[1] The main aim of this study is to find and classify hotspots of stratospheric gravity waves on a global scale. The analysis is based on a 9 year record (2003 to 2011) of radiance measurements by the Atmospheric Infrared Sounder (AIRS) aboard NASA's Aqua satellite. We detect gravity waves based on 4.3 mm brightness temperature variances. Our method focuses on peak events, i.e., strong gravity wave events for which the local variance considerably exceeds background levels. We estimate the occurrence frequencies of these peak events for different seasons and time of day and use the results to find local maxima or "hotspots." In addition, we use AIRS radiances at 8.1 mm to simultaneously detect convective events, including deep convection in the tropics and mesoscale convective systems at middle latitudes. We classify the gravity wave sources based on seasonal occurrence frequencies for convection, but also by means of time series analyses and topographic data. Our study reproduces well-known hotspots of gravity waves, e.g., the Andes and the Antarctic Peninsula. However, the high horizontal resolution of the AIRS observations also allows us to locate numerous mesoscale hotspots, which are partly unknown or poorly studied so far. Most of these mesoscale hotspots are found near orographic features like mountain ranges, coasts, lakes, deserts, or isolated islands. This study will help to select promising regions and seasons for future case studies of gravity waves.Citation: Hoffmann, L., X. Xue, and M. J. Alexander (2013), A global view of stratospheric gravity wave hotspots located with Atmospheric Infrared Sounder observations,

[1] The Atmospheric Infrared Sounder (AIRS) on board the National Aeronautics and Space Administration's (NASA's) Aqua satellite has been continuously measuring mid-infrared nadir and sub-limb radiance spectra since summer of 2002. These measurements are utilized to retrieve three-dimensional stratospheric temperature distributions by applying a new fast forward model for AIRS and an accompanying optimal estimation retrieval processor. The retrieval scheme presented in this article does not require simultaneous observations of microwave instruments like the AIRS operational analyses. Instead, independent retrievals are carried out at the full horizontal sampling capacity of the instrument. Horizontal resolution is enhanced by a factor 3 in along-and across-track directions compared with the AIRS operational data. The total retrieval error of the individual temperature measurements is 1.6 to 3.0 K in the altitude range from 20 to 60 km. Retrieval noise is 1.4 to 2.1 K in the same vertical range. Contribution of a priori information to the retrieval results is less than 1% to 2% and the vertical resolution of the observations is about 7 to 15 km. The temperature measurements are successfully compared with ECMWF operational analyses and AIRS operational Level 2 data. The new temperature data set is well suited for studies of stratospheric gravity waves. We present AIRS observations of small-scale gravity waves induced by deep convection near Darwin, Australia, in January 2003. A strong mountain wave event over the Andes in June 2005 is analyzed in detail. Temperature perturbations derived from the new data set are compared with results from the AIRS operational Level 2 data and coincident measurements of the High Resolution Dynamics Limb Sounder (HIRDLS). The new retrieval does not show response to wave perturbations if the vertical wavelength is below 10 km. For 15 km vertical wavelength, the amplitudes are damped by a factor of two. For vertical wavelengths of greater than 20 km, AIRS shows very similar wave structure to HIRDLS and also has the advantage of providing horizontal phase front information. Data from the new full-resolution retrieval are far more suitable for gravity wave studies than results from the AIRS operational analysis.Citation: Hoffmann, L., and M. J. Alexander (2009), Retrieval of stratospheric temperatures from Atmospheric Infrared Sounder radiance measurements for gravity wave studies,

