Stratospheric Wind Research Articles

Generation of a Quasi-Biennial Oscillation in an NWP Model Using a Stochastic Gravity Wave Drag Parameterization

Abstract Many operational numerical weather prediction (NWP) systems now extend into the stratosphere and are beginning to be used to generate forecasts beyond conventional 5–10-day periods out to seasonal time scales. Past observational and modeling studies have shown that the quasi-biennial oscillation (QBO) in equatorial stratospheric winds can play an important role in stratosphere–troposphere dynamical coupling over these longer time scales. Consequently, stratosphere-resolving NWP models used to generate seasonal forecasts should contain the necessary physics to generate and maintain the QBO. This study describes several key modifications that were necessary to produce a QBO in a high-altitude NWP model, which include an increase in model vertical resolution, implementation of a computationally efficient stochastic gravity wave drag parameterization, and reductions in the amount of horizontal and vertical diffusion in the stratosphere. Results from a 10-yr free-running model simulation with these modifications show that the westerly QBO phase produces lower temperatures and stronger westerly flow in the Northern Hemisphere (NH) winter polar stratosphere compared to the easterly QBO phase. Ensembles of 120-day simulations over the December–March period show that these modifications replace persistent easterly flow in the equatorial lower stratosphere with a more realistic transition from easterly to westerly flow. The resulting changes in planetary wave propagation produce a statistically significant response in the dynamics of the NH extratropical stratosphere consistent with the Holton–Tan relationship. The westerly shift in equatorial winds also produces a significant response in the NH extratropical troposphere, where the sea level pressure differences in winter resemble the positive phase of the northern annular mode.

A method for establishing a long duration, stratospheric platform for astronomical research

During certain times of the year at middle and low latitudes, winds in the upper stratosphere move in nearly the opposite direction than the wind in the lower stratosphere. Here we present a method for maintaining a high-altitude balloon platform in near station-keeping mode that utilizes this stratospheric wind shear. The proposed method places a balloon-borne science platform high in the stratosphere connected by a lightweight, high-strength tether to a "tug" vehicle located in the lower or middle stratosphere. Using aerodynamic control surfaces, wind-induced aerodynamic forces on the tug can be manipulated to counter the wind drag acting on the higher altitude science vehicle, thus controlling the upper vehicle's geographic location. We describe the general framework of this station-keeping method, some important properties required for the upper stratospheric science payload and lower tug platforms, and compare this station-keeping approach with the capabilities of a high altitude airship and conventional tethered aerostat approaches. We conclude by discussing the advantages of such a platform for a variety of missions with emphasis on astrophysical research.

Observations of planetary waves in the mesosphere-lower thermosphere during stratospheric warming events

Abstract. This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere–lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150° W to 25° E and latitudes from 51 to 66° N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000–2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal.

Incorporating atmospheric uncertainties into estimates of the detection capability of the IMS infrasound network

To monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT), a dedicated network is being deployed. Multi-year observations recorded by the International Monitoring System (IMS) infrasound network confirm that its detection capability is highly variable in space and time. Today, numerical modeling techniques provide a basis to better understand the role of different factors describing the source and the atmosphere that influence propagation predictions. Previous studies estimated the radiated source energy from remote observations using frequency dependent attenuation relation and state-of-the-art specifications of the stratospheric wind. In order to account for a realistic description of the dynamic structure of the atmosphere, model predictions are further enhanced by wind and temperature error distributions as measured in the framework of the ARISE project. In the context of the future verification of the CTBT, these predictions quantify uncertainties in the spatial and temporal variabilities of the IMS infrasound network performance in higher resolution, and will be helpful for the design and prioritizing maintenance of any arbitrary infrasound monitoring network.

