A new mechanism for spontaneous imbalance exciting large-area gravity waves

2021 ◽  
Author(s):  
Markus Geldenhuys ◽  
Peter Preusse ◽  
Isabell Krisch ◽  
Christoph Zülicke ◽  
Jörn Ungermann ◽  
...  
Keyword(s):  
Numerical Experiment ◽  
Gravity Waves ◽  
General Circulation ◽  
General Circulation Models ◽  
Large Area ◽  
Mountain Waves ◽  
Geostrophic Balance ◽  
Research Aircraft ◽  
The Difference ◽  
Infrared Radiances

<p>In order to improve global atmospheric modelling, the trend is towards including source-specific gravity waves (GWs) in general circulation models. In a case study, we search for the source of a GW observed over Greenland on 10 March 2016 using the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) onboard the German research aircraft HALO. GLORIA is a remote sensing instrument where the measured infrared radiances are converted into a 3D temperature field through tomography. <br>We observe a GW packet between 10 and 13km that covers ∼1/3 of the Greenland mainland. GLORIA observations indicate a horizontal (vertical) wavelength of 330km (2km) and a temperature amplitude of 4.5K. Slanted phase fronts indicate intrinsic propagation against the jet but the GW packet propagates (ground-based) with the wind. To find the GW source, 3D GLORIA observations, GROGRAT raytracer, ERA5 data, and an ECMWF numerical experiment are used. The numerical experiment with a smoothed topography indicates virtually no GWs suggesting that the GW field in the full model is caused by the orography. However, these are not mountain waves. A favourable area for spontaneous GW emission is identified within the jet exit region by the cross-stream ageostrophic wind speed, which indicates when the flow is not in geostrophic balance. Backtracing experiments (using GROGRAT) trace into the jet and imbalance regions. The difference between the full and the smooth-topography experiment is the change in wind components by the compression of air above Greenland. These accelerations and decelerations in the jet cause the jet to become out of geostrophic balance, which excites GWs by spontaneous adjustment. We present, to the best of our knowledge, the first observational evidence of GWs by this topography-jet mechanism.</p>

scholarly journals Orographically-Induced Spontaneous Imbalance within the Jet Causing a Large Scale Gravity Wave Event

2021 ◽  
Author(s):  
Markus Geldenhuys ◽  
Peter Preusse ◽  
Isabell Krisch ◽  
Christoph Zülicke ◽  
Jörn Ungermann ◽  
...  
Keyword(s):  
Group Velocity ◽  
Gravity Wave ◽  
Large Scale ◽  
Middle Atmosphere ◽  
Large Temperature ◽  
Large Area ◽  
Mountain Waves ◽  
Geostrophic Balance ◽  
The Difference ◽  
The Impact

Abstract. To better understand the impact of gravity waves (GWs) on the middle atmosphere in the current and future climate, it is essential to understand their excitation mechanisms and to quantify their basic properties. Here a new process for GW excitation by orography-jet interaction is discussed. In a case study, we identify the source of a GW observed over Greenland on 10 March 2016 during the POLSTRACC (POLar STRAtosphere in a Changing Climate) aircraft campaign. Measurements were taken with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument deployed on the High Altitude Long Range (HALO) German research aircraft. The measured infrared limb radiances are converted into a 3D observational temperature field through the use of inverse modelling and limited angle tomography. We observe GWs along a transect through Greenland where the GW packet covers ≈ 1/3 of the Greenland mainland. GLORIA observations indicate GWs between 10 and 13 km altitude with a horizontal wavelength of 330 km, a vertical wavelength of 2 km and a large temperature amplitude of 4.5 K. Slanted phase fronts indicate intrinsic propagation against the wind, while the the ground-based propagation is with the wind. The GWs are arrested below a critical layer above the tropospheric jet. Compared to its intrinsic horizontal group velocity (25–72 ms−1) the GW packet has a slow vertical group velocity of 0.05–0.2 ms−1. This causes the GW packet to propagate long distances while spreading over a large area while remaining constrained to a narrow vertical layer. Not only orography is a plausible source, but also out of balanced winds in a jet exit region and wind shear. To identify the GW source, 3D GLORIA observations are combined with a gravity wave raytracer, ERA5 reanalysis, and high-resolution numerical experiments. In a numerical experiment with a smoothed orography, GW activity is quite weak indicating that the GWs in the realistic orography experiment are due to orography. However, analysis shows that these GWs are not mountain waves. A favourable area for spontaneous GW emission is identified in the jet by the cross-stream ageostrophic wind, which indicates when the flow is out of geostrophic balance. Backwards raytracing experiments trace into the jet and regions where the Coriolis and the pressure gradient forces are out of balance. The difference between the full and a smooth-orography experiment is investigated to reveal the missing connection between orography and the out of balance jet. We find that this is flow over a broad area of elevated terrain which causes compression of air above Greenland. The orography modifies the wind flow over large horizontal and vertical scales, resulting in out of balance geostrophic components. The out of balance jet then excites GWs in order to bring the flow back into balance. This is the first observational evidence of GW generation by such an orography-jet mechanism.

