Search

Atmospheric gravity waves (GWs) impact the circulation and variability of the atmosphere. Sub‐grid scale GWs, which are too small to be resolved, are parameterized in weather and climate models. However, some models are now available at resolutions at which these waves become resolved and it is important to test whether these models do this correctly. In this study, a GW resolving run of the European Center for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS), run with a 1.4 km average grid spacing (TCo7999 resolution), is compared to observations from the Atmospheric Infrared Sounder (AIRS) instrument, on NASA's Aqua satellite, to test how well the model resolves GWs that AIRS can observe. In this analysis, nighttime data are used from the first 10 days of November 2018 over part of Asia and surrounding regions. The IFS run is resampled with AIRS's observational filter using two different methods for comparison. The ECMWF ERA5 reanalysis is also resampled as AIRS, to allow for comparison of how the high resolution IFS run resolves GWs compared to a lower resolution model that uses GW drag parametrizations. Wave properties are found in AIRS and the resampled models using a multi‐dimensional S‐Transform method. Orographic GWs can be seen in similar locations at similar times in all three data sets. However, wave amplitudes and momentum fluxes in the resampled IFS run are found to be significantly lower than in the observations. This could be a result of horizontal and vertical wavelengths in the IFS run being underestimated. Plain Language Summary: Small‐scale atmospheric waves, known as gravity waves (GWs), transport energy and momentum and affect the dynamics of the atmosphere. GWs in a high resolution run of the European Center for Medium‐Range Weather Forecasts Integrated Forecasting System (IFS) weather model are compared to those in observations from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite, to test how well these waves are resolved in the model. Nighttime data are compared over part of Asia and surrounding regions, during the first 10 days of November 2018. Since the high resolution IFS run has a higher vertical resolution, and a significantly higher horizontal resolution than the satellite observations, the model is resampled as if the satellite was viewing the model atmosphere. This removes GWs with horizontal and vertical wavelengths outside of the ranges that can be seen in the observations, allowing the data sets to be compared. Gravity waves formed by wind flowing over mountain ranges can be seen at similar times and in similar locations in the IFS run and observations, but wave amplitudes in the resampled IFS run are found to be significantly lower. Key Points: A kilometer‐scale Integrated Forecasting System (IFS) run is resampled as Atmospheric Infrared Sounder (AIRS) using two different methods to allow for comparison of gravity wave (GW) propertiesGWs can be seen in the resampled IFS run and AIRS at similar times and locationsMean amplitudes in the resampled IFS run are found to be significantly lower than in the observations by a factor of ∼2.77 [ABSTRACT FROM AUTHOR]

