[1] We proposed a broad spectral data-analyzing method to study the altitude and seasonal variability of gravity wave (GW)-associated dynamics in the lower atmosphere at a midlatitude by using the radiosonde data from Miramar Nas (32.87°N, 117.15°W), California, during 1998California, during -2008. Generally, the presented primary statistical features of GW parameters and their seasonal variation are consistent with previous radiosonde observations by using the conventional hodograph analysis method based on monochromatic GW theory. These consistencies suggest our proposed analyzing method is feasible in extracting GW parameters from the radiosonde data. More interesting, our analyses can reveal the altitude variations of GW parameters, which have been seldom reported in previous radiosonde observations. Similar to previous observations, most seasonal and height variability of GW parameters is closely connected with tropospheric jet, suggesting the important role of the jet in determining GW parameters as well as the lower atmospheric dynamics and thermal structure. Mainly due to the broad spectral nature of the observed GWs, there are also some differences between our results and previous studies based on monochromatic GW extraction. By using the perturbation of vertical ascent rate, we directly calculated GW momentum and heat fluxes, which can only be indirectly derived from the hodograph analysis. Furthermore, the directly derived heat flux can explain the frequently observed tropospheric inversion in winter. Besides GW parameters, turbulent energy dissipation rate and diffusion coefficient were also derived. The derived turbulence parameters and their altitude variations are in good agreement with radar observations reported elsewhere.

[1] Gravity wave activities and background dynamical structure in the troposphere and lower stratosphere (TLS) over five stations at latitudes from 10°N to 40°N were statistically studied by using the data from Radiosonde observation on a twice daily basis at 0800 and 2000 LT. The background dynamical structure exhibits evident latitudinal and seasonal variations and has a profound influence on inertial gravity waves in the TLS. In the analyses of inertial gravity waves, according to the background structures, the observation height coverage is divided into two segments, which are the tropospheric (0-10 km) and lower-stratospheric segments (18-25 km). The observational results indicate that the tropospheric jet is the most important excitation source for gravity waves both in the troposphere and lower stratosphere, and it plays different roles in determining the morphology of gravity waves in these two segments. The jet-excited gravity waves in the troposphere can propagate both upward and downward, and only part of the upward-propagating waves can penetrate into the stratosphere because of the Doppler shifting by the jet, while in the lower stratosphere, gravity waves excited by the tropospheric jet propagate upward. Most differences between the tropospheric and lowerstratospheric results can be explained from the linear dispersion relations of gravity waves and the Doppler shifting by the strong tropospheric jet. However, such an explanation is qualitative rather than quantitative. Generally, the observations reveal that the tropospheric gravity waves are mainly controlled by their excitation sources, implying the wave characteristics may be regarded as (at least an indicator of) the wave excitation source characteristics. These results suggest that in order to attain a more realistic source parameterization for gravity waves propagating in the middle and upper atmosphere, more attention should be paid to the gravity waves in the troposphere. Moreover, the causes of the pronounced peak of lower-stratospheric gravity wave intensity at tropical latitudes revealed by previous observations are also discussed.Citation: Zhang, S. D., and F. Yi (2007), Latitudinal and seasonal variations of inertial gravity wave activity in the lower atmosphere over central China,

Abstract. The latitudinal and seasonal variations of gravity wave (GW) potential energy density (E P ), kinetic energy density (E K ), and total energy density (E T ), i.e, the sum of potential and kinetic energy densities in the tropospheric (typically 2-10 km) and lower stratospheric (typically 18-25 km) segments have been derived from 10 years (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) of radiosonde observations over 92 United States stations in the Northern Hemisphere. The latitudinal variation of E P in the lower stratosphere is in good agreement with satellite observations. However, E K and E T in the lower stratosphere are different from satellite observations and the difference is believed to be linked with the latitudinal dependence of GW sources. Our analysis reveals that GW energy properties exhibit distinctive latitudinal and seasonal variations. The upward-propagating GW energy in the troposphere is larger than that in the lower stratosphere at low latitudes but the opposite holds true at high latitudes. The transition latitude, where the upward-propagating energies in the two altitude regions are the same, occurs at 35 • N throughout the year. So striking differences between GW activity in the troposphere and lower stratosphere are not likely explained only by the background wind Doppler shifting due to strong tropospheric jets. Our analysis indicates that the region around tropopause, roughly from 10 km to 18 km, is an important source region, especially at latitudes below 35 • N. Our studies strongly suggest that in order to fully understand the global GW activity in the lower atmosphere, the GW kinetic energy and its geographical and seasonal variations should be included, and more attention should be given to GWs in the troposphere and GW sources within the intermediate region, especially the upper troposphere.

[1] An observational study of nonlinear interaction between the quasi 2 day wave (QTDW) and the diurnal and semidiurnal tides from meteor radar measurements at Maui is reported. The diurnal and semidiurnal tides show a short-term variation with the QTDW activity. The variation of amplitude of the semidiurnal tide is opposite to that of the QTDW. The minimum amplitudes of the diurnal tide appear several days later than the maximum amplitudes of the QTDW, and the diurnal tide obviously strengthens when the QTDW drops to small amplitudes. The bispectrum analysis shows significant nonlinear interactions among the QDTW and the tidal components. The two quasi 16 h modes with periods of 16.2 h and 15.8 h generated in the interactions of the QTDW with the diurnal and semidiurnal tides can clearly be distinguished because of the slight deviation of the QTDW period from 48 h. The bicoherence spectrum demonstrates that the QTDW and the semidiurnal tide have quite strong levels of coherence, indicating that the nonlinear interaction is a mechanism responsible for the variability of the semidiurnal tide. Although there is also some interaction between the QTDW and the diurnal tide, their coherence level is low. When the QTDW drops to very weak amplitudes, the background wind decreases and reverses. During this time, the diurnal tide holds large amplitudes. These results support the notion that the variability of the diurnal tide is mainly attributable to the strong QTDW-induced changes in the background atmosphere, which was shown in the modeling study by Chang et al. (2011). Hence, both the nonlinear interaction and the background flow changes are responsible for the observed variation of the diurnal tide.

