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The ICON model with upper atmosphere extension (UA-ICON) running with the Atmosphere in the Earth System (AES) physics package suffers from a weak stratospheric polar vortex, though this package is intended for simulations of ICON on the seasonal to climatological time scales. For example, UA-ICON (AES) data from model free runs predict an unrealistically high number of SSWs per decade. However, due to the weak polar vortex, the simulated SSWs are of too weak intensity concerning the polar cap averaged temperature and mid-latitude zonal mean, zonal wind anomalies compared to observations. The climatology and variability of the Northern hemispheric (NH) stratospheric winter circulation widely improve when applying ICON and UA-ICON with the physics package for numerical weather prediction (NWP). However, whereas the NWP physics package is implemented technically for UA-ICON, it is not well-tuned for the upper mesosphere/ lower thermosphere (UMLT) region. This concerns, e.g. the temperatures in the summer mesopause regions at both hemispheres, which are much too warm, or the zonal mean wind structures in the lower thermosphere, which do not show the wind reversals from westerly to the easterly zonal wind in winter and vice versa in summer. These problems are caused mainly by an underrepresentation of the non-orographic gravity wave drag (NGWD), implemented with the parametrization based on Warner and McIntyre. The orographic gravity wave drag (OGWD) implemented with a parametrization based on Lott and Miller acts mainly in the stratosphere, but OGWD tuning also influences temperatures in the mesopause., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The way the large-scale flow determines the energy of the nonorographic mesoscale inertia–gravity waves (IGWs) is theoretically significant and practically useful for source parameterization of IGWs. The relations previously developed on the f plane for tropospheric sources of IGWs including jets, fronts, and convection in terms of associated secondary circulations strength are generalized for application over the globe. A low-pass spatial filter with a cutoff zonal wavenumber of 22 is applied to separate the large-scale flow from the IGWs using the ERA5 data of ECMWF for the period 2016–19. A comparison with GRACILE data based on satellite observations of the middle stratosphere shows reasonable representation of IGWs in the ERA5 data despite underestimates by a factor of smaller than 3. The sum of the energies, which are mass-weighted integrals in the troposphere from the surface to 100 hPa, as given by the generalized relations is termed initial parameterized energy. The corresponding energy integral for the IGWs is termed the diagnosed energy. The connection between the parameterized and diagnosed IGW energies is explored with regression analysis for each season and six oceanic domains distributed over the globe covering the Northern and Southern Hemispheres and the tropics. While capturing the seasonal cycle, the domain area-average seasonal mean initial parameterized energy is weaker than the diagnosed energy by a factor of 3. The best performance in regression analysis is obtained by using a combination of power and exponential functions, which suggests evidence of exponential weakness.

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 thefplane. 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.

We present a new version of the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) with specified dynamics. We utilize a spectral method that nudges only the large-scale flow to MERRA-2 reanalysis. The nudged HIAMCM simulates gravity waves (GWs) down to horizontal wavelengths of about 200 km from the troposphere to the thermosphere like the free-running model, including the generation of secondary and tertiary GWs. Case studies show that the simulated large-scale GWs are consistent with those in the reanalysis, while the medium-scale GWs compare well with observations in the northern winter 2016 stratosphere from the Atmospheric InfraRed Sounder (AIRS). GWs having wavelengths larger than about 1350 km can be described with the nonlinear balance equation. The GWs relevant in the stratosphere, however, have smaller scales and require a different approach. We propose that the GW amplification due to kinetic energy transfer from the large-scale flow combined with GW potential energy flux convergence helps to identify the mesoscale GW sources due to spontaneous emission. The GW amplification is strongest in the region of maximum large-scale vertical wind shear in the mid-stratosphere. Maps of the time-averaged stratospheric GW activity simulated by the HIAMCM and computed from AIRS satellite data show a persistent hot spot over Europe during January 2016. At about 40 km, the average GW amplitudes are maximum in the region of fastest large-scale flow. We argue that refraction of GWs originating in the troposphere, as well as GWs from spontaneous emission in the stratosphere contribute to this effect.

