Ch. Jacobi

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.

Climatology of the semidiurnal tide at 52–56°N from ground-based radar wind measurements 1985–1995

Abstract Long-term wind measurements carried out at 6 northern midlatitude sites (Saskatoon, Sheffield, Juliusruh, Collm, Obninsk, Kazan) are investigated to establish a climatology of the semidiurnal tide in the mesopause region for the narrow latitudinal range between 52°N and 56°N. Comparison of zonal and meridional components shows that in general the horizontal components are circularly polarized. Intercomparison of amplitudes and phases generally shows good agreement between the results from the different measuring systems. The results are compared with an empirical model of the semidiurnal tide. The longitudinal variation of the semidiurnal tide is small in summer, but the tidal amplitudes in winter are larger at Saskatoon and Kazan, compared with the results from the other sites. The possible influence of wave–tidal interaction in the stratosphere on the interannual variability of this difference is discussed.

Planetary wave activity obtained from long-period (2–18 days) variations of mesopause region winds over Central Europe (52 °N, 15 °E)

Abstract Daily analyses of the zonal and meridional prevailing wind at the mesopause region around 95 km height are investigated with respect to long-period variations with periods ranging between 2 and 18 d that may with some restrictions be interpreted as planetary wave activity. A mean climatology of these variations taken from the years 1983–1995 is presented. The mean annual variability of the signal shows good correspondence with some results known from literature, but a strong interannual variability of the wave activity is found. Time series of the zonal component of the planetary wave activity show an increase during the regarded period, which is in accordance with results from literature.

Extra long period (20–40 day) oscillations in the mesospheric and lower thermospheric winds: observations in Canada, Europe and Japan, and considerations of possible solar influences

Abstract An extra long period (20–40 day) oscillation has been identified in the mesospheric and lower thermospheric (60–100 km) winds observed simultaneously by radars (MF, LF) at four sites from 70°N to 30°N in the northern hemisphere during the winter of 1995/1996. A long-term (1980–1999) investigation of this oscillation at Saskatoon and Collm is also carried out to obtain climatological and statistical characteristics. Spectral analysis has shown that this oscillation is a common feature of the winter (November–March) atmosphere, having strong amplitudes throughout the mesosphere (∼10 m/s ) and lower thermosphere (∼5 m/s ) , and being much stronger at mid-low latitudes. Although the oscillation has a climatology similar to the long period normal mode planetary waves (10–16 day), the phases at the various sites are very similar, and not consistent with a freely propagating wave. Comparisons with geomagnetic/solar wind parameters and solar UV radiation suggest that the oscillation could be related to the short-term solar rotation period (ca. 27 days) in some way. However the range of observed wind periods is very broad and this raises questions about this interpretation. Nevertheless the inter-annual variations of this 20–40 day oscillation indicate a weak 11-year solar cycle correlation in the mesosphere (positive) and the lower thermosphere (negative). Also, the cross-correlation between the winds and solar radiation shows significant quasi 27-day correlation and the wind lags behind the solar radiation a few days in the mesosphere. In general it is implied that the atmosphere could react to the solar activity in an indirect way due to certain dynamical mechanisms.

Response of the mesopause region dynamics to the February 2001 stratospheric warming

Abstract The response of the mesosphere/lower-thermosphere (MLT) region to a major stratospheric warming in Europe during winter 2000/2001 has been investigated using mesopause-region winds measured by meteor radar or the LF-D1 method over three stations (Castle Eaton, 52°N; Collm, 52°N; and Esrange 68°N). The vertical wind structure measured over the three sites, and its time evolution, are found to be quite similar despite the different techniques used in the measurements. The effects of stratospheric warming are very clear, and are similar over both the mid-latitude and high-latitude sites. The warming resulted in a reversal of both the zonal and meridional wind. In the zonal component, this reversal was apparently associated with a planetary-wave oscillation with a period of ∼10 days. The effect was most conspicuous in the vertical prevailing wind gradients. The mesopause-region effects thus seem to be the results of a superposition of an intensifying planetary wave and a slow overall decrease in the strength of the zonal prevailing winds.

Numerical simulation of tides, Rossby and Kelvin waves with the COMMA-LIM model

Abstract A 48-layer version of the COMMA-LIM (Cologne Model of the Middle Atmosphere at Leipzig Institute for Meteorology) three-dimensional global mechanistic model of the Earths atmosphere from 0 km to 135 km with logarithmic pressure height coordinates was developed. The model is capable of reproducing the global structures and propagation of different planetary waves in the middle atmosphere. The contribution of gravity waves, tides, Rossby and Kelvin waves on the zonally averaged momentum budget of the mesosphere / lower thermosphere region is investigated.

Long-term trends in the mesopause wind field obtained from LF D1 wind measurements at Collm, Germany

Abstract The wind field of the upper mesopause region over Central Europe (52° N, 15°E) has been recorded automatically since the 1970s by D1 LF wind measurements at the Collm Observatory of the University of Leipzig. The measured time series are analysed with respect to long-term trends and solar cycle dependencies. Using data from different periods, the results partly differ, which is possibly due to oscillations at very long time scales.

Non-linear interaction of the quasi 2-day wave and long-term oscillations in the summer midlatitude mesopause region as seen from LF D1 wind measurements over Central Europe (Collm, 52oN, 15oE)

Abstract From night-time mesopause region wind measurements over Central Europe, the day-to-day variability of the measured quasi 2-day wave is investigated with respect to its correlation with summer planetary wave events. A 14-year dataset of summer winds is used. The results indicate that the 2-day wave is modulated by planetary waves, especially by the 16-day wave. Secondary frequencies that are expected, if a non-linear interaction between the waves is assumed, are found from regression analysis. Additionally, these secondary oscillations are modulated by the duration of the quasi 2-day wave event itself. Tertiary frequencies that belong to this modulation are also found from periodogram analysis. The long-term dataset analysis shows that the non-linear interaction is a regular phenomenon that is frequently found in connection with planetary wave activity in the summer upper mesosphere and lower thermosphere.

A long-term comparison of saskatoon MF radar and collm LF D1 mesosphere-lower thermosphere wind measurements

Abstract Monthly mean profiles of mean winds and semidiurnal amplitudes and phases in the height range 70–110 km measured at Saskatoon (52°N; 107°W) and Collm (52°N; 15°E) between 1983 and 1998 have been investigated. The seasonal variations of both mean winds and tidal amplitudes and phases are similar at both stations, but there are regular differences that lead to quasi-stationary non-zonal structures. The winter semidiurnal tidal amplitudes over Saskatoon are about twice as large than over Collm at the respective height range, but the vertical phase structure is the same. The characteristic mean wind structures as, for example, the summer southward wind maximum or the summer mesospheric easterly jet are found to be about 5–10 km lower over Saskatoon than over Collm. The mean winter vertical prevailing wind gradients are stronger over Saskatoon than over Collm. On a decadal time scale the variations of the wind parameters are similar, but on the interannual time scale variations differ; in winter this indicates a possible connection with the mean stratospheric circulation.