scholarly journals Intercomparison of Gravity Waves in Global Convection-Permitting Models

2019 ◽  
Vol 76 (9) ◽  
pp. 2739-2759 ◽  
Author(s):  
Claudia Christine Stephan ◽  
Cornelia Strube ◽  
Daniel Klocke ◽  
Manfred Ern ◽  
Lars Hoffmann ◽  
...  
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Primary Source ◽  
General Circulation Models ◽  
Systematic Change ◽  
Model Formulation ◽  
Difficult Challenge ◽  
Atmospheric Gravity ◽  
Coarse Grids ◽  
Horizontal Grid

AbstractLarge uncertainties remain with respect to the representation of atmospheric gravity waves (GWs) in general circulation models (GCMs) with coarse grids. Insufficient parameterizations result from a lack of observational constraints on the parameters used in GW parameterizations as well as from physical inconsistencies between parameterizations and reality. For instance, parameterizations make oversimplifying assumptions about the generation and propagation of GWs. Increasing computational capabilities now allow GCMs to run at grid spacings that are sufficiently fine to resolve a major fraction of the GW spectrum. This study presents the first intercomparison of resolved GW pseudomomentum fluxes (GWMFs) in global convection-permitting simulations and those derived from satellite observations. Six simulations of three different GCMs are analyzed over the period of one month of August to assess the sensitivity of GWMF to model formulation and horizontal grid spacing. The simulations reproduce detailed observed features of the global GWMF distribution, which can be attributed to realistic GWs from convection, orography, and storm tracks. Yet the GWMF magnitudes differ substantially between simulations. Differences in the strength of convection may help explain differences in the GWMF between simulations of the same model in the summer low latitudes where convection is the primary source. Across models, there is no evidence for a systematic change with resolution. Instead, GWMF is strongly affected by model formulation. The results imply that validating the realism of simulated GWs across the entire resolved spectrum will remain a difficult challenge not least because of a lack of appropriate observational data.

scholarly journals Comparison of the Northwestern Pacific Summer Climate Simulated by AMIP II AGCMs

2012 ◽  
Vol 25 (17) ◽  
pp. 6036-6056 ◽  
Author(s):  
Minghong Zhang ◽  
Shuanglin Li ◽  
Jian Lu ◽  
Renguang Wu
Keyword(s):  
General Circulation ◽  
Diabatic Heating ◽  
General Circulation Models ◽  
Northwestern Pacific ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Summer Climate ◽  
Walker Cell ◽  
Wide Range ◽  
Atmosphere Model ◽  
The Difference

Abstract This study examines the skills in simulating interannual variability of northwestern Pacific (NWP) summer climate in 12 atmospheric general circulation models (AGCMs) attending the Atmospheric Model Intercomparison Project phase 2 (AMIP II). The models show a wide range of skills, among those version 1 of the Hadley Centre Global Atmosphere Model (HadGAM1) showed the highest fidelity and thus may be a better choice for studying East Asian–NWP summer climate. To understand the possible causes for the difference among the models, five models {HadGAM1; ECHAM5; the Geophysical Fluid Dynamics Laboratory Atmosphere Model, version 2.1 (AM2.1); Model for Interdisciplinary Research on Climate 3.2, high-resolution version [MIROC3.2(hires)]; and the fourth-generation National Center for Atmospheric Research Community Atmosphere Model (CAM3)} that have various skill levels, ranging from the highest to the moderate to the minor, were selected for analyses. The simulated teleconnection of NWP summer climate with sea surface temperatures (SSTs) in the tropical Indian and Pacific Oceans was first compared. HadGAM1 reproduces suppressed (intensified) rainfall during El Niño (La Niña) events and captures well the remote connection with the tropical Indian Ocean, while the other models either underestimate [ECHAM5, AM2.1, MIROC3.2(hires)] or fail to reproduce (CAM3) these teleconnections. The Walker cell and diabatic heating were further compared to shed light on the underlying physical mechanisms for the difference. Consistent with the best performance in simulating interannual rainfall, HadGAM1 exhibits the highest-level skill in capturing the observed climatology of the Walker cell and diabatic heating. These results highlight the key roles of the model’s background climatology in the Walker cell and diabatic heating, thus providing important clues to improving the model’s ability.

