On the tropospheric response to transient stratospheric momentum torques

An idealised model is used to examine the tropospheric response to sudden stratospheric warmings (SSWs), by imposing transient stratospheric momentum torques tailored to mimic the wave-forcing impulse associated with spontaneously-occurring SSWs. Such an approach enables us to examine both the ∼2-3-week forcing stage of an SSW during which there is anomalous stratospheric wave-activity convergence, as well as the recovery stage during which the wave forcing abates and the stratosphere radiatively recovers over 2-3 months. It is argued that applying a torque is better suited than a heating perturbation for examining the response to SSWs, due to the meridional circulation that is induced to maintain thermal-wind balance (i.e., the ‘Eliassen adjustment’); an easterly torque yields downwelling at high latitudes and equatorward flow below, similar to the wave-induced circulation that occurs during spontaneously-occurring SSWs, whereas a heating perturbation yields qualitatively opposite behavior and thus cannot capture the initial SSW evolution. During the forcing stage, the meridional circulation in response to an impulse comparable to the model’s internal variability is able to penetrate down to the surface and drive easterly-wind anomalies via Coriolis torques acting on the anomalous equatorward flow. During the recovery stage, after which the tropospheric flow has already responded, the meridional circulation associated with the stratosphere’s radiative recovery provides the persistent stratospheric forcing that drives the high-latitude easterly anomalies, which is then augmented by synoptic-wave feedbacks that drive and amplify the annular-mode response. In comparison, planetary waves are found to play a relatively small role.

Unsteady Linearisation of Bed Shear Stress for Idealised Storm Surge Modelling

The modelling of time-varying shallow flows, such as tides and storm surges, is complicated by the nonlinear dependency of bed shear stress on flow speed. For tidal flows, Lorentz’s linearisation circumvents nonlinearity by specifying a (steady) friction coefficient r based on a tide-averaged criterion of energy equivalence. However, this approach is not suitable for phenomena with episodic and irregular forcings such as storm surges. Here, we studied the implications of applying Lorentz’s energy criterion in an instantaneous sense, so that an unsteady friction coefficient r(t) adjusts to the temporal development of natural wind-driven flows. This new bed-stress parametrisation was implemented in an idealised model of a single channel, forced by time-varying signals of wind stress (acting over the entire domain) and surface elevation (at the channel mouth). The solution method combines analytical solutions of the cross-sectionally averaged linearised shallow-water equations, obtained in the frequency domain, with an iterative procedure to determine r(t). Model results, compared with a reference finite-difference solution retaining the quadratic bed shear stress, show that this new approach accurately captures the qualitative and quantitative aspects of the surge dynamics (height and timing of surge peaks, sloshing, friction-induced tide-surge interaction) for both synthetic and realistic wind forcings.

Varieties of Constitutionalism in Asia

Western liberal constitutionalism, the dominant vision of which is rights-oriented and court centric is hegemonic to the discourse. While important, this is a particular rather than total expression of constitutionalism which conflates constitutionalism with liberalism and treat this as paradigmatic. This may obscure all other non-liberal models or fuel their dismissal as sham constitutions. A third space must be found, without collapsing anti-constitutionalist, illiberal despotism with no objective limits on power into the sphere of constitutionalism, and without equating judicial review with constitutionalism. To that end, this article seeks to advance the project of pluralising understandings of constitutionalism, drawing from the rich variety of constitutional experiments in Asia, with its multiplicity of religions, political ideologies government systems, cultures, and levels of economic development. Against an idealised model of liberal constitutionalism deployed as an organisational tool to highlight features of non-liberal constitutionalism in their full variety, the article examines three typologies of constitutionalism: religious, socialist and communitarian. In so doing, the idea of normatively desirable and defensible constitutionalist models is investigated, through an inquiry that goes beyond text and courts to other sites of constitutional practice and governance.

