Variation Of Gravity Wave Activity Research Articles

Observation of Gravity Waves at the Tropical Tropopause Using Superpressure Balloons

AbstractTropical gravity wave activity is investigated using measurements of momentum fluxes gathered during Strateole‐2 superpressure balloon flights. The data set consists of eight balloon flights performed in the deep tropics from November 2019 to February 2020. The flights lasted for 2–3 months each, and in‐situ meteorological data were collected every 30 s. The relation between gravity waves and deep convection is investigated using geostationary satellite data from the NOAA/NCEP GPM_MERGIR satellite data product, at 1 h resolution. The amplitude of gravity wave momentum fluxes shows a clear dependence on the distance to the nearest convective system, with a strong decay as distance to convection increases. The largest momentum‐flux values (>mPa) are only found less than 200 km away from deep convection. The sensitivity of the wave flux to distance from convection is stronger for high frequency gravity waves (periods shorter than 60 min). Lower frequency waves tend to a non‐zero, background value away from convection, supporting some background value in gravity‐wave drag parameterizations. On the other hand, the wide range of momentum flux values observed close to the convection emphasizes the intermittent nature of the gravity‐wave source. The large scale variation of gravity‐wave intermittency within the equatorial belt is also studied. The results highlight spatial variations of gravity wave activity, with the highest momentum flux recorded over land.

Investigation of gravity wave activity based on operational radiosonde data from 13 years (1997-2009): Climatology and possible induced variability

Atmospheric gravity waves (GWs) are important for the dynamics of the atmosphere. The analysis of 13 years of routine radiosonde data from Prague (50.01°N, 14.27°E) with temporal highly resolved temperature, pressure and wind measurements is presented in order to derive a climatology of gravity wave activity in the lower stratosphere. An annual cycle with a maximum during winter and a minimum during summer is identified. Gravity wave activity is twice as high during winter as during summer. Winter periods are investigated by wavelet analysis. They show similar periods in vertical flux of horizontal momentum and pressure variance time series. These features may be attributed to planetary waves. When analyzing individual years, maxima of gravity wave activity and vertical flux of horizontal momentum often appears together with minima in surface pressure. We speculate therefore that at least parts of the interannual variations of gravity wave activity may due to cyclones.

Lidar observations of gravity wave activity in the middle atmosphere over Davis (69°S, 78°E), Antarctica

AbstractA 16 month series of lidar measurements in the stratosphere and mesosphere‐lower thermosphere (MLT) region over Davis Station (69°S, 78°E) in Antarctica is used to study gravity waves. The unprecedentedly large number of observations totaling 2310 h allows us to investigate seasonal variations in gravity wave activity in great detail. In the stratosphere the gravity wave potential energy density (GWPED) is shown to have a large seasonal variation with a double peak in winter and minimum in summer. We find conservative wave propagation to occur between 29 and 41 km altitude in winter as well as in summer, whereas smaller energy growth rates were observed in spring and autumn. These results are consistent with selective critical‐level filtering of gravity waves in the lower stratosphere. In the MLT region the GWPED is found to have a semiannual oscillation with maxima in winter and summer. The structure of the winter peak is identical to that in the stratosphere, suggesting that the gravity wave flux reaching the MLT region is controlled by the wind field near the tropopause level.

Seasonal variation of gravity wave activity at midlatitudes from 7 years of COSMIC GPS and Rayleigh lidar temperature observations

AbstractSeven year series of gravity wave (GW) potential energy at midlatitude stratosphere (10 to 50 km) is constructed by combining temperature profiles provided by Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS satellite constellation and Rayleigh lidar operating at Haute Provence observatory in Southern France. The combined series are used to evaluate the representation of GW in Modern Era Retrospective‐Analysis for Research and Applications reanalysis. The seasonal and zonal variabilities of GW activity are diagnosed using zonal wind and wind divergence provided by ERA‐Interim reanalysis. The spatiotemporal distribution of GW activity is found strongly dependent on the zonal wind variation, wind divergence, and topography. We show that anomalies in the wind divergence can serve as a proxy for locating synoptic‐scale enhancements of GW. The analysis provides evidence for orographic GW excitation, and the results are compatible with geostrophic adjustment being an additional source of stratospheric GW. The seasonal GW variability can be largely explained by interaction with the mean flow and wave propagation.

