kirk
Predictability of the North Atlantic thermohaline circulation (THC)
variability as simulated in the GFDL coupled ocean-atmosphere general
circulation model is established for a set of ensemble experiments. The
ensembles consist of identical oceanic initial conditions underneath a model
atmosphere chosen randomly from the model climatology. This experimental design
is based on the separation in time scales present in the model which motivates
the assumption that the predictability deduced from these ensembles provides an
upper limit to the model's THC predictability. The climatology is taken from a
multi-century model integration whose THC variability has power concentrated at
the 40-60 year time scale. A linear stochastic perspective is shown to be
generally...
2.
A linear thermohaline oscillator driven by stochastic atmospheric
forcing - Griffies, Stephen M.; Tziperman, Eli
The interdecadal variability of a stochastically forced four-box model of the
oceanic meridional thermohaline circulation (THC) is described and compared to
the THC variability in the coupled ocean-atmosphere GCM of Delworth, Manabe,
and Stouffer (1993). The box model is placed in a linearly stable thermally
dominant mean state under mixed boundary conditions. A linear stability
analysis of this state reveals one damped oscillatory THC mode in addition to
purely damped modes. The variability of the model under a moderate amount of
stochastic forcing, meant to emulate the random variability of the atmosphere
affecting the coupled model's interdecadal THC variability, is studied. A
linear interpretation, in which the damped oscillatory...
3.
A Scaling Theory for Horizontally Homogeneous, Baroclinically Unstable
Flow on a Beta-Plane - Held, Isaac M.; Larichev, Vitaly D.
The scaling argument developed by Larichev and Held (1995) for eddy amplitudes
and fluxes in a horizontally homogeneous, two-layer model on an f-plane is
extended to a beta-plane. In terms of the non-dimensional number x =
U/(beta*lambda^2), where lambda is the deformation radius and U is the mean
thermal wind, the result for the RMS eddy velocity V, the characteristic
wavenumber of the energy-containing eddies and of the eddy-driven jets k, and
the magnitude of the eddy diffusivity for potential vorticity D, in the limit
x>>1, are as follows: V/U ~ x ; k*lambda ~ 1/x ; D/(U*lambda) ~ x^2. Numerical
simulations provide qualitative support for this scaling,...
4.
The stability of a zonally averaged thermohaline circulation model - Schmidt, G. A.; Mysak, L. A.
A combination of analytical and numerical techniques are used to efficiently
determine the qualitative and quantitative behaviour of a one-basin zonally
averaged thermohaline circulation ocean model. In contrast to earlier studies
which use time stepping to find the steady solutions, the steady state
equations are first solved directly to obtain the multiple equilibria under
identical mixed boundary conditions. This approach is based on the
differentiability of the governing equations and especially the convection
scheme. A linear stability analysis is then performed, in which the normal
modes and corresponding eigenvalues are found for the various equilibrium
states. Resonant periodic solutions superimposed on these states are predicted
for various types of forcing....
5.
Interpretation of TOVS Water Vapor Radiances Using a Random Strong Line
Model - Soden, Brian J.; Bretherton, Francis P.
This study illustrates the application of a random strong line (RSL) model of
radiative transfer to the interpretation of satellite observations of the
upwelling radiation in the 6.3 micron water vapor absorption band. The model,
based upon an assemblage of randomly overlapped, strongly absorbing, pressure
broadened lines, is compared to detailed radiative transfer calculations of the
upper (6.7 micron) tropospheric water vapor radiance and demonstrated to be
accurate to within ~ 1.2 K. Similar levels of accuracy are found when the model
is compared to detailed calculations of the middle (7.3 micron) and lower (8.3
micron) tropospheric water vapor radiance, provided that the emission from the
underlying surface is...
6.
Mechanisms of Seasonal - ENSO interaction - Tziperman, Eli; Zebiak, Steve; Cane, Mark
The mechanisms of interaction between the seasonal cycle and ENSO are
investigated using the Zebiak and Cane ENSO prediction model. The most dominant
seasonal effect is found to be due to the wind divergence field, as determined
by the seasonal motion of the ITCZ, through its effect on the atmospheric
heating. The next order seasonal effects are due to the seasonality of the
background SST and ocean upwelling velocity, and the corresponding mechanisms
are analyzed. It is suggested that the seasonal forcing has a first order
effect on ENSO's dynamics. Important aspects of the seasonal forcing may be
included in idealized delayed oscillator ENSO models by making the...
7.
Hadley circulations and large scale motions of moist convection in the
two dimensional numerical model - Satoh, Masaki
As a tool for understanding the meridional circulation of the atmosphere, a
two-dimensional ( latitude -- height ) numerical model is used to clarify the
relationship between the Hadley circulation and large-scale motions associated
with moist convection. The model is based on the primitive equations including
the moist process, and two kinds of coordinates are used: the spherical
coordinate and the Cartesian coordinate with a uniform rotation. The surface
temperature is externally fixed and the troposphere is cooled by the radiation;
unstable stratification generates large-scale convective motions.
Dependencies on the surface temperature difference from north to south Delta
T_s are investigated. The numerical results show that a systematic...
8.
Entropic "sound" in the atmosphere - Apostol, B. -F.; Stefan, S.; Apostol, M.
It is shown that small, local disturbances of entropy in the atmosphere may
give rise to "sound" waves propagating with a velocity which depends on the
amplitude ratio of the local relative variations of temperature and volume.
This velocity is much smaller than the mean molecular velocity and the usual,
adiabatic sound velocity.
9.