The paper presents a new method for the decomposition of the horizontal wind divergence among linear waves on the sphere: inertia‐gravity (IG), mixed Rossby‐gravity (MRG), Kelvin and Rossby waves. The work is motivated by the need to quantify the vertical velocity and momentum fluxes in the tropics where the distinction between the Rossby and gravity regime, present in the extratropics, becomes obliterated. The method leads to divergence power spectra as a function of latitude and pressure. The spectra follow the same power laws as the vertical kinetic energy spectra for different wave types. The key novel aspect of the work is the coexistence of wave types at the same zonal wavenumbers. Applied to ERA5 data in August 2018, the new method reveals that the zonally‐integrated Kelvin wave divergence makes up about 19% of the upper‐troposphere divergence within 10°S–10°N. The MRG wave divergence has four times smaller magnitude. The relative roles of the two waves vary with scale. Overall small roles of the Kelvin and MRG waves in the tropical divergence is explained by decomposing their kinetic energies into rotational and divergent parts. The beta effects produces less then 5% of the tropospheric divergence associated with Rossby waves. The majority of divergence belongs to IG modes and is nearly equipartitioned between the eastward‐ and westward‐propagating IG modes in the upper troposphere, whereas stratospheric partitioning depends on the background flow. Plain Language Summary: The horizontal circulation is commonly discussed using divergence and vorticity instead of the two wind components. The advantage is that vorticity and divergence characterize the Rossby wave and inertia‐gravity (IG) wave dynamics, respectively, thereby offering an easier understanding of wave processes. The two wave regimes are well separated in extratropics also in the wavenumber and frequency space. However, there is no regime separation in the tropics where IG waves shape variability at all scales including low‐frequency variability. Furthermore, two additional waves are present, Kelvin waves and mixed Rossby‐gravity (MRG) waves that span the wavenumber and frequency space between Rossby and IG waves. This study aims at a better understanding of partitioning between divergent and rotational components of equatorial waves. In particular, Kelvin and MRG waves have significant variances at the same scales as Rossby and IG waves calling for an approach that quantifies their divergence at the same wavenumbers. This paper develops such an approach for the spherical atmosphere and demonstrates it on the ERA5 data in August 2018. One important result is that the Kelvin and MRG wave constitute up to approximately 25% of divergence in the tropical upper troposphere, mainly in Kelvin waves. Key Points: A new method couples divergence with wave circulation as a function of latitude and pressure levelDivergence quantification accounts for wave types sharing the same zonal scale, which is crucial for disentangling tropical flowsApplied to ERA5, the new method highlights vortical nature of mixed Rossby‐gravity waves and large‐scale Kelvin waves [ABSTRACT FROM AUTHOR]

A unique set of relaxation experiments with a forecast model initialized during December–January 1999–2019 is used to explore tropical influence on the Northern Hemisphere polar stratosphere and stratosphere–troposphere coupling, to quantify predictability benefits due to the perfect knowledge of the tropical variability. On average, predictability of the polar stratosphere, as represented by the 50 hPa geopotential height anomalies north of 50°N (Z50), increases from 17 days in freely running (control) forecasts to 21 days in the tropical relaxation experiments. At sub‐seasonal time‐scales, a statistically significant improvement in weekly mean skill scores can be demonstrated in 14%–20% of individual forecast ensembles, mostly in cases when the skill of the corresponding control forecast is worse than average. In these forecasts, root‐mean‐square errors and forecast spread of Z50 during forecast weeks 3–5 are decreased by 10%–15%. Stratospheric improvements are detected during periods of both vortex strengthening and vortex weakening, including most major Sudden Stratospheric Warmings that occurred during the study period, via modulation of the upward wave activity fluxes. An active Madden–Julian Oscillation (MJO) is found in most of these events with MJO phase 5–7 preceding vortex weakening and MJO phase 3–4 preceding vortex strengthening. Forecasts with improved stratospheric circulation also have improved tropospheric circulation during and after the periods when improvements are detected in the stratosphere. We attribute these improvements to both stratosphere–troposphere coupling and tropospheric tropical teleconnections. [ABSTRACT FROM AUTHOR]

Long‐term high‐resolution temperature data of the Compact Rayleigh Autonomous Lidar (CORAL) is used to evaluate temperature and gravity wave (GW) activity in ECMWF Integrated Forecasting System (IFS) over Río Grande (53.79°S, 67.75°W), which is a hot spot of stratospheric GWs in winter. Seasonal and altitudinal variations of the temperature differences between the IFS and lidar are studied for 2018 with a uniform IFS version. Moreover, interannual variations are considered taking into account updated IFS versions. We find monthly mean temperature differences <2 K at 20–40 km altitude. At 45–55 km, the differences are smaller than 4 K during summer. The largest differences are found during winter (4 K in May 2018 and −10 K in August 2018, July 2019 and 2020). The width of the difference distribution (15th/85th percentiles), the root mean square error, and maximum differences between instantaneous individual profiles are also larger during winter (>±10 K) and increase with altitude. We relate this seasonal variability to middle atmosphere GW activity. In the upper stratosphere and lower mesosphere, the observed temperature differences result from both GW amplitude and phase differences. The IFS captures the seasonal cycle of GW potential energy (Ep) well, but underestimates Ep in the middle atmosphere. Experimental IFS simulations without damping by the model sponge for May and August 2018 show an increase in the monthly mean Ep above 45 km from only ≈10% of the Ep derived from the lidar measurements to 26% and 42%, respectively. GWs not resolved in the IFS are likely explaining the remaining underestimation of the Ep. Key Points: Monthly mean temperature differences between the IFS and ground‐based lidar measurements in the middle atmosphere are largest in winterWintertime temperature differences are related to middle atmosphere gravity wave activity and amplitude and phase differences are importantDamping by the model sponge and unresolved gravity waves reduce gravity wave potential energy in the middle atmosphere in the IFS [ABSTRACT FROM AUTHOR]

