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waveclouds

Role of gravity waves in the Southern Hemispheric circulation and climate

WASCLIM - a ROMIC II Project (2019-2022)

WASCLIM is a project of the research initiative
Role Of the Middle atmosphere In Climate (ROMIC-II)

ROMIC-II (project call in German) is the follow-up of the research initiative ROMIC. The two central research questions addressed in ROMIC-II are:

A. How does the variability of the sun induce changes in the tropospheric climate via changes in the circulation, physical processes, and chemical composition of the middle atmosphere?

B. How does tropospheric forcing, in particular anthropogenic influences, feed back to the troposphere and change the climate by its impact in the middle atmosphere?

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Atmospheric Gravity Waves

 Convection, fronts, flow over mountains, and spontaneous adjustments occurring at the tropospheric jet streams generate vertically propagating gravity waves (GWs) in the troposphere and lower stratosphere (Smith 1979, Gill 1982, Baines 1995, Fritts and Alexander 2003, Sutherland 2010, Nappo 2012, Plougonven and Zhang 2014). Generally, we differentiate between orographic GWs and non-orographic GWs (OGWs and NOGWs). Through their far-field interactions, GWs constitute an important coupling mechanism in the Earth’s atmosphere and the momentum deposition is described as orographic and non-orographic GW drag (OGWD, NOGWD), respectively. The associated redistribution of momentum and energy controls the global middle atmospheric circulation (Dunkerton 1978, Lindzen 1981, Alexander et al. 2010).

The Problem: missing gravity wave drag at 60° S

 Comparisons of stratospheric zonal-mean absolute GW momentum fluxes (GWMFs) from global circulation model (GCM) predictions and global satellite data reveal a large variability between currently used GW parametrizations. Some features are reproduced reasonably well in terms of magnitudes and the latitudinal dependence, but there are still large deviations. Particular deviations are the "gap" in simulated zonal mean GWMFs near 60° S in winter (Geller et al. 2013) and the large simulated maxima above mountainous regions which are not observed by satellites such as SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) and HIRDLS (High Resolution Dynamics Limb Sounder) (Geller et al. 2013). Most GCMs exhibit large systematic biases in the simulation of the Southern Hemisphere (SH) polar stratospheric circulation. These biases are known as the "cold pole bias" occurring, e.g., in the Canadian Middle Atmosphere Model (CMAM) or the Whole Atmosphere Community Climate Model (WACCM) (SPARC CCMVal 2010, Butchart et al. 2011) as they underestimate the polar stratospheric temperatures. Due to isolation of the cold stratospheric air and the weak planetary wave activity, there are significant late-biases in the simulated spring-time breakups of the SH vortex. Most likely, the simplified representations of GWs is a major cause of the biases in the simulation of SH stratospheric climate.
Summarizing and referring to a statement by T. G. Shepherd, the problem of parameterizing unresolved GWs and their sensitive interaction with large-scale dynamics is among the most uncertain aspects of climate modeling (Shepherd 2014).

 OGWs likely play an important role but also other processes could explain the "missing drag at 60° S" (de la Cámara et al. 2016). Generally, the physical origin of the observed belt of enhanced GWMF around 60° S can be explained by different mechanisms: (i) The downwind advection and meridional refraction of OGWs from the Southern Andes and the Antarctic Peninsula into the PNJ (Sato et al. 2009). (ii) Unresolved OGWs from small islands (Alexander et al. 2009, Alexander and Grimsdell 2013). (iii) The generation of secondary GWs generated locally in the breaking region of these primary OGWs (Hindley et al. 2015). (iv) NOGWs from sources associated with winter storm tracks over the Southern Ocean (Hendricks et al. 2014). (v) a zonally uniform distribution of small amplitude waves from non-orographic mechanisms such as spontaneous adjustment and jet instability around the edge of the stratospheric PNJ (Hindley et al. 2015).

