Christoph Jacobi

Global and Seasonal Variations of Stratospheric Gravity Wave Activity Deduced from the CHAMP/GPS Satellite

Abstract Global analyses of gravity wave (GW) activity in the stratosphere are presented using radio occultation data from the Challenging Minisatellite Payload (CHAMP) satellite. Temperature profiles obtained from CHAMP/ GPS radio occultations are first compared with ground-based instruments. In general, good agreement is found between these different techniques. Monthly mean values of potential energy Ep, being a measure of GW activity, which is estimated with radiosonde observations, are compared with CHAMP/GPS data and it is found that radiosonde-observed Ep values are slightly higher than those estimated with radio occultations. Strong diurnal variation of GW activity has been found. From the global morphology of GW activity, large Ep values are noticed, besides at tropical latitudes, even at midlatitudes during winter, but not during equinoxes. This suggests that wave activity at stratospheric heights is not only modulated due to orography (mountain/lee waves) but mainly depends on seasonal variatio...

Comparative study of interannual changes of the mean winds and gravity wave activity in the middle atmosphere over Japan, Central Europe and Canada

Abstract Results are presented of measurements of the mean wind and its variances attributed to the intensity of internal gravity waves (IGWs) with the MU radar at Shigaraki, Japan, from 1986 to 1999, with the low-frequency ionospheric drift method Dl at Collm, Germany and with the medium frequency (MF) radar at Saskatoon, Canada, between 1983 and 1999 at altitudes 65–80, 80–110 and 60– 100 km , respectively. The amplitudes of eastward–westward seasonal variations of the mean zonal wind in the mesosphere and lower thermosphere are different in different years. The mean wind and IGW intensities show substantial seasonal, quasi-biennial and interannual variations, which may be different at different altitudes. The measurements show differences in the interannual changes, IGW intensities at different locations. Comparison of interannual changes of the wind and drift velocity variances reveal substantial differences between Saskatoon and Collm and Shigaraki. Such differences in seasonal and interannual changes in the wind perturbation variances may be attributed to the changes in the strengths of IGW sources in the atmosphere at different altitudes and locations and with the conditions of wave propagation in the lower, middle and upper atmosphere. Among the possible reasons for these changes could be solar activity, the eruption of the Pinatubo volcano in June 1991 and, probably, interannual changes of the temperature of oceans in tropics (El Nino events) on the circulation of the middle and upper atmosphere.

Analysis of Gravity Waves from Radio Occultation Measurements

In the height range 10–30 km atmospheric gravity waves lead to periodic perturbations of the background temperature field in the order of 2–3 K, which can be resolved in temperature profiles derived from radio occultation measurements. Due to the spherical symmetry assumption in the retrieval algorithm and the low horizontal resolution of the measurement weakening in the amplitude and phase shift of the waves occurs. The influence of the geometric wave parameters and the measurement geometry on a homogeneous spectrum of plane gravity waves in the range 100–1000 km horizontal and 1–10 km vertical wavelength is investigated with a 2D-model ranging ±1000 km around the tangent point and 10–50 km in height. The investigation shows that with radio occultation measurements more than 90% of the simulated wave spectrum can be resolved with relative amplitudes above the 1/e level. Considering the total variance, about 80% is retrieved in the worst case when the GPS receiver scans perpendicular to the wave crests and about 88% as mean value when the view angle of the receiver through the gravity waves is arbitrarily oriented. More realistic wave spectra lead to slightly smaller values.

Influence of anthropogenic climate gas changes on the summer mesospheric/lower thermospheric meridional wind

A model study is performed to simulate the effect of the observed atmospheric CO 2 increase and O 3 decrease on the northern hemisphere (NH) summer middle atmosphere meridional circulation. It is found that, due to a reduction of gravity wave filtering through the stratospheric / mesospheric easterly jet, the mesopause region meridional wind at midlatitudes decreases by up to 2 m s -1 , i.e. about 25%, if the middle atmospheric CO 2 content is increased by 10% and O 3 is reduced up to 10% at high latitudes reflecting the changes of atmospheric composition over the last 30 years. The model results are in good agreement with midlatitude long-term radar wind measurements at that altitude.

Global distribution of the migrating terdiurnal tide seen in sporadic E occurrence frequencies obtained from GPS radio occultations

Global Positioning System radio occultation measurements by FORMOsa SATellite mission-3/Constellation Observing System for Meteorology, Ionosphere and Climate satellites were used to analyse the characteristics of the 8-h oscillation in sporadic E (ES) layers. Six-year averages based on the 3-monthly mean zonal means from December 2006 to November 2012 were constructed for the amplitude of the terdiurnal oscillation in the occurrence frequency of ES. A global distribution from 60° S to 60° N is given, revealing two peaks above 100 km during solstice with one maximum at low and midlatitudes (approximately 10° to 40°) in each hemisphere. During equinox, the global distribution is marked by two dominant peaks centred at midlatitudes, while an additional weak maximum is located at very low southern latitudes. The seasonal characteristics around 110 km reveal large values during equinox at low and midlatitudes (<40° N), while further peaks occur in April at >40° S and in July near 30° S. The pattern around 90 km is dominated by a broad peak between 20° and 30° S from March to September. Comparisons with the terdiurnal oscillation in the neutral atmosphere derived from zonal wind and vertical zonal wind shear simulated with a circulation model of the middle atmosphere, as well as with satellite observations of the terdiurnal tide in temperature, fit quite well for the results above 100 km, but do not show agreement for lower altitudes.