Ionising feedback from an O star formed in a filament

We explore a simple semi-analytic model for what happens when an O star (or cluster of O stars) forms in an isolated filamentary cloud. The model is characterised by three configuration parameters: the radius of the filament, $R_{_{\rm FIL}}$, the mean density of H2 in the filament, $n_{_{\rm FIL}}$, and the rate at which the O star emits ionising photons, $\dot{\cal N}_{_{\rm LyC}}$. We show that for a wide range of these configuration parameters, ionising radiation from the O star rapidly erodes the filament, and the ionised gas from the filament disperses into the surroundings. Under these circumstances the distance, L, from the O star to the ionisation front (IF) is given approximately by L(t) ∼ 5.2 pc$\, [R_{_{\rm FIL}}/0.2\, {\rm pc}]^{-1/6}$$\, [n_{_{\rm FIL}}/10^4\, {\rm cm^{-3}}]^{-1/3}$$\, [\dot{\cal N}_{_{\rm LyC}}/10^{49}\, {\rm s}^{-1}]^{1/6}$ [t/Myr]2/3, and we derive similar simple power-law expressions for other quantities, for example the rate at which ionised gas boils off the filament, $\dot{M}_{_{\rm IF}}(t)$, and the mass, $M_{_{\rm SCL}}(t)$, of the shock-compressed layer (SCL) that is swept up behind the IF. We show that a very small fraction of the ionising radiation is expended locally, and a rather small amount of molecular gas is ionised and dispersed. We discuss some features of more realistic models, and the extent to which they might modify or invalidate the predictions of this idealised model. In particular we show that, for very large $R_{_{\rm FIL}}$ and/or large $n_{_{\rm FIL}}$ and/or low $\dot{\cal N}_{_{\rm LyC}}$, continuing accretion onto the filament might trap the ionising radiation from the O star, slowing erosion of the filament even further.

Diagnosing the vorticity balances of the Weddell Gyre

<p>Antarctic Bottom Water formed in the Weddell Sea is transported by the Weddell Gyre (WG) into the Antarctic Circumpolar Current (ACC). From here, this water is exported to the world ocean and influences the global overturning circulation. Studying the dynamics of the WG could therefore improve our understanding of the Southern Ocean carbon and energy budget.</p><p>The dynamics of the WG in a NEMO global model is investigated at various resolutions. The WG transport is largest at intermediate resolution (R4) and only the low-resolution model (R1) has a transport close to observations. We attempt to identify the physical processes responsible for this difference by studying the vorticity diagnostics. These physical processes include (but are not limited to) wind stress curl, lateral friction and bottom pressure torques.  </p><p>A textbook understanding of gyres relies on the idea of vorticity balance and this idea is extended to identify the physical processes spinning the WG up and down. We integrate the vorticity diagnostics outputted by NEMO over the area enclosed by the WG streamlines. These integrations are equal to the work done by separate forces on fluid parcels circulating around the gyre.</p><p>In the future we also hope to apply this analysis to an idealised model representing the Weddell Sea. This model will also use NEMO but have analytic forcing, bathymetry and a prescribed ACC.</p>

Tropospheric response to stratospheric momentum torques

<p>An idealised model is used to examine the tropospheric response to stratospheric momentum torques with an emphasis on the response to high-latitude sudden stratospheric warmings (SSWs). Previous related studies have generally imposed such torques in models that lack a key element of realism; for instance, models that do not have a realistic stratosphere, models without stationary planetary waves (i.e., without topography), and models that do not have a troposphere and so precludes any investigation of a downward impact. The idealised moist model of an atmosphere (MiMA) used here overcomes these three shortcomings and is hence well-suited to study the downward impact of extreme events in the stratosphere in a more realistic setup. In particular, we impose transient zonally-symmetric momentum forcing to various latitude bands in the stratosphere, spun-off from a free-running control run (CTRL). In addition to varying the latitudinal location of the forcing, we vary the depth, duration and magnitude to examine the sensitivity of the tropospheric response. Preliminary results show that in contrast to thermally-forced SSWs for which the initial 'Eliassen adjustment' (i.e., the meridional circulation response during the forcing period) is opposite to that found during free-running SSWs, the momentum-forced events here, produce a meridional circulation that mimics that found in the free-running events. This meridional circulation immediately transfers the imposed momentum forcing to the surface, projecting onto the tropospheric Northern Annular Mode (NAM) and initiating a synoptic-wave feedback, a process that takes much longer to develop in the thermally-forced SSWs. Hence, a sudden and strong enough wave forcing (approximated here by an imposed momentum torque) can induce a meridional circulation that penetrates deep into the troposphere and immediately initiate a tropospheric NAM response. The applicability of these experiments to the real atmosphere will be discussed via comparing the evolution of the forced events to free-running SSWs identified in CTRL.</p>

