Search

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.

The instability of the Northern Hemisphere polar vortex is mainly caused by the breaking of planetary-scale (Rossby) waves (RWs). However, gravity waves (GWs) may also play an important role in polar vortex preconditioning before breakdown events. Moreover, GWs affect dynamics in the stratosphere by altering the upward propagation of RWs at short time scales and therefore indirectly influence polar vortex stability. Due to the coarse spatial resolution of global chemistry-climate models, current efforts in climate research rely on simulations where the majority of the GW spectrum is parameterized. In the present study, we apply a recently developed method for detecting strong orographic gravity wave (OGW) drag events in the lower stratosphere above the Himalayas. For this, we use a specified dynamics simulation of the chemistry-climate model CMAM (Canadian Middle Atmosphere Model) spanning the period 1979–2010. We show that strong OGW drag events above the Himalayas are associated with anomalously increased upward RW propagation in the stratosphere. This, in turn, is associated with an increase of the refractive index in the mid-latitude lower stratosphere, a region where the OGW drag dominates. Our results also illustrate that OGW strong events have the potential to alter ozone variability via changes of mixing in the surf zone and advection from lower latitudes. Altogether, we detail a preconditioning process of the polar vortex morphology by GWs above the Himalayas and how this relates to the proximity to polar vortex breakdown.

The gravity waves play a crucial role in driving and shaping the middle atmospheric circulation. The Upper-Atmospheric extension of the ICOsahedral Non-hydrostatic (UA-ICON) general circulation model was recently developed with satisfying performances in both idealized test cases and climate simulations, however the sensitivity of the circulation to the parameterized orographic and non-orographic gravity wave drag remains largely unexplored. Using UA-ICON and ICON-NWP, the sensitivity of the dynamics and circulation to both orographic and non-orographic parameterized gravity waves effects are investigated. ICON-NWP stands for the numerical-weather prediction mode of the ICON model (see Zängl et al, 2015, QJRMetSoc), with a model top at about 80 km altitude. The UA-ICON mode differs from ICON-NWP in deep-atmosphere dynamics (instead of shallow-atmosphere dynamics) and upper-atmosphere physics parameterizations being switched on. In addition, the model top is at about 150 km. The sensitivity experiments involve employing repeated annual cycle sea surface temperatures, sea ice, and greenhouse gases under year 1988. This year is selected as both El-Nino southern oscillation and pacific decadal oscillation are in their neutral phase and no explosive volcano eruption has occurred and hence conditions in this year can serve as a useful proxy for the multi-year mean condition and an estimate of its internal variability. For both UA-ICON and ICON-NWP, we perform simulations where in the control (CTL) simulation both orographic and non-orographic gravity wave drags are switched on. The other two experiments are identical to the control simulation except that either orographic (OGWD-off) or b) non-orographic (NGWD-off) gravity wave drags are switched off. The analysis include comparisons between CTL and OGWD-off and NGWD-off simulations and include wave-mean flow interaction diagnostics (Eliassen-Palm flux and its divergence and refractive index of Rossby waves) and mass stream function of the Brewer-Dobson circulation. We also investigate the sudden stratospheric warming frequency and polar vortex morphology in order to understand whether a missing gravity wave forcing can further amplify or curtail the effects of future climate. We present our goal, method as well as first results and discuss possible further analysis.

The recent release of the long-term ERA5 reanalysis data spanning from 1950 to present offers new opportunities for analysing trends and variability of stratospheric dynamics. For the first time, a 60 year period (1960-2020) can be analysed in reanalysis data and compared with chemistry-climate model simulations. The analyses of stratospheric circulation trends and seasonalities over this long time period can help us to better understand the long-term evolution of the Brewer-Dobson circulation (BDC), and the related inter-model differences and model dependencies. Therefore, this way an improved credibility in future projections of the BDC can be obtained.We find that the global trend patterns of the temperature, zonal wind and residual vertical velocity agrees well between ERA5 and the multi model mean. However, differences occur in the width and altitude of the maximum trend. The tropical upwelling mass flux time series in the lower stratosphere of models and reanalysis disagrees at the beginning of the period, but they converge after around 1980. The agreement of the time series increases with altitude, where the QBO dominates the signal. Moreover, we find a generally good agreement in the zonal wind trends, although some differences are detected in the subtropical jet strength and upward shift, as well as in the polar vortex region where the models exhibit larger changes than ERA5. Another striking difference is the temperature trend in the tropical upper troposphere/lower stratosphere, where models show a more extended warming trend into the lower stratosphere. In this presentation, we show these results, put them in relation to what had been shown in previous studies for other time periods and discuss possible explanations for the differences as well as implications for the further evolution of the BDC.

The regional climate model RegCM4 extended with the land surface model CLM4.5 was coupled to the chemistry transport model CAMx to analyze the impact of urban meteorological forcing on surface fine aerosol (PM2.5) concentrations for summer conditions over the 2001–2005 period, focusing on the area of Europe. Starting with the analysis of the meteorological modifications caused by urban canopy forcing, we found a significant increase in urban surface temperatures (up to 2–3 K), a decrease of specific humidity (by up to 0.4–0.6 gkg−1), a reduction of wind speed (up to −1 ms−1) and an enhancement of vertical turbulent diffusion coefficient (up to 60–70 m2s−1). These modifications translated into significant changes in surface aerosol concentrations that were calculated by a “cascading” experimental approach. First, none of the urban meteorological effects were considered. Then, the temperature effect was added, then the humidity and the wind, and finally, the enhanced turbulence was considered in the chemical runs. This facilitated the understanding of the underlying processes acting to modify urban aerosol concentrations. Moreover, we looked at the impact of the individual aerosol components as well. The urbanization-induced temperature changes resulted in a decrease of PM2.5 by −1.5 to −2 µg m−3, while decreased urban winds resulted in increases by 1–2 µg m−3. The enhanced turbulence over urban areas resulted in decreases of PM2.5 by −2 µg m−3. The combined effect of all individual impact depends on the competition between the partial impacts and can reach up to −3 µg m−3 for some cities, especially when the temperature impact was stronger in magnitude than the wind impact. The effect of changed humidity was found to be minor. The main contributor to the temperature impact is the modification of secondary inorganic aerosols, mainly nitrates, while the wind and turbulence impact is most pronounced in the case of primary aerosol (primary black and organic carbon and other fine particle matter). The overall as well as individual impacts on secondary organic aerosol are very small, with the increased turbulence acting as the main driver. The analysis of the vertical extent of the aerosol changes showed that the perturbations caused by urban canopy forcing, besides being large near the surface, have a secondary maximum for turbulence and wind impact over higher model levels, which is attributed to the vertical extent of the changes in turbulence over urban areas. The validation of model data with measurements showed good agreement, and we could detect a clear model improvement in some areas when including the urban canopy meteorological effects in our chemistry simulations.