The Northern Hemisphere stratospheric polar vortex (SPV), a band of fast westerly winds over the Pole extending from approximately 10 to 50 km altitude, is a key driver of European winter weather. Extremely weak polar vortex states, so called sudden stratospheric warmings (SSWs), are on average followed by dry and cold weather in Northern Europe, as well as wetter weather in Southern Europe. However, the surface response of SSWs varies greatly between events, and it is not well understood which factors modulate this difference. Here, we address the role of the timing of SSWs within the cold season (December–March) for the temperature and precipitation response in Europe. Given the limited sample size of SSWs in the observations, hindcasts of the seasonal forecasting model SEAS5 from the European Centre for Medium-Range Weather Forecasts are analysed. First, we evaluate key characteristics of stratosphere–troposphere coupling in SEAS5 against reanalysis data and find them to be reasonably well captured by the model, justifying our approach. We then show that in SEAS5, early winter (December and January) SSWs are followed by more pronounced surface impacts compared to late winter (February and March) SSWs. For example, in Scotland, the low precipitation anomalies are roughly twice as severe after early winter SSWs than after late winter SSWs. The difference in the response cannot be explained by more downward propagating SSWs in early winter, or by different monthly precipitation climatologies. Instead, we demonstrate that the differences result from stronger SPV anomalies associated with early winter SSWs. This is a statistical artefact introduced through the commonly used SSW event definition, which involves an absolute threshold, and, therefore, leads to stronger SPV anomalies during early winter SSWs when the stratospheric mean state is stronger. Our study highlights the sensitivity of surface impacts to SSW event definition. [ABSTRACT FROM AUTHOR]

Convectively generated gravity waves (CGGWs) are important for numerical weather prediction due to their effect on the quasi‐biennial oscillation (QBO) in the stratosphere. Using global ECMWF IFS simulations at TCo7999 (or 1.25 km), TCo2559 (or 3.9 km) and TCo1279 (or 7.8 km) horizontal resolutions, sensitivity of resolved CGGWs to the horizontal resolution and to the explicit versus parametrized representation of deep convection is elucidated during the westerly shear phase of the QBO. Parametrized deep convection is found to inhibit CGGWs, resulting in a twofold reduction in CGGW forcing. When deep convection is explicitly resolved, the total CGGW forcing is almost unchanged across the horizontal resolutions. However, the contribution of long and mesoscale CGGWs (with horizontal wavelengths 100 km ≤λh<1,900 km) to the total CGGW forcing decreases and the contribution of smaller‐scale CGGWs (with λh<100 km) increases as the horizontal resolution increases. At the maximum CGGW forcing altitude, at TCo7999 resolution 43% of the total CGGW forcing is due to long and mesoscale waves, whereas at TCo2559 and TCo1279 resolutions their contribution is 70% and 90%, respectively. While CGGW forcing from long and mesoscale waves is similar at TCo7999 resolution with explicit deep convection and at TCo1279 resolution with parametrized deep convection, CGGW forcing from these waves is artificially enhanced at TCo1279 and TCo2559 resolutions with explicit deep convection. This is due to the explicit deep convection being too strong and having too much variance for 100 km ≤λh<1,900 km. Therefore, parametrizations of deep convection and CGGWs (to account for forcing from waves with λh<100 km) are required even at TCo2559 resolution. Additionally, resolved CGGW forcing at TCo7999 resolution is examined for the easterly shear phase of the QBO; similar to the westerly shear phase, the smaller‐scale waves contribute >55% to the total CGGW forcing at the maximum CGGW forcing altitude. [ABSTRACT FROM AUTHOR]