scholarly journals Synoptic Responses to Mountain Gravity Waves Encountering Directional Critical Levels

2007 ◽  
Vol 64 (3) ◽  
pp. 828-848 ◽  
Author(s):  
Armel Martin ◽  
François Lott
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Large Scale ◽  
Baroclinic Instability ◽  
General Circulation Models ◽  
Small Scale ◽  
Mountain Range ◽  
Critical Levels ◽  
Sensitivity Tests ◽  
Large Scale Flow

Abstract A heuristic model is used to study the synoptic response to mountain gravity waves (GWs) absorbed at directional critical levels. The model is a semigeostrophic version of the Eady model for baroclinic instability adapted by Smith to study lee cyclogenesis. The GWs exert a force on the large-scale flow where they encounter directional critical levels. This force is taken into account in the model herein and produces potential vorticity (PV) anomalies in the midtroposphere. First, the authors consider the case of an idealized mountain range such that the orographic variance is well separated between small- and large-scale contributions. In the absence of tropopause, the PV produced by the GW force has a surface impact that is significant compared to the surface response due to the large scales. For a cold front, the GW force produces a trough over the mountain and a larger-amplitude ridge immediately downstream. It opposes somehow to the response due to the large scales of the mountain range, which is anticyclonic aloft and cyclonic downstream. For a warm front, the GW force produces a ridge over the mountain and a trough downstream; hence it reinforces the response due to the large scales. Second, the robustness of the previous results is verified by a series of sensitivity tests. The authors change the specifications of the mountain range and of the background flow. They also repeat some experiments by including baroclinic instabilities, or by using the quasigeostrophic approximation. Finally, they consider the case of a small-scale orographic spectrum representative of the Alps. The significance of the results is discussed in the context of GW parameterization in the general circulation models. The results may also help to interpret the complex PV structures occurring when mountain gravity waves break in a baroclinic environment.

scholarly journals Temporal variability of tidal and gravity waves during a record long 10 day continuous lidar sounding

2017 ◽  
Author(s):  
Kathrin Baumgarten ◽  
Michael Gerding ◽  
Gerd Baumgarten ◽  
Franz-Josef Lübken
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Middle Atmosphere ◽  
Wave Interaction ◽  
Weather Conditions ◽  
General Circulation Models ◽  
Driving Mechanism ◽  
Lidar Measurement ◽  
Large Variability ◽  
Tidal Waves

Abstract. Gravity waves (GW) as well as solar tides are a key driving mechanism for the circulation in the Earth's atmosphere. The propagation of gravity waves is strongly infected by tidal waves as they modulate the mean background wind field and vice versa, which is not yet fully understood and not implemented in many circulation models. The daylight capable Rayleigh-Mie-Raman (RMR) lidar at Kühlungsborn (54° N, 12&deg E) typically provides temperature data to investigate both wave phenomena during one full day or several consecutive days in the middle atmosphere between 30 and 75 km altitude. Outstanding weather conditions in May 2016 allowed for an unprecedented 10-day continuous lidar measurement which shows a large variability of gravity waves and tides on time scales of days. Using a 1-dimensional spectral filtering technique, gravity and tidal waves are separated according to their specific periods or vertical wavelengths, and their temporal evolution is studied. During the measurement a strong 24 h-wave occurs only between 40 and 60 km and vanishes after a few days. The disappearance is related to an enhancement of gravity waves with periods of 4–8 h. Wind data provided by ECMWF are used to analyze the meteorological situation at our site. The local wind structure changes during the observation period, which leads to different propagation conditions for gravity waves in the last days of the measurement and therefore a strong GW activity. The analysis indicates a further change in wave-wave interaction resulting in a minimum of the 24 h tide. The observed variability of tides and gravity waves on timescales of a few days clearly demonstrates the importance of continuous measurements with high temporal and spatial resolution to detect interaction phenomena, which can help to improve parametrization schemes of GW in general circulation models.

