The 8-h tide in the mesosphere and lower thermosphere over Collm (51.3° N; 13.0° E), 2004–2011

The horizontal winds in the mesosphere and lower thermosphere (MLT) at heights of about 80–100 km have been measured continuously since summer 2004 using an all-sky 36.2 MHz VHF meteor radar at Collm, Germany (51.3° N, 13° E). A climatology of the 8-h solar tide has been constructed from these data. The amplitude shows a seasonal behaviour with maximum values during the equinoxes, and it is generally increasing with altitude. The largest amplitudes are measured in autumn, partly reaching values up to 15 m s⁻¹. The phase, defined as the time of maximum eastward or northward wind, respectively, has earlier values in winter and later ones in summer. Except for summer, the phase difference between the zonal and meridional components is close to +2 h, indicating circular polarization of the tidal components. The vertical wavelengths are short in summer (~20 km) but significantly longer during the rest of the year. The terdiurnal tide is generally assumed to originate from either a terdiurnal component of solar heating or nonlinear interaction between the diurnal and semidiurnal tide. Analysing monthly means reveals positive correlation during the spring maximum, but negative correlation in autumn

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

International 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

Unexpectedly small semidiurnal tidal wind amplitudes in the mid-latitudemesopause region during September 2002

International audienceThe mesopause region monthly mean winds and semidiurnal tidal amplitudes and phases over Central Europe have been measured at Collm Observatory since September 1982. The regular annual cycle of the semidiurnal tidal amplitudes show maximum values during late August and September. In contrast to that, in autumn 2002 no enhancement of the tidal amplitudes was measured, while the autumn tidal phase transition occurred unusually early. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides; climatology

EUV-TEC proxy to describe ionospheric variability using satellite-borne solar EUV measurements

An updated version of a proxy, termed EUV-TEC, describing the global total primary photoionisation is calculated from satellite-borne EUV measurements assuming a model atmosphere consisting of four major atmospheric constituents. Regional number densities of the background atmosphere are taken from the NRLMSISE-00 climatology. For calculation the Lambert-Beer law is used to describe the decrease of the radiation along their way through the atmosphere. The EUV-TEC proxy thus describes the ionospheric response to solar EUV radiation and its variability. EUV-TEC is compared against the global mean total electron content (TEC), a fundamental ionospheric parameter created from vertical TEC maps derived from GPS data. Strong correlation between these indices is found on different time scales. Results show that the EUV-TEC proxy represents the ionsopheric variability better than the conventional solar index F10.7 does, especially during high and moderate solar activity

Parameters of internal gravity waves in the mesosphere-lower thermosphere region derived from meteor radar wind measurements

International audienceA procedure of revealing parameters of internal gravity waves from meteor radar wind measurements is presented. The method is based on dividing the measuring volume into different parts and, using wavelet analysis, calculating the phase progression of frequency peaks in the vertical and horizontal direction. Thus, the distribution of vertical and horizontal wavelengths and directions of IGW energy propagation, using meteor radar data, has been obtained. The method was applied to a 4-month data set obtained in July and August, 1998 and 1999. As expected, the majority of waves have been found to propagate upwards, although a considerable number seem to propagate downwards as well. High-frequency (intrinsic periods T* of less than 2 h) waves are dominating. The distribution of waves over the course of an average day is only weakly structured, with weak maxima in the morning and evening

Impact of intermittent gravity wave activity on the middle atmospheric circulation during boreal winter

Simulations of the circulation in the middle atmosphere during northern winter performed with a nonlinear, mechanistic, global circulation model show that the upper mesospheric jet is greatly overestimated and also the position with respect to latitude and height does not correspond to observations. Apart from that also the winter wind reversal in the mesopause region, evoked by breaking gravity waves (GWs), is located too low around 80km, but is observed to be usually around 100 km. These discrepancies are planned to be eliminated by modifying the distribution of GW amplitudes driving the GW parameterization. This distribution is currently based on potential GW energy data derived from GPS radio occultation measurements and has to be replaced by a distribution based on momentum flux estimates applying midfrequency approximation. The results show a weaker mesospheric jet more realistically tilted towards lower latitudes with height. Also the meridional circulation extending from the summer to the winter pole decelerates and less GWs are propagating into the mesosphere. By additionally varying the GW amplitudes in magnitude and time, the wind reversal is shifted upwards and the mesospheric jet is slowed down.Simulationen der Zirkulation der mittleren Atmosphäre während des nordhemisphärischen Winters unter Verwendung eines nicht-linearen mechanistischen globalen Zirkulationsmodells ergaben beim Vergleich mit Messungen, dass der simulierte, mesosphärische Jet stark überschätzt wird und dessen Position von den Beobachtungen abweicht. Die in der Mesopausenregion einsetzende Windumkehr, hervorgerufen durch brechende Schwerewellen, befindet sich in etwa 80 km anstatt in 100 km. Diese Diskrepanzen sollen eliminiert werden. Hierfür wird die Verteilung der Schwerewellenamplituden, die die Schwerewellenparametrisierung innerhalb des Modells antreibt, am oberen Rand der Troposphäre modifiziert. Diese basiert derzeit auf global beobachteten, zonal gemittelten Daten der potentiellen Energie von Schwerewellen abgeleitet aus GPS Radiookkultationsmessungen und soll durch eine auf Impulsflüssen basierende Verteilung ersetzt werden. Das Modellexperiment zeigt, dass der mesosphärische Jet mit der Höhe in Richtung niedriger Breiten geneigt ist und abgebremst wird. Zudem schwächt die Meridionalzirkulation vom Sommer- zum Winterpol leicht ab und weniger Schwerewellen dringen bis in die Mesosphäre vor. Zusätzlich wird durch zeitliche und unterschiedlich starke Variation der Schwerewellenamplitude die Windumkehr verlagert und der mesosphärische Jet abgebremst