We assess the value of machine learning as an accelerator for the parameterization schemes of operational weather forecasting systems, specifically the parameterization of nonorographic gravity wave drag. Emulators of this scheme can be trained to produce stable and accurate results up to seasonal forecasting timescales. Generally, networks that are more complex produce emulators that are more accurate. By training on an increased complexity version of the existing parameterization scheme, we build emulators that produce more accurate forecasts. For medium range forecasting, we have found evidence that our emulators are more accurate than the version of the parametrization scheme that is used for operational predictions. Using the current operational CPU hardware, our emulators have a similar computational cost to the existing scheme, but are heavily limited by data movement. On GPU hardware, our emulators perform 10 times faster than the existing scheme on a CPU. Plain Language Summary: The ability of computers to construct models from data (machine learning) has had significant impacts on many areas of science. Here, we use this ability to construct a model of an element of a numerical weather forecasting system. This element captures one physical process in the model, a part of the model that describes the propagation of large‐scale waves through the atmosphere, but the long‐term aim would be to make many models each capturing a process. The goal is that the computer‐generated model will perform the task more efficiently than the existing model. Testing is then carried out to ensure that our computer model performs as accurately as the existing model. This is a challenging step, as learning is carried out over short time periods (seconds), but forecasts need to be accurate over years. Our computer‐generated models produce accurate forecasts on all tested timescales. On current computers, they are not faster, but will be if weather forecasting centers invest in computers with graphics processing units. Key Points: Nonorographic gravity wave drag parametrization can be accurately emulated with a neural networkThese emulators produce accurate and stable forecasts over long timescalesNeural networks can reduce the cost of an increased complexity scheme [ABSTRACT FROM AUTHOR]

Planetary waves (PWs) and gravity waves (GWs) are the key drivers of middle atmospheric circulation. Insufficient observations and inaccurate model representation of GWs limit our understanding of their stratospheric contributions, especially during the Antarctic polar vortex breakdown. This study employs the strength of the high‐resolution ERA5 reanalysis in resolving a broad spectrum of GWs in southern midlatitudes and its ability to estimate their forcing during the breakdown period. Most of the resolved southern hemisphere GWs deposit momentum around 60°S over the Southern Ocean. Further, a zonal momentum budget analysis during the breakdown period reveals that the resolved GW forcing in ERA5 provides as much as one‐fourth of the necessary wind deceleration at 60°S, 10 hPa. The parameterized GW drag, mostly from non‐orographic sources, provides more than half of the wind deceleration. Both findings highlight the key role of GWs in the vortex breakdown and discuss possibilities for further stratospheric GW analysis. Plain Language Summary: Strong flow over mountains during winters and instabilities in the southern hemisphere troposphere can excite gravity waves that propagate from near‐surface all the way to atmospheric heights of 50–80 km. At these heights, they dissipate momentum and decelerate the strong eastward winds. Knowing the structure and extent of the forcing by these waves can help to better understand their role in driving the stratospheric and mesospheric circulation. However, the net forcing by these waves is not accurately known on account of limited observations and because global climate models cannot sufficiently resolve them. This study illustrates that the high‐resolution ERA5 reanalysis, which forms a natural bridge between observations and free running climate models, can be used to estimate the mean forcing due to such gravity waves and can help assess their role in springtime deceleration of polar vortex. Such an analysis was not possible using previous reanalysis datasets due to low resolution. The findings show that, indeed, gravity wave forcing can provide a large fraction of the deceleration needed to slow down the strong westerly winds in late winters. Key Points: ERA5 partially resolves gravity waves allowing evaluation of their contribution to the springtime polar vortex breakdownClimatology of the gravity wave forcing highlights relevance of gravity wave refraction and oblique propagation and suggests deceleration of up to −8 m/s/day around 60°SDeceleration of the vortex due to gravity wave dissipation can be as high as the total necessary deceleration for the vortex breakdown [ABSTRACT FROM AUTHOR]

