The interaction between the tropopause inversion layer (TIL) and the inertial gravity wave (IGW) activities is first presented by using a high vertical resolution radiosonde data set at a midlatitude station, Boise, Idaho (43.57°N, 116.22°W), for the period 1998-2008. The tropopause-based vertical coordinate is used for the TIL detection, and for meticulously studying the IGW variation around the TIL, the broad spectral method is used for the IGW extraction. Generally, the TIL at the midlatitude station is stronger and thicker in winter and spring, which is consistent with previous studies. Our study confirmed the intense interaction between the TIL and IGW. It is found that the TIL not only could inhibit the upward propagation of IGWs from below but also imply the possible excitation links between the TIL and IGW. The results also indicate that the enhanced wind shear layer just 1 km above the tropopause may result in instability and finally leads to the IGW breaking and intensive turbulence. Subsequently, the IGW-induced intensive turbulence leads to strong wave energy dissipation and a downward heat flux. This downward heat transportation could significantly cool the tropopause, while it has only negligible thermal effect on the atmosphere above the tropopause. Then, the IGW-induced cooling at the tropopause makes the tropopause colder and sharper and finally forms the TIL. These suggest besides previously proposed mechanisms that IGWs also contribute greatly to the formation of TIL, which is consistent with a recent related simulation study.

[1] Atmospheric waves and their interactions in the thermospheric neutral wind are studied based on Arecibo incoherent scatter radar observations. Our analysis suggests that the thermospheric atmosphere is usually disturbed by certain types of waves, including tides, gravity waves, and planetary waves, of which the diurnal tide is almost always the dominant disturbance. Strong interactions (defined as the coexistence of strong positive and negative correlations among the interacting waves) between the diurnal tide and gravity waves are frequently observed during the entire observation period. These strong interactions can persist for several days, although they are highly intermittent. Moreover, the sum and difference interactions between the diurnal tide and gravity waves always occur simultaneously and the energy exchange between the interacting waves is sometimes reversible. In addition to tide-gravity wave interactions, tide-planetary wave and tide-tide interactions are also found in our data. In tide-planetary wave interactions, the tidal oscillations are modulated at the interacting planetary wave periods. A combination of bispectral and correlation analyses verifies the occurrence of nonlinear interactions among different tidal components in the middle thermosphere. Moreover, during tide-tide interactions, the energy transfer trend changes very frequently, indicating that tidal energy is frequently redistributed among different tidal components. Generally, our study provides proof of strong tide-gravity wave, tide-planetary wave, and tide-tide interactions in the middle thermosphere, which has rarely been reported to date.

Abstract.Starting from a set of fully nonlinear equations, this paper studies that two initial gravity wave packets interact to produce a third substantial packet in a nonisothermal and dissipative atmosphere. The effects of the inhomogeneous temperature and dissipation on interaction are revealed. Numerical experiments indicate that significant energy exchange occurs through the nonlinear interaction in a nonisothermal and dissipative atmosphere. Because of the variability of wavelengths and frequencies of interacting waves, the interaction in an inhomogeneous temperature field is characterised by the nonresonance. The nonresonant three waves mismatch mainly in the vertical wavelengths, but match in the horizontal wavelengths, and their frequencies also tend to match throughout the interaction. Below 80 km, the influence of atmospheric dissipation on the interaction is rather weak due to small diffusivities. With the further propagation of wave above 80 km, the exponentially increasing atmospheric dissipation leads to the remarkable decay and slowly upward propagation of wave energy. Even so, the dissipation below 110 km is not enough to decrease the vertical wavelength of wave. The dissipation seems neither to prevent the interaction occurrence nor to prolong the period of wave energy exchange, which is different from the theoretical prediction based on the linearised equations. The match relationship and wave energy evolution in numerical experiments are helpful in further investigating interaction of gravity waves in the middle atmosphere based on experimental observations.