Presence Of Atmospheric Gravity Waves Research Articles

Study of atmospheres in the solar system, from stellar occultation or planetary transit

Stellar occultations and transits occur when a planetary body passes in front of a star (including our Sun). For objects with an atmosphere, refraction plays an essential role to explain the drops of flux and the aureoles observed during these events. This can be used to derived key parameters of the atmospheres, such as their density, pressure and temperature profiles, as well as the presence of atmospheric gravity waves and zonal winds. Here we derive from basic principles the equations that rule the ray propagation in planetary atmospheres, and we show how they can be used to derive the physical parameters of these atmospheres.

Investigating the Development of Distinctive Subauroral Flow Channels During the November 7–8, 2004 Superstorm

AbstractWe investigate the development of large (∼3,000 m/s) subauroral flows occurring during the November 7–8, 2004 Superstorm by utilizing multisatellite and multiinstrument measurements from the Magnetosphere (M), topside Ionosphere (I), and Thermosphere (T). We present eight scenarios (Sc‐1–Sc‐8) depicting the development of large SubAuroral Polarization Streams (SAPS) in the initial phase (Sc‐1–Sc‐3) and recovery phase (Sc‐5), and structured SAPS (Sc‐4, Sc‐6–Sc‐7) and Double‐peak SubAuroral Ion Drifts (DSAID) in the recovery phase (Sc‐8). Magnetospheric observations reveal dayside magnetopause reconnection increasing both the magnetospheric convection electric field and the large‐scale Field‐Aligned Currents (FACs), long‐lasting (∼5 h) secondary dipolarizations during the initial phase, and a series of dipolarization events during the recovery phase. Ionospheric observations reveal the development of these large subauroral flows (i) during forward and reverse plasma convections—when the topside ionosphere received earthward energy from the magnetosphere and when the diversion of cross‐tail currents in the stretched magnetotail activated the Substorm Current Wedge (SCW)—and (ii) in a short‐circuited system that acted as a voltage generator. Thermospheric observations reveal earthward energy deposition from the ionosphere that drove wind surges—implying the presence of Atmospheric Gravity Waves (AGWs) and Traveling Ionospheric Disturbances (TIDs)—propagating equatorward/poleward and passing through the trough region, and further enhancing the subauroral flows by deepening the trough. From these results we conclude that the strong M‐I‐T coupling occurring under superstorm conditions played a significant role in the enhancement of the subauroral flows investigated.

Observational signatures of unusual outgoing longwave radiation (OLR) and atmospheric gravity waves (AGW) as precursory effects of May 2015 Nepal earthquakes

Earthquake preparation processes may start 1–30 days before its actual occurrence. Measurements of outgoing longwave radiation (OLR) and detection of the presence of atmospheric gravity waves (AGW) in very low frequency (VLF) radio signals can be used as tools to identify such processes. We studied these signals monitored prior to a recent major earthquake that occurred in Nepal at southeast of Kodari on May 12, 2015 at 12:50 pm local time (07:05 UTC) with Richter scale magnitude of M = 7.3 and depth 10 km (6.21 miles). It was preceded by another major earthquake on April 25, 2015 with magnitude M = 7.9. First, to study the effects of seismic events on OLR, we used NOAA/IR daily (two degree gridded) data from April 16 to May 30, 2015 and followed the method of Eddy field calculation mean to find pre-seismic anomalies. We found singularities in Eddy field OLR curves around the earthquake epicenter starting 3 days prior to the earthquake days and disappearance of such singularities after the events. Such singularities can be associated with a large amount of energy released by the earthquakes. Second, we analyzed very low frequency (VLF) data recorded at Ionospheric and Earthquake Research Centre (IERC) of Indian Centre for Space Physics transmitted from JJI (22.2 kHz) station of Japan. We looked for the presence of atmospheric gravity waves in the ionosphere which can be considered as an important factor in finding seismo-ionospheric correlations. We performed both fast Fourier transform (FFT) and wavelet analysis on the signal and found significant presence of such waves (periods of almost 1 h) four days before the earthquake.

Imaging of atmospheric gravity waves in the stratosphere and upper mesosphere using satellite and ground‐based observations over Australia during the TWPICE campaign

During the Tropical Warm Pool International Cloud Experiment (TWPICE) an intense tropical low was situated between Darwin and Alice Springs, Australia. Observations made on 31 January 2006 by the Atmospheric Infrared Sounder instrument on the NASA Aqua satellite imaged the presence of atmospheric gravity waves (AGWs), at approximately 40 km altitude, with horizontal wavelengths between 200 and 400 km that were originating from the region of the storm. Airglow images obtained from Alice Springs (about 600 km from the center of the low) showed the presence of similar waves with observed periods of 1 to 2 h. The images also revealed the presence of 30‐ to 45‐km‐horizontal‐wavelength AGWs with shorter observed periods of near 15 to 25 min. Ray tracing calculations show that (1) some of the long wavelength waves traveled on rays, without ducting, to the altitudes where the observations were obtained, and (2) shorter‐period waves rapidly reached 85 km altitude at a horizontal distance close to the storm, thus occurring over Alice Springs only if they were trapped or ducted. The mesospheric inversion layer seen in the measured temperature data almost forms such a trapped region. The winds therefore critically control the formation of the trapped region. Wind profiles deduced from the available data show the plausibility for the formation of such a trapped region. Variations in the wind, however, would make ideal trapped region conditions short‐lived, and this may account for the sporadic nature of the short‐period wave observations.

