the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Emulating lateral gravity wave propagation in a global chemistry-climate model (EMAC v2.55.2) through horizontal flux redistribution
Sebastian Rhode
Hella Garny
Peter Preusse
Petr Pisoft
Aleš Kuchar
Patrick Jöckel
Astrid Kerkweg
Bastian Kern
Abstract. The columnar approach of gravity wave (GW) parameterisations in weather and climate models has been identified as a potential reason for dynamical biases in middle atmospheric dynamics. For example, GW momentum flux (GWMF) discrepancies between models and observations at 60° S arising through the lack of horizontal orographic GW propagation is suspected to cause deficiencies in representing the Antarctic polar vortex. However, due to the decomposition of the model domains onto different computing tasks for parallelisation, communication between horizontal grid boxes is computationally extremely expensive, making horizontal propagation of GWs unfeasible for global chemistry-climate simulations.
To overcome this issue, we here present a simplified solution approximating horizontal GW propagation through redistribution of the GWMF at one single altitude by means of tailor-made redistribution maps. To generate the global redistribution maps averaged for each grid box, we use a parameterisation describing orography as a set of mountain ridges with specified location, orientation and height combined with a ray-tracing model describing lateral propagation of so-generated mountain waves. In the global chemistry-climate model (CCM) EMAC (ECHAM MESSy Atmospheric Chemistry), these maps then allow us to redistribute the GW momentum flux horizontally at one level obtaining an affordable overhead of computing resources. The results of our simulations show GWMF and drag patterns which are horizontally more spread-out than with the purely columnar approach, GWs now also are present above the ocean and regions without mountains. In this paper, we provide a detailed description of how the redistribution maps are computed and how the GWMF redistribution is implemented in the CCM. Moreover, an analysis shows why 15 km is the ideal altitude for the redistribution. First results with the redistributed orographic GWMF provide clear evidence that the redistributed GW drag in the Southern Hemisphere has the potential to modify and improve Antarctic polar vortex dynamics, thereby paving the way for enhanced credibility of CCM simulations and projections of polar stratospheric ozone.
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Roland Eichinger et al.
Status: open (until 13 Jun 2023)
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RC1: 'Comment on egusphere-2023-270', Anonymous Referee #1, 28 May 2023
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Orographic gravity waves (OGWs) propagate obliquely in reality while current parameterization schemes assume vertical propagation which is a long-stanging known problem. Using the approach of horizontal flux redistribution, this work studies the parameterization of lateral gravity wave propagation in a global chemistry-climate model. This is an important step toward accurate representation of OGWs in climate models. The paper is scientific sounding and worthy publication. But there are still some issuses to be clarified. So I recomment major revsion.
- Why are MWs assumed to be perpendicular to the source ridge? In reality, the winds can be oblique to the ridge.
- L136: The resolution of ERA5 is 0.25 degree, right?
- Do you mean there are many ridges within a grid cell? I’m confused about this because, in OGWD parameterization, only a dominant ridge is considered within one grid cell.
- Remove “it was used” which appears to be redundant.
- About Figure 3. Firstly, why the GWMF of NO_HOR is less than those redistributed ones in the lower altitudes (especially below 10 km)? Wouldn’t it be greater in the absence of lateral propagation of OGWs? Secondly, why are the GWMFs different for different Htar? Taking Htar = 40 km and 45 km for example, they should be the same below 40 km since no lateral propagation below this level for both cases, right?
- About the implementation in L264-273. When applying the lateral propagation, how far can the parameterized OGWs propagate in the horizontal direction? In other words, is there an upper limit of horizontal distance for the lateral propagation of OGWs? Moreover, can you talk about the potential influence of applying the redistribution only at one single altitude level? Clearly, the OGWs propagate more and more laterally with height. Applying at single level means omitting the lateral propagation below this level and underestimation above this level. Is it possible to apply the lateral propagation at full altitudes? In this case, Zoro is not a dynamic parameter (L309) and you don’t have to calculate Eq. (5), right?
- Why the map is 4D? Is it time-varying and spatially different?
- Ok, the authors stated that the map is temporally constant. How about using a time-varying map, at least monthly variation? (see the notably monthly variation in Fig.4a)
- Is it possible that the difference between the total GW drag may be due to the different wind circulations in these two experiments which determine the wave source and breaking?
Citation: https://doi.org/10.5194/egusphere-2023-270-RC1
Roland Eichinger et al.
Roland Eichinger et al.
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