In order to reduce uncertainties in modeling the stratospheric circulation, global observations of gravity wave momentum flux (GWMF) vectors are required for comparison with distributions of resolved ...and parametrized GWMF in global models. For the first time, we derive GWMF vectors globally from data of a nadir‐viewing satellite instrument: we apply a 3‐D method to an Atmospheric Infrared Sounder (AIRS) temperature data set that was optimized for gravity wave (GW) analysis. For January 2009, the resulting distributions of GW amplitudes and of net GWMF highlight the importance of GWs in the polar vortex and the summertime subtropics. Net GWMF is preferentially directed opposite to the background wind, and, interestingly, it is dominated by large‐amplitude GWs of relatively long horizontal wavelength. For convective GW sources, these large horizontal scales are in contradiction with traditional thoughts. However, the observational filter effect needs to be kept in mind when interpreting the results.
Key Points
A first global distribution of net gravity wave momentum flux is derived from satellite observations
Net gravity wave momentum flux (net GWMF) is directed prevalently opposite to the background wind
Waves of large‐amplitude and above‐average horizontal scale dominate the net GWMF distribution
On 15 January 2022, the Hunga Tonga–Hunga Ha'apai volcano erupted violently. This exceptional event excited a manifold of atmospheric waves. Here, we focus on the mesoscale part of the wave spectrum. ...About 8.5 hr after the eruption a strong atmospheric gravity wave (GW) was observed in the stratosphere by the satellite instruments Atmospheric Infrared Sounder (AIRS) and Microwave Limb Sounder (MLS) in the vicinity of Tonga. By ray‐tracing, we confirm the eruption as the source of this GW event. We determine the wave characteristics of the GW in terms of horizontal and vertical wavelengths and GW momentum flux. The strength of the GW is compared to the usual Southern Hemisphere flux values during this week. The event is comparable to the strongest convective events considering MLS, and exceptionally strong considering AIRS, which observes faster waves only.
Plain Language Summary
Atmospheric gravity waves (GWs) are small‐ and mesoscale waves typically generated by convective events, jet instabilities, or flow over orography. On 15 January 2022, an explosive eruption of the Hunga Tonga–Hunga Ha'apai volcano occurred. This eruption was a source of strong GWs of a wide range of wavelengths and phase speeds. We here focus on mesoscale waves and in particular on vertical wavelengths with the potential for interaction with the background wind, that is, vertical scales usually considered for driving atmospheric circulation. Observations by two different types of satellite instruments (limb sounding and nadir sounding) give us the unique opportunity for a detailed study of this GW event originating from a source process that has rarely been considered so far. Tracing the wave backward in time using a wave‐propagation model confirms the volcanic eruption as the wave source. Horizontal and vertical wavelengths of the GW are 500 and 20 km, respectively. Amplitudes and momentum flux indicate that the event is outstanding in particular for the higher phase speeds. We consider this rare wave event highly interesting for future more detailed observational studies, as well as for atmospheric modeling.
Key Points
On 15 January 2022, a wide variety of gravity wave scales were excited by the eruption of the Hunga Tonga‐Hunga Ha'apai volcano
We here focus on characterizing and quantifying the mesoscale response and confirm by backtracing the eruption as source of these waves
Compared to usual conditions amplitudes and momentum flux of the eruption wave event are strong for Microwave Limb Sounder and exceptional for Atmospheric Infrared Sounder
In this work absolute values of gravity wave (GW) momentum flux are derived from global temperature measurements by the satellite instruments High Resolution Dynamics Limb Sounder (HIRDLS) and ...Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). Momentum fluxes in the stratosphere are derived for both instruments and for SABER in the whole mesosphere. The large‐scale atmospheric background state is removed by a two‐dimensional Fourier decomposition in longitude and time, covering even planetary‐scale waves with periods as short as 1–2 days. Therefore, it is possible to provide global distributions of GW momentum flux from observations for the first time in the mesosphere. Seasonal as well as longer‐term variations of the global momentum flux distribution are discussed. GWs likely contribute significantly to the equatorward tilt of the polar night jet and to the poleward tilt of the summertime mesospheric jet. Our results suggest that GWs can undergo large latitudinal shifts while propagating upward. In particular, GWs generated by deep convection in the subtropical monsoon regions probably contribute significantly to the mesospheric summertime wind reversal at mid‐ and high latitudes. Variations in the GW longitudinal distribution caused by those convectively generated GWs are still observed in the mesosphere and could be important for the generation of the quasi two‐day wave. Indications for quasi‐biennial oscillation (QBO) induced variations of GW momentum flux are found in the subtropics. Also variations at time scales of about one 11‐year solar cycle are observed and might indicate a negative correlation between solar flux and GW momentum flux.