Gravity wave activity in the troposphere and lower stratosphere: An observational study of seasonal and interannual variations

AbstractAn 11 year (1998‐2008) temperature and wind data set obtained over Annette Island (55.03°N, 131.57°W) is used to examine gravity wave activity and their seasonal and interannual variations in gravity wave activity. Vertical wave number spectra of normalized temperature and wind fluctuations are calculated and are compared with the predictions of gravity wave saturation models. Results indicate that there are serious discrepancies between our measurements and earlier observational results at present stage of study of vertical wave number spectra. The correlation coefficients between tropospheric and stratospheric temperature spectral parameters are very small, suggesting that the result is in agreement with previous studies. Time series of total wave energy reveal clear seasonal and interannual variations. Maximum wave energy amplitudes occur near winter of each year in the troposphere and near summer of each year in the stratosphere. Specifically, the maximum wave energy amplitudes in the troposphere show a close correspondence with the maximum occurrence rate of dynamical instability. In addition, the maximum wave energy amplitudes in the stratosphere also show a close correspondence with the maximum occurrence rate of convective instability.

Research using high (and higher) resolution radiosonde data

High vertical resolution radiosonde data (HVRRD), which began to become available in the early 1990s, have had value in gravity wave and tropical wave studies [Hamilton and Vincent, 1995]. For example, routine analysis of upper troposphere‐lower stratosphere temperature and wind has led to better understanding of seasonal and geographic variations in gravity wave activity and spectral characteristics. Since then, HVRRD have been exploited for much broader research applications in fields where HVRRD provide the highest resolution available for observational parameters. With recent upgrades to the National Oceanic Atmospheric Administration's (NOAA) network of U.S. upper air stations providing yet higher vertical resolution, new research applications of operational sounding data are emerging. A repository of long‐term, routine, very high resolution in situ observations of fundamental atmospheric parameters, including temperature, water vapor, and wind velocity, is now available, for which the full research potential has yet to be realized.

Seasonal variations of gravity wave activity in the lower stratosphere over an Antarctic Peninsula station

An 8 year series of 965 high-resolution radiosonde soundings over Rothera (67 degrees S, 68 degrees W) on the Antarctic Peninsula are used to study gravity wave characteristics in the lower stratosphere. The gravity wave energy is shown to have a seasonal variation with peaks at the equinoxes; the largest peak is around the spring equinox. During the winter months and extending into the spring, there is both an enhancement in the downward propagating wave activity and a reduction in the amount of critical-level filtering of upward propagating mountain waves. The horizontal propagation directions of the gravity waves were determined using hodographs. It was found that there is a predisposition toward northward and westward propagating waves above Rothera. This is in agreement with previous observations of gravity wave momentum flux in the wintertime mesosphere over Rothera. These results are consistent with a scenario whereby the stratospheric gravity wavefield above Rothera is determined by a combination of wind flow over topography-generating waves from below, and sources such as the edge of the polar stratospheric vortex-generating waves from above, especially during winter and spring.

A Brief Study on the Characteristics of Tropospheric Wind Energy over Wuhan Based on Certain Energy Materials

By analysing the wind data observed by radiosonde during 2000-2002 in Wuhan, zonal and meridional wind velocity, wind shear and energy were discussed. A winter maximum and a summer minimum of the zonal wind velocity which pace up and down between the height of 10-15km was found. The weaker variation of the meridional wind reflects the seasonal variation of gravity wave activity in the lower atmosphere. The energy variation show a trend stronger in winter and spring then in other seasons. A winter maximum and a summer minimum in the troposphere and lower stratosphere was also found, which ranged in similar height range as wind velocity. As a result, a strong correlation between horizontal wind velocity and kinetic energy can be infered.

Seasonal variations of gravity wave activity and spectra derived from sodium temperature lidar

Gravity wave measurements were carried out by a newly developed sodium temperature lidar at São José dos Campos (23°S, 46°W). The seasonal variations of gravity wave activity and spectra derived from temperature data are investigated. The total temperature perturbation and temperature vertical power spectra, Ft(m) at 2π/(8 km), 2π/(4 km), and 2π/(2 km) all show that the semiannual maxima occur near the equinoxes, which is consistent with our earlier study (Yang et al., 2006) using only sodium concentration data. The vertical structure of the mean temperature variance could be related to the vertical distribution of N2. Large nightly variability of the temperature perturbation spectral slope was also found as in other sodium concentration lidar studies. The comparison between temperature and density perturbations induced by quasi‐random waves was investigated, and the agreement was found to be good on most (∼78%) nights. Good agreement between monochromatic wave‐induced temperature perturbations and sodium concentration perturbations has also been found, and the wave parameters derived from temperature data and sodium concentration data are comparable.