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. 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.

Tropopause folds are known as areas of enhanced stratosphere-troposphere exchange. These exchange processes are governed by turbulent mixing in the upper-tropospheric and lower-stratospheric shear zones around the tropopause jet. Since the 1970s, turbulence is also predicted to enhance the ageostrophic circulation around the jet, which leads to the formation of the tropopause fold in an upper-level jet-front system. This claim was recently confirmed by a numerical weather prediction study using the ECMWF-IFS.With our balloon-borne turbulence measuring instrument LITOS, we recently sounded a deep and a medium tropopause fold with astonishing results: in both cases, the strength of turbulence in the lower stratospheric shear layer was three orders of magnitude higher compared to the upper tropospheric shear layer, reaching severe turbulence strengths in the deep-fold case. This has not been reported before, potentially because hardly any observational turbulence study covering both shear layers exists in the literature. In our study, we also quantitatively compare turbulence induced PV changes with PV profiles from the IFS and assess the meteorological situation using further IFS data. Additionally, we investigate mixing processes from tracer-tracer correlations of ozone and water vapour along the flight track of our instrument.

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.

Quantification of inertia–gravity waves (IGWs) generated by upper-level jet–surface front systems and their parameterization in global models of the atmosphere relies on suitable methods to estimate the strength of IGWs. A harmonic divergence analysis (HDA) that has been previously employed for quantification of IGWs combines wave properties from linear dynamics with a sophisticated statistical analysis to provide such estimates. A question of fundamental importance that arises is how the measures of IGW activity provided by the HDA are related to the measures coming from the wave–vortex decomposition (WVD) methods. The question is addressed by employing the nonlinear balance relations of the first-order δ–γ, the Bolin–Charney, and the first- to third-order Rossby number expansion to carry out WVD. The global kinetic energy of IGWs given by the HDA and WVD are compared in numerical simulations of moist baroclinic waves by the Weather Research and Forecasting (WRF) Model in a channel on the f plane. The estimates of the HDA are found to be 2–3 times smaller than those of the optimal WVD. This is in part due to the absence of a well-defined scale separation between the waves and vortical flows, the IGW estimates by the HDA capturing only the dominant wave packets and with limited scales. It is also shown that the difference between the HDA and WVD estimates is related to the width of the IGW spectrum.

The vertical coupling between the stratosphere and the mesosphere is diagnosed from polar cap temperatures averaged over 60°–90°N with a new method: the joint occurrence of a warm stratosphere at 10 hPa and a cold mesosphere at 0.01 hPa. The investigation of an 11-yr-long dataset (2004–15) from Aura-MLS observations shows that such mesospheric coupling days appear in 7% of the winter. During major sudden stratospheric warming events mesospheric couplings are present with an enhanced average daily frequency of 22%. This daily frequency changes from event to event but broadly results in five of seven major warmings being classified as mesospheric couplings (2006, 2008, 2009, 2010, and 2013). The observed fraction of mesospheric coupling events (71%) is compared with simulations of the Kühlungsborn Mechanistic Circulation Model (KMCM), the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), and the Whole Atmosphere Community Climate Model (WACCM). The simulated fraction of mesospheric coupling events ranges between 57% and 94%, which fits the observations. In searching for causal relations weak evidence is found that major warming events with strong intensity or split vortices favor their coupling with the upper mesosphere. More evidence is found with a conceptual model: an effective vertical coupling between 10 and 0.01 hPa is provided by deep zonal-mean easterlies at 60°N, which are acting as a gravity-wave guide. The explained variance is above 40% in the four datasets, which indicates a near-realistic simulation of this process.