Abstract. The European Centre for Medium-Range Weather Forecasts' (ECMWF's) next-generation reanalysis ERA5 provides many improvements, but it also confronts the community with a “big data” challenge. Data storage requirements for ERA5 increase by a factor of ∼80 compared with the ERA-Interim reanalysis, introduced a decade ago. Considering the significant increase in resources required for working with the new ERA5 data set, it is important to assess its impact on Lagrangian transport simulations. To quantify the differences between transport simulations using ERA5 and ERA-Interim data, we analyzed comprehensive global sets of 10-day forward trajectories for the free troposphere and the stratosphere for the year 2017. The new ERA5 data have a considerable impact on the simulations. Spatial transport deviations between ERA5 and ERA-Interim trajectories are up to an order of magnitude larger than those caused by parameterized diffusion and subgrid-scale wind fluctuations after 1 day and still up to a factor of 2–3 larger after 10 days. Depending on the height range, the spatial differences between the trajectories map into deviations as large as 3 K in temperature, 30 % in specific humidity, 1.8 % in potential temperature, and 50 % in potential vorticity after 1 day. Part of the differences between ERA5 and ERA-Interim is attributed to the better spatial and temporal resolution of the ERA5 reanalysis, which allows for a better representation of convective updrafts, gravity waves, tropical cyclones, and other meso- to synoptic-scale features of the atmosphere. Another important finding is that ERA5 trajectories exhibit significantly improved conservation of potential temperature in the stratosphere, pointing to an improved consistency of ECMWF's forecast model and observations that leads to smaller data assimilation increments. We conducted a number of downsampling experiments with the ERA5 data, in which we reduced the numbers of meteorological time steps, vertical levels, and horizontal grid points. Significant differences remain present in the transport simulations, if we downsample the ERA5 data to a resolution similar to ERA-Interim. This points to substantial changes of the forecast model, observations, and assimilation system of ERA5 in addition to improved resolution. A comparison of two Lagrangian trajectory models allowed us to assess the readiness of the codes and workflows to handle the comprehensive ERA5 data and to demonstrate the consistency of the simulation results. Our results will help to guide future Lagrangian transport studies attempting to navigate the increased computational complexity and leverage the considerable benefits and improvements of ECMWF's new ERA5 data set.

The January 2022 Hunga Tonga–Hunga Ha’apai eruption was one of the most explosive volcanic events of the modern era1,2, producing a vertical plume that peaked more than 50 km above the Earth3. The initial explosion and subsequent plume triggered atmospheric waves that propagated around the world multiple times4. A global-scale wave response of this magnitude from a single source has not previously been observed. Here we show the details of this response, using a comprehensive set of satellite and ground-based observations to quantify it from surface to ionosphere. A broad spectrum of waves was triggered by the initial explosion, including Lamb waves5,6 propagating at phase speeds of 318.2 ± 6 m s−1 at surface level and between 308 ± 5 to 319 ± 4 m s−1 in the stratosphere, and gravity waves7 propagating at 238 ± 3 to 269 ± 3 m s−1 in the stratosphere. Gravity waves at sub-ionospheric heights have not previously been observed propagating at this speed or over the whole Earth from a single source8,9. Latent heat release from the plume remained the most significant individual gravity wave source worldwide for more than 12 h, producing circular wavefronts visible across the Pacific basin in satellite observations. A single source dominating such a large region is also unique in the observational record. The Hunga Tonga eruption represents a key natural experiment in how the atmosphere responds to a sudden point-source-driven state change, which will be of use for improving weather and climate models.

[1] Convective gravity waves are an important driver of the equator-to-pole circulation in the stratospheric summer hemisphere, but their nature is not well known. Previous studies showing tight relationships between deep convection and convective waves mainly focus on tropical latitudes. For midlatitudes most analyses are based on case studies. Here we present a new multiyear occurrence frequency analysis of convective waves at midlatitudes. The study is based on radiance measurements made by the Atmospheric Infrared Sounder (AIRS) satellite experiment during the North American thunderstorm season, May to August, in the years [2003][2004][2005][2006][2007][2008]. For this study we optimized an existing algorithm to detect deep convection in AIRS data to be applicable at midlatitudes. We also present a new detection algorithm for gravity waves in AIRS data based on a variance filter approach for 4.3 mm brightness temperatures. The new algorithm can detect plane wave perturbations in the altitude range from 20 to 65 km with vertical wavelengths larger than 15 km and horizontal wavelengths from 50 to 1000 km. By analyzing spatial and temporal correlations of the individual AIRS observations, it can be shown that more than 95% of the observed gravity waves in a core region over the North American Great Plains are related to deep convective clouds, i.e., are likely being classified appropriately as convective waves. We conclude that the core region is a good location to observe and characterize the properties of convective waves at midlatitudes. The statistical analyses presented here are also valuable to validate parameterization schemes for convective gravity waves. For completeness, it should be mentioned that our analyses cover not only the U.S. Midwest but the North American continent as well as the surrounding ocean regions in general. Our analysis also reveals interesting details about tropical convection and related gravity wave activity, as well as the capability of the AIRS instrument to observe these.