Quasi-two-day wave in an unstable summer atmosphere - some numerical results on excitation and propagation

Based on numerical calculations we demonstrate that small changes in the smooth climatological background atmosphere may lead to an unstable mean zonal wind distribution in the summer middle atmosphere. We relate these changes to small ones because locations and power of the main circulation structures are conserved, except for the acceleration of the easterly jet in the stratosphere/mesosphere. The instability forces oscillations propagating westward with a period of about 2 days and zonal wave numbers s=3 and/or 4. There are variations in the mean zonal wind distribution due to the excitation and transient propagation of these waves, and the numerical results correspond to features of these variations observed in experimental studies. The growing waves tend to remove the source of excitation. This process is effective enough to reduce the strong easterly jet and to remove the strong negative gradient of the zonal mean potential vorticity in the region of the instability. Therefore, when these parameters are calculated as mean values over a long time interval, the obtained values are too small to provide the instability. Strong 2-day waves, in turn, are unstable and can generate secondary waves with longer periods and lower zonal wave numbers. This effect is only significant for extremely strong 2-day waves. Another process is found to be more effective to produce secondary waves. We demonstrated that the 2-day wave with s=3 forced by nonlinear interaction between the 10-14 day planetary waves and the 2-day wave of zonal wave number 4 is unstable. This wave instability generates secondary waves with amplitudes that are large enough to be observed by ground-based radars, for example

Some numerical results on the quasi-two-day wave excitation andpropagation in the unstable summer middle atmosphere

Mit Hilfe numerischer Simulationen wird gezeigt, dass manche Änderungen des klimatologischen Hintergrundwindfeldes zu instabilem mittleren Zonalwind in der mittleren Atmosphäre Sommerhemisphäre führen. Diese Instabilität treibt Oszillationen mit einer Periode um 2 Tage an, welche eine zonale Wellenzahl von s = 3 oder 4 aufweisen. Beobachtete Variationen des mittleren Windes stehen in Verbindung mit diesen numerisch gefundenen Schwingungen. Starke 2-Tage-Wellen wiederum sind instabil und können daher Wellen längerer Periodendauer und kleinerer Wellanzahl anregen. Dieser Effekt ist jedoch nur für sehr starke 2-Tage-Wellen signifikant. Effektiver ist ein Prozess, bei dem nichtlineare Wechselwirkung zwischen einer 10-14-Tage-Welle und der 2-Tage-Welle der zonalen Wellenzahl 4 eine neue quasi-2-Tage-Welle mit einer Periodendauer von 55-60 Stunden anregt. Diese Welle generiert sekundäre Wellen effektiver als die ursprüngliche 2-Tage-Wellen; die sekundären Wellen können beobachtet werden.Basing on numerical calculations we have demonstrated that some changing of the climatological background atmosphere could lead to an unstable mean zonal wind distribution in the summer middle atmosphere. This instability forces oscillations propagating westward with a period of about 2 days and zonal wavenumbers s = 3 and/or 4. There are variations in the mean zonal wind distribution due to the excitation and transient propagation of these waves and numerical results correspond to features of these changes obtained in experimental studies. Strong 2-day waves in turn are unstable and can generate secondary waves with longer periods and lower zonal wavenumbers. This effect is significant only for very strong 2-day waves. It is shown that the 2-day wave with s=3 forced by non-linear interaction between 10-14 day planetary waves and the 2-day wave of zonal wave number 4 is unstable. This wave generates secondary waves of lower zonal wavenumbers more easily than the primary 2-day waves and these secondary waves may be observed

Radar observations of geomagnetic disturbance effects on midlatitude mesosphere/lower thermosphere dynamics

Zeitreihen von Monatsmittelwerten des Windes in der Mesosphäre/unteren Thermosphäre über Collm werden auf mögliche Korrelationen mit der Nordatlantischen Oszillation (NAO) und der Südlichen Oszillation (SO) hin untersucht. Während eine positive Korrelation bis in die 1990er Jahre existiert, schwächt sich diese in der Folge ab und kehrt sich teilweise um. Da NAO und SO gekoppelt sind, erfolgen diese Änderungen etwa zur selben Zeit. Die Änderung der Kopplung steht wahrscheinlich in Verbindung mit einer generellen Änderung der Dynamik der mittleren Atmosphäre

Structural changes in lower ionosphere wind trends at midlatitudes

Long-term variability of the mesosphere/lower thermosphere (lower E region ionosphere) since 1970 has been analyzed using wind data series obtained at Collm (52° N, 15° E) using the LF drift method and at Obninsk (55° N, 37° E) applying VHF meteor radar. Applying piecewise linear trend analysis with a priori unknown number and positions of breakpoints shows that trend models with breakpoints are generally to be preferred against straight lines. There is a strong indication for a change of trends in wind parameters around 1975–1980. Similar changes are also found in the lower atmosphere, e.g., in tropospheric temperatures. This indicates a coupling between atmospheric layers at time scales of decades

