Pedram Hassanzadeh

Responses of midlatitude blocks and wave amplitude to changes in the meridional temperature gradient in an idealized dry GCM

The response of atmospheric blocks and the wave amplitude of midlatitude jets to changes in the midlatitude to pole, near-surface temperature difference (ΔT), is studied using an idealized dry general circulation model (GCM) with Held-Suarez forcing. Decreasing ΔT results in slower zonal winds, a mean state with reduced meridional gradient of the 500 hPa geopotential height (Z500), a smaller variance of Z500 anomalies, and a robust decrease in blocks and meridional amplitude of waves. Neglecting the decrease of variance associated with reduced ΔT would lead to the incorrect expectation that mean states with smaller Z500 gradients produce more blocks and higher wave amplitudes. Our results suggest further investigation of the hypothesis that reduced ΔT due to Arctic Amplification would increase blocking events and wave amplitude, hence leading to more midlatitude extreme weather events.

Zombie Vortex Instability. I. A Purely Hydrodynamic Instability to Resurrect the Dead Zones of Protoplanetary Disks

There is considerable interest in hydrodynamic instabilities in dead zones of protoplanetary disks as a mechanism for driving angular momentum transport and as a source of particle-trapping vortices to mix chondrules and incubate planetesimal formation. We present simulations with a pseudo-spectral anelastic code and with the compressible code Athena, showing that stably stratified flows in a shearing, rotating box are violently unstable and produce space-filling, sustained turbulence dominated by large vortices with Rossby numbers of order 0.2-0.3. This Zombie Vortex Instability (ZVI) is observed in both codes and is triggered by Kolmogorov turbulence with Mach numbers less than 0.01. It is a common view that if a given constant density flow is stable, then stable vertical stratification should make the flow even more stable. Yet, we show that sufficient vertical stratification can be unstable to ZVI. ZVI is robust and requires no special tuning of boundary conditions, or initial radial entropy or vortensity gradients (though we have studied ZVI only in the limit of infinite cooling time). The resolution of this paradox is that stable stratification allows for a new avenue to instability: baroclinic critical layers. ZVI has not been seen in previous studies of flows in rotating, shearing boxes because those calculations frequently lacked vertical density stratification and/or sufficient numerical resolution. Although we do not expect appreciable angular momentum transport from ZVI in the small domains in this study, we hypothesize that ZVI in larger domains with compressible equations may lead to angular transport via spiral density waves.

Blocking variability: Arctic Amplification versus Arctic Oscillation

To predict future changes in blocking and the resulting weather extremes, some studies have proposed the negative phase of Arctic Oscillation (−AO) as an analogue for Arctic amplification because of similarities between their mean states: reduced midlatitude-to-pole temperature gradients and weakened, equatorward shifted jet streams. Using well-controlled modeling experiments, we show that blocking variations associated with mean state anomalies are opposite depending on whether these anomalies are driven by the internal dynamics as in AO or forced externally as in Arctic amplification. While blocking increases and its latitudinal-distribution shifts poleward in −AO, we find opposite responses when a mean state identical to the −AO mean state is externally forced. Findings suggest that the observed blocking-AO relationship is a correlation which does not imply that the −AO mean state causes increased blocking and should not be employed as a prototype for Arctic amplification. Furthermore, results urge for a careful consideration of causality before using internal variability to predict low-frequency response to external forcings.

Three-dimensional vortices generated by self-replication in stably stratified rotating shear flows.

A previously unknown instability creates space-filling lattices of 3D vortices in linearly-stable, rotating, stratified shear flows. The instability starts from an easily-excited critical layer. The layer intensifies by drawing energy from the background shear and rolls-up into vortices that excite new critical layers and vortices. The vortices self-similarly replicate to create lattices of turbulent vortices. The vortices persist for all time. This self-replication occurs in stratified Couette flows and in the dead zones of protoplanetary disks where it can de-stabilize Keplerian flows.

The universal aspect ratio of vortices in rotating stratified flows: theory and simulation

We derive a relationship for the vortex aspect ratio

Efficient Calculation of Radiation Heat Transfer in Participating Media

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

(vertical half-thickness over horizontal length scale) for steady and slowly evolving vortices in rotating stratified fluids, as a function of the Brunt-Vaisala frequencies within the vortex

A perspective on climate model hierarchies

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The Linear Response Function of an Idealized Atmosphere. Part II: Implications for the Practical Use of the Fluctuation–Dissipation Theorem and the Role of Operator’s Nonnormality

and in the background fluid outside the vortex

Data-driven reduced modelling of turbulent Rayleigh–Bénard convection using DMD-enhanced fluctuation–dissipation theorem

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