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- ItemOn the longitudinal structure of the transient day-to-day variation of the semidiurnal tide in the mid-latitude lower thermosphere - I. Winter season(München : European Geopyhsical Union, 2001) Merzlyakov, E.G.; Portnyagin, Yu.I.; Jacobi, C.; Mitchell, N.J.; Muller, H.G.; Manson, A.H.; Fachrutdinova, A.N.; Singer, W.; Hoffmann, P.The longitudinal structure of the day-to-day variations of semidiurnal tide amplitudes is analysed based on coordinated mesosphere/lower thermosphere wind measurements at several stations during three winter campaigns. Possible excitation sources of these variations are discussed. Special attention is given to a nonlinear interaction between the semidiurnal tide and the day-to-day mean wind variations. Data processing includes the S-transform analysis which takes into account transient behaviour of secondary waves. It is shown that strong tidal modulations appear during a stratospheric warming and may be caused by aperiodic mean wind variations during this event.
- ItemStudy of the tidal variations in mesospheric temperature at low and mid latitudes from WINDII and potassium lidar observations(Göttingen : Copernicus GmbH, 2004) Shepherd, M.; Fricke-Begemann, C.Zonal mean daytime temperatures from the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) and nightly temperatures from a potassium (K) lidar are employed in the study of the tidal variations in mesospheric temperature at low and mid latitudes in the Northern Hemisphere. The analysis is applied to observations at 89 km height for winter solstice, December to February (DJF), at 55° N, and for May and November at 28° N. The WINDII results are based on observations from 1991 to 1997. The K-lidar observations for DJF at Kühlungsborn (54° N) were from 1996-1999, while those for May and November at Tenerife 28° N were from 1999. To avoid possible effects from year-to-year variability in the temperatures observed, as well as differences due to instrument calibration and observation periods, the mean temperature field is removed from the respective data sets, assuming that only tidal and planetary scale perturbations remain in the temperature residuals. The latter are then binned in 0.5 h periods and the individual data sets are fitted in a least-mean square sense to 12-h and 8-h harmonics, to infer semidiurnal and terdiurnal tidal parameters. Both the K-lidar and WINDII independently observed a strong semidiurnal tide in November, with amplitudes of 13 K and 7.4 K, respectively. Good agreement was also found in the tidal parameters derived from the two data sets for DJF and May. It was recognized that insufficient local time coverage of the two separate data sets could lead to an overestimation of the semidiurnal tidal amplitude. A combined ground-based/satellite data set with full diurnal local time coverage was created which was fitted to 24 h+ 12 h+8 h harmonics and a novel method applied to account for possible differences between the daytime and nighttime means. The results still yielded a strong semidiurnal tide in November at 28° N with an amplitude of 8.8 K which is twice the SD amplitude in May and DJF. The diurnal tidal parameters were practically the same at 28° N and 55° N, in November and DJF, respectively, with an amplitude of 6.5 K and peaking at ∼9h. The diurnal and semidiurnal amplitudes in May were about the same, 4 K, and 4.6 K, while the terdiurnal tide had the same amplitudes and phases in May and November at 28° N. Good agreement is found with other experimental data while models tend to underestimate the amplitudes.
- ItemDerivation of turbulent energy dissipation rate with the Middle Atmosphere Alomar Radar System (MAARSY) and radiosondes at Andøya, Norway(München : European Geopyhsical Union, 2016) Li, Qiang; Rapp, Markus; Schrön, Anne; Schneider, Andreas; Stober, GunterWe present the derivation of turbulent energy dissipation rate ε from a total of 522 days of observations with the Middle Atmosphere Alomar Radar SYstem (MAARSY) mesosphere–stratosphere–troposphere (MST) radar running tropospheric experiments during the period of 2010–2013 as well as with balloon-borne radiosondes based on a campaign in the summer 2013. Spectral widths are converted to ε after the removal of the broadening effects due to the finite beam width of the radar. With the simultaneous in situ measurements of ε with balloon-borne radiosondes at the MAARSY radar site, we compare the ε values derived from both techniques and reach an encouraging agreement between them. Using all the radar data available, we present a preliminary climatology of atmospheric turbulence in the UTLS (upper troposphere and lower stratosphere) region above the MAARSY site showing a variability of more than 5 orders of magnitude inherent in turbulent energy dissipation rates. The derived ε values reveal a log-normal distribution with a negative skewness, and the ε profiles show an increase with height which is also the case for each individual month. Atmospheric turbulence based on our radar measurements reveals a seasonal variation but no clear diurnal variation in the UTLS region. Comparison of ε with the gradient Richardson number Ri shows that only 1.7 % of all the data with turbulence occur under the condition of Ri < 1 and that the values of ε under the condition of Ri < 1 are significantly larger than those under Ri > 1. Further, there is a roughly negative correlation between ε and Ri that is independent of the scale dependence of Ri. Turbulence under active dynamical conditions (velocity of horizontal wind U > 10 m s−1) is significantly stronger than under quiet conditions (U < 10 m s−1). Last but not least, the derived ε values are compared with the corresponding vertical shears of background wind velocity showing a linear relation with a corresponding correlation coefficient r = 58 % well above the 99.9 % significance level. This implies that wind shears play an important role in the turbulence generation in the troposphere and lower stratosphere (through the Kelvin–Helmholtz instability).
- ItemA case study of gravity waves in noctilucent clouds(München : European Geopyhsical Union, 2004) Dalin, P.; Kirkwood, S.; Moström, A.; Stebel, K.; Hoffmann, P.; Singer, W.We present a case study of a noctilucent cloud (NLC) display appearing on 10-11 August 2000 over Northern Sweden. Clear wave structures were visible in the clouds and time-lapse photography was used to derive the parameters characterising the gravity waves which could account for the observed NLC modulation. Using two nearby atmospheric radars, the Esrange MST Radar data and Andoya MF radar, we have identified gravity waves propagating upward from the upper stratosphere to NLC altitudes. The wave parameters derived from the radar measurements support the suggestion that gravity waves are responsible for the observed complex wave dynamics in the NLC.