A Mechanism for the Effect of Tropospheric Jet Structure on the Annular Mode–Like Response to Stratospheric Forcing

2012 ◽  
Vol 69 (7) ◽  
pp. 2152-2170 ◽  
Author(s):  
Isla R. Simpson ◽  
Michael Blackburn ◽  
Joanna D. Haigh
Keyword(s):  
Momentum Flux ◽  
General Circulation ◽  
General Circulation Models ◽  
Lower Latitude ◽  
Mean Flow ◽  
Zonal Winds ◽  
Feedback Strength ◽  
Poleward Shift ◽  
Climate Forcings ◽  
Jet Structure

Abstract For many climate forcings the dominant response of the extratropical circulation is a latitudinal shift of the tropospheric midlatitude jets. The magnitude of this response appears to depend on climatological jet latitude in general circulation models (GCMs): lower-latitude jets exhibit a larger shift. The reason for this latitude dependence is investigated for a particular forcing, heating of the equatorial stratosphere, which shifts the jet poleward. Spinup ensembles with a simplified GCM are used to examine the evolution of the response for five different jet structures. These differ in the latitude of the eddy-driven jet but have similar subtropical zonal winds. It is found that lower-latitude jets exhibit a larger response due to stronger tropospheric eddy–mean flow feedbacks. A dominant feedback responsible for enhancing the poleward shift is an enhanced equatorward refraction of the eddies, resulting in an increased momentum flux, poleward of the low-latitude critical line. The sensitivity of feedback strength to jet structure is associated with differences in the coherence of this behavior across the spectrum of eddy phase speeds. In the configurations used, the higher-latitude jets have a wider range of critical latitude locations. This reduces the coherence of the momentum flux anomalies associated with different phase speeds, with low phase speeds opposing the effect of high phase speeds. This suggests that, for a given subtropical zonal wind strength, the latitude of the eddy-driven jet affects the feedback through its influence on the width of the region of westerly winds and the range of critical latitudes on the equatorward flank of the jet.

scholarly journals The Impact of the State of the Troposphere on the Response to Stratospheric Heating in a Simplified GCM

2010 ◽  
Vol 23 (23) ◽  
pp. 6166-6185 ◽  
Author(s):  
Isla R. Simpson ◽  
Michael Blackburn ◽  
Joanna D. Haigh ◽  
Sarah N. Sparrow
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Lower Latitude ◽  
Mean Flow ◽  
Magnitude Response ◽  
Stratospheric Temperature ◽  
The Mean ◽  
Climate Forcings ◽  
The Impact ◽  
Jet Structure

Abstract Previous studies have made use of simplified general circulation models (sGCMs) to investigate the atmospheric response to various forcings. In particular, several studies have investigated the tropospheric response to changes in stratospheric temperature. This is potentially relevant for many climate forcings. Here the impact of changing the tropospheric climatology on the modeled response to perturbations in stratospheric temperature is investigated by the introduction of topography into the model and altering the tropospheric jet structure. The results highlight the need for very long integrations so as to determine accurately the magnitude of response. It is found that introducing topography into the model and thus removing the zonally symmetric nature of the model’s boundary conditions reduces the magnitude of response to stratospheric heating. However, this reduction is of comparable size to the variability in the magnitude of response between different ensemble members of the same 5000-day experiment. Investigations into the impact of varying tropospheric jet structure reveal a trend with lower-latitude/narrower jets having a much larger magnitude response to stratospheric heating than higher-latitude/wider jets. The jet structures that respond more strongly to stratospheric heating also exhibit longer time scale variability in their control run simulations, consistent with the idea that a feedback between the eddies and the mean flow is both responsible for the persistence of the control run variability and important in producing the tropospheric response to stratospheric temperature perturbations.