scholarly journals Hierarchy of climate and hydrological uncertainties in transient low flow projections

2015 ◽  
Vol 12 (12) ◽  
pp. 12649-12701 ◽  
Author(s):  
J.-P. Vidal ◽  
B. Hingray ◽  
C. Magand ◽  
E. Sauquet ◽  
A. Ducharne
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Internal Variability ◽  
General Circulation Models ◽  
Low Flow ◽  
Small Scale ◽  
Total Uncertainty ◽  
Low Flows ◽  
The Difference ◽  
Flow Variables

Abstract. This paper proposes a methodology for estimating the transient probability distribution of yearly hydrological variables conditional to an ensemble of projections built from multiple general circulation models (GCMs), multiple statistical downscaling methods (SDMs) and multiple hydrological models (HMs). The methodology is based on the quasi-ergodic analysis of variance (QE-ANOVA) framework that allows quantifying the contributions of the different sources of total uncertainty, by critically taking account of large-scale internal variability stemming from the transient evolution of multiple GCM runs, and of small-scale internal variability derived from multiple realizations of stochastic SDMs. The QE-ANOVA framework was initially developed for long-term climate averages and is here extended jointly to (1) yearly anomalies and (2) low flow variables. It is applied to better understand possible transient futures of both winter and summer low flows for two snow-influenced catchments in the southern French Alps. The analysis takes advantage of a very large dataset of transient hydrological projections that combines in a comprehensive way 11 runs from 4 different GCMs, 3 SDMs with 10 stochastic realizations each, as well as 6 diverse HMs. The change signal is a decrease in yearly low flows of around −20 % in 2065, except for the most elevated catchment in winter where low flows barely decrease. This signal is largely masked by both large- and small-scale internal variability, even in 2065. The time of emergence of the change signal on 30 year low-flow averages is however around 2035, i.e. for time slices starting in 2020. The most striking result is that a large part of the total uncertainty – and a higher one than that due to the GCMs – stems from the difference in HM responses. An analysis of the origin of this substantial divergence in HM responses for both catchments and in both seasons suggests that both evapotranspiration and snowpack components of HMs should be carefully checked for their robustness in a changed climate in order to provide reliable outputs for informing water resource adaptation strategies.

scholarly journals Seasonal Forecasts of Canadian Winter Precipitation by Postprocessing GCM Integrations

2008 ◽  
Vol 136 (3) ◽  
pp. 769-783 ◽  
Author(s):  
Hai Lin ◽  
Gilbert Brunet ◽  
Jacques Derome
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
General Circulation Models ◽  
Winter Precipitation ◽  
Second Phase ◽  
Seasonal Forecasts ◽  
Large Area ◽  
Ensemble Forecasts ◽  
The North ◽  
Ensemble Mean

Abstract In the second phase of the Canadian Historical Forecasting Project (HFP2), four global atmospheric general circulation models (GCMs) were used to perform seasonal forecasts over the period of 1969–2003. Little predictive skill was found from the uncalibrated GCM ensemble seasonal predictions for the Canadian winter precipitation. This study is an effort to improve the precipitation forecasts through a postprocessing approach. Canadian winter precipitation is significantly influenced by two of the most important atmospheric large-scale patterns: the Pacific–North American pattern (PNA) and the North Atlantic Oscillation (NAO). The time variations of these two patterns were found to be significantly correlated with those of the leading singular value decomposition (SVD) modes that relate the ensemble mean forecast 500-mb geopotential height over the Northern Hemisphere and the tropical Pacific SST in the previous month (November). A statistical approach to correct the ensemble forecasts was formulated based on the regression of the model’s leading forced SVD patterns and the observed seasonal mean precipitation. The performance of the corrected forecasts was assessed by comparing its cross-validated skill with that of the original GCM ensemble mean forecasts. The results show that the corrected forecasts predict the Canadian winter precipitation with statistically significant skill over the southern prairies and a large area of Québec–Ontario.