The Project

 The project WASCLIM investigates the impact of GWs on the SH stratospheric circulation and tropospheric climate by means of field observations, high-resolution numerical modelling, the analysis of meteorological reanalyses, and climate modelling. The development and implementation of a simple parametrization for the horizontal propagation of GWs, and, eventually, climate sensitivity simulations will quantify the resulting effects on tropospheric climate and climate change. In the project, long-term ground-based measurements with autonomous temperature-lidar CORAL (Compact Rayleigh Autonomous Lidar) at the southern tip of South America are continued. They are complemented by episodic airborne measurements, amongst others, with the new ALIMA (Airborne Lidar for Studying the Middle Atmosphere) instrument and the infrared limb imager GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) onboard the German research aircraft HALO (High Altitude and Long Range Research Aircraft) and ground-based meteor radar observations.

 The region of the Andes and the Antarctic Peninsula is a hotspot of stratospheric GW activity. A data set for different GW sources will be assembled and high-resolution ICON (= global forecast model of the German Weather Service) simulations will complement observational data in a consistent way. This data set together with the drag climatology derived from the reanalyses will be used to develop a source-transfer parametrization mimicking the horizontal GW propagation. Moreover, ICON simulations will be used to evaluate its performance in EMAC (= global chemistry climate model of German research institutions) by means of nudged EMAC simulations for the specific episode of the aircraft campaign. The statistical mean representation of GWs in the modified EMAC model will be conducted based on long-term reanalyses-data.




Latest news of the project (updated 2. Sept 2021)

upcoming: A virtual scientific workshop on "gravity waves in the Southern Andes region" is scheduled for 28.-30. September 2021.

♦ The ROMIC-II Kick-off meeting was held on 26. February 2021 (virtual meeting).

♦ The virtual SouthTRAC-Gravity Waves meeting was held on 26./27./28. October 2020.

♦ The first SouthTRAC UTLS Chemistry and Transport Workshop was hosted virtually on 22./23. September 2020.

♦ The first data workshop for the gravity wave part of the SouthTRAC campaign took place on 20./21. January 2020 at DLR in Oberpfaffenhofen.

♦ Airborne observations were sucessfully completed in the framework of the SouthTRAC campaign in September to November 2019. For more details, visit the SouthTRAC website.

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Work packages

The project tasks are grouped into five scientific work packages and one workpackage coordinating their entity.

WP0: Project Coordination (DLR)
WP1: Observations (DLR, FZJ, KIT, IAP)
WP2: Mesoscale modelling (DLR)
WP3: Reanalyses (LMU)
WP4: Parametrization (FZJ)
WP5: Climate modelling (DLR)

Halo

WP1: Observations

In this work package, ground-based and airborne observations are organized and conducted in the region of the Southern Andes, the Drake Passage, and the Antarctic Peninsula. The aim is the detailed probing and quantification of GW activity and GWMFs in the troposphere and middle atmosphere. Ground-based remote sensing and radiosonde observations at the southern tip of South America focus on OGWs and their temporal development. Mesospheric wind measurements are relevant to quantify tidal motions and Doppler shifting of ground-based GW measurements. Remote-sensing and in-situ measurements onboard HALO extend the observational volume allowing the investigation of NOGW sources and the horizontal propagation of OGWs and NOGWs, i.e., the spatial structure and extent of the GW field. The airborne measurements took place in the framework of the SouthTRAC campaign in September to November 2019. Key instruments on HALO were the upward looking Rayleigh lidar ALIMA covering the middle atmosphere/lower thermosphere, and the infrared limb imager GLORIA covering the upper troposphere and lower stratosphere region. This unique combination of two cutting edge instruments offers new insights in the complex GW structures in the atmosphere.