Vertical group and phase velocities of ionospheric waves derived from the MU radar

The middle‐ and upper‐atmosphere (MU) radar (34.85°N, 136.10°E) was operated in the incoherent scatter power‐only mode to observe the ionosphere during 17 June 2001. Pronounced 200‐ to 300‐min. waves in the echo power appeared in the F2 region during 0240–1250 local time on 17 June 2001. A procedure of Fourier analyses is conducted to derive power spectra, vertical phase, and group velocities of the pronounced waves. Results show that the center frequency of the waves is a function of the wave number. The opposite directions of the vertical phase and group velocities imply the presence of atmospheric gravity waves.

Interannual variations of the mean wind and gravity wave variances in the middle atmosphere over Hawaii

Using simple numerical filters, wind variances with time scales of 0.1 –1 h and 1 –5 h are estimated from medium frequency radar wind observations at altitudes 70 –94 km over Hawaii (22°N, 160°W) during the years 1990–2000. The results show interannual variations of the mean wind and variances. The variances can be attributed to the presence of atmospheric gravity waves (GWs). At altitudes below 80 km a maximum of GW intensity was observed between the years 1992 and 1996. An average seasonal cycle in wind variances is calculated by compositing over the 11 years of data. The monthly residual values, obtained after subtraction of the average seasonal variations show quasi-biennial (∼27 month) and longer period oscillations. Maximum variances are observed in 1993 and 1998–1999 at altitudes above ∼84 km . A correlation is found between GW wind variances and the Southern Oscillation Index, which in turn reflects El Niño activity in the tropical Pacific. However, more analysis with several El Niño events is required to establish the correlation with greater confidence.

The influence of atmospheric gravity waves evolving in horizontal shear flow on the ionosphere F2-layer

The ionosphere F2-layer height profile of the electron density is obtained by using the presence of atmospheric gravity waves (GW) evolving in horizontal shear flow. The possible responsibility of the GW evolving in the horizontal shear flow for the formation of large scale travelling ionospheric disturbance (TID) with small periods is shown.

The Influence of Gravity Waves on Radiometric Measurements: A Case Study

During the 1978 PHOENIX experiment at the Boulder Atmospheric Observatory in Colorado, the presence of atmospheric gravity waves was detected by various independent remote sensing instruments. Fluctuations in the zenith atmospheric radiation were measured at 22.235 and 55.45 GHz in the water vapor and oxygen absorption bands and compared with corresponding fluctuations of surface pressure and the height of FM-CW radar echo returns. These fluctuations are explained, qualitatively and quantitatively, in terms of an internal gravity wave generated by wind shear above the boundary layer. The analysis shows that the oscillations at 22.235 GHz are essentially due to fluctuations of water vapor in the antenna beam while those at 55.45 GHz are due to temperature variations.

An observational study of lee waves using radiosonde data

Corby (1957) showed that radiosonde data could be used to detect departures of the ver- tical velocity of the instrument, of a few m sec-l, from a mean value. Occasionally marked periodic variations of the vertical velocity along the path of the balloon suggested the presence of atmospheric gravity waves, almost certainly due to orography, extending through considerable depths of the atmosphere. Corby limited his study to levels below 40 000 ft; however, on some occasions the waves which he inferred maintained considerable vertical velocities up to this level and may have extended considerably higher. I n the present investigation improved British radiosonde data have been examined for indications of small-scale air motions at all levels. Some emphasis has been placed on motions in the lower stratosphere, which have considerable practical interest in view of the imminent operation of supersonic aircraft there. DOI: 10.1111/j.2153-3490.1972.tb01587.x

An observational study of lee waves using radiosonde data

Corby (1957) showed that radiosonde data could be used to detect departures of the ver- tical velocity of the instrument, of a few m sec-l, from a mean value. Occasionally marked periodic variations of the vertical velocity along the path of the balloon suggested the presence of atmospheric gravity waves, almost certainly due to orography, extending through considerable depths of the atmosphere. Corby limited his study to levels below 40 000 ft; however, on some occasions the waves which he inferred maintained considerable vertical velocities up to this level and may have extended considerably higher. I n the present investigation improved British radiosonde data have been examined for indications of small-scale air motions at all levels. Some emphasis has been placed on motions in the lower stratosphere, which have considerable practical interest in view of the imminent operation of supersonic aircraft there.