The European Arctic: A Laboratory for Seismoacoustic Studies

The International Monitoring System (IMS) for verifying compliance with the Comprehensive Nuclear‐Test‐Ban Treaty (CTBT) comprises sensors associated with four monitoring technologies: seismic, infrasound, hydroacoustic, and radionuclide. The so‐called waveform technologies (seismic, infrasound, and hydroacoustic) are used to detect and locate events that could constitute treaty violations. All four technologies may be employed to investigate the nature of events, with the network of radionuclide sensors in place to provide evidence of a nuclear explosion. Historical, political, and technical issues surrounding the CTBT are discussed by Dahlman et al. (2009, 2011). The global IMS infrasound network (Fig. 1) is primarily to detect signals generated by atmospheric nuclear tests. Figure 1. Status of the International Monitoring System infrasound network in August 2014. Filled symbols are certified stations sending data to the International Data Center (IDC) in Vienna. White symbols indicate the treaty coordinates of stations planned or under construction. The IMS infrasound network has been deployed over the last two decades (Brachet et al. , 2010; Christie and Campus, 2010), and only with the network approaching completion has a realistic picture of its detection capability emerged (Le Pichon et al. , 2009; Green and Bowers, 2010). The detectability of atmospheric signals is governed by a seasonally varying wind‐determined anisotropy. In the northern summer, the stratospheric winds blow predominantly east to west, facilitating the detection of infrasound at stations west of sources and inhibiting the detection at stations east of sources. In the northern winter, the winds blow in the opposite direction changing the sense of high and low detectability. The reverse patterns occur in the southern hemisphere. There is increasing interest in using infrasound for probing atmospheric structure (e.g., Lalande et al. , 2012), and the broader properties and applications of infrasound are discussed by Evers and Haak (2009) and Hedlin et al. (2012). In October …

The statistics of precipitation and droughts during opposite phases of the quasi-biennial oscillation of atmospheric processes and its relation to the yield in the European part of Russia

As a result of the analysis of the data from weather stations and the values of Palmer Drought Severity Index (PDSI), territorial regularities are revealed in the distribution of total ten-day precipitation, frequency of severe atmospheric droughts in May and June, and spring wheat yield in the European part of Russia depending on the phase of the quasi-biennial oscillation of equatorial stratospheric wind (QBO) in 1953–2010. It is demonstrated that the variations can be explained by differences in the statistics of synoptic cyclonic and anticyclonic vortices during different QBO phases. The spatial structure of differences in ten-day precipitation and frequency of severe droughts on the European part of Russia during opposite phases of the QBO agrees with differences in spring wheat yield. The QBO was considered in the modeling of spring wheat yield in the Rostov oblast, and this allowed improvement of prognostic values of yield by 14%. The results of the simulation of spring wheat yield with account of the quasi-biennial oscillation of stratospheric equatorial wind as a predictor with the relative spatiotemporal stability of values can be used for working out new methods for forecasting the crop yield.

Northern Hemisphere stratospheric winds in higher midlatitudes: longitudinal distribution and long-term trends

Abstract. The Brewer–Dobson circulation (mainly meridional circulation) is very important for stratospheric ozone dynamics and thus for the overall state of the stratosphere. There are some indications that the meridional circulation in the stratosphere could be longitudinally dependent, which would have an impact on the ozone distribution. Therefore, we analyse here the meridional component of the stratospheric wind at northern middle latitudes to study its longitudinal dependence. The analysis is based on the NCEP/NCAR-1 (National Centers for Environmental Prediction and the National Center for Atmospheric Research), MERRA (Modern Era-Retrospective Analysis) and ERA-Interim (European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis Interim) reanalysis data. The well-developed two-core structure of strong but opposite meridional winds, one in each hemisphere at 10 hPa at higher northern middle latitudes, and a less pronounced five-core structure at 100 hPa are identified. In the central areas of the two-core structure the meridional and zonal wind magnitudes are comparable. The two-core structure at 10 hPa is almost identical for all three different reanalysis data sets in spite of the different time periods covered. The two-core structure is not associated with tides. However, the two-core structure at the 10 hPa level is related to the Aleutian pressure high at 10 hPa. Zonal wind, temperature and the ozone mixing ratio at 10 hPa also exhibit the effect of the Aleutian high, which thus affects all parameters of the Northern Hemisphere middle stratosphere. Long-term trends in the meridional wind in the "core" areas are significant at the 99% level. Trends of meridional winds are negative during the period of ozone depletion development (1970–1995), while they are positive after the ozone trend turnaround (1996–2012). Meridional wind trends are independent of the sudden stratospheric warming (SSW) occurrence and the quasi-biennial oscillation (QBO) phase. The influence of the 11-year solar cycle on stratospheric winds has been identified only during the west phase of QBO. The well-developed two-core structure in the meridional wind illustrates the limitations of application of the zonal mean concept in studying stratospheric circulation.