Three-Dimensional Monitoring of the Polar Ionosphere with Ground- and Space-Based GPS

Integrated electron density measurements along GPS signal ray paths provide a useful data source for 3-dimensional ionospheric reconstruction. Since a substantial data amount is required for the application of the inverse problem, ground-based inversions are restricted to regions with high ground receiver density. The additional incorporation of space-based GPS data from LEO satellites allows the monitoring in areas less densely covered by ground receivers. We present a case study of 3-dimensional reconstruction for Nov. 2001 in the North polar region. Input data are calibrated TEC derived from IGS and CHAMP measurements. The results show increased electron densities near the geomagnetic pole. The comparison of the reconstruction results with measurements of the Langmuir Probe onboard CHAMP shows a good agreement along the CHAMP orbit. The large-scale structure of the ionospheric observations are closely related to the present geomagnetic conditions.

Observations and Ray Tracing of Gravity Waves: Implications for Global Modeling

Vertical coupling by atmospheric waves is essential for the wind and temperature structure of the middle atmosphere. In particular, momentum carried by atmospheric gravity waves (GWs) governs the global circulation in the mesosphere and is for instance the reason for the cold summer mesopause. However, the small horizontal scales of GWs (tens to thousands of km) are challenging both global modeling and observations from satellite. Further, due to the small scales involved, there is a severe lack of understanding about GWs themselves, as well as dynamical phenomena involving GWs. Until recently, global observations of GWs were sparse and little was known about the global distribution of GWs, as well as their seasonal variation. Therefore, several projects in the priority program Climate And Weather of the Sun-Earth System (CAWSES) of the Deutsche Forschungsgemeinschaft (DFG) have addressed a number of the most pressing problems. Global distributions of GW activity and momentum fluxes have been derived from observations with number of satellite instruments, resulting in the first multi-year global data sets of GW parameters, covering time scales from seasonal variations up to the duration of almost a full 11-year solar cycle. In addition, seasonal and tidal variations of sporadic E layers in the ionosphere were studied in Global Positioning System (GPS) radio occultation data. Satellite observations of GWs and sporadic E layers were complemented by ground-based observations (radar and low-frequency (LF) drift measurements). All these observations, as well as accompanying modeling activities provided important constraints for GW parameterizations. Further activities addressed important aspects of GW propagation usually neglected in global modeling: GW ray tracing studies revealed the importance of non-vertical propagation of GWs and first steps were undertaken to develop an improved GW parameterization based on GW ray tracing techniques.

A possible connection of mid-latitude mesosphere/lower thermosphere zonal winds and the southern oscillation

Abstract Collm (52°N, 15°E) mid-latitude mesosphere/lower thermosphere (MLT) zonal winds are investigated with respect to a possible influence of the Southern Oscillation (SO). It is found that in winter the prevailing wind is positively correlated with the Southern Oscillation index (SOI), which qualitatively fits to the response of the low-latitude stratosphere on the SO. In summer, however, there is a negative correlation between MLT zonal winds and SOI. A possible mechanism, which includes slightly enhanced planetary wave propagation to the MLT in summers of low SOI is discussed.

Mesospheric and Lower Thermospheric Winds at Middle Europe and Northern Scandinavia during the Leonid 1999 Meteor Storm

We report observations of winds in the mesosphere and lower thermosphere during the Leonid meteor storm of November 17/18, 1999. The observations were obtained at five radar sites in Middle Europe and Northern Scandinavia using meteor radars in Germany and Northern Sweden, Medium Frequency (MF) radars in Germany and Northern Norway and Low Frequency (LF) wind measurements in Germany. We present hourly means of zonal and meridional winds covering the altitude range 82 km to 106 km. At mid-latitudes (52° – 54°N) we observe strong eastward and southward directed winds during the storm phase of the Leonid shower in the early morning hours of November 18 whereas eastward and northward directed winds are dominating at high latitudes (67° – 69°N). Strong semidiurnal and weaker diurnal tidal oscillations are observed in the wind field at both latitudes at altitudes above 90 km.

Characterization of a Double Mesospheric Bore Over Europe

Observations of a pair of mesospheric bore disturbances that propagated through the nighttime mesosphere over Europe are presented. The observations were made at the Padua Observatory, Asiago (45.9°N, 11.5°E) by the Boston University all-sky imager on 11 March 2013. The bores appeared over the north-west horizon, approximately 30 minutes apart, and propagated towards the south-east. Using additional satellite and radar data, we present evidence indicating the bores originated in the mesosphere from a single, larger-scale mesospheric disturbance propagating through the mesopause region. Furthermore, the large-scale mesospheric disturbance appeared to be associated with an intense weather disturbance that moved southeastwards over the United Kingdom and Western Europe during 10 and 11 March.