Commonly, wave quantities are maintained in zonal mean averages. Hence, local wave phenomena remain unclear. Here, we introduce a diagnostic tool for studies of wave packets locally. The "Unified Wave Diagnosis" (UWaDi) uses the Hilbert Transform to obtain a complex signal from a real-valued function and estimates the amplitude and wave number locally. Operational data from the European Centre for Medium-Range Weather Forecasts is used to perform the analysis. Restrictions on gravity wave propagation due to model sponge layers are identified well above the 10 hPa altitude. From a minor stratospheric warming in January 2016 three cases for vertical gravity wave propagation in different background wind conditions are selected. It is shown that zonal mean wind quantities cannot reveal local "valves" allowing gravity waves to propagate into the mid-stratosphere. The unexpected finding of high gravity wave activity at the minor warming of 30 January 2016 is related to strong planetary wave activity and a strong local "pump". Accordingly, the advantages of a local wave packet analysis are demonstrated for profiles up to the model sponge layer.

The impact of moisture on inertia–gravity wave generation is assessed for an idealized unstable baroclinic wave using the Weather Research and Forecasting Model (WRF) in a channel on the f plane. The evolution of these waves in a moist simulation is compared with a dry simulation. The centers of action for inertia–gravity wave activity are identified as the equatorward-moving upper-level front and the poleward-progressing upper-level jet–surface front system. Four stratospheric wave packets are found, which are significantly more intense in the moist simulation and have slightly higher frequency. They are characterized by their structure and position during the baroclinic wave life cycle and are related to forcing terms in jet, front, and convection systems. By exploring the time series of mass and energy, it is shown that the release of latent heat leads to a change in enthalpy, an increase in the eddy kinetic energy, and an intensification of the inertia–gravity wave energy. The ratio of the inertia–gravity wave energy to the eddy kinetic energy is estimated to be about 1/200 for the moist simulation, which is 3 times larger than that for the dry simulation. An empirical parameterization scheme for the inertia–gravity wave energy is proposed, based on the fast large-scale ageostrophic flow associated with the jet, front, and convection. The diagnosed stratospheric inertia–gravity wave energy is well captured by this parameterization in six WRF simulations with different moisture and resolutions. The approach used to construct the parameterization may serve as a starting point for state-dependent nonorographic gravity wave drag schemes in general circulation models.

Long-term observations from medium-frequency and meteor radars (1993–2012) and rocket soundings (1979–1990 and 2002–2007) are used to study mesosphere and lower thermosphere (MLT) zonal wind variations in relation to the stratospheric winds over northern low latitudes. The combined data set provides a complete height profile of amplitude of semiannual oscillation (SAO) up to 100 km, with an exception around 75–80 km. The SAO signal has maxima around 50 km and 82 km and a minimum around 65 km. The MLT zonal winds show remarkable interannual variability during northern hemispheric spring equinox and much less during fall equinox. Zonal wind mesospheric spring equinox enhancements (MSEE) appear with a periodicity of 2–3 years, suggesting a modulation by the quasi-biennial oscillation, which we identified with the strength of stratospheric westward winds. Out of 20 years of observations, the stratospheric westward winds are strong during 11 years (non-MSEE) and weak during 9 years. Six of these 9 years show large MLT winds (MSEE), and 3 years (1999, 2004, and 2006) show small MLT winds (missing MSEE). These unexpected small winds occur in years with global circulation anomalies associated with strong sudden stratospheric warmings and an early spring transition of zonal winds. With the proposed three MSEE classes, we take into account local and global forcing factors.