Gravity waves in the equatorial thermosphere and their relation to lower atmospheric variability

Abstract Using a general circulation model that contains the region from the ground surface to the upper thermosphere, we have examined characteristics of gravity waves in the equatorial thermosphere. At an altitude of 150 km, the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength λ x ≈ 2000 km), 40 (λ x ≈ 1000 km) and 80 (λ x ≈ 500 km) are 3, 1.5 and 1 h, respectively. For individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes due to dissipation processes in the thermosphere, such as molecular viscosity and ion drag force, indicating that gravity waves with a larger horizontal phase velocity (larger vertical wavelength) can penetrate into the thermosphere. The longitudinal variation of gravity wave activity in the equatorial thermosphere and upward propagation of gravity waves from the lower atmosphere were also studied. The longitudinal distribution of gravity wave activity in the thermosphere is quite similar to that of gravity wave activity in the lower atmosphere and the cumulus convective activity in the tropical troposphere. Our results indicate that the strong energy flux due to gravity waves from the enhanced cumulus convective activity propagates upward into the upper thermosphere. The relation between the wind fluctuation associated with gravity waves and the ionospheric variation is discussed. Fluctuations of the neutral zonal wind with periods of 1–2 h are significant in the 200- to 300-km height region, and its amplitude sometimes exceeds 50 m s−1. These results suggest that upward propagating gravity waves can affect the ionospheric variation in the F-region.

Seasonal variations in gravity wave activity at three locations in Brazil

Abstract. Using the variance in meteor radar winds as a measure of gravity wave activity, we investigate the temporal variations in gravity waves at three locations in Brazil: São João do Cariri (7.3° S, 36.4° W), Cachoeira Paulista (22.7° S, 45.0° W) and Santa Maria (29.7° S, 53.7° W). The technique used is that of Hocking (2005) which makes it possible to separate the zonal and meridional components of the fluctuating wind velocity. We find that the seasonal variation of the fluctuating wind is similar to that of the amplitude of the diurnal tide, showing a predominantly semi-annual variation, stronger at Cachoeira Paulista and Santa Maria than at the quasi-equatorial station, Cariri. Both with respect to the seasonal trend and shorter term variations, strong coupling between gravity wave activity and tides is indicated by a remarkably close correlation between the fluctuating velocity and the vertical shear in the tidal winds. It is not clear as to whether this is caused by gravity wave forcing of the tides or whether it results from in situ generation of gravity waves by tidal wind shear.

Seasonal and interannual variations of gravity wave activity in the low-latitude mesosphere and lower thermosphere over Tirunelveli (8.7° N, 77.8° E)

Abstract. The Mesosphere and Lower Thermosphere (MLT) winds acquired by medium frequency (MF) radar at Tirunelveli (8.7° N, 77.8° E) for the years 1993–2007 are used to study seasonal and interannual variabilities of gravity wave (GW) variances in the altitude region 84–94 km. The GW variances in zonal and meridional winds show semiannual oscillation with maximum variance during March–April and August–September and minimum during June–July and November–December months. The wind variances, in general, are observed to be enhanced during and after the year 1998 and they undergo large interannual variability, in particular, during spring equinox months. An enhancement of GW variances is observed during spring equinox months of the years 2000, 2004 and 2006. These larger GW enhancements, most of the times, coincide with eastward phase of zonally averaged stratospheric QBO at 30 hPa over equator and sudden stratospheric warming occurred at high latitudes. From the zonal and meridional variances, the perturbation ellipses are calculated and they show that the predominant direction of propagation of gravity waves is in SE-NW plane.

Polar mesosphere and lower thermosphere dynamics: 1. Mean wind and gravity wave climatologies

Mean wind and gravity wave climatologies are presented for the polar mesosphere and lower thermosphere (MLT). The data were derived using MF radars at Davis (69°S, 78°E) and Syowa (69°S, 40°E) in the Antarctic and Poker Flat (65°N, 147°W) and Andenes (69°N, 16°E) in the Arctic. The dynamics of the Antarctic MLT are found to be significantly different from the Arctic MLT. Summer maxima in both the westward and equatorward winds occur closer to the solstice in the Antarctic than in the Arctic. The greater symmetry around the solstice suggests radiative effects may play a greater role in controlling the state of the Antarctic MLT than in the Arctic, where dynamical effects appear to be more important. Gravity wave observations also suggest that wave drag may be greater in the Arctic than in the Antarctic. The equatorward flow near the mesopause persists later in summer in the Arctic than in the Antarctic, as do observations of polar mesospheric clouds and polar mesospheric summer echoes. All three phenomena begin at about the same time in each hemisphere, but end later in the Arctic than in the Antarctic. It is proposed that the magnitude of the meridional winds can be used as a proxy for gravity wave driving and the consequent adiabatic cooling in the MLT. Seasonal variations in gravity wave activity are predominately combinations of annual and semiannual components. Significant hemispheric differences are observed for both the timing and magnitude of these seasonal variations.

