In this study, global (501S-501N) distribution of water vapor is investigated using COSMIC GPS RO measurements. Detailed comparisons have been made between COSMIC and high resolution GPS radiosonde measurements across 13 tropical stations and model outputs (ERA-Interim, NCEP, and JRA-25 reanalyses data sets). In comparison with independent techniques like radiosonde (Väisälä), it is found that COSMIC GPS RO wet profiles are accurate up to 7-8 km (assuming radiosonde as standard technique). In general, comparisons with corresponding seasonal means of model outputs are qualitatively in good agreement, although they differ quantitatively especially over convective regions of South America, Africa, and Indonesia. In tropical latitudes, the COSMIC specific humidity values are higher than the model outputs. Among various model outputs, ERA-Interim data set show near realistic features to that observed by COSMIC GPS RO measurements. Large asymmetry in the specific humidity distribution is observed between northern and southern hemispheres.

Abstract. The newly-installed MFR (medium frequencyradarAnnual climatologies involving both height and frequency versus time contour plots for periods from 8 h to 30 days, show that the changes with longitude are very significant and distinctive, often exceeding the local latitudinal variations. Comparisons with models and the recent UARS-HRDI global analysis of tides are discussed. The fits of the horizontal wave numbers of the longer period oscillations are provided in unique frequency versus time contour plots and shown to be consistent with the expected dominant modes. Annual climatologies of planetary waves (16 day, 2 day) and gravity waves reveal strong seasonal and longitudinal variations.

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Abstract. The 16-day wave climatology at Yamagawa (31.2 • N, 130.6 • E) is investigated by analyzing the mesosphere and lower thermosphere (MLT) wind data collected with the recently installed MF radar. We have utilized the continuous data gathered during the five-year period 1996-2000. The wave climatology clearly shows some seasonal variations. The period of late autumn-spring is marked with larger wave activity, with the strongest waves being observed in the winter months. The maximum amplitude observed at Yamagawa is about 20 m/s, which is comparatively larger than the amplitudes observed at mid-latitude stations. The height dependence of the 16-day wave suggests that the maximum amplitude is observed at altitudes below 80 km. The summer months are characterized with much weaker wave activity. The vertical wavelength appears to be larger in the winter months and shorter in the summer months. The present analysis again confirms that the 16-day wave is highly sensitive to the background mean winds. Eastward motion of the background winds is a more favourable condition for the 16-day wave penetration to the MLT heights. The wave features show some signs of interannual variability. Overall, the observed features of the 16-day wave at Yamagawa, which is located at the edge of the subtropical latitudes, show some correspondence with the results reported for mid-latitude stations.

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HF Doppler observations of vertical prereversal enhancement in vertical plasma plamm drifts in the equatorial F region at drifts. In this paper we present the diurnal, Trivandrum (dip: 0.9øS), conducted during 1984-seasonal, solar, and magnetic activity variations 1986, are presented. The observations reveal a of the vertical plamm drifts at Trivandrum based fairly consistent diurnal variation pattern with on the observations conducted during the period the drift velocities directing u•rd during day 1984-1986. and downward during night. The diurnal pattern is dcminated by the characteristic prereversal Measurement Technique and Observations enhancement that exhibits the most striking seasonal, solar, and magnetic activity effects. The HF Doppler radar used in the present Under high solar activity, the average peak investigation offers a simple and yet valuable velocity _c• the enhancement is found_lto drop frcm means for gaining information on ionospheric •O40 m s in equinox to •10 m s •n sunmet. plasma motions [Balan et al., 1982]. The systemLikewise, the average equinoctial peak velocity consists of a high frequency pulse transmitter drops almost by a factor of 3 as the solar with a peak power of 3 kw, a phase coherent activity index (S10.7) falls frcm 125 to 70 receiver with quadrature detection, a frequency units. The peak velocity has a unique magnetic synthezsizer with stability better than one part activity dependence; it drops by more than a in 10', a multichannel strip chart recorder, and factor of 2 as the activity changes frcm quiet to transmitting and receiving antennas. The moderate but assumes well above the quiet day tranmuitter RF and the receiver injection value for high activity.The corresponding frequencies as well as the modulation and gate variations for the other times of the day are pulses are all derived frcm the same master nowhere as pronounced as for the period of the synthesizer, rendering the system phase coherent. prereversal enhancauent. The observations The system was operated at a frequency of 5.5 MHz presented here are found to be in broad agreement with a pulse width of 100 •s and a pulse with those reported for Jicamarca and are also in repetition frequency of 50 Hz to probe the accordance with the model predictions based on bottomside F region. The quadrature signals,A cos the F region dynamo. 0 and A sin 0, have been recorded in the same manner as reported by Jayachandran et al. [1987].

E_ region real height determinations for 2.2 MHz has been carried out using standard electron density (N(h)) profiles from Maeda's rocket observations and the International Reference Ionosphere (IRI) models for application to the Saskatoon (52°N, 107°W) MF radar. A comparative study of the virtual and real heights for the totally reflected signals, for both solar maximum and minimum conditions, reveals that these heights increase from solar maximum to minimum. The group retardations from rocket N(h) are found to be lower than that of the IRI models in all the seasons. During summer the noon time virtual heights are very similar for the IRI model and MF radar data, but during the winter the IRI virtual heights are considerably lower than the observed heights. The derived group retardations are added to the mean wind and tidal climatologies for Saskatoon. This enables the prediction of the heights up to which the winds and tidal data can be taken without any correction for group retardation.