12 research outputs found
Low-frequency suppression of Southern Hemisphere tropospheric eddy heat flux
This paper analyzes the variability of the zonal cospectrum of Southern Hemisphere tropospheric eddy heat flux in reanalysis data. It is shown that the reduced spectral power of low-frequency eddy heat flux variability largely arises from the anticorrelation in the eddy heat transports by different zonal wave numbers. Although the most plausible mechanism for this relation invokes baroclinicity as a mediating agent, this hypothesis does not seem to be supported by the observed variability of baroclinicity. Low-frequency baroclinicity variability is primarily driven by the mean meridional circulation, with only a minor role for the eddy heat flux
Model Hierarchies for Understanding Atmospheric Circulation
This is the final version. Available from Wiley via the DOI in this record.In this review, we highlight the complementary relationship between simple and comprehensive models in addressing key scientific questions to describe Earth’s atmospheric circulation. The systematic
representation of models in steps, or hierarchies, connects our understanding from idealized systems
to comprehensive models, and ultimately the observed atmosphere. We define three interconnected
principles that can be used to characterize the model hierarchies of the atmosphere. We explore
the rich diversity within the governing equations in the dynamical hierarchy, the ability to isolate
and understand atmospheric processes in the process hierarchy, and the importance of the physical
domain and resolution in the hierarchy of scale.
We center our discussion on the large scale circulation of the atmosphere and its interaction with
clouds and convection, focusing on areas where simple models have had a significant impact. Our
confidence in climate model projections of the future is based on our efforts to ground the climate
predictions in fundamental physical understanding. This understanding is, in part, possible due to
the hierarchies of idealized models that afford the simplicity required for understanding complex
systems.Natural Environment Research Council (NERC)US National Science FoundationUS Department of Energy Office of Biological and Environmental ResearchNatural Science and Engineering Research Council of CanadaAustralian Research CouncilSimons FoundationGerman Ministry of Education and Research (BMBF)FONA: Research for Sustainable DevelopmentState Research Agency of Spai
A rotating annulus driven by localized convective forcing: a new atmosphere-like experiment
We present an experimental study of flows in a
cylindrical rotating annulus convectively forced by local heating
in an annular ring at the bottom near the external wall
and via a cooled circular disk near the axis at the top surface
of the annulus. This new configuration is distinct from
the classical thermally-driven annulus analogue of the atmosphere
circulation, in which thermal forcing is applied
uniformly on the sidewalls, but with a similar aim to investigate
the baroclinic instability of a rotating, stratified
flow subject to zonally symmetric forcing. Two vertically
and horizontally displaced heat sources/sinks are arranged
so that, in the absence of background rotation, statically unstable
Rayleigh-BĂ©nard convection would be induced above
the source and beneath the sink, thereby relaxing strong constraints
placed on background temperature gradients in previous
experimental configurations based on the conventional
rotating annulus. This better emulates local vigorous convection
in the tropics and polar regions of the atmosphere
whilst also allowing stably-stratified baroclinic motion in
the central zone of the annulus, as in midlatitude regions in
the Earth’s atmosphere. Regimes of flow are identified, depending
mainly upon control parameters that in turn depend
on rotation rate and the strength of differential heating. Several
regimes exhibit baroclinically unstable flows which are
qualitatively similar to those previously observed in the classical
thermally-driven annulus, However, in contrast to the
classical configuration, they typically exhibit more spatiotemporal
complexity. Thus, several regimes of flow demonstrate the equilibrated co-existence of, and interaction between,
free convection and baroclinic wave modes. These
new features were not previously observed in the classical
annulus and validate the new setup as a tool for exploring
fundamental atmosphere-like dynamics in a more realistic
framework. Thermal structure in the fluid is investigated and
found to be qualitatively consistent with previous numerical
results, with nearly isothermal conditions respectively above
and below the heat source and sink, and stably-stratified,
sloping isotherms in the near-adiabatic interior
The impact of baroclinic eddy feedback on the persistence of jet variability in the two-layer model
Although it is well known that the persistence of extratropical jet shifts is enhanced by a positive eddy feedback, the dynamics of this feedback is still debated. Two types of mechanisms have been proposed: barotropic mechanisms rely on changes in upper-level propagation and baroclinic mechanisms rely on the coupling between barotropic and baroclinic flow. Recent studies have suggested that barotropic models can capture key aspects of the observed jet variability but the role of baroclinic dynamics has been less explored. This study investigates the temporal relations between barotropic and baroclinic anomalies and their eddy forcings during the internal variability of the simple two-layer quasigeostrophic model. A large correlation is found between barotropic and baroclinic anomalies and between the meridional and vertical components of the Eliassen-Palm divergence, especially at low frequency. The low-frequency variability is consistent with the baroclinic mechanism: persistent upper-level eddy momentum convergence is associated with (and precedes) persistent anomalies in the poleward eddy heat flux. In contrast, at high frequency, poleward heat flux anomalies are associated with eddy momentum divergence aloft and both eddy forcings have same-sign contributions to the upper-level eddy potential vorticity (PV) flux. In this limit the eddy PV flux is associated with wave activity transience as effective diffusivity is too small to dissipate the wave-mean flow interaction term. The large correlation between barotropic and baroclinic anomalies implies that the low-frequency variability of barotropic flow may be affected by thermal damping when this damping is sufficiently strong. For example, zonal index persistence drops drastically in our model when baroclinicity shifts are prevented by strong thermal restoration. © 2014 American Meteorological Society.Peer Reviewe
Atmospheric Dynamics of Terrestrial Planets
The solar system presents us with a number of planetary bodies with shallow atmospheres that are sufficiently Earth-like in their form and structure to be termed “terrestrial.” These atmospheres have much in common, in having circulations that are driven primarily by heating from the Sun and radiative cooling to space, which vary markedly with latitude. The principal response to this forcing is typically in the form of a (roughly zonally symmetric) meridional overturning that transports heat vertically upward and in latitude. But even within the solar system, these planets exhibit many differences in the types of large-scale waves and instabilities that also contribute substantially to determining their respective climates. Here we argue that the study of simplified models (either numerical simulations or laboratory experiments) provides considerable insights into the likely roles of planetary size, rotation, thermal stratification, and other factors in determining the styles of global circulation and dominant waves and instability processes. We discuss the importance of a number of key dimensionless parameters, for example, the thermal Rossby and the Burger numbers as well as nondimensional measures of the frictional or radiative timescales, in defining the type of circulation regime to be expected in a prototypical planetary atmosphere subject to axisymmetric driving. These considerations help to place each of the solar system terrestrial planets into an appropriate dynamical context and also lay the foundations for predicting and understanding the climate and circulation regimes of (as yet undiscovered) Earth-like extrasolar planets. However, as recent discoveries of “super-Earth” planets around some nearby stars are beginning to reveal, this parameter space is likely to be incomplete, and other factors, such as the possibility of tidally locked rotation and tidal forcing, may also need to be taken into account for some classes of extrasolar planet