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From March to December 2014, Loon LLC flew 107 super-pressure balloons in the lower stratosphere over the Southern Ocean. Their GPS sampling frequency of 1Hz allowed them to sample motion associated with atmospheric internal gravity waves generated by the Andes mountains, small islands, and non-orographic sources such as fronts. Analyzing the balloons’ data time series using wavelets, we present distributions of the gravity waves’ momentum fluxes, phase speeds, and wavelengths, both in the time-mean and as they vary from month to month. Many climate models parameterize gravity waves using a phase speed-momentum flux relationship, so we focus on the relationship between those quantities.

Gravity waves (GW) are one of the key drivers of middle atmospheric circulation. They drive the mesospheric circulation and the QBO - the predominant mode of tropical stratospheric variability - and play an important role in the wintertime polar vortex variability. These waves manifest over spatial scales as small as O(10 m) to as large as O(1000 km) and thus, are often poorly resolved in global climate models on account of limited model grid resolution and grid-scale numerical dissipation. Moreover, the parameterizations used to represent the momentum forcing associated with GW are highly idealized and often not well-tuned due to limited observations. These parameterizations simplify GW representation by ignoring certain aspects of their dynamical evolution, including lateral propagation, transience, and intermittency, which results in an inaccurate model representation of their interaction with the synoptic and mean flow. The present work focuses on creating a new, data-driven emulator for GW representation in climate models. The emulator employs an artificial neural network that learns the local and non-local gravity wave evolution by training on high-resolution reanalysis (ERA5) and ECMWF’s global 1km-resolution climate model that respectively partly and fully resolve the mesoscale portion of the atmospheric gravity wave spectrum. We test the performance of the data-driven emulators by embedding them in an intermediate-complexity climate model and assessing their momentum contribution towards driving a suite of middle atmospheric processes including the springtime disintegration of the Antarctic polar vortex, the quasi-biennial oscillation of the tropical stratospheric winds, and the sudden stratospheric warmings.

The winter jet stream in the North Atlantic has been shown to preferentially occur at three distinct latitudes [Woolings et al., 2010; Woolings et al., 2018], which we will call the three Atlantic “jet regimes.” Distinct physical mechanisms may be responsible for each of the three jet regimes—for example, the northernmost jet regime is strongly linked to the Greenland tip jet [White et al., 2019]. We seek to investigate the role of stratospheric and CO2 forcing, such as from sudden stratospheric warmings (SSWs), strong polar vortex events (SPVs), and anthropogenic global warming, on the Atlantic jet in the context of these jet regimes.To do so, we use a “jet latitude index” (JLI), which is determined by finding the latitude of the peak zonal winds over some latitude range, averaged over some longitude range, to show that sudden stratospheric warmings (SSWs) impact the likelihood that the Atlantic jet will be in any particular jet regime. These calculations are performed in the ECMWF Interim Reanalysis (ERAI) data set, an in-house 200-year Whole Atmosphere Community Climate Model (WACCM) run, and in a subset of CMIP6 models. We seek to investigate how changes in the composite response of the jet over the Atlantic associated with SSWs, SPVs, and greenhouse gas forcing, are borne out in the context of the three Atlantic jet regimes. We find that, following SSWs, the northern regime becomes less frequent, and the southern regime becomes more frequent, while the jet latitude peaks of the regimes do not notably shift. Following SPVs, the northern regime becomes more frequent, the southern regime becomes less frequent, and again, the peak latitudes do not shift. Under CO2 forcing, we do not find a consistent signal from model to model, and we test whether these differences may be related to model differences in local meridional temperature gradients over the Atlantic.

We present a novel, single-column gravity wave parameterization (GWP) that uses machine learning to emulate a physics-based GWP. An artificial neural network (ANN) is trained with output from an idealized atmospheric model and tested in an offline environment, illustrating that an ANN can learn the salient features of gravity wave momentum transport directly from resolved flow variables. We demonstrate that when trained on the westward phase of the Quasi-Biennial Oscillation, the ANN can skillfully generate the momentum fluxes associated with the eastward phase. We also show that the meridional and zonal wind components are the only flow variables necessary to predict horizontal momentum fluxes with a globally and temporally averaged R^2 value over 0.8. State-of-the-art GWPs are severely limited by computational constraints and a scarcity of observations for validation. This work constitutes a significant step towards obtaining observationally validated, computationally efficient GWPs in global climate models.

The effects of wave–wave interactions on sudden stratospheric warming formation are investigated using an idealized atmospheric general circulation model, in which tropospheric heating perturbations of zonal wave numbers 1 and 2 are used to produce planetary-scale wave activity. Zonal wave–wave interactions are removed at different vertical extents of the atmosphere in order to examine the sensitivity of stratospheric circulation to local changes in wave–wave interactions. We show that the effects of wave–wave interactions on sudden warming formation, including sudden warming frequencies, are strongly dependent on the wave number of the tropospheric forcing and the vertical levels where wave–wave interactions are removed. Significant changes in sudden warming frequencies are evident when wave–wave interactions are removed even when the lower-stratospheric wave forcing does not change, highlighting the fact that the upper stratosphere is not a passive recipient of wave forcing from below. We find that while wave–wave interactions are required in the troposphere and lower stratosphere to produce displacements when wave number 2 heating is used, both splits and displacements can be produced without wave–wave interactions in the troposphere and lower stratosphere when the model is forced by wave number 1 heating. We suggest that the relative strengths of wave number 1 and 2 vertical wave flux entering the stratosphere largely determine the split and displacement ratios when wave number 2 forcing is used but not wave number 1.

