Baumgarten G.,Leibniz Institute of Atmospheric Physics
Atmospheric Measurement Techniques | Year: 2010
A direct detection Doppler lidar for measuring wind speed in the middle atmosphere up to 80 km with 2 h resolution was implemented in the ALOMAR Rayleigh/Mie/Raman lidar (69° N, 16° E). The random error of the line of sight wind is about 0.6 m/s and 10 m/s at 49 km and 80 km, respectively. We use a Doppler Rayleigh Iodine Spectrometer (DoRIS) at the iodine line 1109 (∼532.260 nm). DoRIS uses two branches of intensity cascaded channels to cover the dynamic range from 10 to 100 km altitude. The wind detection system was designed to extend the existing multi-wavelength observations of aerosol and temperature performed at wavelengths of 355 nm, 532 nm and 1064 nm. The lidar uses two lasers with a mean power of 14 W at 532 nm each and two 1.8 m diameter tiltable telescopes. Below about 49 km altitude the accuracy and time resolution is limited by the maximum count rate of the detectors used and not by the number of photons available. We report about the first simultaneous Rayleigh temperature and wind measurements by lidar in the strato- and mesosphere on 17 and 23 January 2009. Copyright 2010 by the American Geophysical Union.
Lubken F.-J.,Leibniz Institute of Atmospheric Physics
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2014
In this paper some classical concepts regarding scattering of radio waves on turbulent structures in the ionosphere are summarized. Spectral representations according to Batchelor and Driscoll & Kennedy are compared and the role of various potential tracer gradients is elucidated. Systematic similarities and differences in the representation of the impact of these tracers on scatter intensity are investigated. The importance of turbulence and background parameters for radar volume reflectivities is discussed. This study highlights the importance of measuring these parameters as completely and reliably as possible when interpreting the strength of backscattered radar signal in terms of turbulent and atmospheric background parameters. © 2013 Elsevier Ltd.
Becker E.,Leibniz Institute of Atmospheric Physics
Space Science Reviews | Year: 2012
This review recapitulates the concept of the wave-driven residual circulation in the stratosphere and mesosphere. The residual circulation is defined as the conventional mean meridional circulation corrected by the quasi-linear Stokes drift due to atmospheric waves. Only when the zonal-mean primitive equations are transformed using the residual circulation, they reflect the causality arising from the Eliassen-Palm (EP) theorem. The EP theorem states that the proper wave-mean flow interaction, defined as the EP flux divergence, vanishes for waves that are linear, conservative, and steady. In the real atmosphere, this theorem is violated mainly due to wave breaking and turbulence. The resulting EP flux divergence then drives a residual circulation which causes the observed substantial deviations from some hypothetical radiatively determined state. With regard to this dynamical control we discuss the different contributions of Rossby waves and gravity waves. Recapitulation of Lindzen's theory of gravity-wave saturation allows us to interpret various phenomena in the upper mesosphere such as interhemispheric coupling or modulations of the gravity-wave driven branch of the residual circulation by solar proton effects and thermal tides. In addition we discuss the relative importance of changes in radiative transfer and tropospheric gravity-wave sources on the long-term temperature trends in the summer mesosphere. © 2011 Springer Science+Business Media B.V.
Latteck R.,Leibniz Institute of Atmospheric Physics |
Bremer J.,Leibniz Institute of Atmospheric Physics
Journal of Geophysical Research: Atmospheres | Year: 2013
Polar mesosphere summer echoes (PMSE) are strong enhancements of received signal power at very high radar frequencies occurring at altitudes between about 80 and 95km at polar latitudes during summer. PMSE are caused by inhomogeneities in the electron density of the radar Bragg scale within the plasma of the cold summer mesopause region in the presence of negatively charged ice particles. Thus, the occurrence of PMSE contains information about mesospheric temperature and water vapor content but also depends on the ionization due to solar electromagnetic radiation and precipitating high energetic particles. Continuous and homogeneous observations of PMSE have been done on the North-Norwegian Island Andøya (69.3°N, 16.0°E) from 1994 until 2008 using the ALOMAR SOUSY and the ALWIN radar at 53.5MHz. In 2009, the Leibniz-Institute of Atmospheric Physics in Kühlungsborn, Germany started the installation of the Middle Atmosphere ALOMAR Radar System (MAARSY) at the same location. The observation of mesospheric echoes could be continued in spring 2010 starting with an initial stage of expansion of MAARSY and is carried out with the completed installation of the radar since May 2011. Since both the ALWIN radar and MAARSY are calibrated, the received echo strength of PMSE from 14 years of mesospheric observations (1999-2012) could be converted into absolute signal power. This data series could be extended to the years 1994 until 1997 on the basis of signal-to-noise ratio values derived during the years between 1994 and 2008. The PMSE occurrence rate is positively correlated with the geomagnetic Ap index (significance level χ=85-95%), however, is not correlated with the solar Lyman α radiation. Using different regression analysis methods, the PMSE occurrence rates show a significant positive trend during the time interval from 1994 until 2012 (χ=95-99%). Key Points Long-term changes of PMSE from 19 years of continuous VHF radar observationsPMSE occurrence is positively correlated with solar and geomagnetic activityPMSE occurrence frequencies show a positive trend with a high significance level ©2013. American Geophysical Union. All Rights Reserved.
Lubken F.-J.,Leibniz Institute of Atmospheric Physics |
Berger U.,Leibniz Institute of Atmospheric Physics
Journal of Geophysical Research: Atmospheres | Year: 2011
Latitudinal and interhemispheric differences of model results on trends in mesospheric ice layers and background conditions are analyzed. The model nudges to European Centre for Medium-Range Weather Forecasts data below ∼45 km. Greenhouse gas concentrations in the mesosphere are kept constant. Temperature trends in the mesosphere mainly come from shrinking of the stratosphere and from dynamical effects. Water vapor increases at noctilucent cloud (NLC) heights and decreases above due to increased freeze drying caused by temperature trends. There is no tendency for ice clouds in the Northern Hemisphere for extending farther southward with time. Trends of NLC albedo are similar to satellite measurements, but only if a time period longer than observations is considered. Ice cloud trends get smaller if albedo thresholds relevant to satellite instruments are applied, in particular at high polar latitudes. This implies that weak and moderate NLC is favored when background conditions improve for NLC formation, whereas strong NLC benefits less. Trends of ice cloud parameters are generally smaller in the Southern Hemisphere (SH) compared to the Northern Hemisphere (NH), consistent with observations. Trends in background conditions have counteracting effects on NLC: temperature trends would suggest stronger ice increase in the SH, and water vapor trends would suggest a weaker increase. Larger trends in NLC brightness or occurrence rates are not necessarily associated with larger (more negative) temperature trends. They can also be caused by larger trends of water vapor caused by larger freeze drying, which in turn can be caused by generally lower temperatures and/or more background water. Trends of NLC brightness and occurrence rates decrease with decreasing latitude in both hemispheres. The latitudinal variation of these trends is primarily determined by induced water vapor trends. Trends in NLC altitudes are generally small. Stratospheric temperature trends vary differently with altitude in the NH and SH but add up to similar trends at mesospheric cloud heights. Copyright 2011 by the American Geophysical Union.