286 research outputs found
Thermodynamic derivation and use of a nonequilibrium canonical ensemble
A thermodynamic expression for the analog of the canonical ensemble for
nonequilibrium systems is described based on a purely information theoretical
interpretation of entropy. As an application, it is shown that this
nonequilibrium canonical distribution implies some important results from
nonequilibrium thermodynamics, specifically, the fluctuation theorem and the
Jarzynski-equality.Comment: 4 pages, 0 figure
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The chaos machine: analogue computing rediscovered (1)
Analogue computers provide actual rather than virtual representations of model systems. They are powerful and engaging computing machines that are cheap and simple to build. This two-part Retronics article helps you build (and understand!) your own analogue computer to simulate the Lorenz butterfly that's become iconic for Chaos theory
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The role of heat-flux–temperature covariance in the evolution of weather systems
Local diabatic heating and temperature anomaly fields need to be positively correlated for the diabatic heating to maintain a circulation against dissipation. Here we quantify the thermodynamic contribution of local air–sea heat exchange on the evolution of weather systems using an index of the spatial covariance between heat flux at the air–sea interface and air temperature at 850 hPa upstream of the North Atlantic storm track, corresponding with the Gulf Stream extension region. The index is found to be almost exclusively negative, indicating that the air–sea heat fluxes act locally as a sink on potential energy. It features bursts of high activity alternating with longer periods of lower activity. The characteristics of these high-index bursts are elucidated through composite analysis and the mechanisms are investigated in a phase space spanned by two different index components. It is found that the negative peaks in the index correspond with thermodynamic activity triggered by the passage of a weather system over a spatially variable sea-surface temperature field; our results indicate that most of this thermodynamically active heat exchange is realised within the cold sector of the weather systems
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An adiabatic mechanism for the reduction of jet meander amplitude by potential vorticity filamentation
The amplitude of ridges in large-amplitude Rossby waves have been shown to decrease systematically with lead time during the first 1-5 days of operational global numerical weather forecasts. These models also exhibit a rapid reduction in the isentropic gradient of potential vorticity (PV) at the tropopause during the first 1-2 days of forecasts. This paper identifies a mechanism linking the reduction in large-scale meander amplitude on jet streams to declining PV gradients. The mechanism proposed is that a smoother isentropic transition of PV across the tropopause leads to excessive PV filamentation on the jet flanks and a more lossy waveguide. The approach taken is to analyse Rossby wave dynamics in a single-layer quasi-geostrophic model. Numerical simulations show that the amplitude of a Rossby wave propagating along a narrow but smooth PV front do indeed decay transiently with time. This process is explained in terms of the filamentation of PV from the jet core and associated absorption of wave activity by the critical layers on the jet flanks, and a simple method for quantitatively predicting the magnitude of the amplitude reduction without simulation is presented. Explicitly-diffusive simulations are then used to show that the combined impact of diffusion and the adiabatic rearrangement of PV can result in a decay rate of Rossby waves which is 2-4 times faster than could be expected from diffusion acting alone. This predicted decay rate is sufficient to explain the decay observed in operational weather forecasting models
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Flow-dependent predictability of the North Atlantic Jet
The North Atlantic eddy-driven jet is a major component of the large-scale flow in the northern hemisphere. Here we present evidence from reanalysis and ensemble forecast data for systematic flow-dependent predictability of the jet during northern hemisphere winter (DJF). It is found that when the jet is weakened or split it is both less persistent and less predictable. The lack of predictability manifests itself as the onset of an anomalously large instantaneous rate of spread of ensemble forecast members as the jet becomes weakened. This suggests that as the jet weakens or splits it enters into a state more sensitive to small differences between ensemble forecast members, rather like the sensitive region between the wings of the Lorenz attractor
Latitudinal storm track shift in a reduced two-level model of the atmosphere
The eddy-driven jet stream and storm tracks in the mid-latitude atmosphere
are known to shift in latitude on various timescales, but the physical
processes that cause these shifts are still unclear. In this study, we
introduce a minimal dynamical system derived from the classical Phillips
two-level model with the goal of elucidating the essential mechanisms
responsible for the interaction between eddies and mean flow. Specifically, we
aim to understand the link between the structure of the eddies and the shift of
the latitudinal maximum of the zonal flow. By varying the horizontal shape of
the eddies, we find three distinct dynamical regimes whose occurrence depends
on the intensity of the external baroclinic forcing: a purely zonal flow, a
barotropic eddy regime with net poleward momentum flux, and a baroclinic eddy
regime with both net poleward momentum and temperature flux. For weak
baroclinic forcing, the classical zonal flow solution with latitudinal maximum
at the centre of the beta-channel is found. For strong forcing, if eddies are
southwest-northeast tilted and zonally elongated, the system is in the
baroclinic eddy regime, resulting in a poleward shift of the jet. The
intermediate barotropic eddy regime also features a poleward shifted jet, yet
with eddies structurally distinct from the baroclinic regime. Changing the
parameters yields transitions between the regimes that can be either continuous
or discontinuous in terms of the properties of the atmosphere. The findings of
this study also provide insights into the properties of the storm track change
between the jet entrance and jet exit regions of the North Atlantic.Comment: 15 pages, 4 figure
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Marginal stability and predator-prey behaviour within storm tracks
A predator-prey relationship between storm track intensity and growth rate is revealed in reanalysis data for the North Atlantic and North Pacific, as well as in an idealised global circulation model with a zonally asymmetric heating dipole. Averaging in the phase space of these two quantities reveals that both quantities oscillate on approximately monthly timescales. These oscillations occur due to quasi-periodic bursts in storm track activity that reduce excess baroclinicity and bring the flow back towards a state that is marginally stable to those bursts. Many detailed properties of these oscillations are reproduced well by a two-dimensional dynamical system, especially in respect of the North Atlantic storm track which is more zonally constrained compared to the North Pacific. It is predicted and observed that on average stronger storm events occur less frequently but grow on a shorter timescale. The results suggest that nonlinearly oscillating behaviour around a state of baroclinic neutrality is a general feature of localised storm tracks, and they offer a new perspective on the study of baroclinic instability
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The transient atmospheric response to a reduction of sea-ice cover in the Barents and Kara seas
The observed reduction of Arctic sea-ice has drawn a lot of interest for its potential impact on mid-latitude weather variability. One of the outstanding challenges is to achieve a deeper understanding of the dynamical processes involved in this mechanism. To progress in this area, we have designed and performed an experiment with an intermediate complexity atmospheric model. The experiment shows a transient atmospheric response to a surface diabatic heating in the Barents and Kara seas leading to an anomalous circulation first locally, then over the polar region and finally over the Euro-Atlantic sector. A hypothesis that explains the mechanisms for the propagation of the signal is put forward. The discussion of this hypothesis provides an insight into the nature of the link between sea-ice forcing and the modes of internal variability of the atmosphere. We demonstrate that after removal of sea ice in the Barents and Kara seas, first the linear atmospheric response dominates and is confined in the proximity of the heating area, then a large-scale response, associated also to eddy-feedback, is found and finally anomalies reach the lower-stratosphere and show a hemispheric pattern in the troposphere. These results identify the drivers of the tropospheric connection between sea-ice variability and the North Atlantic Oscillation and highlight the role of the lower stratosphere
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