First GCM simulations of Uranus and Neptune atmospheres with realistic radiative transfer

2020 ◽  
Author(s):  
Gwenaël Milcareck ◽  
Sandrine Guerlet ◽  
Jan Vatant d'Ollone ◽  
Aymeric Spiga ◽  
Ehouarn Millour
Keyword(s):  
Radiative Transfer ◽  
Zonal Wind ◽  
Seasonal Variations ◽  
General Circulation ◽  
Internal Heat ◽  
General Circulation Models ◽  
Heat Fluxes ◽  
Wind Speeds ◽  
Wind Structure ◽  
Jet Structure

<p>Uranus and Neptune’s atmospheres are active worlds, with vigorous meteorological activity and strong zonal winds occurring despite small absorbed solar radiation and internal heat fluxes. A few 3-D General Circulation Models (GCM) of their atmospheres exist in the literature, focusing mostly on understanding their zonal jet structure [1,2] or the evolution of large disturbances [3,4].</p> <p>Building a complete and realistic GCM is a challenging task, given the long orbital and radiative timescales involved, along with the rather high spatial and temporal resolution needed when solving the atmospheric equations of motion on the rotating sphere. For this reason, existing GCMs include crude representation of radiative transfer (a simple relaxation scheme to an equilibrium temperature profile) and/or neglect seasonal variations.</p> <p> </p> <p>We are currently developing a GCM for Uranus and Neptune’s atmospheres, building on our existing expertise on Jupiter and Saturn GCMs [5,6]. Compared to other existing GCMs for ice giants, our model includes state-of-the art parametrization of radiative transfer. The radiation scheme is a full radiative transfer using correlated-k distributions. Seasonal variations of the incoming solar flux are taken into account. Opacity sources include gaseous opacity from methane, ethane, acetylene, H2-H2, H2-He continua along with opacity from two aerosol layers: one optically thick cloud with a base at the 2-bar level and one optically thin haze layer with a base at 300 mbar. These layers are consistent with the putative H2S and CH4 clouds reported by many observational studies (eg [7,8]).</p> <p> </p> <p>Simulations at radiative-equilibrium are discussed in a companion abstract [9] ; in this one we focus on dynamical aspects. We will present results from first 3D GCM simulations performed at a horizontal resolution up to 256x192 in longitude x latitude (corresponding to 1.4°x0.9°), extending from 3 bars to 0.3 mbar. A broad equatorial retrograde jet develop on both Uranus and Neptune and two prograde jets emerge near 50° latitude in the Neptune simulation. This is in qualitative agreement with the observed zonal wind structure on Neptune, although the zonal jet wind speeds are much smaller than the observed ones. We are able to show that acceleration by eddies is an important contributor to the two prograde jets in the Neptune simulation.</p> <p>However, the Uranus simulation does not exhibit high-latitude prograde jets that have been reported by cloud-tracking observations. In other words, the zonal jet structure currently obtained in our simulations differs significantly between the two planets, which is puzzling and at odds with their qualitatively similar observed zonal wind structures. This might indicate that important processes governing the atmospheric circulation of ice giants is missing in our GCM.</p> <p>Another outcome of these simulations is that all tropospheric zonal jets are slowed down to near zero wind speeds in the lower stratosphere. The reason behind this behaviour is under investigation, as is the associated meridional circulation.</p> <p> </p> <p>Next steps will include the study of the role of Uranus and Neptune respective axial tilts and internal heat fluxes (or lack thereof) on their circulation. Furthermore, our GCM is still lacking important processes, such as latent heat release from water and other condensing species, and is lacking a realistic parametrization for convective processes. This might explain the observation-model mismatches in their zonal wind structure and will be the subject of future developments.</p> <p> </p> <p> </p> <p>[1] Lian and Showman, Icarus, Vol. 207, Issue 1, p. 373-393, 2010.</p> <p>[2] Liu and Schneider, Journal of the Atmospheric Sciences, Vol. 67, issue 11, pp. 3652-3672, 2010.</p> <p>[3] Lebeau and Dowling, Icarus, Vol. 132, Issue 2, pp. 239-265, 1998.</p> <p>[4] Hammel et al., Icarus, Vol. 201, Issue 1, p. 257-271, 2006.</p> <p>[5] Spiga et al., Icarus, Vol. 335, article id. 113377, 2020.</p> <p>[6] Guerlet et al., accepted in Icarus, 2020.</p> <p>[7] Irwin et al., Icarus, Vol. 227, p. 37-48, 2014.</p> <p>[8] Sromovsky et al., Icarus, Vol. 317, p. 266-306, 2018.</p> <p>[9] Vatant d’Ollone et al., EPSC 2020</p>

