R.R. Clark

Empirical wind model for the upper, middle and lower atmosphere

Abstract The HWM90 thermospheric wind model has been revised in the lower thermosphere and extended into the mesosphere, stratosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, local time (tides), and longitude (stationary wave 1), with a cubic spline interpolation in altitude. The model represents a smoothed compromise between the original data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Overall root mean square differences between dar.a and model values are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal winds, and 10 m/s and 5 m/s respectively for meridional winds.

Global study of northern hemisphere quasi-2-day wave events in recent summers near 90 km altitude

Abstract We attempt to find the northern hemisphere zonal wavenumber for a striking quasi-2-day wave “event” or “burst” observed near 90 km altitude in the summer of 1992. A unique set of data on the upper atmosphere from nine radar sites is analysed (spacings ∼400– ∼ 12,000 km), and compared with expectations from models. The 2-day wave phase comparison, which finds zonal wavenumber m = 4, is conclusive. Determination of n, which defines the meridional wave amplitude structure, is not attempted, as the sites here have only a small latitude spread (21°N to 55°N). Also the amplitude seems to be unstable showing some sort of modulation which is not simultaneous at all sites. Finally, the radars have not been “calibrated” against each other in terms of wind speed. This calibration would have to be done before small differences in wave amplitude could be believed. A similar event in 1991 for which fewer sites are available is also discussed. Here the choice between m = 3 and 4 is not as clear.

Global-scale tidal variability during the PSMOS campaign of June-August 1999: interaction with planetary waves

During the PSMOS Global-scale tidal variability experiment campaign of June 1-August 31, 1999, a network of radars made measurements of winds, waves and tides in the mesosphere/lower-thermosphere region over a wide range of latitudes. Clear evidence was found that fluctuations in tidal amplitudes occur on a global scale in both hemispheres, and that at least some of these fluctuations are periodic in nature. Modulation of the amplitude of the 12 h tide was particularly evident at periods of 10 and 16 days, suggesting a non-linear interaction with planetary waves of those periods to be responsible. In selected cases, the secondary waves predicted from non-linear theory could be identified and their zonal wave numbers determined. In some, but not all, cases the longitudinal structure of the secondary waves supports the theory of planetary-wave/tidal interaction being responsible for the observed tidal modulation. It was noted also that beating between a 12.4-lunar and the solar tide could produce a near 16-day modulation of the 12 h tide amplitude that is frequently observed in late summer.

Global-scale tidal structure in the mesosphere and lower thermosphere during the PSMOS campaign of June-August 1999 and comparisons with the global-scale wave model

Observations of mean winds and semidiurnal and diurnal tides in the mesosphere/lower-thermosphere (MLT) region were made during the 3-month Planetary-Scale Mesopause Observing System Summer 1999 campaign. Data from 22 ground-based radars (and from two other instruments with measurements for the same period but in 1998) allow us to investigate the ability of the GSWM-00 to simulate the solar tides in the mesopause region (90-95 km). Here we have found that the GSWM-00 provides an increasingly reasonable estimate of most of the tidal characteristics in the MLT region. However, the representation of the 24 h tide appears superior to that of the 12 h tide. Some of these discrepancies are studied in detail. In particular, the observations reveal significant 12 h tidal amplitudes at high latitudes in the Northern Hemisphere summer. There is evidence for relation between the longitudinal variability of the mean zonal wind and the tidal characteristics seen from the radar wind measurements in the summer middle latitudes and a quasi-stationary planetary wave with zonal wave number one.

Hemispheric properties of the two-day wave from mesosphere-lower-thermosphere radar observations

Abstract Measurements of the pseudo 2-day wave have been made at many of the mesosphere-lower-thermosphere radars. Comparisons are made here between measurements taken at Saskatoon MF radar (52°N, 107°W) and two meteor radars, one at Christmas Island (2°N, 157°W) and the other at Durham (43°N, 71°W). Although results averaged for 10 days or longer agree with previous measurements (i.e. larger amplitudes and more phase stability in late summer), when the wave is analyzed over 2–4 days as a 48 h component, interesting phase properties emerge and the wave is seen over more of the year. The wave is amplitude and phase modulated, making the interpretation of results obtained over long time frames (20 days or more) difficult. There is strong evidence of solar influence on the 2-day wave.

