Many attempts have been undertaken to construct empirical wind models of the middle and upper atmosphere that include the stratosphere, the mesosphere and the upper mesosphere/lower thermosphere region. The most widely used models are the COSPAR International Reference Atmosphere 72 (CIRA-72, 1972) and the Fleming et al. (1988) model, which is a part of CIRA-86 model. Since the CIRA model concerns areas far above the greatest heights for standard radiosondes the CIRA-72 model was mainly based on rocket data, including only sparse meteor radar and ionospheric drift data. In that model the zonal wind below 85 km was calculated from the thermal (gradient) wind equation and the related temperature was determined from satellite radiance measurements. Above 85 km wind data were calculated from the mass spectrometer and incoherent scatter (MSIS-83) empirical model temperatures (Hedin et al., 1991). This method is not a direct way of wind determination. In addition the reliability of the gradient winds is also questionable due to the low accuracy of the satellite temperature measurements in the upper mesosphere, and the absence of direct temperature data between 85 and 100 km in the MSIS model. It is useful to note that the Fleming et al. (1988) model does not contain information about meridional winds.
However, it is known that, unlike in the stratosphere, the prevailing meridional winds in the upper mesosphere/lower thermosphere are only 1.5-2 times slower than the zonal winds. A well-known meridional wind model for this region (Groves, 1969) has been developed utilizing sporadic rocket wind data obtained at only few sites in the Northern Hemisphere (NH). As a result, this model presented a quite schematic picture of the height-latitudinal structure of a zonal mean meridional wind field at 60-110 km for separate seasons. In this case the circulation in the Southern Hemisphere (SH) was believed to be a mirror image of NH wind systems for the corresponding season, only with the opposite sign. Nastrom et al. (1982) have also developed an empirical model of the meridional circulation at 95 km for NH summer. Their analysis has shown that at all measurement sites the prevailing meridional wind was predominantly southward independent of longitude. Therefore it was concluded that the prevailing wind in the lower thermosphere (LT) is mainly ageostrophic. This conclusion of meridional wind ageostrophicity for different seasons was made by Portnyagin (1986) who analysed ground-based meridional wind observations for sites located in two narrow latitudinal belts and situated far apart in longitude. A model known as ``interim new CIRA'' contained a set of radar derived direct wind measurements for 14 locations (Manson et al., 1985). Much useful information about global wind structures in the upper mesosphere/lower thermosphere region was obtained. In Manson et al. (1991) comparisons between satellite-derived gradient winds from the Fleming et al. (1988) model and radar-derived winds were made. It was found that overall the agreement for the zonal winds at the particular observational sites was rather good (but not complete). The comparison of meridional winds revealed significant ageostrophy. The first attempt to develop a global height-latitude model of meridional winds from ground-based radar measurements was undertaken by Manson et al. (1987). However due to insufficient data (only nine sites were used) the authors only succeeded in constructing the height-latitude cross-sections of meridional wind fields for two months and for a limited latitude range in both hemispheres.
The radar-based Global Empirical Wind Model (GEWM) for the upper mesosphere/lower thermosphere (70-110 km) was constructed in 1984 (Portnyagin, 1984) and updated in the following years (Portnyagin, 1986, 1987; Portnyagin and Solovjova, 1992, 2000, Portnyagin et al., 1995, 2003). The last version of the model, basically referring the to 1990-2000 period, also includes data from the HRDI experiment on board UARS. The analytic empirical horizontal wind model (HWM93), using the height interval 70-110 km, has been recently developed by Hedin et al. (1996). The model is based not only on the CIRA-86 tabulations, but also on the selected historical rocket data, previous rocket data based tabulations, meteor radar and MF radar data, and lower thermosphere incoherent scatter data. However the data used for constructing of the model for the upper mesosphere/lower thermosphere region (80-100 km) were obtained at the limited number of stations (see Table 1 in Hedin et al., 1996).
Recently direct wind observations from the wind imaging interferometer (WINDII) and the high-resolution Doppler imager (HRDI) on board the Upper Atmosphere Research Satellite (UARS) have provided the principal new global wind data set for the upper mesosphere/lower thermosphere region. The corresponding empirical prevailing zonal wind models (see, e.g., Wang et al., 1997) and the prevailing meridional wind model (Fauliot et al., 1997) were constructed. Fleming et al. (1996) and Portnyagin et al. (1999) concluded that, in general, the space-based zonal wind models exhibited significant differences relative to the ground-based models. However, Fauliot et al. (1997) have stated that the WINDII-based prevailing meridional wind model are similar to the ground-based Portnyagin et al. (1995) model.
It may be concluded that with the GEWM a zonal mean wind model for the mesosphere/lower thermosphere is available. However, still several problems should be solved to create a consistent update wind model. The GEWM model is mainly based on MF and MR radar data. These are different methods with somewhat different wind results when measured at the same point. This creates non-homogeneous set of the wind data. Thus, there is need of systematic study of these differences and subsequent homogenisation of the data. Introducing space-based data into the model requires the same study for space-based and ground-based measurements. Data obtained during several years are used for construction of a climatic wind model. Frequently it is not possible to use the data obtained during the same years. There are long-term trends and inter-annual wind variability, which are possibly different at different latitudes and longitudes. For a consistent climatic wind model one needs to understand the distribution of these trends and how the used climatic wind values reflect the real atmospheric wind distribution.
 Below the stratopause, analyses data (UKMO, ECMWF, NCAR/NCEP) are available that, although the winds derived from these data are not purely empirical, give good estimates of the mean circulation there. The wind models for the mesosphere, however, still need to be improved. In addition, only few information is available for non-zonal structures in the middle atmosphere, namely stationary planetary waves, and the variability in the time scale of planetary waves. Only recently, Fahrutdinova et al. (2003) accomplished the comparison for some midlatitude sites (Kazan, Collm, Saskatoon) using UKMO and radar data. They showed significant longitudinal distinctions in the altitudinal and seasonal structure of the circulation, thus indicating their non-zonal character.
Concerning tides, which become a prominent feature of middle atmosphere circulation above 70 km, only few climatologies are available. The probably first attempt to construct a global picture of the diurnal and semidiurnal tide has bee undertaken by Manson et al. (1989). More recently, updated measurements from MF radars at different latitudes have been presented (Manson et al., 1999), while Jacobi et al. (1999) presented semidiurnal tides from a narrow (middle) latitude band, but for different longitudes. An empirical model, as the GEWM based upon radar winds, have been presented by Portnyagin and Solovjova (1998), while Forbes et al. 1994) presented tides calculated from an assimilative approach. The HWM93 (Hedin et al., 1996) also contains information about tides. Global pictures of the tide were also derived from UARS measurements (Burrage et al., 1995, Khattatov et al., 1997a,b). However, as has been found from comparison of long-term measurements at different longitudes, the tides are very variable in both temporal and spatial sense, so that the models thus far are not yet satisfactory, and updated measurements should be included.
It may be concluded that there is still need for a comprehensive reference mode that contains mean winds, planetary waves, tidal amplitudes and phases, and information about the variability of the winds, including trends and long-period variations. Since the database in several regions is sparse (mesosphere, thermosphere), pure empirical models constructed using only one method are only possible for selected regions. Therefore, a combined approach may be taken into account, that includes empirical models for the regions well covered with data, but an assimilative approach for the mesosphere and the lower thermosphere region.