Sensitivity of the Latitude of the Surface Westerlies to Surface Friction

2007 ◽  
Vol 64 (8) ◽  
pp. 2899-2915 ◽  
Author(s):  
Gang Chen ◽  
Isaac M. Held ◽  
Walter A. Robinson
Keyword(s):  
Kinetic Energy ◽  
Momentum Flux ◽  
Phase Speed ◽  
Surface Friction ◽  
Water Model ◽  
Shallow Water Model ◽  
Mean Flow ◽  
Zonal Winds ◽  
Adjustment Time ◽  
Critical Latitude

The sensitivity to surface friction of the latitude of the surface westerlies and the associated eddy-driven midlatitude jet is studied in an idealized dry GCM. The westerlies move poleward as the friction is reduced in strength. An increase in the eastward phase speed of midlatitude eddies is implicated as playing a central role in this shift. This shift in latitude is mainly determined by changes in the friction on the zonal mean flow rather than the friction on the eddies. If the friction on the zonal mean is reduced instantaneously, the response reveals two distinctive adjustment time scales. In the fast adjustment over the first 10–20 days, there is an increase in the barotropic component of zonal winds and a substantial decrease in the eddy kinetic energy; the shift in the surface westerlies and jet latitude occurs in a slower adjustment. The space–time eddy momentum flux spectra suggest that the key to the shift is a poleward movement in the subtropical critical latitude associated with the faster eastward phase speeds in the dominant midlatitude eddies. The view is supported by simulating the upper-tropospheric dynamics in a stochastically stirred nonlinear shallow water model.

scholarly journals Vertical resolution dependence of gravity wave momentum flux simulated by an atmospheric general circulation model

2014 ◽  
Vol 7 (6) ◽  
pp. 7559-7573
Author(s):  
S. Watanabe ◽  
K. Sato ◽  
Y. Kawatani ◽  
M. Takahashi
Keyword(s):  
Gravity Wave ◽  
Momentum Flux ◽  
General Circulation ◽  
Circulation Model ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Vertical Resolution ◽  
Time Step ◽  
Atmospheric General Circulation ◽  
Wave Momentum

Abstract. The dependence of the gravity wave spectra of energy and momentum flux on the horizontal resolution and time step of atmospheric general circulation models (AGCMs) has been thoroughly investigated in the past. In contrast, much less attention has been given to the dependence of these gravity wave parameters on models' vertical resolutions. The present study demonstrates the dependence of gravity wave momentum flux in the stratosphere and mesosphere on the model's vertical resolution, which is evaluated using an AGCM with a horizontal resolution of about 0.56°. We performed a series of sensitivity test simulations changing only the model's vertical resolution above a height of 8 km, and found that inertial gravity waves with short vertical wavelengths simulated at higher vertical resolutions likely play an important role in determining the gravity wave momentum flux in the stratosphere and mesosphere.

scholarly journals Seasonal Variations of High-Frequency Gravity Wave Momentum Fluxes and Their Forcing toward Zonal Winds in the Mesosphere and Lower Thermosphere over Langfang, China (39.4° N, 116.7° E)