Long-term changes and trends of mesosphere/lower thermosphere winds over Collm, Germany

We analyse 43 years of mesosphere/lower thermosphere (MLT) horizontal winds obtained from a joint analysis of low frequency (LF) spaced receiver lower ionospheric drift measurements from 1979 through 2006 and very high frequency (VHF) meteor radar wind observations since 2004 at Collm (51°N, 13°E). Due to limitations of the earlier LF measurements, we restrict ourselves to the analysis of monthly mean winds near 90 km, which represents the height of maximum meteor activity as well as LF reflections in the MLT. In the 1980s and 1990s, we observe mainly positive trends of the zonal prevailing wind throughout the year, while the meridional winds tend to decrease in magnitude in both summer and winter. We also analyse interannual variability, in particular with respect to a possible signature of NAO and ENSO. These signals, however, are relatively weak and not stable throughout the time of observations.Wir analysieren 43 Jahre Messungen horizontaler Winde in der Mesosphäre/unteren Thermosphäre (MLT) über Collm (51°N, 13°E), die aus einer gemeinsamen Analyse von Langwellen (LF) -Driftmessungen in der unteren Ionosphäre von 1979 bis 2006 und VHF-Meteorradar-Windbeobachtungen seit 2004 gewonnen wurden. Aufgrund der Einschränkungen der früheren LF-Messungen beschränken wir uns auf die Analyse der mittleren monatlichen Windgeschwindigkeiten bei 90 km, welches die Höhe maximaler Meteorraten sowie die mittleren nächtlichen LF-Reflexionshöhen in der MLT darstellt. In den 1980er und 1990er Jahren beobachten wir das ganze Jahr über hauptsächlich positive Trends des zonalen mittleren Windes, während die Stärke des meridionalen Windes sowohl im Sommer als auch im Winter tendenziell abnimmt. Wir analysieren auch die Variabilität von Jahr zu Jahr, insbesondere im Hinblick auf eine mögliche Signatur von NAO und ENSO. Diese Signale sind jedoch relativ schwach und nicht über die gesamte Beobachtungszeit stabil nachweisbar

Piecewise linear trend detection in mesosphere/lower thermosphere wind time series

A piecewise linear model is developed to detect climatic trends and possible structural changes in time series with a priori unknown number and positions of breakpoints. The initial noise is allowed to be interpreted by the first- and second-order autoregressive models. The goodness of fit of candidate models, if the residuals are accepted as normally distributed white noise, is evaluated using the Schwarz Bayesian Information Criterion. The uncertainties of all modeled trend parameters are estimated using the Monte-Carlo method. The model is applied to the mesosphere/lower thermosphere winds obtained at Collm (52°N, 15°E) during 1960-2007. A persistent increase after ~1980 is observed in the annual mean zonal wind based on the primary model while only a weak positive trend arises in the meridional component. Major trend breakpoints are identified around 1968-71 and 1976-79 in both the zonal and meridional winds

Gravity wave flux modulation by planetary waves in a circulation model

Mit Hilfe eines Zirkulationsmodells der mittleren Atmosphäre wird die Ausbreitung der Quasi-Zwei-Tage-Welle simuliert. Das Modell verfügt über eine aktuelle Schwerewellenparametrisierung und ermöglicht daher die detaillierte Beschreibung der Wechselwirkung planetarer Wellen mit Schwerewellen. Bei Anwesenheit der Quasi-Zwei-Tage-Welle wird der Schwerewellenfluss mit der Periode von zwei Tagen und der räumlichen Struktur der Quasi- Zwei-Tage-Welle moduliert. Modellergebnisse zeigen, dass sich die Quasi-Zwei-Tage-Welle nicht gut in die untere Thermosphäre ausbreitet. Phasenvergleiche zwischen Quasi-Zwei-Tage-Welle und Divergenz des Eliassen-Palm-Flusses der Schwerewellen zeigen, dass dies eine Folge sekundärer Anregung der Quasi-Zwei-Tage-Welle durch brechende Schwerewellen ist, die außer Phase mit der Originalwelle erfolgt