scholarly journals Temporal variability of tidal and gravity waves during a record long 10-day continuous lidar sounding

2018 ◽  
Vol 18 (1) ◽  
pp. 371-384 ◽  
Author(s):  
Kathrin Baumgarten ◽  
Michael Gerding ◽  
Gerd Baumgarten ◽  
Franz-Josef Lübken
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Middle Atmosphere ◽  
Weather Conditions ◽  
General Circulation Models ◽  
Driving Mechanism ◽  
Lidar Measurement ◽  
Large Variability ◽  
Tidal Waves ◽  
Measurement Period

Abstract. Gravity waves (GWs) as well as solar tides are a key driving mechanism for the circulation in the Earth's atmosphere. The propagation of gravity waves is strongly affected by tidal waves as they modulate the mean background wind field and vice versa, which is not yet fully understood and not adequately implemented in many circulation models. The daylight-capable Rayleigh–Mie–Raman (RMR) lidar at Kühlungsborn (54∘ N, 12∘ E) typically provides temperature data to investigate both wave phenomena during one full day or several consecutive days in the middle atmosphere between 30 and 75 km altitude. Outstanding weather conditions in May 2016 allowed for an unprecedented 10-day continuous lidar measurement, which shows a large variability of gravity waves and tides on timescales of days. Using a one-dimensional spectral filtering technique, gravity and tidal waves are separated according to their specific periods or vertical wavelengths, and their temporal evolution is studied. During the measurement period a strong 24 h wave occurs only between 40 and 60 km and vanishes after a few days. The disappearance is related to an enhancement of gravity waves with periods of 4–8 h. Wind data provided by ECMWF are used to analyze the meteorological situation at our site. The local wind structure changes during the observation period, which leads to different propagation conditions for gravity waves in the last days of the measurement period and therefore a strong GW activity. The analysis indicates a further change in wave–wave interaction resulting in a minimum of the 24 h tide. The observed variability of tides and gravity waves on timescales of a few days clearly demonstrates the importance of continuous measurements with high temporal and spatial resolution to detect interaction phenomena, which can help to improve parametrization schemes of GWs in general circulation models.

scholarly journals Application of the Compressible, Nonhydrostatic, Balanced Omega Equation in Estimating Diabatic Forcing for Parameterization of Inertia–Gravity Waves: Case Study of Moist Baroclinic Waves Using WRF

2019 ◽  
Vol 77 (1) ◽  
pp. 113-129
Author(s):  
Mahnoosh Haghighatnasab ◽  
Mohammad Mirzaei ◽  
Ali R. Mohebalhojeh ◽  
Christoph Zülicke ◽  
Riwal Plougonven
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Diabatic Heating ◽  
Wrf Model ◽  
Vertical Motion ◽  
General Circulation Models ◽  
Baroclinic Waves ◽  
Diabatic Forcing ◽  
Omega Equation ◽  
Inertia Gravity Waves

Abstract The parameterization of inertia–gravity waves (IGWs) is of considerable importance in general circulation models. Among the challenging issues faced in studies concerned with parameterization of IGWs is the estimation of diabatic forcing in a way independent of the physics parameterization schemes, in particular, convection. The requirement is to estimate the diabatic heating associated with balanced motion. This can be done by comparing estimates of balanced vertical motion with and without diabatic effects. The omega equation provides the natural method of estimating balanced vertical motion without diabatic effects, and several methods for including diabatic effects are compared. To this end, the assumption of spatial-scale separation between IGWs and balanced flows is combined with a suitable form of the balanced omega equation. To test the methods constructed for estimating diabatic heating, an idealized numerical simulation of the moist baroclinic waves is performed using the Weather Research and Forecasting (WRF) Model in a channel on the f plane. In overall agreement with the diabatic heating of the WRF Model, in the omega-equation-based estimates, the maxima of heating appear in the warm sector of the baroclinic wave and in the exit region of the upper-level jet. The omega-equation-based method with spatial smoothing for estimating balanced vertical motion is thus presented as the proper way to evaluate diabatic forcing for parameterization of IGWs.

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