Atmosphere ◽  
2020 ◽  
Vol 11 (11) ◽  
pp. 1253
Author(s):  
Caixia Tian ◽  
Xiong Hu ◽  
Yurong Liu ◽  
Xuan Cheng ◽  
Zhaoai Yan ◽  
...  
Keyword(s):  
Seasonal Variations ◽  
High Frequency ◽  
Momentum Flux ◽  
Lower Thermosphere ◽  
Mean Flow ◽  
Zonal Winds ◽  
Short Period ◽  
Momentum Fluxes ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere

Meteor radar data collected over Langfang, China (39.4° N, 116.7° E) were used to estimate the momentum flux of short-period (less than 2 h) gravity waves (GWs) in the mesosphere and lower thermosphere (MLT), using the Hocking (2005) analysis technique. Seasonal variations in GW momentum flux exhibited annual oscillation (AO), semiannual oscillation (SAO), and quasi-4-month oscillation. Quantitative estimations of GW forcing toward the mean zonal flow were provided using the determined GW momentum flux. The mean flow acceleration estimated from the divergence of this flux was compared with the observed acceleration of zonal winds displaying SAO and quasi-4-month oscillations. These comparisons were used to analyze the contribution of zonal momentum fluxes of SAO and quasi-4-month oscillations to zonal winds. The estimated acceleration from high-frequency GWs was in the same direction as the observed acceleration of zonal winds for quasi-4-month oscillation winds, with GWs contributing more than 69%. The estimated acceleration due to Coriolis forces to the zonal wind was studied; the findings were opposite to the estimated acceleration of high-frequency GWs for quasi-4-month oscillation winds. The significance of this study lies in estimating and quantifying the contribution of the GW momentum fluxes to zonal winds with quasi-4-month periods over mid-latitude regions for the first time.

The Tropospheric Response to Tropical and Subtropical Zonally Asymmetric Torques: Analytical and Idealized Numerical Model Results

2012 ◽  
Vol 69 (1) ◽  
pp. 214-235 ◽  
Author(s):  
Tiffany A. Shaw ◽  
William R. Boos
Keyword(s):  
Temperature Gradient ◽  
Northern Hemisphere ◽  
General Circulation ◽  
Large Scale ◽  
Critical Line ◽  
Circulation Model ◽  
General Circulation Models ◽  
Kelvin Wave ◽  
Meridional Temperature Gradient ◽  
Poleward Shift

Abstract The tropospheric response to prescribed tropical and subtropical zonally asymmetric torques, which can be considered as idealizations of vertical momentum transfers by orographic gravity waves or convection, is investigated. The linear analytical Gill model response to westward upper-tropospheric torques is compared to the response to a midtropospheric heating, which is a familiar point of reference. The response to an equatorial torque projects onto a Kelvin wave response to the east that is of opposite sign to the response to the east of the heating at upper levels. In contrast, the torque and heating both produce Rossby gyres of the same sign to the west of the forcing and the zonal-mean streamfunction responses are identical. When the forcings are shifted into the Northern Hemisphere, the streamfunction responses have opposite signs: there is upwelling in the Southern (Northern) Hemisphere in response to the torque (heating). The nonlinear response to westward torques was explored in idealized general circulation model experiments. In the absence of a large-scale meridional temperature gradient, the response to an equatorial torque was confined to the tropics and was qualitatively similar to the linear solutions. When the torque was moved into the subtropics, the vorticity budget response was similar to a downward control–type balance in the zonal mean. In the presence of a meridional temperature gradient, the response to an equatorial torque involved a poleward shift of the midlatitude tropospheric jet and Ferrel cell. The response in midlatitudes was associated with a poleward shift of the regions of horizontal eddy momentum flux convergence, which coincided with a shift in the upper-tropospheric critical line for baroclinic waves. The shift in the critical line was caused (in part) by the zonal wind response to the prescribed torque, suggesting a possible cause of the response in midlatitudes. Overall, this hierarchy of analytical and numerical results highlights robust aspects of the response to tropical and subtropical zonally asymmetric torques and represents the first step toward understanding the response in fully comprehensive general circulation models.