In an attempt to advance the understanding of the Earth's weather and climate by representing deep convection explicitly, we present a global, four‐month simulation (November 2018 to February 2019) with ECMWF's hydrostatic Integrated Forecasting System (IFS) at an average grid spacing of 1.4 km. The impact of explicitly simulating deep convection on the atmospheric circulation and its variability is assessed by comparing the 1.4 km simulation to the equivalent well‐tested and calibrated global simulations at 9 km grid spacing with and without parametrized deep convection. The explicit simulation of deep convection at 1.4 km results in a realistic large‐scale circulation, better representation of convective storm activity, and stronger convective gravity wave activity when compared to the 9 km simulation with parametrized deep convection. Comparison of the 1.4 km simulation to the 9 km simulation without parametrized deep convection shows that switching off deep convection parametrization at a too coarse resolution (i.e., 9 km) generates too strong convective gravity waves. Based on the limited statistics available, improvements to the Madden‐Julian Oscillation or tropical precipitation are not observed at 1.4 km, suggesting that other Earth system model components and/or their interaction are important for an accurate representation of these processes and may well need adjusting at deep convection resolving resolutions. Overall, the good agreement of the 1.4 km simulation with the 9 km simulation with parametrized deep convection is remarkable, despite one of the most fundamental parametrizations being turned off at 1.4 km resolution and despite no adjustments being made to the remaining parametrizations. Plain Language Summary: We present the world's first global simulation of an entire season of the Earth's atmosphere with 1.4 km average grid spacing and the top of the modeled atmosphere as high as 80 km. Albeit only a single realization due to its considerable computational cost, the resulting model output provides a reference and guidance for future simulations. For illustration we compare to simulations at 9 km grid spacing that represent the state of the art in numerical weather prediction and are still considerably finer when compared to models that are used for climate projections today. Thanks to its unprecedented detail, the simulation output will support future model development and satellite mission planning and may be seen as a prototype contribution to a future digital twin of our Earth. Key Points: A unique simulation with 1.4 km average grid spacing is presented for model development and process evaluationThe 1.4 km simulation shows remarkable fidelity with respect to the well‐calibrated simulation at 9 km with parametrized deep convectionSwitching off deep convection at a too coarse resolution (9 km) generates too strong convective gravity waves [ABSTRACT FROM AUTHOR]

Using a nonlinear global primitive equation model, spontaneous inertia‐gravity wave (IGW) emission is investigated in an idealized representation of the stratospheric polar night. It is shown that IGWs are spontaneously emitted in the interior of the fluid in a jet exit region that develops around a nonlinear Rossby wave critical layer. Two key ingredients for the generation are identified: the presence of a Rossby wave guide on the polar night jet; and a zero wind line on the jet flank that gives rise to nonlinear Rossby wave breaking and strong distortion of the flow. The emission of IGWs appears here as a quasi‐steady process that begins at a well‐defined time when the flow deformation becomes large enough. Part of the emitted IGWs undergoes wave capture by the cat's‐eye flow in a Rossby wave critical layer. Another part – in the form of a well‐defined IGW packet – escapes the wave capture limit, and propagates away into the far field. The propagating wave packet is numerically well‐converged to increases in both vertical and horizontal resolution and thus provides an ideal test bed for understanding IGW emission and informing non‐orographic gravity wave drag parametrization design. [ABSTRACT FROM AUTHOR]

