87 research outputs found
Quasi-two-day wave in an unstable summer atmosphere - some numerical results on excitation and propagation
No full textInternational audienceBased 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
Mesosphere/lower thermosphere prevailing wind model
No full textCopyright © 2004 COSPAR. Published by Elsevier Ltd.The mesosphere/lower thermosphere (MLT) wind data from the 46 ground-based (GB) MF and meteor radar (MR) stations, located at the different latitudes over the globe, and the space-based (SB) HRDI data were used for constructing of the empirical global climatic 2-D prevailing wind model at 80–100 km heights for all months of the year. The main data set is obtained during 1990–2001 period. It is shown that the three datasets (MF, MR, HRDI) are mainly well correlated. However, a certain systematic bias between the GB and SB data at 96 km exists, as well as that between the MF and MR data higher 88 km. Simple correction factors are proposed to minimize these biases. The 2-D distant-weighted least-square interpolation procedure for some arbitrary collection of points was used for drawing model contour plots. The model is available in the computer readable form and may be used for construction of the new CIRA model.Yu. Portnyagin, T. Solovjova, E. Merzlyakov, J. Forbes, S. Palo, D. Ortland, W. Hocking, J. MacDougall, T. Thayaparan, A. Manson, C. Meek, P. Hoffmann, W. Singer, N. Mitchell, D. Pancheva, K. Igarashi, Y. Murayama, Ch. Jacobi, D. Kuerschner, A. Fahrutdinova, D. Korotyshkin, R. Clark, M. Taylor, S. Franke, D. Fritts, T. Tsuda, T. Nakamura, S. Gurubaran, R. Rajaram, R. Vincent, S. Kovalam, P. Batista, G. Poole, S. Malinga, G. Fraser, D. Murphy, D. Riggin, T. Aso and M. Tsutsumihttp://www.elsevier.com/wps/find/journaldescription.cws_home/644/description#descriptio
Some numerical results on the quasi-two-day wave excitation andpropagation in the unstable summer middle atmosphere
No full textMit 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
Mesosphere/lower thermosphere winds measured with nearby SKiYMET meteor radars at Collm and Juliusruh, and comparison with Kazan winds
No full text© 2019 URSI Landesausschuss in der Bundesrepublik Deutschland e.V. Mesosphere/lower thermosphere winds have been measured by VHF meteor radars at Collm (51.3°N, 13.0°E) and Juliusruh (54.6°N, 13.4°E) for more than a decade. We compare hourly winds from the two radars. We find that the mean meridional wind difference is small and amounts to less than 2 or 3 m/s in winter or summer, respectively. Zonal hourly wind differences are also smaller than 4 m/s. The standard deviations of the differences are in the order of 15 m/s, and they slightly increase with height. The hourly winds at the two sites are strongly correlated, and the remaining differences are probably mainly due to gravity waves and mesoscale wind structures. We also included meteor radar winds from the Kazan radar (56°N, 49°E), which was installed in 2015. Comparison of the hourly mean Juliusruh winds with those from the Kazan meteor radar also show mostly small mean differences, but the hourly winds are uncorrelated. The individual differences are much larger, due to the influence of tides and planetary waves at the different longitudes of these stations
First results of meteor radar lower thermosphere wind measurements at Dixon, Arctic (73.5゜N, 80゜E)
No full textP(論文)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.departmental bulletin pape
A comparison of optical and radar measurements of mesospheric winds and tides
No full textOptical measurements of mesospheric winds by Fabry‐Perot spectrometers, FPSs, at Mawson, 67.6°S 62.9°E, and Davis, 68.6°S 78.0°E, Antarctica are compared with similar measurements obtained using a spaced‐antenna MF radar at Davis. The FPSs observed the OH emission. Different analysis procedures, used to determine the mean wind, and amplitude and phase of the semidiurnal tide, have been compared. At these latitudes the diurnal tide is weak and the semi‐diurnal tide, although highly variable in amplitude, is usually the dominant periodicity. When comparing the amplitude and phase of the semidiurnal tide good agreement is obtained between measurements by the two instruments
Structural changes in lower ionosphere wind trends at midlatitudes
No full textLong-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
The 1.5-5-day eastward waves in the upper stratosphere-mesosphere as observed by the Esrange meteor radar and the SABER instrument
No full textData of neutral meridional wind obtained by the meteor radar at Esrange and data of temperature and pressure measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) spacecraft were studied with respect to a day-to-day atmospheric variability with periods from 1.5 to 5 days. The detailed analysis was carried out for February 2004. Perturbations of the atmospheric parameters at the examined periods appeared mainly as eastward propagating waves of zonal wavenumbers 1 and 2. We suggested that these waves excited by the jet instability on both flanks of the polar night jet in the upper stratosphere and mesosphere interact non-linearly with each other, and this interaction generates secondary waves. The radar observed both primary and secondary waves at mesospheric heights. The data analysis supports this suggestion. Under conditions of weaker instability observed in February 2003 the perturbations of atmospheric parameters of periods from 1.5 to 5 days had smaller amplitudes at heights of the mesosphere than those in February 2004. It was found that the Eliassen-Palm fluxes calculated for the waves generated by the jet instability were mainly downward directed. This result suggests a possible dynamical influence of the mesospheric layers on the lower atmospheric levels
Radar observations of geomagnetic disturbance effects on midlatitude mesosphere/lower thermosphere dynamics
No full textZeitreihen 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
Influence of geomagnetic disturbances on mean winds and tides in the mesosphere/lower thermosphere at midlatitudes
No full textObservations of upper mesosphere/lower thermosphere (MLT) wind have been performed at Collm (51.3N, 13.0E) and Kazan (56N, 49E), using two SKiYMET all-sky meteor radars with similar configuration. Daily vertical profiles of mean winds and tidal amplitudes have been constructed from hourly horizontal winds. We analyse the response of mean winds and tidal amplitudes to geomagnetic disturbances. To this end, we compare winds and amplitudes for very quiet (Ap≤5) and unsettled/disturbed (Ap≥20) geomagnetic conditions. Zonal winds in both the mesosphere and lower thermosphere are weaker during disturbed conditions for both summer and winter. The summer equatorward meridional wind jet is weaker for disturbed geomagnetic conditions. Tendencies for geomagnetic effects on mean winds over Collm and Kazan qualitatively agree during most of the year. For the diurnal tide, amplitudes in summer are smaller in the mesosphere and greater in the lower thermosphere, but no clear tendency is seen for winter. Semidiurnal tidal amplitudes increase during geomagnetic active days in summer and winter. Terdiurnal amplitudes are slightly reduced in the mesosphere during disturbed days, but no clear effect is visible for the lower thermosphere. Overall, while there is a noticeable effect of geomagnetic variability on the mean wind, the effect on tidal amplitudes, except for the semidiurnal tide, is relatively small and partly different over Collm and Kazan
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