scholarly journals Compensation between Resolved and Unresolved Wave Drag in the Stratospheric Final Warmings of the Southern Hemisphere

2015 ◽  
Vol 72 (11) ◽  
pp. 4393-4411 ◽  
Author(s):  
Guillermo Scheffler ◽  
Manuel Pulido
Keyword(s):  
Gravity Wave ◽  
Southern Hemisphere ◽  
Momentum Flux ◽  
General Circulation ◽  
Planetary Wave ◽  
Polar Vortex ◽  
General Circulation Models ◽  
Wave Drag ◽  
Stratospheric Polar Vortex ◽  
Sensitivity Experiments

Abstract The role of planetary wave drag and gravity wave drag in the breakdown of the stratospheric polar vortex and its associated final warming in the Southern Hemisphere is examined using reanalyses from MERRA and a middle-atmosphere dynamical model. The focus of this work is on identifying the causes of the delay in the final breakdown of the stratospheric polar vortex found in current general circulation models. Sensitivity experiments were conducted by changing the launched momentum flux in the gravity wave drag parameterization. Increasing the launched momentum flux produces a delay of the final warming date with respect to the control integration of more than 2 weeks. The sensitivity experiments show significant interactions between planetary waves and unresolved gravity waves. The increase of gravity wave drag in the model is compensated by a strong decrease of Eliassen–Palm flux divergence (i.e., planetary wave drag). This concomitant decrease of planetary wave drag is at least partially responsible for the delay of the final warming in the model. Experiments that change the resolved planetary wave activity entering the stratosphere through artificially changing the bottom boundary flux of the model also show an interaction mechanism. Gravity wave drag responds via critical-level filtering to planetary wave drag perturbations by partially compensating them. Therefore, there is a feedback cycle that leads to a partial compensation between gravity wave and planetary wave drag.

scholarly journals Impact of increased resolution on long-standing biases in HighResMIP-PRIMAVERA climate models

2021 ◽  
Author(s):  
Eduardo Moreno-Chamarro ◽  
Louis-Philippe Caron ◽  
Saskia Loosveldt Tomas ◽  
Oliver Gutjahr ◽  
Marie-Pierre Moine ◽  
...  
Keyword(s):  
General Circulation ◽  
Cloud Cover ◽  
Climate Models ◽  
General Circulation Models ◽  
Coupled Models ◽  
Near Surface ◽  
Warm Bias ◽  
Zonal Winds ◽  
Cloud Radiative Effect ◽  
Improved Model

Abstract. We examine the impacts of increased resolution on four long-standing biases using five different climate models developed within the PRIMAVERA project. Atmospheric resolution is increased from ~100–200 km to ~25–50 km, and ocean resolution is increased from ~1° (i.e., eddy-parametrized) to ~0.25° (i.e., eddy-present). For one model, ocean resolution is also increased to 1/12° (i.e., eddy-rich). Fully-coupled general circulation models and their atmosphere-only versions are compared with observations and reanalysis of near-surface temperature, precipitation, cloud cover, net cloud radiative effect, and zonal wind over the period 1980–2014. Both the ensemble mean and individual models are analyzed. Increased resolution especially in the atmosphere helps reduce the surface warm bias over the tropical upwelling regions in the coupled models, with further improvements in the cloud cover and precipitation biases particularly over the tropical South Atlantic. Related to this and to the improvement in the precipitation distribution over the western tropical Pacific, the double Intertropical Convergence Zone bias also weakens with resolution. Overall, increased ocean resolution from ~1° to ~0.25° offers limited improvements or even bias degradation in some models, although an eddy-rich ocean resolution seems beneficial for reducing the biases in North Atlantic temperatures and Gulf Stream path. Despite the improvements, however, large biases in precipitation and cloud cover persist over the whole tropics as well as in the upper-troposphere zonal winds at mid-latitudes in coupled and atmosphere-only models at higher resolutions. The Southern Ocean warm bias also worsens or persists in some coupled models. And a new warm bias emerges in the Labrador Sea in all the high-resolution coupled models. The analysis of the PRIMAVERA models therefore suggests that, to reduce biases, i) increased atmosphere resolution up to ~25–50 km alone might not be sufficient and ii) an eddy-rich ocean resolution might be needed. The study thus adds to evidence that further improved model physics and tuning might be necessary in addition to increased resolution to mitigate biases.