Key Points
Gravity wave momentum flux global distributions in both strato‐ and mesosphere
Interaction of gravity waves with zonal wind jets studied
Discussion of variations on different timescales: seasonal, QBO, solar cycle
The spectral distribution in terms of horizontal and vertical wavenumber is deduced for gravity wave (GW) momentum flux observations from space for the first time. The paper focuses on High ...Resolution Dynamics Limb Sounder (HIRDLS) observations in the subtropics and compares spectra over particularly strong deep convective forcing with a background inferred from regions of less momentum flux. The deep convective spectra are strongly enhanced at high horizontal wavenumbers over a wide range of vertical wavenumbers and phase speeds. The observations indicate that GWs generated by deep convection are an important contribution on top of the background spectrum due to other GW sources (at least about 20–30% on zonal average is from convective GWs). The implications for global modeling are discussed.
Key Points
Gravity wave spectra are derived from satellite in the summertime subtropics
Over deep convection the spectrum is enhanced at short horizontal wavelengths
Momentum flux of convective gravity waves is important on zonal average
It is known that atmospheric dynamics in the tropical stratosphere have an influence on higher altitudes and latitudes as well as on surface weather and climate. In the tropics, the dynamics are ...governed by an interplay of the quasi-biennial oscillation (QBO) and semiannual oscillation (SAO) of the zonal wind. The QBO is dominant in the lower and middle stratosphere, and the SAO in the upper stratosphere/lower mesosphere. For both QBO and SAO the driving by atmospheric waves plays an important role. In particular, the role of gravity waves is still not well understood. In our study we use observations of the High Resolution Dynamics Limb Sounder (HIRDLS) satellite instrument to derive gravity wave momentum fluxes and gravity wave drag in order to investigate the interaction of gravity waves with the SAO. These observations are compared with the ERA-Interim reanalysis. Usually, QBO westward winds are much stronger than QBO eastward winds. Therefore, mainly gravity waves with westward-directed phase speeds are filtered out through critical-level filtering already below the stratopause region. Accordingly, HIRDLS observations show that gravity waves contribute to the SAO momentum budget mainly during eastward wind shear, and not much during westward wind shear. These findings confirm theoretical expectations and are qualitatively in good agreement with ERA-Interim and other modeling studies. In ERA-Interim most of the westward SAO driving is due to planetary waves, likely of extratropical origin. Still, we find in both observations and ERA-Interim that sometimes westward-propagating gravity waves may contribute to the westward driving of the SAO. Four characteristic cases of atmospheric background conditions are identified. The forcings of the SAO in these cases are discussed in detail, supported by gravity wave spectra observed by HIRDLS. In particular, we find that the gravity wave forcing of the SAO cannot be explained by critical-level filtering alone; gravity wave saturation without critical levels being reached is also important.
Global model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) are analyzed for resolved gravity waves (GWs). Based on fitted 3-D wave vectors of individual waves and using the ...ECMWF global scale background fields, backward ray tracing from 25 km altitude is performed. Different sources such as orography, convection and winter storms are identified. It is found that due to oblique propagation waves spread widely from narrow source regions. Gravity waves which originate from regions of strong convection are frequently excited around the tropopause and have in the ECMWF model low phase and group velocities as well as very long horizontal wavelengths compared to other models and to measurements. While the total amount of momentum flux for convective GWs changes little over season, GWs generated by storms and mountain waves show large day-to-day variability, which has a strong influence also on total hemispheric fluxes; from one day to the next the total hemispheric flux may increase by a factor of 3. Implications of these results for using the ECMWF model in predicting, analyzing and interpreting global GW distributions as well as implications for seamless climate prediction are discussed.