References

Burrage, M.D., et al., 1995, J. Atmos. Solar-Terr. Phys. 59, 497-509.
CIRA-72, A. Verlag Akademisches Berlin, 1972.
Fahrutdinova, A.N., et al., 2003, COSPAR 34th Assembly, Adv. Space Res., in press.
Fauliot, V., et al., 1997, Ann Geophysicae 15, 1221-1231.
Fleming, E. L., et al., 1988, NASA Technical Memorandum 100697.
Forbes, J.M., et al., 1994: J. Atmos. Terr. Phys. 56, 1237-1249.
Groves, G. V., 1969, J. British Interplanet. Soc. 22, 285-307.
Hedin, A. E., et al.,1991, J. Geophys. Res. 96, 7657-7688.
Hedin, A. E., et al. 1996, J. Atmos. Terr. Phys. 58, 1421-1447.
Jacobi, Ch., et al., 1999, J. Atmos. Solar-Terr. Phys. 61, 975-991.
Khattatov, B.V., et al., 1997a, J. Geophys. Res. 102, 4405-4422.
Khattatov, B.V., et al., 1997b, J. Geophys. Res. 102, 4423-4435.
Manson, A.H., et al., 1985, Handbook for MAP 16, 239-268.
Manson, A.H., et al., 1987, Adv. Space Res. 7, 143-153.
Manson, A.H., et al., 1991, J. Atmos. Sci., 48, 411-428.
Manson, A.H., et al., 1989, J. Atmos. Solar-Terr. Phys. 51, 579-593.
Manson, A. H, et al., 1999, J. Atmos. Solar-Terr. Phys. 61, 809 - 828.
Nastrom, G.D., et al., 1982, Geophys. Res. Lett. 9, 139-307.
Portnyagin, Yu.I., 1984, Handbook for MAP 10, 134-142.
Portnyagin, Yu.I., 1986, J. Atmos. Terr. Phys. 48, 1099-1109.
Portnyagin, Yu.I., 1987, J. Meteorol Hydrol. 4, 6-14.
Portnyagin, Yu.I., and T. V. Solovjova, 1992, J. Meteorol Hydrol. 10, 28-35.
Portnyagin, Yu.I., and T.V. Solovjova, 1998, Adv. Space Res. 20, 1731-1752.
Portnyagin, Yu.I., et al., 1995, J. Atmos. Terr. Phys. 57, 967-977.
Portnyagin, Yu.I., et al., 1999, Earth, Planets Space 51, 701-710.
Portnyagin, Yu.I., et al., 2003: Adv. Space Res., in press.
Wang, D.Y., et al., 1997, J. Geophys. Res., 102, 6729-6745.

 

Homepage

last modification: 27.4.2004