[1] Mesospheric coolings and wind reversals during sudden stratospheric warmings are studied in a 570 day permanent January simulation with the Kuhlungsborn Mechanistic Circulation Model (KMCM). The model is run at a high resolution in combination with an advanced parameterization of turbulence, allowing for an explicit description of gravity-wave effects. Self-generated major sudden stratospheric warmings are quantified with benchmark tests of Charlton and Polvani (2007) and are found to be nearly realistic. Extension of the tests into the mesosphere gives rise to a benchmark diagram for the entire middle atmosphere, showing that the mesospheric cooling is robust while the mesospheric wind response is variable. The same features appear in a composite constructed from five stratospheric warming events. The zonal-mean structures indicate an old stratopause descending with about 6 km per day during about 5 days before the central date. About 10 days after the central date, an elevated new stratopause develops in the upper mesosphere and subsequently progresses downward, which is driven by westward gravity waves. Temporal characteristics are deduced from 7 day smoothed daily time series from representative stratospheric and mesospheric zonal-mean quantities. A correlation analysis shows that the temperature signals are anticorrelated, while the wind signals are uncorrelated. The begin, peak, and end times of the events are diagnosed with two objective methods. Both methods reveal nearly equal-time appearance of stratospheric warming and wind deceleration (within 2 days), followed by mesospheric cooling with a delay of less than 5 days. This rules out mesospheric precursors of sudden stratospheric warmings in our simulation.

The link between poleward-breaking Rossby waves and stratospheric inertia–gravity waves is examined. With a visual inspection of Ertel’s potential vorticity maps based on ECMWF analyses it was found that Rossby wave–breaking events occurred over northern Europe in about 40% of the winter days in 1999–2003. The majority of them were breaking poleward downstream. A total of 10 field campaigns were performed in the winters of 1999–2002 at Kühlungsborn, Germany (54°N, 12°E). They are related to such events and can be considered as representative for poleward-breaking Rossby waves. Inertia–gravity wave properties are diagnosed from radiosonde observations. They appeared to be shallower, slower, and stronger than the climatological mean for the north German lowlands. Hence, Rossby wave–breaking events are linked with strong stratospheric inertia–gravity wave activity. A novel parameterization of inertia–gravity wave generation and propagation is proposed. The stratospheric inertia–gravity wave action in the 16–20-km height range was parameterized with the synoptic-scale cross-stream ageostrophic wind, which accounts for imbalances in the upper-tropospheric jet streak. This empirical relationship is supported with quasigeostrophic theory. Effects of damping and critical level absorption are taken into account with Wentzel–Kramers–Brillouin theory. For verification of the parameterization with homogeneous meteorological fields in space and time, the 10 field campaigns were hindcasted with the nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model. About 80% of the variance in inertia–gravity wave action was found to be explained. For the 10 campaigns a close link was found between the poleward-breaking Rossby waves and the strong stratospheric inertia–gravity waves. The role of the polar vortex was twofold: first, it forced the poleward-oriented Rossby waves to break downstream and to form strong tropospheric jet streaks generating inertia–gravity waves. Second, the strong winds in the stratosphere favored the upward propagation of the inertia–gravity waves. The proposed new parameterization of inertia–gravity wave generation and propagation was validated and can be used to deduce mesoscale wave intensity from synoptic flow characteristics during poleward Rossby wave–breaking events.

Poleward-breaking Rossby waves often induce an upper-level jet streak over northern Europe. Dominant inertia–gravity wave packets are observed downstream of this jet. The physical processes of their generation and propagation, in such a configuration, are investigated with a mesoscale model. The study is focused on an observational campaign from 17 to 19 December 1999 over northern Germany. Different simulations with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) have been performed. For a high-resolution process study, three domains were set up that encompass the evolution of Rossby waves and that of inertia–gravity waves. To minimize the impact of model damping, the horizontal and vertical resolution has been adjusted appropriately. With a novel statistical approach, the properties of inertia–gravity wave packets have been estimated. This method uses the horizontal divergence field and takes into account the spatial extension of a wave packet. It avoids the explicit treatment of the background field and works for arbitrary wavelength. Two classes of inertia–gravity waves were found: subsynoptic waves with a horizontal wavelength of about 500 km and mesoscale waves with a horizontal wavelength of about 200 km. The subsynoptic structures were also detected in radiosonde observations during this campaign. The similarity between simulated and observed wavelengths and amplitudes suggests that the simulations can be considered as near realistic. Spontaneous radiation from unbalanced flow is an important process of inertia–gravity wave generation. Synoptic-scale imbalances in the exit region of the upper-tropospheric jet streak were identified with the smoothed cross-stream Lagrangian Rossby number. In a number of simulations with different physics, it was found that the inertia–gravity wave activity was related to the tropospheric jet, orography, and moist convection. The upward propagation of inertia–gravity waves was favored during this event of a poleward-breaking Rossby wave. The presence of the polar vortex induced background winds exceeding the critical line. Consequently, the activity of inertia–gravity waves in the lower stratosphere increased by an order of magnitude during the case study. The successful simulation of the complex processes of generation and propagation showed the important role of poleward Rossby wave breaking for the appearance of inertia–gravity waves in the midlatitudes.