Mean winds of the upper middle atmosphere (∼70–110 km) from the global radar network: Comparisons with CIRA 72, and new rocket and satellite data

Abstract A substantial quantity of wind data have been assembled from radar systems since CIRA-72 was formed: most of these radars include height ranging, and operate on a regular and even continuous basis. Systems include meteor and MF (medium frequency) Radars: an MST (mesosphere-stratosphere-troposphere) Radar (meteor mode); and an LF (low frequency) drift system. Latitudes represented are near 20° N/S, 35° N/S, 45° N/S, 50°N, 65° N/S. In all cases tidal oscillations were calculated so that corrected mean winds (zonal, meridional) are available - the meridional was not included in CIRA-72. Means for groups of years near 1980 are available, as well as individual recent years (1983, 1984) to allow assessment of secular trends: revised and improved analysis has been completed for several stations. Height-time cross-sections have been formed for each observatory: heights are typically ∼75–110 km, with time resolution of 7–30 days. Such detailed cross-sections were almost unknown before 1972. Comparisons with CIRA-72 are shown, and these emphasize the differences between hemispheres (NH, SH) in the radar winds. Other new winds from rockets and satellite radiances are contrasted with the radar set. There are important differences with the satellite-derived geostrophic winds (1973–78): possible explanations involve secular trends, longitudinal variations, and ageostrophy.

Bispectral analysis of mesosphere winds

Abstract Spectral components exist in the measured winds whose presence is not easily explained through known production mechanisms. Since the atmosphere is non-linear it is likely that non-linear interactions between tides and planetary waves will result in modulation of the tides with the associated production of new spectral components, possibly at the sum and/or difference frequency of the original components. This process has been suggested in the literature and spectral analysis of mesosphere winds has produced spectral components with this sum and difference relation. In an effort to show that these spectral components have properties that are consistent with this non-linear production process, meteor winds measured with the UNH Meteor radar at Durham (71 °W, 43 °N) were subjected to bispectral analysis. Results which involve interactions between tidal components only are expected and easily explained. However, other results are surprising and it is shown that some interactions that are known to occur do not show up in the analysis.

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Abstract Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations. The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [ Forbes and Vial (1989), J. atmos. lerr. Phys . 51 , 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed. The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [ Forbes. S and Hagan (1988), Planet. Space Sci . 36 , 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.

Semidiurnal tide in the 80–150 km region: an assimilative data analysis

Abstract A set of tabulated functions called ‘Hough Mode Extensions’ (HMEs), which represent numerical extensions of classical Hough modes into the viscous regime of the thermosphere, are used to least-squares fit a climatological data base of tidal measurements. The data base consists of monthly average vertical profiles of semidiurnal amplitudes and phases at 17 radar sites accessing some part of the 80–150 km height region. The radars are distributed between 78 S and 70 N latitude, and each one provides measurements of one or more of the following: eastward wind, southward wind, perturbation temperature. As a result of the fitting process, a single complex normalizing coefficient is derived for each month and for each of the four HMEs, designated (2,2), (2,3), (2,4) and (2,5) after their classical Hough function designations. Once the complex coefficients are derived, reconstruction by weighted superposition of the HMEs results in globally continuous specifications of semidiurnal horizontal and vertical wind, temperature, pressure, and density throughout the 80–150 km height region. The tidal variations in density, in particular, provide greater accuracy for several aerospace applications. The methodology developed here can also be utilized to derive tidal lower boundary conditions for Thermospheric General Circulation Models (TGCMs), or as a basis for future empirical model development. Comparisons are also made with HME coefficients and global tidal fields from the Forbes and Vial [(1989) J. atmos. terr. Phys. 51 , 649] numerical tidal model.

Tidal Winds from the Mesosphere, Lower Thermosphere Global Radar Network during the second LTCS Campaign: December 1988

Winds and tides were measured by nine MLT (mesosphere, lower thermosphere) radars with locations between 70°N and 78°S, including an equatorial station at Christmas Island, 2°N (Avery et al., 1990). The mean winds were eastward (westward) in the northern (southern) hemisphere mesosphere, consistent with midwinter circulations. For the 12-hour (semidiurnal) tide, observations and the model of Forbes and Vial (1989) were in generally good agreement: in both cases northward components were closer to being in phase in the two hemispheres, and winter wavelengths were shorter than those of the mid-latitude summer. Major differences were large (small) amplitudes at 70°N for model (observations); and poor agreement of equatorial tidal profiles. For the 24-hour (diurnal tide), the radar observations and model of Forbes and Hagan (1988) were in useful agreement in the summer hemisphere. However, the short (long) wavelengths at mid (high) latitudes of the models winter hemisphere were not observed during LTCS (Lower Thermosphere Coupling Study) 2, nor in climatologies for December. Suggestions as to the reasons for this disparity are presented.