Interdecadal variability and oceanic thermohaline adjustment - Greatbatch, Richard J.; Peterson, K. Andrew
Changes in the strength of the thermohaline overturning circulation are
associated, by geostrophy, with changes in the east-west pressure difference
across an ocean basin. The tropical-polar density contrast and the east-west
pressure difference are connected by an adjustment process. In flat-bottomed
ocean models the adjustment is associated with viscous, baroclinic Kelvin wave
propagation. Weak-high latitude stratification leads to the adjustment having
an interdecadal timescale. We reexamine model interdecadal oscillations in the
context of the adjustment process, for both constant flux and mixed surface
boundary conditions. Under constant surface flux, interdecadal oscillations are
associated with the passage of a viscous Kelvin wave around the model domain.
Our results suggest the oscillations...
10.
Sub-Suns and Low Reynolds Number Flow - Katz, J. I.
The phenomenon called the ``sub-Sun'' is the specular reflection of sunlight
by horizontally oriented plates of ice. Although well-known in meteorological
optics, the hydrodynamics of the orientation is not quantitatively understood.
I review the theory of torques on objects at low Reynolds numbers, define
coefficients $C_o$, $C_p$, and $C_\psi$ which describe the orienting torques on
discs, rods, and hexagonal prisms, and report here the results of experiments
to measure $C_o$ and $C_p$.
11.
Estimating the Attractor Dimension of the Equatorial Weather System - Tiong, Melvin Leok Boon
The correlation dimension and limit capacity serve theoretically as lower and
upper bounds, respectively, of the fractal dimension of attractors of dynamic
systems. In this paper, we show that estimates of the correlation dimension
grow rapidly with increasing noise level in the time-series, while estimates of
the limit capacity remain relatively unaffected. It is therefore proposed that
the limit capacity be used in studies of noisy data, despite its heavier
computational requirements. An analysis of Singapore wind data with the limit
capacity estimate revealed a surprisingly low dimension (~2.5). It is suggested
that further studies be made with comprehensive equatorial weather data.
12.
A Hamiltonian weak-wave model for shallow-water flow - Nore, Caroline; Shepherd, Theodore G.
A reduced dynamical model is derived which describes the interaction of weak
inertia-gravity waves with nonlinear vortical motion in the context of rotating
shallow-water flow. The formal scaling assumptions are (i) that there is a
separation in timescales between the vortical motion and the inertia-gravity
waves, and (ii) that the divergence is weak compared to the vorticity. The
model is Hamiltonian, and possesses conservation laws analogous to those in the
shallow-water equations. Unlike the shallow-water equations, the energy
invariant is quadratic. Nonlinear stability theorems are derived for this
system, and its linear eigenvalue properties are investigated in the context of
some simple basic flows.
13.
Estimating the Fractal Dimension, K_2-entropy, and the Predictability of
the Atmosphere - Raidl, Ales
The series of mean daily temperature of air recorded over a period of 215
years is used for analysing the dimensionality and the predictability of the
atmospheric system. The total number of data points of the series is 78527.
Other 37 versions of the original series are generated, including ``seasonally
adjusted'' data, a smoothed series, series without annual course, etc. Modified
methods of Grassberger and Procaccia are applied. A procedure for selection of
the ``meaningful'' scaling region is proposed. Several scaling regions are
revealed in the ln C(r) versus ln r diagram. The first one in the range of
larger ln r has a gradual slope and the...
14.
A Stochastic Diffusion Model of Climate Change - Pelletier, Jon D.
We present a model for variations in atmospheric temperature from time scales
of one day to one million years based on a stochastic diffusion (random walk)
model of the turbulent transport of heat energy vertically in a coupled
atmosphere-ocean model. The predictions of the model are supported by station
records and paleoclimatic proxy data of temperature variations.
15.
Kardar-Parisi-Zhang model for the fractal structure of cumulus cloud
fields - Pelletier, Jon D.
We model the ascent of warm, moist air in the Earth's atmosphere by turbulent
convection and expansion with the KPZ equation, familiar in the physics
literature on surface growth. Clouds form in domains where the interface
between the rising air and its surrounding air achieves an elevation higher
than that necessary for condensation. The model predictions are consistent with
the perimeter fractal dimension and the cumulative frequency-size distribution
of cumulus cloud fields observed from space.
16.
Analysis and modeling of scale-invariance in plankton abundance - Pelletier, Jon D.
The power spectrum, $S$, of horizontal transects of plankton abundance are
often observed to have a power-law dependence on wavenumber, $k$, with exponent
close to -2: $S(k)\propto k^{-2}$ over a wide range of scales. I present power
spectral analyses of aircraft lidar measurements of phytoplankton abundance
from scales of 1 to 100 km. A power spectrum $S(k)\propto k^{-2}$ is obtained.
As a model for this observation, I consider a stochastic growth equation where
the rate of change of plankton abundance is determined by turbulent mixing,
modeled as a diffusion process in two dimensions, and exponential growth with a
stochastically variable net growth rate representing a fluctuating environment.
The model...
17.
Decaying Turbulence and the Dynamics of Diffusing Vortices with
Conservation Laws - Sire, Clement
In this letter, I solve a model for the dynamics of vortices in a decaying
two-dimensional turbulent fluid. The model describes their effective diffusion,
and the merging of pairs of vortices of same vorticity sign, when they get too
close. The merging process is characterized by the conservation of energy and
of the quantity $Nr^n$, where $r$ is the mean vortex radius, and $N$ their
number. $n=4$ corresponds to a constant peak vorticity, and $n=2$ to a constant
kurtosis. I found the scaling laws for various physical quantities ($r$,
enstrophy, kurtosis...), and for instance, it is shown that $N\sim
(t/\ln(t))^{-\frac{2n}{3n-4}}$ for $n>2$, and $N\sim t^{-2}$ for $n=2$, in...
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