A theoretical framework is presented that describes the transfer of momentum, heat and mass through the air–sea interface. It is based on the steady-state balance of turbulent kinetic energy, which allows the consistent derivation of turbulent velocity, and dissipation rate across the air–sea interface. The theory is of K – L -type with particular accuracy near the interface, which is considered to be fixed and aerodynamically smooth. Stationarity, horizontal homogeneity, and the invariance of the vertical fluxes have to be assumed. Mean profiles are discussed as results of numerical integration of the balance equations and as analytical two-or three-layer approximations. A wide range of field and laboratory data is used to determine four dimensionless tuning parameters. It is demonstrated how the stratification changes the wind and Schmidt number dependence of the gas transfer velocity. For realistic environmental conditions, profiles of velocity, temperature and carbon dioxide concentration are presented. They extend from some height in the air through the interface down to some depth in the sea. While the major transfer resistance of momentum lies within the turbulent atmospheric layer, the temperature profile is influenced by the skin layers at both sides. The flux of carbon dioxide is mainly controlled by the skin of the ocean. During calm situations the transfer is influenced by stratification changing to exclusively molecular transports for stable stratification but to free convection for unstable stratification. For passive tracers such as carbon dioxide a corresponding parameterisation of the transfer velocity is presented.

The near surface temperature profile is described by a combination of theories for the molecular skin layer (Soloviev and Schlussel, 1994) and the turbulent bulk layer (Monin and Ozmidov, 1985). The formulation presented by Soloviev et al. (1994) describes the temperature difference through the thermal molecular boundary layer, whereas similarity theories is appropriate for the turbulent bulk layers below. A combination of both theories allows for the relation of bulk temperatures at arbitrary depth of the well-mixing near-surface layer to the skin temperature of the infrared layer (10 μm thickness, from where thermal radiation is emitted into the atmosphere). In an example is shown, that for usual night-time conditions (surface heat flux of Q0 = 100 W/m2, 10-meter wind velocity varying between U10 = 0 … 25 m/s), the skin layer forms between 62 … 88 % of the oceanic temperature signal at 1 m depth. The contributions of the skin to the overall temperature signal are large for strong cooling (free convection) and strong winds (breaking of long surface waves).

[1] Mercury in the environment deserves serious concern because of the mobility of volatile elemental mercury (Hg0) in the atmosphere, in combination with the harmful effect of Hg compounds on human health and the ecosystem. A major source of global atmospheric mercury is presumed to be oceanic Hg0 emission. However, available Hg0 surface water data to reliably estimate the ocean's mercury emissions are sparse. In this study, high-resolution surface water and air measurements of Hg0 were carried out between Europe and South Africa in November 2008 and between South America and Europe in April–May 2009. On each cruise a strong enrichment of Hg0 in tropical surface water was determined that apparently followed the seasonal shift of the Intertropical Convergence Zone (ITCZ). A combination of a high Hg0 production rate constant and the actual low wind speeds, which prevented emission, probably caused the accumulation of Hg0 in surface waters of the ITCZ. Hg0 emissions in the tropics were significant only if wind speed variability on a monthly scale was considered, in which case the observed significant decline of total Hg in tropical surface waters during the northern winter could be explained. In the midlatitudes, increased autumn Hg0 emissions were calculated for November in the Northern Hemisphere and for May in the Southern Hemisphere; conversely, emissions were low during both the northern and the southern spring. Mercury removal from surface waters by Hg0 emission and sinking particles was comparable to its supply through wet and dry deposition.