icon

WP2: Mesoscale modelling

In this work package, numerical simulations of orographic and non-orographic GW events are performed using the global ICON model of DWD (Zängl et al. 2015). Locally enhanced model resolution of around 1 km can be realized by nesting directly in ICON and allows ICON to resolve the GW spectrum to a large part. Temporal evolution and spatial distribution of GWMF can be computed from ICON simulations and justified against the observations and their analysis. This is done for the global ICON data which rely on orographic and non-orographic GW parameterizations, and for the highly resolved nested data providing reliable 4D (time and space) GW information. In this manner, the ICON data will help to establish a connection of the tasks and results in the simplified model world (WP3, WP4 and WP5) to real world observations (WP1).

reanalyses

WP3: GW fluxes and associated mean flow changes in reanalyses

The general goal of this work package is to provide a bridge between the detailed GW analyses from the field campaign and the general representation of GWs in climate models (WP5). Such a bridge is necessary to transform the knowledge gained by the case study as part of the field campaign to a statistical characterisation needed to improve the GW parameterisation in climate models. To do so, we will quantitatively assess GWMF and the associated forcing of the mean flow in the SH polar stratosphere based on several ECMWF reanalysis products, as well as operational analyses. Missing and/or misrepresented drag will be inferred both directly from analysis increments due to data assimilation, as well as indirectly from the residual of the momentum budget. The results are expected to both put the field observations into the perspective of the past record, as well as provide a baseline for the improved GW parameterisation in EMAC based on observationally-constrained global model runs.

reanalyses

WP4: Budgets of lateral propagation for GWs from different sources: development of a parametrization

In this work package, we seek an efficient way to represent horizontal GW propagation in a GCM and thus reproducing the high GWMF in the SH PNJ by more realistic processes than in previous studies. The GWMF contained in the PNJ is given by the strength of the source, the horizontal and vertical propagation path and whether it reaches the PNJ, and the dissipation and filtering which the GWs experiences. In a conventional parametrization implemented in a GCM, parametrized GWs propagate only vertically. Ray-tracing handles horizontal propagation. However, in order to significantly reduce the high computational cost that comes along with climate simulations, we follow a different approach. Our approach is to perform ray-tracing offline, evaluate the results statistically and compile look-up tables. For each orographic source, these will provide a probability distribution µ that expresses statistically to which locations at an evaluation height GWs propagate. In other words, we will spread GWs from a given source to all locations in the lower stratosphere where µ is positive.

emac

WP5: Global Modelling with EMAC

In this work package, the aim is to improve the representation of GWs in global models using observationally based results from the other work packages. GWMF and drag from observations, high-resolution modelling, and reanalyses will serve as basis for the evaluation of GW activity in the global model. While high-resolution model data can be compared to nudged EMAC simulations for the specific episode of the campaign, the statistically mean representation of GWs in the model can only be evaluated with long-term data of GW forcing as will be provided from reanalysis data. The simplified horizontal propagation parameterization developed in WP4 will be implemented in EMAC. The impact of GW modifications on the simulation of the climatological mean circulation of the stratosphere, the response of stratospheric circulation to anthropogenic forcing, and finally, the dynamical downward coupling of stratospheric circulation change to the tropospheric climate will be studied. In particular, we will focus on the problem of the current "gap" in GWD around 60° S by means of 1) comparison to observational data to identify the nature of the "gap" in more detail, and 2) dedicated sensitivity studies with respect to modifications of the GW parametrizations. We aim to evaluate the hypothesis that the missing GWD at 60° S in the stratosphere is due to neglecting horizontal propagation of OGWs. As alternative hypothesis, we test whether additional sources of GWs (orographic or non-orographic) around 60° S are needed to explain the missing GWD at 60° S. Thereby, we will investigate whether similar effects on the mean stratospheric circulation are obtained by altering GW propagation (re-distribution of GWD with same sources) or sources.

Research Institutions


DLR

German Aerospace Center

Institute of Atmospheric Physics


FZJ

Forschungszentrum Jülich

Institute of Energy and Climate Research



LMU

Ludwig-Maximilians-Universität München

Meteorological Institute Munich


IAP

Leibniz-Institute of Atmospheric Physics


KIT

Karlsruhe Institute of Technology

Institute of Meteorology and Climate Research


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