This paper presents characteristics of quasi-two-day waves (QTDWs) in the mesosphere and lower thermosphere (MLT) between 52° S and 52° N from 2002 to 2011 using TIMED/SABER temperature data. Spectral analysis suggests that dominant QTDW components at mid-high latitudes of the Southern Hemisphere (SH) and the Northern Hemisphere (NH) are (2.13, W3) and (2.04, W4), respectively. The most remarkable QTDW is (2.13, W3), which happened in the southern summer of 2002–2003 at 32° S from 60 to 90 km in altitude. Its downward phase propagation indicates upward propagation of the wave energy and a potential source region below 60 km. Analysis of horizontal wind fields in the same period shows the westward and southward propagation of (2.13, W3) and a possible reflection region above 90 km. Fundamental parameters of QTDWs present significant interhemispheric differences and interannual variations in statistical analysis. Amplitudes in the SH are twice larger than that in the NH, and vertical wavelengths are a little longer in the SH. QTDWs may endure stronger dissipation in southern summer because of shorter durations of their attenuation stages. Impact of the equatorial quasi-biennial-oscillation (QBO) on QTDWs can extend to mid-high latitudes of both hemispheres. It seems easier for QTDWs to propagate upward in the equatorial QBO's westerly phase in the lower stratosphere and easterly phase in the middle stratosphere. Interannual variations of QTDW strength may be influenced by solar activity as well. Strengths of QTDWs appear to be stronger (weaker) in the solar maximum (minimum)

[1] Starting from a set of fully nonlinear equations, the whole process of a gravity wave excited through resonant interaction under sum resonant conditions is clearly exhibited. In the whole interaction, the wavelength and frequency of the excited wave are in agreement with the values derived from the sum resonant conditions. And the energy growth of the excited wave is mainly from the primary wave. Moreover, strong energy exchange in the interaction shows that the nonlinear interaction may play a significant role in determining the atmospheric wave spectrum and transporting the momentum to higher altitudes. A discussion on the resonant and nonresonant interactions is also presented, and the detuning degree of interaction is proposed, which may be applied to measure whether or not the effective energy exchange occurs in the nonlinear interactions of gravity waves.

[1] Applying a second-order numerical scheme, nonresonant interactions of gravity waves in a compressible atmosphere are investigated. The numerical results show that the nonresonant interaction of the gravity waves does occur, and an apparent energy exchange among the interacting waves can be observed, which indicates that the gravity waves with different spatial and temporal scales can extensively interact. In the nonresonant interaction, the wave energy generally tends to transfer from the primary wave with the highest frequency to the secondary and excited waves, which is different from that in the resonant interaction. Under the same initial energy of the secondary wave, the final energy of the excited wave is almost proportional to the initial energy of the primary wave. When the initial energies of the primary waves are identical, the final energy of the excited wave increases slowly with the increasing initial energy of the secondary wave. Similar to that in the resonant interaction, in the nonresonant interaction, wave vector and frequency of the excited wave show temporal variability. The wave vectors of the interacting triad do not satisfy the matching conditions which should be obeyed in the weak interaction theory. We also investigate the effects of the background wind field and viscous dissipation on the nonresonant interaction. The positive and negative background winds show tendencies to strengthen and weaken the wave energy transfer, respectively. This also differs from the weak interaction theory, in which only a Doppler shift can be caused. The primary effect of the viscosity is to dissipate the energy of the interacting waves. The viscosity can hardly prevent the nonresonant excitation, suggesting that the restriction of amplitude threshold on the interaction in the presence of viscosity predicted by the weak interaction approximation seems to be rather loose. As in the resonant interaction, the principal energy exchange is finished before the waves depart from each other, which indicates that the interacting characteristic time does exist in the nonresonant interaction. The characteristic time and evolutions of wavelength and intrinsic frequency for the excited wave are insensitive to the small initial wave amplitudes and Gaussian packet widths of the interacting waves, horizontal background wind field, and viscosity. However, the characteristic time is relevant to the wavelengths of the interacting waves.

Relative to many investigations of inertial gravity waves (IGWs) in the Antarctic, IGW activity in the Arctic region was paid less attention to. We use radiosonde observations at the Ny‐Alesund station (78.9°N, 11.9°E) from April 2012 to June 2016 to study the IGW characteristics in the lower stratosphere over the Arctic. The observation reveals a prevailing eastward zonal background wind below 20 km and an obvious annual cycle of the background temperature from the troposphere to the lower stratosphere, which is different from the results in the middle and low latitudes. By combining Lomb‐Scargle spectrum and hodograph technique, case study demonstrates that the lower stratospheric IGWs exhibit a feature of freely propagating waves. Statistical analysis indicates that the IGWs have dominant horizontal (vertical) wavelength of 50–1,050 km (1–4 km) and ratio (1–2.5) of the intrinsic to inertia frequencies. Wave energy exhibits an annual oscillation with the maximum in winter and the minimum in summer. In winter, the downward propagating waves increase to about 20% due to polar stratospheric vortex. Because of the lower atmospheric filtering, the IGWs display a dominant direction of westward propagation, thus have a mean vertical flux of −0.647 mPa for the zonal momentum, which indicates that the IGWs can put a westward drag on the atmospheric wind field over the Arctic as they break and dissipate. All the vertical wavenumber spectra have spectral slopes from −2.23 to −2.99 close to the universal spectrum index of −3.