Annual and semiannual harmonics of wind in the Northern stratosphere, mesosphere, and lower thermosphere

Based on the UK MetOffice gridded analysis in the altitudes from the tropopause to the mesopause of the Northern Hemisphere and the meteor radar observations in the mesosphere/lower thermosphere over Kazan (56°N49°E) and Collm (51°N13°E), the annual and semiannual harmonics of the horizontal wind components in the stratosphere, mesosphere, and lower thermosphere are studied for the period 2004–2013. The maxima of the amplitude of the annual harmonics of zonal wind are much more pronounced than the respective maxima for meridional wind. In contrast, the magnitudes of the maxima of the semiannual harmonics are comparable between zonal and meridional wind. The annual harmonics of horizontal wind in the studied layer typically reaches maximum in January–February. The semiannual harmonics of the components of horizontal wind in the stratosphere–lower thermosphere layer basically attains it first maximum in spring or in early summer. The results, included in the present paper, may be used for climate models validation.

Middle-atmospheric zonal and meridional wind profiles from polar, tropical and midlatitudes with the ground-based microwave Doppler wind radiometer WIRA

Abstract. WIRA is a ground-based microwave Doppler spectroradiometer specifically designed for the measurement of profiles of horizontal wind in the upper stratosphere and lower mesosphere region where no other continuously running measurement technique exists. A proof of principle has been delivered in a previous publication. A technical upgrade including a new high-frequency amplifier and sideband filter has improved the signal to noise ratio by a factor of 2.4. Since this upgrade the full horizontal wind field comprising zonal and meridional wind profiles is continuously measured. A completely new retrieval based on optimal estimation has been set up. Its characteristics are detailed in the present paper. Since the start of the routine operation of the first prototype in September 2010, WIRA has been measuring at four different locations at polar, mid- and tropical latitudes (67°22' N/26°38' E, 46°57' N/7°26' E, 43°56' N/5°43' E and 21°04' S/55°23' E) for time periods between 5.5 and 11 months. The data presented in this paper are daily average wind profiles with typical uncertainties and resolutions of 10 to 20 m s−1 and 10 to 16 km, respectively. A comparison between the data series from WIRA and European Centre for Medium-Range Weather Forecasts (ECMWF) model data revealed agreement within 10% in the stratospheric zonal wind. The meridional wind profiles agree within their error bars over the entire sensitive altitude range of WIRA. However, significant differences in the mesospheric zonal wind speed of up to 50% have been found.