Estimation of Gravity Wave Momentum Flux and Phase Speeds from Quasi-Lagrangian Stratospheric Balloon Flights. Part II: Results from the Vorcore Campaign in Antarctica

2008 ◽  
Vol 65 (10) ◽  
pp. 3056-3070 ◽  
Author(s):  
Albert Hertzog ◽  
Gillian Boccara ◽  
Robert A. Vincent ◽  
François Vial ◽  
Philippe Cocquerez
Keyword(s):  
Gravity Wave ◽  
Momentum Flux ◽  
General Circulation ◽  
General Circulation Models ◽  
Companion Paper ◽  
Wave Drag ◽  
Mountain Waves ◽  
Long Duration ◽  
Momentum Fluxes ◽  
Wave Momentum

The stratospheric gravity wave field in the Southern Hemisphere is investigated by analyzing observations collected by 27 long-duration balloons that flew between September 2005 and February 2006 over Antarctica and the Southern Ocean. The analysis is based on the methods introduced by Boccara et al. in a companion paper. Special attention is given to deriving information useful to gravity wave drag parameterizations employed in atmospheric general circulation models. The balloon dataset is used to map the geographic variability of gravity wave momentum fluxes in the lower stratosphere. This flux distribution is found to be very heterogeneous with the largest time-averaged value (28 mPa) observed above the Antarctic Peninsula. This value exceeds by a factor of ∼10 the overall mean momentum flux measured during the balloon campaign. Zonal momentum fluxes were predominantly westward, whereas meridional momentum fluxes were equally northward and southward. A local enhancement of southward flux is nevertheless observed above Adélie Land and is attributed to waves generated by katabatic winds, for which the signature is otherwise rather small in the balloon observations. When zonal averages are performed, oceanic momentum fluxes are found to be of similar magnitude to continental values (2.5–3 mPa), stressing the importance of nonorographic gravity waves over oceans. Last, gravity wave intermittency is investigated. Mountain waves appear to be significantly more sporadic than waves observed above the ocean.

scholarly journals A Hybrid Approach for Model Order Reduction of Barotropic Quasi-Geostrophic Turbulence

Fluids ◽  
2018 ◽  
Vol 3 (4) ◽  
pp. 86 ◽  
Author(s):  
Sk. Rahman ◽  
Omer San ◽  
Adil Rasheed
Keyword(s):  
Neural Network ◽  
General Circulation ◽  
Hybrid Approach ◽  
Single Layer ◽  
General Circulation Models ◽  
Projection Methods ◽  
Ocean Model ◽  
Galerkin Projection ◽  
Mean Flow ◽  
Modeling Framework

We put forth a robust reduced-order modeling approach for near real-time prediction of mesoscale flows. In our hybrid-modeling framework, we combine physics-based projection methods with neural network closures to account for truncated modes. We introduce a weighting parameter between the Galerkin projection and extreme learning machine models and explore its effectiveness, accuracy and generalizability. To illustrate the success of the proposed modeling paradigm, we predict both the mean flow pattern and the time series response of a single-layer quasi-geostrophic ocean model, which is a simplified prototype for wind-driven general circulation models. We demonstrate that our approach yields significant improvements over both the standard Galerkin projection and fully non-intrusive neural network methods with a negligible computational overhead.

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