A set of coupled nonlinear reaction-diffusion equations was formulated for the oxidation of CO on low-index plane Pt surfaces and solved for their spatiotemporal behaviour using continuation techniques and the method of global connections. A 2-variable model for Pt(111), consisting of the CO and O coverages as a function of time and space, gave rise to reaction fronts. In order to model the behaviour of Pt(111) or Pt(100) a third equation describing the respective adsorbate-driven phase transition had to be added. The resulting 3-variable system predicts extended regions of excitability, where pulses can be triggered. The front — respectively pulse — velocities as well as the critical radii for the nucleation were computed and compared to experimental data where available.

A meteorological case study for the impact of inertia-gravity waves on surface meteorology is presented. The large-scale environment from 17 to 19 December 1999 was dominated by a poleward breaking Rossby wave transporting subtropical air over the North Atlantic Ocean upward and north-eastward. The synoptic situation was characterized with an upper tropospheric jet streak passing Northern Europe. The unbalanced jet spontaneously radiated inertia-gravity waves from its exit region. Near-inertial waves appeared with a horizontal wavelength of about 200 km and an apparent period of about 12 h. These waves transported energy downwards and interacted with large-scale convection. This configuration is simulated with the nonhydrostatic Fifth-Generation Mesoscale Model. Together with simplified runs without orography and moisture it is demonstrated that the imbalance of the jet (detected with the cross-stream ageostrophic wind) and the deep convection (quantified with the latent heat release) are forcing inertia-gravity waves. This interaction is especially pronounced when the upper tropospheric jet is located above a cold front at the surface and supports deep frontal convection. Weak indication was found for triggering post-frontal convection by inertia-gravity waves. The realism of model simulations was studied in an extended validation study for the Baltic Sea region. It included observations from radar (DWDPI, BALTRAD), satellite (GFZGPS), weather stations (DWDMI) and assimilated products (ELDAS, MESAN). The detected spatio-temporal patterns show wind pulsations and precipitation events at scales corresponding to those of inertia-gravity waves. In particular, the robust features of strong wind and enhanced precipitation near the front appeared with nearly the same amplitudes as in the model. In some datasets we found indication for periodic variations in the post-frontal region. These findings demonstrate the impact of upper tropospheric jet-generated inertia-gravity waves on the dynamics of the boundary layer. It also gives confidence to models, observations and assimilation products for covering such processes. In an application for the Gotland Basin in the Baltic Sea, the implications of such mesoscale events on air-sea interaction and energy and water budgets are discussed.

The evolution of the axial component of the atmospheric angular momentum vector (AAM) is investigated for September 2002, using ECMWF analyses. The components of the conservation equation of AAM for southern polar cap volume are calculated in order to examine the importance of their contributions to the AAM tendency during the unexpected break-up of the southern polar vortex in the last week of September. The AAM tendency for the mean polar cap (90°–65° S) oscillates with a period of 5 d until the break-up of the polar vortex, after that it is negative for about 9 d. The polar friction torque induced by katabatic winds is weakly positive and nearly constant. For this mean polar cap, the mountain torque at the surface is mainly positive and larger than the friction torque. The strong changes of the mountain torque are caused by shifts of surface pressure patterns induced by a Rossby wave train touching the orography of Antarctica. The fluxes of AAM are northwards and reduce the AAM of the polar cap volume. The changes of AAM fluxes are mainly determined by the evolution of momentum fluxes due to transient Rossby waves in the upper troposphere and lower stratosphere. The vortex break-up is associated with a strong decrease of the amount of mountain torque and an increase in the convergence of relative AAM fluxes.