COLLM UPPER ATMOSPHERE WIND DATA 52N, 15E

The wind field of the upper mesopause region is continually observed by daily D1 radio wind measurements in the LF range, using the ionospherically reflected sky wave of commercial radio transmitters on three measuring paths (177, 225 and 270 kHz). The measurements are carried out according to the closely-spaced receiver technique. A modified form of the similar-fade method is used to interpret the wind measurements (see e.g. Schminder and Kürschner, 1992, 1994). The data are combined to half-hourly zonal and meridional mean wind values on each frequency. Including the results of the individual measurements on each of the three frequencies, combined with a weighting function based on an estimate of the "chaotic velocity" (Sprenger and Schminder, 1969; Schminder and Kürschner, 1992), mean values are calculated that refer to a reflection point at 52N, 15E. Since during the day the absorption of the sky wave is too large, the daily measuring period is in summer restricted to night and twilight, while in the most part of the winter measurements are possible during the whole day.

The reflection height is measured on 177 kHz using travel time differences between the ground wave and the reflected sky wave. The differences are obtained using side-band phase comparsons of both wave components in the modulation frequency range near 1.8 kHz (Kürschner et al., 1987). Since the measurements are inhomogeneously distributed in time, a multiple regression analysis is used to determine estimates of the daily prevailing wind as well as the tidal wind field components from the half-hourly mean values of the measured zonal and meridional wind components. The spectral selectivity of the separation of prevailing and tidal wind was improved through fitting the measured values for the two horizontal wind components as a vector, assuming clockwise circularly polarized tidal wind components (Kürschner, 1991)

Note that the reflection height is not directly used in this kind of analysis. Since the total reflection measurements provide wind values only at one height (which is varying through the night) a direct estimation of daily wind profiles is not possible. However, if the measured half-hourly wind components of several days are taken together, a mean wind profile can be calculated using a modified form of Eq. 1 with height-dependent coefficients (e.g. Schminder and Kürschner, 1988, 1990, 1992, 1994). These results may depend on the degree of height-dependence. Please contact Christoph Jacobi, if you are interested.

The reflection height ranges roughly between 85 and 105 km on a monthly average, but individual values can be much higher, especially in winter after midnight or under geomagnetically disturbed conditions. After sunrise h´ decreases rapidly due to ionisation in the D-region. Since absorption during the day is large, partially no measurements are possible then. In the late afternoon h´ rises slowly to its nighttime values. Additionally, in winter after midnight very high values of h´ are found as a result of the split of the reflected sky wave into the ordinary and the extraordinary component. As a consequence of these diurnal reflection height variations not all of the half-hourly measurements for the regression analysis can be used, since especially in summer large gradients of the prevailing wind would influence the results of the analysis due to apparent wind variations while the reflection height changes. Therefore only those half-hourly mean wind values are included, when the mean monthly reflection height has values that are sufficiently close to the mean nighttime value of about 95 km. In Table 1 the respective time intervals, from which the data are taken, are listed for each month of the year. Using only these intervals it is possible to reduce the error due to height variations. However, it has to be kept in mind, that especially during high geomagnetic activity the reflection height may strongly vary. Thus, since geomagnetic and solar activity are correlated and the latter exhibits an 11-year cycle, long-term trend analyses may be influenced by this effect and especially estimates of possible solar cycle dependencies of planetary wave activity have to be interpreted with care.

We als calculate monthly means from monthly median half-hourly values. For these daily periods of 17 - 5 UT are used.

REFERENCES

Kürschner, D. (1991) Ein Beitrag zur statistischen Analyse hochatmosphärischer Winddaten aus bodengebundenen Messungen. Zeitschrift für Meteorologie 41, 262 - 266.

Kürschner, D., Schminder, R., Singer, W. and Bremer, J. (1987) Ein neues Verfahren zur Rea- lisierung absoluter Reflexionshöhenmessungen an Raumwellen amplitudenmodulierter Rund- funksender bei Schrägeinfall im Langwellenbereich als Hilfsmittel zur Ableitung von Windprofilen in der oberen Mesopausenregion. Zeitschrift für Meteorologie 37, 322 - 332.

Schminder, R. and Kürschner, D. (1988) Mean winds and tides in the 85- to 110-km region over Central Europe in 1983 - 1986. Journal of Geophysical Research 93, 2493 - 2497.

Schminder, R. and Kürschner, D. (1990) Winds and tides in the upper mesopause region over Central Europe in 1988. Zeitschrift für Meteorologie 40, 316 - 321.

Schminder, R. and Kürschner, D. (1992) Höhen-Zeit-Schnitte der Windfelder (Grund- und Gezeitenwind) zwischen 85 und 105 km Höhe über Mitteleuropa für 1990 aus funktechnischen D1-Windmessungen am Observatorium Collm. Kleinheubacher Berichte 35, 137 - 145.

Schminder, R. and Kürschner, D. (1994) Permanent monitoring of the upper mesosphere and lower thermosphere wind fields (prevailing and semidiurnal tidal components) obtained from LF D1 measurements in 1991 at the Collm Geophysical Observatory. Journal of Atmospheric and Terrestrial Physics 56, 1263 - 1269.

Schminder, R., Singer, W., Kürschner, D., Hoffmann, P. and Keuer, D. (1994) Permanent monitoring of the upper atmosphere wind field over Central Europe by a joint analysis of LF wind profiler and MF radar measurements at Collm and Juliusruh (Germany). Meteorologische Zeitschrift, N.F. 3, 297 - 300.

Sprenger, K. and Schminder, R. (1969) On some relationships between the correlation analysis and similar-fade analysis results of drift measurements in the lower ionosphere. Journal of At- mospheric and Terrestrial Physics 31, 1085 - 1098.