First results of meteor radar lower thermosphere wind measurements at Dixon, Arctic (73.5゜N, 80゜E)

Results of simultaneous wind measurements by the identical meteor radars at Dixon (73.5°N, 80°E) and Obninsk (55°N, 37°E) are presented for the time interval from November 12, 1999 to July 31, 2000. A number of features were observed which require comprehensive investigation on the basis of long-term wind measurements in the high-latitude lower thermosphere. The observed semidiurnal tide phases at Dixon are close to those published for Troms0, providing some evidence for predominance of the migrating semidiurnal tide for semidiurnal oscillations at this latitude. Highly coherent oscillations in tidal amplitudes and prevailing winds were also revealed, as well as time intervals with non-significant semidiurnal tide during which oscillations with periods different from but close to 12 h were observed

The summertime 12-h wind oscillation with zonal wavenumber <i>s</i> = 1 in the lower thermosphere over the South Pole

International audienceMeteor radar measurements of winds near 95 km in four azimuth directions from the geographic South Pole are analyzed to reveal characteristics of the 12-h oscillation with zonal wavenumber one (s=1). The wind measurements are confined to the periods from 19 January 1995 through 26 January 1996 and from 21 November 1996 through 27 January 1997. The 12-h s=1 oscillation is found to be a predominantly summertime phenomenon, and is replaced in winter by a spectrum of oscillations with periods between 6 and 11.5 h. Both summers are characterized by minimum amplitudes (5?10 ms?1) during early January and maxima (15?20 ms?1) in November and late January. For 10-day means of the 12-h oscillation, smooth evolutions of phase of order 4?6 h occur during the course of the summer. In addition, there is considerable day-to-day variability (±5?10 ms?1 in amplitude) with distinct periods (i.e., ~5 days and ~8 days) which suggests modulation by planetary-scale disturbances. A comparison of climatological data from Scott Base, Molodezhnaya, and Mawson stations suggests that the 12-h oscillation near 78°S is s=1, but that at 68°S there is probably a mixture between s=1 and other zonal wavenumber oscillations (most probably s=2). The mechanism responsible for the existence of the 12-h s=1 oscillation has not yet been identified. Possible origins discussed herein include in situ excitation, nonlinear interaction between the migrating semidiurnal tide and a stationary s=1 feature, and thermal excitation in the troposphere

High- and mid-latitude quasi-2-day waves observed simultaneouslyby four meteor radars during summer 2000

International audienceResults from the analysis of MLT wind measurements at Dixon (73.5°N, 80°E), Esrange (68°N, 21°E), Castle Eaton (UK) (53°N, 2°W), and Obninsk (55°N, 37°E) during summer 2000 are presented in this paper. Using S-transform or wavelet analysis, quasi-two-day waves (QTDWs) are shown to appear simultaneously at high- and mid-latitudes and reveal themselves as several bursts of wave activity. At first this activity is preceded by a 51?53h wave with S=3 observed mainly at mid-latitudes. After a short recess (or quiet time interval for about 10 days near day 205), we observe a regular sequence of three bursts, the strongest of them corresponding to a QTDW with a period of 47?48h and S=4 at mid-altitudes. We hypothesize that these three bursts may be the result of constructive and destructive interference between several spectral components: a 47?48h component with S=4; a 60-h component with S=3; and a 80-h component with S=2. The magnitudes of the lower (higher) zonal wave-number components increase (decrease) with increasing latitude. The S-transform or wavelet analysis indicates when these spectral components create the wave activity bursts and gives a range of zonal wave numbers for observed bursts from about 4 to about 2 for mid- and high-latitudes. The main spectral component at Dixon and Esrange latitudes is the 60-h oscillation with S=3. The zonal wave numbers and frequencies of the observed spectral components hint at the possible occurrence of the nonlinear interaction between the primary QTDWs and other planetary waves. Using a simple 3-D nonlinear numerical model, we attempt to simulate some of the observed features and to explain them as a consequence of the nonlinear interaction between the primary 47?48h and the 9?10day waves, and the resulting linear superposition of primary and secondary waves. In addition to the QTDW bursts, we also infer forcing of the 4-day wave with S=2 and the 6?7day wave with S=1, possibly arising from nonlinear decoupling of the 60-h wave with S=3. The starting mechanism for this decoupling is the Rossby wave instability (e.g. Baines, 1976). This result is consistent with the day-to-day wind variability during the observed QTDW events. An interesting feature of the final stage of the observed QTDW activity in summer 2000 is the occurrence of strong 4?5 day waves with S=3. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides; general or miscellaneous