Tbe spatial and temporal evohltion of gravity wave packet interactions is studied numerically. It is shown that Igrougb tile resonant parametric excitation an upgoing gravity wave packet can cause the growth of two secondary waves from noise level up to a significanl inlensity in several bours The prbnary wave packet is apparently debrmed as it decays. Tile energy transfer among tile intenlcling waves is no longer reversible since their amplitudes are Iocalised Therefore the characteristic time for the interactions is of a parlicular significance: it represents a time during whicb the principal energy transfer arises. Beyond the cbaracteristic time the net energy transfer among the interacting waves becomes rather weak, but the local change in the wave energy densities can be Conlsiderable Only a part of the initial energy of the primary wave packet is transferred to the secondary waves during the parametric excilation. The amounts of energy, which each of the two secondary waves extract from the primary wave, are ditthrent, exhibiting a parameter preference in the energy transfer. 1he parametric excitation process can be completed in The propagation lime, For the resonant interaction with two gravity wave packets initially baying large amplitudes, the evolution tale is faster than that in the parametric excitation The primary wave packet can lose most of its energy and finally be reduced to a small fluetuation. The viscous dissipation not only decreases the wave energies but also strongly affects the local energy transfer among the interacting gravity wave packets.

Data obtatmed from the medlun-frequency radar at Wuhan (30°N, 114°E} from Ⅱ to 25 February and frum 28 February to 13 March have been used to study the mean wind and tidal ogcillations at mesopause (8O-98km) region over Wuhan to 13 Minter The observed zonal and meridional mean winds show obvious temporal variation and usually decrease widl the mcreasing heighls in most observational time. the zonal mean wind is eastward A Aynamic spectral analysis of disturbance amplutudc illustratcs a promincnt peak at a period of 24h al all hcighls and alines! uninterrupted, and there are occasionally shghlly stong senlldiurnal and weak lerdiurnal tidal disturbarces, indicating that in Ihe mid-lalitude regions. the diurnal tide is Ihe dominant tidal disturbance at tnesopausc in winter. The temporal variation of tidal ampbtudes and their wave kinematical cncrgies iffustrates that there may bc sigmificant resonant interaction among the diurnal, semidiurnal and terdiurnal rides A quantitative compariosn with Ihe global-scale wave model (GSWM) demonslrates that the observed diuulal and semidiurnal tidal amplitudes are less 3nd larger than lhe results of GSWM-02. respectively Tbe essentially linear GSWM carmot predicl Ihe decrease ot diurnal tidal anplitude at higher heigbts. which muy be due to several possible nonlinear processes. such at wave-Wave interaction aad wave break. Although the height vanations of tidal phases are Consistenly well with the prediction of GSWM. there are evident pbase differenees between our observation and tile model. The profiles of the diunanl tidal phases show nbvious decreasc trond with Ihe increasiong height, indicalmg the observed diurnal tide is propagating upward The vertical wavelength of Ihe observed diurnal tide is estimated to be about 37km. and die eorresponding downward phase velocity is about 15km-1h.

Based on lidar measurements beteen March and September 2001, the charccteristics of sporadic Na layers (Na, layers) over wuhan (30.5

We examine the evolution of three-gravdy-wave interactions, with one initaly much stronger Ihun the other two, in a dissipative atmosphere, The characteristic time is used as an essential lime ca e for depicting the interactions. In the absense of viscosity, the solutions of the lineatized interaction equations, viz., the paramettic instability, give a good description for the evolution of secondary waves up to one-hail ol the characteristic time Beyond this time, the linear solutions are invalid; the evolution must be obtained by solving the complete interaction equations For given initial wave amplitudes, the characteristic time increases with the increasing wavelength of the primary wave; thus the criterion for the validity of resonant interaction approximation may be better satisfied The viscous dissipation makes the characteristoc time larger, and when severe enough a porlion oftbe seuondary waves, particularly at high wavenumbers, may fail to interacl with the primary wave effectively Additionally. the maxima of the energy densities for the secondary wavve are smaller than those in the absence of viscosity. In the ease of a significalt frequeney mismatch, only a fraction of the primary wave energy lakes part in the hlterchange Copyright.

Winds and waves measured by the Medium Frequency Radars (MFR) at Saskatoon (52N, 107W), Robsart (49N, lg9W), and Sylvan Lake (52N, 114w), and the Fabry-Perot Interferometer (FPI) at Saskatoon are compared. Particular care has been taken to exclude FPI airglow (558 nm, green line) contamination by aurora or scatter from clouds; radar data were setected from high slgnal-to-noise conditions. Comparisons between the MFR/FPI in seasonal wind climatologies (at a radar height of 98km) and wind speed ratios and direction-difference histograms for the Saskatoon systems show excellent agreement. Zonal and meridional wind comparisons (scatter plots) also show no significant differences in mean winds between the FPI and MFR. Differences between instantaneous measurements, as seen on 91 individual days, are categorized into four types of events, depending upon FPI/MFR similarities and differences and the dominance of the solar tide A gravity wave model is then used to demonstrate the effects of waves, with scales from hundreds to several thousand of kilometers, upon such optical and radar systems, each of which has a different inherent spatial averaging. The model is largely successful in explaining the differences between the instantaneous MFR and FPI measurements. The relevance of such comparisons to satellite/ground-based collaborations is briefly discussed.