To understand Earth's climate, climate modelers employ a hierarchy of climate models spanning a wide spectrum of complexity and comprehensiveness. This essay, inspired by the World Climate Research Programme's recent “Model Hierarchies Workshop,” attempts to survey and synthesize some of the current thinking on climate model hierarchies, especially as presented at the workshop. We give a few formal descriptions of the hierarchy and survey the various ways it is used to generate, test, and confirm hypotheses. We also discuss some of the pitfalls of contemporary climate modeling, and how the “elegance” advocated for by Held (2005) has (and has not) been used to address them. We conclude with a survey of current activity in hierarchical modeling, and offer suggestions for its continued fruitful development.

Much research has focused on trends in the Southern Hemispheric circulation in austral summer (December–February) in the troposphere and stratosphere, whereas changes in other seasons have received less attention. Here the seasonality and structure of observed changes in tropospheric and stratospheric winds, temperature, and ozone over the Southern Hemisphere are examined. It is found that statistically significant trends similar to those of the Antarctic summer season are also observed since 1979 in austral fall, particularly May, and are strongest over the Pacific sector of the hemisphere. Evidence is provided for a significant shift in the position of the jet in May over the Pacific, and it is shown that the strengthening and shifting of the jet has rendered the latitudinal distribution of upper-tropospheric zonal wind more bimodal. The Antarctic ozone hole has cooled the lower stratosphere and strengthened the polar vortex. While the mechanism and timing are not fully understood, the ozone hole has been identified as a key driver of the summer season tropospheric circulation changes in several previous observational and modeling studies. It is found here that significant ozone depletion and associated polar cooling also occur in the lowermost stratosphere and tropopause region through austral fall, with spatial patterns that are coincident with the observed changes in stratospheric circulation. It is also shown that radiatively driven temperature changes associated with the observed ozone depletion in May represent a substantial portion of the observed May cooling in the lowermost stratosphere, suggesting a potential for contribution to the circulation changes.

The effects of eddy-eddy interactions on sudden stratospheric warming formation are investigated using an idealized atmospheric general circulation model, in which tropospheric heating perturbations of zonal wave numbers 1 and 2 are used to produce planetary scale wave activity. Eddy-eddy interactions are removed at different vertical extents of the atmosphere in order to examine the sensitivity of stratospheric circulation to local changes in eddy-eddy interactions. We show that the effects of eddy-eddy interactions on sudden warming formation, including sudden warming frequencies, are strongly dependent on the wave number of the tropospheric forcing and the vertical levels where eddy-eddy interactions are removed. Significant changes in sudden warming frequencies are evident when eddy-eddy interactions are removed even when the lower stratospheric wave forcing does not change, highlighting the fact that the upper stratosphere is not a passive recipient of wave forcing from below. We find that while eddy-eddy interactions are required in the troposphere and lower stratosphere to produce displacements when wave number 2 heating is used, both splits and displacements can be produced without eddy-eddy interactions in the troposphere and lower stratosphere when the model is forced by wave number 1 heating. We suggest that the relative strengths of wave numbers 1 and 2 vertical wave flux entering the stratosphere largely determine the split and displacement ratios when wave number 2 forcing is used, but not wave number 1.

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.

An idealized atmospheric general circulation model (AGCM) is used to investigate the sensitivity of model responses to the timing of imposed polar stratospheric cooling, intended to mimic the radiative effects of ozone depletion. The model exhibits circulation responses to springtime cooling that qualitatively match both observations and the responses of comprehensive chemistry climate models. The model's surface response is sensitive to the timing of the cooling, with the onset becoming delayed with later cooling, but with the termination occurring at similar times, suggesting that the meteorology plays an important role. The model's responses do not match the latitudinal structure of the leading annular mode; rather, the response described by the second empirical orthogonal function plays a substantial role, in addition to the first. It is suggested that the imposed cooling, when it delays the final warming, results in an extended period of lower stratospheric variability, which could be an important factor in producing realistic surface responses.

Stratosphere–troposphere interactions are conventionally characterized using the first empirical orthogonal function (EOF) of fields such as zonal-mean zonal wind. Perpetual-winter integrations of an idealized model are used to contrast the vertical structures of EOFs with those of principal oscillation patterns (POPs; the modes of a linearized system governing the evolution of zonal flow anomalies). POP structures are shown to be insensitive to pressure weighting of the time series of interest, a factor that is particularly important for a deep system such as the stratosphere and troposphere. In contrast, EOFs change from being dominated by tropospheric variability with pressure weighting to being dominated by stratospheric variability without it. The analysis reveals separate tropospheric and stratospheric modes in model integrations that are set up to resemble midwinter variability of the troposphere and stratosphere in both hemispheres. Movies illustrating the time evolution of POP structures show the existence of a fast, propagating tropospheric mode in both integrations, and a pulsing stratospheric mode with a tropospheric extension in the Northern Hemisphere–like integration.