Gravity wave activity in the mesopause region from airglow measurements at El Leoncito

Based on almost 1000 nights of OH(6-2) and O 2 b(0-1) airglow data measured during 1998–2002 at El Leoncito (31.8°S, 69.2°W), seasonal variations of gravity wave activity are determined from temperature and intensity variances. The photon-counting technique used permits to derive the individual statistical errors a priori, and to quantitatively isolate the geophysical variance. The two airglow emissions yield results for the altitudes of 87 and 95 km . Tidal activity contains most of the variance (especially between March and October) and therefore must be suppressed to obtain gravity wave activity. There are strong night-to-night variations, from completely quiet to variances of more than 80 K 2 in temperature. The interannual variability was small enough to define a meaningful seasonal climatology. The wave activity variation is semiannual, similar to previous radar wind results, including those at the same latitude, and has its main maximum in southern winter and a weaker maximum in summer. The growth of wave activity from 87 to 95 km , when interpreted in terms of an amplitude growth factor, leads to a value of 1.35±0.01, similar to the one obtained for tides, in a previous paper of ours. From the comparison of intensity and temperature variance for a given emission, mean values of Krassovsky's η are derived (5.60±0.09 for OH, and 5.08±0.08 for O 2), that compare favourably with results obtained from individual wave signatures. This proves the consistency between the wave activities derived from intensities and temperatures.

Diurnal Variation of Gravity Wave Activity at Midlatitudes in the Ionospheric F Region

We investigate the short-term fluctuations in the period range from 15 to 180 minutes in the electron density variations of the F region ionosphere. Electron density profiles obtained at the ionospheric stations of Pruhonice (49.9° N, 14.5° E) and Ebro (40.8° N, 0.5° E) at five minute time sampling have been used for this analysis. The diurnal changes of the activity of the acoustic gravity wave fluctuations (AGW) show a clear enhancement during and several hours after sunrise. The periods of such AGW's are about 60 to 75 minutes and these waves propagates vertically through the ionosphere from a source located at an altitude of 180-220 km. The most likely source for these events seems to be passage of the Solar terminator.

Latitudinal Variations Observed in Gravity Waves with Short Vertical Wavelengths

Knowledge of the latitudinal variations in the occurrence of gravity waves is important for their parameterization in global models. Observations of gravity waves with short vertical scales have shown a pronounced peak in wave activity at tropical latitudes. In this paper, it is shown that such a peak may be a natural consequence of the latitudinal variation in the Coriolis parameter, which controls the lower limit for gravity-wave intrinsic frequencies ω̂. Two distinct but related effects of this parameter on observations of gravity-wave activity are explained and explored with a simple model. The results are also compared to observed latitudinal variations in gravity-wave activity. The authors formally distinguish between observed gravity-wave spectra and what is called gravity-wave “source spectra,” the latter being appropriate for input to gravity-wave parameterizations. The results suggest that the ω̂−5/3 dependence of the gravity-wave energy spectrum commonly assumed as input to parameterizations is likely too steeply sloped. Much more shallowly sloped spectra for gravity-wave parameterization input ∝ω̂−0.6 − ω̂−0.7 show better agreement with observations. The results also underscore the potential importance of intermittency in gravity-wave sources to the interpretation of gravity-wave observations.

Seasonal variation of gravity wave activity at 5–20 km observed with VHF radar at White Sands Missile Range, New Mexico

Wind observations at altitudes from 5 to 20 km obtained from the VHF wind profiler radar at the White Sands Missile Range (WS), New Mexico, are used to construct a climatology of gravity wave intensities over WS during the nearly 6‐year period from January 1991 through September 1996. The kinetic energy densities for both high‐frequency gravity waves (ground‐based period of 6 min to 2 hours) and low‐frequency gravity waves (period from 2 to 21 hours) are considered. These results are compared to earlier results from the middle and upper atmosphere (MU) radar in Japan. Comparisons of gravity wave variability at high and low frequency reveal both similarities and differences between the two sites. The primary differences occur at high frequency. The seasonal cycle in the high‐frequency gravity waves is less pronounced and less regular at WS compared to MU. The magnitude of the high‐frequency kinetic energy density is generally larger at WS compared to MU, and the relationship between the upper tropospheric jet stream winds and the high‐frequency gravity wave activity is not nearly so strong at WS as at MU. The primary high‐frequency gravity wave excitation mechanism at WS appears to change with season. There is evidence of a connection in winter and spring between the wind speeds in the middle troposphere and the high‐frequency gravity wave activity, suggesting a topographic source for these waves. Strong summertime high‐frequency variability at WS in the upper troposphere is most likely linked to waves launched by deep convection during the monsoon season in New Mexico. The results for low‐frequency gravity waves are more consistent between WS and MU. Similarities appear in the kinetic energy density, the nature of the seasonal cycle, and the correlation between the kinetic energy density and the mean winds.