What ensemble size is required for accurate forecasts? Idealised model experiments with very large ensembles

<p>Ensembles of numerical weather prediction models are currently used to represent the forecast uncertainty of forecast variables. However due to the computationally expensive nature of these ensembles, these uncertainties are only known with a large sampling error, and often the underlying distributions are assumed to be gaussian for Data Assimilation purposes. Furthermore, it is unclear how many members are required in an ensemble to obtain a designated level of sampling error. This work endeavours to understand how this error decreases as ensembles become larger, and how the forecast uncertainty evolves over a 24 hour free forecast period, before answering the pressing question of: how many ensembles are required in an NWP ensemble in order to sufficiently resolve the uncertainty? To do this, a simple 1D modified shallow water model which replicates the main features of convection is employed in the form of a massive ensemble with over 100,000 members. The shape of the distributions from this ensemble, which develop significant non-gaussianity, resembles those of the operational NWP ensembles of SCALE-RM and ICON, indicating that this model is sufficiently realistic in representing the forecast uncertainty. The simple model will be used to determine the rate of convergence of different forecast variables as ensemble size increases, and to evaluate the errors resulting from using the small ensemble sizes that are typical in operational NWP.</p>

Impact of Noble-Gas Filler Atoms on the Lattice Thermal Conductivity of CoSb3 Skutterudites: First-Principles Modelling

We present a systematic first-principles modelling study of the structural dynamics and thermal transport in the CoSb3 skutterudites with a series of noble-gas filler atoms. Filling with chemically-inert atoms provides an idealised model for isolating the effects of the fillers from the impact of redox changes to the host electronic structure. A range of analysis techniques are proposed to estimate the filler rattling frequencies, to quantify the separate impacts of the filler on the phonon group velocities and lifetimes, and to show how changes to the phonon spectra and interaction strengths lead to suppressed lifetimes. The noble-gas fillers are found to reduce the thermal conductivity of the CoSb3 framework by up to 15 % primarily by suppressing the group velocities of low-lying optic modes. The filler rattling frequencies are determined by a detailed balance of increasing atomic mass and stronger interactions with the framework, and are found to be a good predictor of the impact on the heat transport. Lowering the rattling frequency below ~1.5 THz by selecting heavy fillers that interact weakly with the framework is predicted to lead to a much larger suppression of the thermal transport, by inducing avoided crossings in the acoustic-mode dispersion and facilitating enhanced scattering and a consequent large reduction in phonon lifetimes. Approximate rattling frequencies determined from the harmonic force constants may therefore provide a useful metric for selecting filler atoms to optimise the thermal transport in skutterudites and other cage compounds such as clathrates.

Impact of Noble-Gas Filler Atoms on the Lattice Thermal Conductivity of CoSb3 Skutterudites: First-Principles Modelling

We present a systematic first-principles modelling study of the structural dynamics and thermal transport in the CoSb3 skutterudites with a series of noble-gas filler atoms. Filling with chemically-inert atoms provides an idealised model for isolating the effects of the fillers from the impact of redox changes to the host electronic structure. A range of analysis techniques are proposed to estimate the filler rattling frequencies, to quantify the separate impacts of the filler on the phonon group velocities and lifetimes, and to show how changes to the phonon spectra and interaction strengths lead to suppressed lifetimes. The noble-gas fillers are found to reduce the thermal conductivity of the CoSb3 framework by up to 15 % primarily by suppressing the group velocities of low-lying optic modes. The filler rattling frequencies are determined by a detailed balance of increasing atomic mass and stronger interactions with the framework, and are found to be a good predictor of the impact on the heat transport. Lowering the rattling frequency below ~1.5 THz by selecting heavy fillers that interact weakly with the framework is predicted to lead to a much larger suppression of the thermal transport, by inducing avoided crossings in the acoustic-mode dispersion and facilitating enhanced scattering and a consequent large reduction in phonon lifetimes. Approximate rattling frequencies determined from the harmonic force constants may therefore provide a useful metric for selecting filler atoms to optimise the thermal transport in skutterudites and other cage compounds such as clathrates.