A severe cold spell with surface temperatures reaching 10 K below its climatology hit Eurasia during late February/early March 2018. This cold spell was associated with a Scandinavian blocking pattern followed by an extreme negative North Atlantic Oscillation (NAO) phase. Here we explore the predictability of this cold spell/NAO event using ensemble forecasts from the Subseasonal‐to‐Seasonal (S2S) archive of the European Centre for Medium‐Range Weather Forecasts. We find that this event was predicted with the observed strength roughly 10 days in advance. However, the probability of the cold spell occurring doubled up to 25 days in advance, when a sudden stratospheric warming (SSW) occurred. Our results indicate that the amplitude of the cold spell was increased by a regime shift to the negative NAO phase at the end of February, which was likely favoured by the SSW. We quantify the contribution of the SSW to the enhanced extended‐range forecast skill for this particular event by running forecast ensembles in which the evolution of the stratosphere is nudged to (a) the observed evolution, and (b) a time‐invariant state. In the experiment with nudging to the observed stratospheric evolution, the probability of a strong cold spell occurring is enhanced to 45%, while it is at its climatological value of 5% when the stratosphere is nudged to a time‐invariant state. These results showing enhanced predictability of surface extremes following SSWs extend previous observational evidence, which is mostly based on composite analyses, to a single event. Our results suggest that it is the subsequent evolution throughout the lower stratosphere following the SSW, rather than the occurrence of the SSW itself, that is crucial in coupling to large‐scale tropospheric flow patterns. However, we caution that probabilistic gain in predictability alone is insufficient to conclude a causal link between the SSW and the cold spell event. [ABSTRACT FROM AUTHOR]

Several recent studies have suggested that the stratosphere can be a source of subseasonal‐to‐seasonal predictability of Southern Hemisphere circulation during the austral spring and early summer seasons, through its influence on the zonal‐mean eddy‐driven jet. We exploit the large sample size afforded by the hindcasts from the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System to address a number of unanswered questions. It is shown that the picture of coherent seasonal variability of the coupled stratosphere‐troposphere system apparent from the reanalysis record during the spring/early summer period is robust to sampling uncertainty and that there is evidence of nonlinearity in the case of the most extreme variations. The effect of El Niño–Southern Oscillation on the eddy‐driven jet during this time of year is found to occur via the stratosphere, with no evidence of a direct tropospheric pathway. A simple two‐state statistical model of the stratospheric vortex is introduced to estimate the subseasonal‐to‐seasonal predictability associated with shifts of the seasonal cycle in the Southern Hemisphere extratropical atmosphere. This simple model, along with a more general model, is subsequently used to interpret skill scores associated with hindcasts made using the full seasonal forecast system. Together, the results provide evidence of tropospheric predictability on subseasonal‐to‐seasonal timescales during this time of year from at least as early as 1 August and show no evidence of a "signal‐to‐noise paradox" between the hindcasts and the reanalysis. Key Points: Southern Hemisphere midlatitude jet variations in spring and early summer are predictable several months aheadThis subseasonal‐to‐seasonal predictability of the Southern Hemisphere jet comes via the stratospheric polar vortexThe observed influence of ENSO on the jet during this time is via this stratospheric pathway [ABSTRACT FROM AUTHOR]

Abstract: The impact of parametrized nonorographic gravity wave drag (NOGWD) on the stratosphere‐troposphere dynamical coupling in atmospheric models is relatively unexplored. Using the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System, we find that changes in NOGWD strength have a substantial impact on the tropospheric eddy‐driven jet (EDJ) in both hemispheres, but the sense of the impact is opposite in the two hemispheres. In the Northern Hemisphere the impact occurs via changes in the amplitude and persistence of stratospheric anomalies. In the Southern Hemisphere it occurs instead via differences in the sensitivity of the EDJ to a given stratospheric anomaly, arising from changes in the seasonal cycle leading up to the polar vortex breakdown. Increasing NOGWD eliminates the springtime phase of the Southern Hemisphere tropospheric semiannual oscillation, resulting in a more equatorward annual‐mean EDJ and showing that the semiannual oscillation cannot be explained entirely from tropospheric mechanisms. [ABSTRACT FROM AUTHOR]