The seasonal variability of the polar stratospheric vortex is studied in a simplified AGCM driven by specified equilibrium temperature distributions. Seasonal variations in equilibrium temperature are imposed in the stratosphere only, enabling the study of stratosphere–troposphere coupling on seasonal time scales, without the complication of an internal tropospheric seasonal cycle. The model is forced with different shapes and amplitudes of simple bottom topography, resulting in a range of stratospheric climates. The effect of these different kinds of topography on the seasonal variability of the strength of the polar vortex, the average timing and variability in timing of the final breakup of the vortex (final warming events), the conditions of occurrence and frequency of midwinter warming events, and the impact of the stratospheric seasonal cycle on the troposphere are explored. The inclusion of wavenumber-1 and wavenumber-2 topographies results in very different stratospheric seasonal variability. Hemispheric differences in stratospheric seasonal variability are recovered in the model with appropriate choices of wave-2 topography. In the model experiment with a realistic Northern Hemisphere–like frequency of midwinter warming events, the distribution of the intervals between these events suggests that the model has no year-to-year memory. When forced with wave-1 topography, the gross features of seasonal variability are similar to those forced with wave-2 topography, but the dependence on forcing magnitude is weaker. Further, the frequency of major warming events has a nonmonotonic dependence on forcing magnitude and never reaches the frequency observed in the Northern Hemisphere.

The authors test the hypothesis that recent observed trends in surface westerlies in the Southern Hemisphere are directly consequent on observed trends in the timing of stratospheric final warming events. The analysis begins by verifying that final warming events have an impact on tropospheric circulation in a simplified GCM driven by specified equilibrium temperature distributions. Seasonal variations are imposed in the stratosphere only. The model produces qualitatively realistic final warming events whose influence extends down to the surface, much like what has been reported in observational analyses. The authors then go on to study observed trends in surface westerlies composited with respect to the date of final warming events. If the considered hypothesis were correct, these trends would appear to be much weaker when composited with respect to the date of the final warming events. The authors find that this is not the case, and accordingly they conclude that the observed surface changes cannot be attributed simply to this shift toward later final warming events.

In previous work, it was established that at high specific impulse operation, plasma acceleration is confined to a small region at the upstream end of the accelerator. Such rapid axial acceleration must be accompanied by viscous losses arising from normal viscous stresses. This effect is studied by including viscous terms in the momentum equation. The resulting higher order formulation is solved by a combination of analytical and numerical methods. Results show that this effect alone can reduce accelerator efficiency by 5% and thrust and specific impulse by about 22%. Accurate knowledge of plasma viscosity values in the accelerator is necessary for computational design. In the regime of high specific impulse operation, accelerator efficiency is strongly influenced by the Reynolds number of the plasma flow.

VIV model test results are presented for a bundle of three parallel pipes all lying in the same plane, similar to a riser with large kill and choke lines. The rigid model was attached to a spring-mounted frame in the MIT towing tank. The horizontal model was towed in the tank and allowed to respond in free vibration to vortex-induced vibration in the cross-flow direction. The angle of attack of the model was varied from 0 to 90 degrees. The model was tested with and without helical strakes. Without strakes the model exhibited significant vibration at 0 and 90 degrees angle of attack. Strakes suppressed VIV at all angles of attack.Copyright © 2009 by ASME

This work presents an exact solution of the equations governing the acceleration of plasma in co-axial MPD accelerators. An important analytical result linking the plasma velocity with the magnetic field is obtained and its practical implications discussed. The non-linear differential equation involving coupling of the electromagnetics and fluid mechanics is solved exactly, and a theoretical upper limit on performance is predicted. The conceptual and practical significance of this as an unstable boundary region separating regimes of power generation and plasma acceleration is discussed. It is also shown that in this limit, current concentration occurs at the upstream end over an extremely narrow region. It is suggested that this is the onset boundary. Motional E.M.F. is shown to play an important role in these phenomena. Numerical results for the accelerator performance are presented using typical values of geometrical and constitutive parameters. Their theoretical and conceptual basis is discussed. Practical applications of these results in engine design and operation are presented.

The North Atlantic atmospheric eddy-driven jet exhibits three “preferred positions,” latitudes where the maximum jet speed occurs more frequently than others. Using an atmospheric general circulation model, Whole Atmospheric Community Climate Model, we explore the extent to which different mountain ranges affect the northern preferred position. The latitude of this preferred position changes only when the latitude of Greenland is changed, and the preferred position disappears when Greenland orography is flattened.We propose that “Greenland tip jet events” create the appearance of a northern “preferred position” in the eddy-driven jet; tip jets are localized zonal jets east of the southern tip of Greenland, created by flow interacting with Greenland orography. In reanalysis data the northern preferred position is strongly associated with tip jet days. A case study with the CAM4 model suggests that biases in the northern preferred position may stem from biases in the position or strength of the climatological eddy-driven jet. &nbsp