Stratospheric temperature perturbations (TP) that have previously been misinterpreted as due to gravity waves are revisited. The perturbations observed by radio occultations during December 2015 had ...peak‐to‐peak amplitudes of 10 K extending from the equator to midlatitudes. The vertically stacked and horizontally flat structures had a vertical wavelength of 12 km. The signs of the TP were 180∘ phase shifted between equatorial and midlatitudes at fixed altitude levels. High‐resolution operational analyses reveal that these shallow temperature structures were caused by inertial instability due to the large meridional shear of the polar night jet at its equatorward flank in combination with Rossby wave breaking. Large stratospheric TP owing to inertial instability do frequently occur in the Northern Hemisphere (Southern Hemisphere) from October to April (April to October) in the 39 years of ECMWF Re‐Analysis‐Interim data. During 10% of the days, TP exceed 5 K (peak to peak).
Plain Language Summary
The stratosphere is the part of the atmosphere between altitudes of ∼15–50 km which contains the ozone layer that shields life from hazardous radiation. We use global stratospheric temperature measurements to learn about the variability of temperatures on vertical scales < 15 km. Usually, it is thought that such variations are caused by waves that are excited by the displacement of air when being lofted upward when, for example, the wind blows over mountains. The air then starts oscillating around its original height level because of gravity. Gravity waves are an important driver of stratospheric winds which, for example, determine the distribution of ozone. We present observations of large stratospheric temperature perturbations which could easily be misinterpreted as gravity waves. Combining the measurements with output of a numerical weather prediction model, we show that the observations are caused by a large‐scale atmospheric instability called inertial instability. Using meteorological data spanning the past 40 years, we quantify when and how often such temperature perturbations of a certain size occur. Our results are important for properly constructing gravity wave climatologies (where inertial instability events must be excluded)—which are in turn an important input for the correct formulation of climate models.
Key Points
The spatial structure and temporal development of stratospheric inertial instability at 30‐45 degrees north is characterized with GPS radio occultation measurements in December 2015
The observed event is among the 0.1% strongest events in the ERA‐Interim climatology; moderate events occur during 10% of the time
Inertial instability events are a possible source of bias for gravity wave climatologies constructed from temperature measurements
Both global scale waves (e.g., Kelvin, equatorial Rossby, or Rossby‐gravity waves) and mesoscale gravity waves contribute to the wind reversals of the quasi biennial oscillation (QBO). The relative ...contributions of the different wave types are highly uncertain. In our work we quantify the contribution of equatorial Kelvin waves to the reversal from stratospheric easterlies to westerlies averaged over two QBO cycles in the period 2002–2006. Our analysis is based on longitude‐time spectra of temperatures measured by the SABER satellite instrument, as well as temperatures from ECMWF operational analyses. Kelvin waves of zonal wavenumber 1–6 and periods longer than 2.5 days are covered. It is found that the contribution of Kelvin waves is about 30–50% of the observed wind reversal and only 20–35% of the expected total wave forcing. The larger part of the wave forcing therefore has to be contributed by other waves, likely mesoscale gravity waves.
One of the most important dynamical processes in the tropical stratosphere is the quasi‐biennial oscillation (QBO) of the zonal wind. Still, the QBO is not well represented in weather and climate ...models. To improve the representation of the QBO in the models, a better understanding of the driving of the QBO by atmospheric waves is required. In particular, the contribution of gravity waves is highly uncertain because of the small horizontal scales involved, and there is still no direct estimation based on global observations. We derive gravity wave momentum fluxes from temperature observations of the satellite instruments HIRDLS and SABER. Momentum flux spectra observed show that particularly gravity waves with intrinsic phase speeds <30m/s (vertical wavelengths <10km) interact with the QBO. Gravity wave drag is estimated from vertical gradients of observed momentum fluxes and compared to the missing drag in the tropical momentum budget of ERA‐Interim. We find reasonably good agreement between their variations with time and in their approximate magnitudes. Absolute values of observed and ERA‐Interim missing drag are about equal during QBO eastward wind shear. During westward wind shear, however, observations are about 2 times lower than ERA‐Interim missing drag. This could hint at uncertainties in the advection terms in ERA‐Interim. The strong intermittency of gravity waves we find in the tropics might play an important role for the formation of the QBO and may have important implications for the parameterization of gravity waves in global models.
Key Points
Satellite observations of gravity waves (GWs) show QBO‐related variations
In the tropics observed GW drag agrees well with the missing drag in ERA‐Interim
GW observations hint at uncertainties in modeled advection terms
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