Orographic Wave Drag over the Southern Ocean: A Linear Theory Perspective

Abstract Recent studies suggest that stratospheric wind biases in global and climate models in the Southern Hemisphere may result from insufficient orographic wave drag, particularly over the Southern Ocean in the latitude belt centered near 60°S. In this study, contributions to the stratospheric wave drag along 60°S from three neighboring orographic wave sources are evaluated using a multiple-layer linear wave model with large-scale wind and stratification profiles derived from the Interim ECMWF Re-Analysis (ERA-Interim) between the years 1991 and 2010. The orographic wave sources include the Patagonian peaks in the southern Andes, the Antarctic Peninsula, and the island of South Georgia. The climatological and dynamical aspects of the wave drag and its dependence on tropospheric winds are investigated. The results suggest that these orographic wave sources may have significant contributions to the stratospheric drag over the Southern Ocean through meridional spreading of the wave momentum flux aloft associated with three-dimensional wave propagation. Among the three locations considered, the wave drag from the Antarctic Peninsula is substantially larger than that from Patagonia and nearly two orders of magnitude larger than that from South Georgia island. The orographic wave drag is in general proportional to the westerly component of the surface winds and becomes virtually zero when the surface winds have an easterly component, associated with critical level absorption between the tropospheric easterlies and prevailing westerlies in the stratosphere. The derived wave drag exhibits substantial temporal variations, including synoptic-scale, month-to-month, and interannual variations.

Wave Activity Events and the Variability of the Stratospheric Polar Vortex

Abstract During Northern Hemisphere winter, polar stratospheric winds and temperatures exhibit significant variability that is due to the vertical propagation of planetary-scale waves. The most dramatic intraseasonal variations in temperature are associated with sudden stratospheric warmings (SSWs), which are wave-breaking events that occur approximately every other year. This paper will introduce the concept of wave activity events (WAEs), which are periods of enhanced pseudomomentum density in the polar stratosphere that occur every year. It will be demonstrated that all SSWs are associated with WAEs; furthermore, minor warmings and many final warmings in the polar spring are also WAEs, and therefore a better understanding of these more frequent wave events can provide additional insights into stratospheric wave-induced variability. Employing the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) for 1979–2011, 119 WAEs are identified and their life cycle is compared with that of the 23 SSWs observed during this period.

Observational evidence for temporary planetary wave forcing of the MLT during fall equinox

AbstractWe present direct observations of zonal wave numbers 1 and 2 planetary wave activity in the mesopause region derived from a longitudinal chain of high‐latitude Northern Hemisphere (51–66°N) Super Dual Auroral Radar Network radars. Over a 9 year period (2000–2008), the planetary wave activity observed shows a consistent increase around the fall equinox. This is shown to be coincident with a minimum in the magnitude of the stratospheric winds and consequently a minimum in the stratospheric gravity wave filtering and the subsequent momentum deposition in the mesopause region. Despite this, the observed meridional winds are shown to be perturbed poleward and mesopause temperatures rise temporarily, suggesting that westward momentum deposition from planetary waves temporarily becomes the dominant forcing on the mesopause region each fall equinox.

Seasonal variations of infrasound detections and their characteristics in the western US

Automatic infrasound detection is analyzed with infrasonic data from the western US for the time period of November 2010 to October 2012. Data from nine University of Utah Seismograph Stations (UUSS) infrasonic arrays are supplemented by three additional arrays in Nevada, operated by Southern Methodist University (SMU). In this study, the detection procedure is based on an adaptive F-detector (AFD) that accounts for both correlated and uncorrelated noise by capturing the time-varying background noise conditions. The adaptation of the detector, characterized by time varying linear remapping of the F-distribution (C-value), depends on the background noise level at the arrays including weather conditions with seasonal variations and local site effects. The infrasound detection catalog consists of 580,177 signals, depending on time and space as well as data availability at individual stations. Based on the Ground-to-Space (G2S) specifications, the number of detections is shown to be correlated with seasonal variations in the stratospheric winds, illustrating the time varying nature of infrasound propagation. These seasonal variations of infrasound detections include the number of detections, correlation value, occurrences in time, phase velocity, and azimuth. The majority of events occur during working hours, Monday through Friday, suggesting a human cause. Based on the automatic detection results, we reviewed the waveforms generated from the known repeated sources such as Dugway Testing Ground (DTG) and Utah Test and Training Range (UTTR) in Utah, and New Bomb (NB) in Nevada. In order to validate the automatic infrasound detection catalog, we document the celerities of automatic infrasound arrivals during the summer and winter using the seismic origin times of NB explosions. Infrasound propagation is dependent on the seasonal variation of wind conditions as well as source location and array distribution.