Seasonal Variation of Gravity Wave Activity in the Lower Ionosphere in Central Europe

The seasonal variation of gravity wave activity at altitudes around 95 km is investigated using digital measurements of low-frequency nighttime radiowave absorption at Průhonice (50°N, 15°E) between 1989 and 1993. The analysis of 5 years of data allows two conclusions to be drawn: (1) under high solar activity conditions, there is no clearly detectable seasonal variation of gravity wave activity; (2) under medium solar activity conditions (1992, 1993), there is a tendency to a pronounced summer maximum.

Gravity waves in the tropical lower stratosphere: An observational study of seasonal and interannual variability

Radiosonde observations made at Cocos Islands (12°S, 97°E) in the Indian Ocean between September 1992 and June 1998 are used to study seasonal and interannual variations in gravity wave activity in the lower stratosphere (18–25 km). The islands are located in a region of generally strong convection that occurs at all times of the year, with the period of strongest convective activity between December and July (wet season). The prevailing zonal winds during the observational period and height range are westward with a quasi‐biennial oscillation (QBO) superimposed. Time series of wave energy show that largest wave amplitudes occur during the wet season when convection is strongest, but a QBO‐like variation is also apparent. Maximum energy densities of about 25 J kg−1 occur early 1993, 1995, and 1997 at the times when the westward shears are largest. Wave energy is found to be propagating upward, and in the horizontal there is considerable azimuthal anisotropy, with predominate eastward propagation against the prevailing wind. Upward fluxes of zonal momentum flux are estimated by combining the temperature and wind information. Fluxes show a similar temporal behavior to the energy. The motion and temperature fields are dominated by waves with vertical wavelengths ∼2 km. Using a Stokes parameter analysis, it is found that the intrinsic frequencies are, on average, 2–3 times the inertial frequency, corresponding to intrinsic periods of 20–25 hours. Horizontal wavelengths between 200 and 2000 km are inferred, with a mean value of about 1000 km. The mean intrinsic phase speeds are about 10 ms−1, but ground‐based phase speeds are centered on 0 ms−1.

Seasonal variation of gravity wave activity in the lower atmosphere observed with the MU radar

In this paper we analyzed the seasonal variation of gravity wave activity in the troposphere and lower stratosphere (5–25 km) as revealed by monthly observations with the MU radar between December 1985 and December 1989. In the lower stratosphere, wind velocity variance due to gravity waves with periods of 5 min‐21 hours showed a clear annual variation with a maximum in winter and a minimum in summer, agreeing fairly well with the seasonal variation of the jet stream intensity. This suggests that the excitation of gravity waves is closely related to the behavior of the background mean winds. The specific kinetic energy per unit volume of short‐period gravity waves (ground‐based period of 5 min‐2 hours) was enhanced in winter and spring, its peak in the profile being located near the peak of the jet stream (∼12 km). On the other hand, the kinetic energy of the long‐period (2–21 hours) component showed clear annual variation with a winter maximum and a summer minimum in the middle troposphere and lower stratosphere. The kinetic energy of the long‐period component was smaller in the lower stratosphere than in the troposphere, which did ot seem to penetrate the lower stratosphere. In the short‐period component the ratio of the horizontal to the vertical wind variances near the jet peak height was observed to be large in winter and small in summer, probably due to the seasonal variation of the Brunt‐Väisälä frequency. The wind variance ratio for the long‐period component in the upper troposphere and lower stratosphere was greatly reduced in winter, possibly due to the Doppler shifting of gravity waves. The vertical flux of zonal momentum for the 5 min‐ to 21‐hour period component showed negative values in the upper troposphere and lower stratosphere, implying that the upward propagating gravity waves in the lower stratosphere mostly traveled westward relative to the background wind. We did not find any significant seasonal variations in the vertical flux of meridional momentum.