Assessing model representations of stratospheric equatorial waves

Some westward‐propagating Rossby‐gravity waves and eastward Kelvin waves can travel up through the atmosphere following their formation in the tropical troposphere. Once they reach the stratosphere, these waves dissipate, releasing their energy. Known as stratospheric equatorial waves, these Rossby‐gravity and Kelvin waves are an important aspect driving the quasi‐biennial oscillation (QBO), a roughly monthly reversal, in the direction of the equatorial stratospheric winds. Similar to other atmospheric systems, such as the El Niño–Southern Oscillation or the North Atlantic Oscillation, the phase of the QBO affects global weather patterns, particularly in North America and western Europe.

Northern winter climate change: Assessment of uncertainty in CMIP5 projections related to stratosphere-troposphere coupling

Future changes in the stratospheric circulation could have an important impact on northern winter tropospheric climate change, given that sea level pressure (SLP) responds not only to tropospheric circulation variations but also to vertically coherent variations in troposphere-stratosphere circulation. Here we assess northern winter stratospheric change and its potential to influence surface climate change in the Coupled Model Intercomparison Project-Phase 5 (CMIP5) multimodel ensemble. In the stratosphere at high latitudes, an easterly change in zonally averaged zonal wind is found for the majority of the CMIP5 models, under the Representative Concentration Pathway 8.5 scenario. Comparable results are also found in the 1% CO2 increase per year projections, indicating that the stratospheric easterly change is common feature in future climate projections. This stratospheric wind change, however, shows a significant spread among the models. By using linear regression, we quantify the impact of tropical upper troposphere warming, polar amplification, and the stratospheric wind change on SLP. We find that the intermodel spread in stratospheric wind change contributes substantially to the intermodel spread in Arctic SLP change. The role of the stratosphere in determining part of the spread in SLP change is supported by the fact that the SLP change lags the stratospheric zonally averaged wind change. Taken together, these findings provide further support for the importance of simulating the coupling between the stratosphere and the troposphere, to narrow the uncertainty in the future projection of tropospheric circulation changes.

Automatic infrasound detection and location of sources in the western United States

A catalog that characterizes sources of regional infrasound observed in the western U.S. (1 November 2010 to 31 October 2012) is produced. Data from nine University of Utah Seismograph Stations infrasonic arrays are supplemented by three additional arrays in Nevada, operated by Southern Methodist University. The detection procedure using an adaptive F-detector provides input into the Bayesian Infrasonic Source Location procedure. The catalog consists of 1510 events with indication of repeated events from many locations such as Dugway Testing Ground, Utah Test and Training Range, and New Bomb. We analyzed the relationship between seasonal variations in the event locations and wind conditions using the Ground-to-Space specifications based on publicly available operational numerical weather prediction data analysis products supplement by empirical models above 80 km. There is significant commonality between this study's bulletin and the Western United States Infrasonic Catalog published by Walker et al. (2011). A previous study utilized infrasound signals detected on the USArray Transportable Array seismic stations (2007–2008). Both results document the vast majority of events that occur during working hours, suggesting a human cause. To illustrate the utility of the event bulletin for exploring atmospheric dynamics, propagation paths of an event detected during the equinox period, when the stratospheric wind is low, were generated using a ray-tracing algorithm. We found that the observations contain stratospheric arrivals, not predicted by ray theory, possibly due to gravity waves increasing the effective jet speed.

Nonmigrating tidal variability in the SABER/TIMED mesospheric ozone

This paper presents for the first time evidence showing nonmigrating tidal variations in the mesospheric ozone (O3) derived from the Sounding of the Atmosphere using Broadband Emission Radiometry/Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (SABER/TIMED) for a full 11 year period, 2002–2012. The O3 tidal fields are extracted from the data by the same method as the temperature tides have been derived. The spatial distribution and seasonal variability of the three strongest nonmigrating O3 tidal variabilities, i.e., SW3, DW2, and DE3, are shown. They demonstrate repeatable presence each year. These O3 tidal variations have large amplitudes at the seasons and latitudes for which the respective temperature (T) tides amplify, i.e., near the equator and during the equinoxes. The phases of the T and O3 tidal signatures are out of phase above 95 km. This phase relationship no longer holds for tidal perturbations below about 90 km. The O3 SW3 and DW2 tidal variations have similar interannual variabilities that appear to follow El Niño–Southern Oscillation variability. The O3 DE3 tidal field, however, has a clear biyearly interannual variability as the biyearly maxima correlate with the westerly phase of the quasi-biennial oscillation in tropical stratospheric winds but only up to 2008.

On the detection of the solar signal in the tropical stratosphere

Abstract. We investigate the relative role of volcanic eruptions, El Niño–Southern Oscillation (ENSO), and the quasi-biennial oscillation (QBO) in the quasi-decadal signal in the tropical stratosphere with regard to temperature and ozone commonly attributed to the 11 yr solar cycle. For this purpose, we perform transient simulations with the Whole Atmosphere Community Climate Model forced from 1960 to 2004 with an 11 yr solar cycle in irradiance and different combinations of other forcings. An improved multiple linear regression technique is used to diagnose the 11 yr solar signal in the simulations. One set of simulations includes all observed forcings, and is thereby aimed at closely reproducing observations. Three idealized sets exclude ENSO variability, volcanic aerosol forcing, and QBO in tropical stratospheric winds, respectively. Differences in the derived solar response in the tropical stratosphere in the four sets quantify the impact of ENSO, volcanic events and the QBO in attributing quasi-decadal changes to the solar cycle in the model simulations. The novel regression approach shows that most of the apparent solar-induced lower-stratospheric temperature and ozone increase diagnosed in the simulations with all observed forcings is due to two major volcanic eruptions (i.e., El Chichón in 1982 and Mt. Pinatubo in 1991). This is caused by the alignment of these eruptions with periods of high solar activity. While it is feasible to detect a robust solar signal in the middle and upper tropical stratosphere, this is not the case in the tropical lower stratosphere, at least in a 45 yr simulation. The present results suggest that in the tropical lower stratosphere, the portion of decadal variability that can be unambiguously linked to the solar cycle may be smaller than previously thought.

Mesosphere and lower thermosphere zonal wind variations over low latitudes: Relation to local stratospheric zonal winds and global circulation anomalies

AbstractLong‐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.

Infrasound signals from the underground nuclear explosions of North Korea

We investigated the infrasound signals from seismic ground motions induced by North Korea's underground nuclear explosions, including the recent third explosion on 2013 February 12. For the third explosion, the epicentral infrasound signals were detected not only by three infrasound network stations (KSGAR, ULDAR and YAGAR) in South Korea but also by two nearby International Monitoring System infrasound stations, IS45 and IS30. The detectability of the signals was limited at stations located on the relatively east side of the epicentre, with large azimuth deviations due to very favourable atmospheric conditions for eastward propagation at stratospheric height in 2013. The stratospheric wind direction was the reverse of that when the second explosion was conducted in 2009 May. The source location of the epicentral infrasound with wave parameters determined at the multiple stations has an offset by about 16.6 km from the reference seismic location. It was possible to determine the infrasonic location with moderate accuracy by the correction of the azimuth deviation due to the eastward winds in the stratosphere. In addition to the epicentral infrasonic signals, diffracted infrasound signals were observed from the second underground nuclear explosion in 2009. The exceptional detectability of the diffracted infrasound was a consequence of the temporal formation of a thin atmospheric inversion layer over the ocean surface when the event occurred.