J. Klostermeyer

Remote sensing of the atmosphere by VHF radar experiments

Problems in atmospheric dynamics, in particular in micro- and mesoscale processes, which can be solved by radar investigations, are summarized. The method of atmospheric radar experiments is described. Some relevant results obtained with the new SOUSY-VHF-Radar are presented, including observations of a warm-front passage, of layered structures, and of vertical and horizontal velocities in the troposphere. A power profile of radar echoes from heights up to the stratosphere proves that the SOUSY-VHF-Radar in its final operating state will be able to investigate structures and dynamics in the mesosphere too.

A simple model of the ice particle size distribution in noctilucent clouds

A timescale analysis indicates that at noctilucent cloud (NLC) heights near 83 km, the growth and decay of ice particles and their transport by tide- and gravity-wave-associated vertical velocity components are the dominant processes that determine the particle size distribution. Other processes including sedimentation are of minor importance, so that the formation of NLCs should, in general, not depend on atmospheric conditions at the mesopause, in agreement with recent findings from simultaneous lidar and rocket experiments. Then a simple NLC model can be constructed consisting essentially of a partial differential equation which describes the temporal behavior of the particle size distribution in particle parcels moving with the wave-associated air velocity. Contrary to previous ones, the present model yields a steady state oscillation within an integration time of 1 day because the optically active ice particles have lifetimes of a few hours only. NLC simulations yield particle concentrations, mean radii, and scatter ratios which are in good agreement with observational results. The model also predicts an occasional coexistence of small populations of optically active particles and large populations of microscopic particles, which may be of basic importance for the interpretation of recent observations indicating a more or less tight coupling between NLCs and simultaneously occurring polar mesosphere summer echoes.

Evidence for ice clouds causing polar mesospheric summer echoes

We compare 22 temperature height profiles derived from falling sphere experiments with simultaneous 50-MHz mesospheric radar echoes obtained during the Middle Atmosphere Co-operation/Summer in Northern Europe campaign in the summer of 1987 above Andenes, Norway. Common features of these observations are as follows: (1) More than 80% of the echoes occur at temperatures below 140 K and more than two thirds of the bottommost echo maxima appear at temperatures that deviate less than 5° from 140 K. (2) We can find no significant correlation between the echo strength or the echo occurrence probability and the local temperature once it is below 140 K. (3) The echoes are not symmetrically distributed around the mesospheric temperature minimum, but there are clearly more echoes below than above the temperature minimum. Equivalently, while the local temperature at the height of the bottommost echoes is about 140 K, it is much lower at the height of the topmost echoes. We compare the height range of the observed polar mesospheric summer echo returns with the height range that can be inferred for an ice cloud from the observed temperature profile by means of a simple stationary nucleation/sedimentation model. For reasonable values of the height-integrated ice particle nucleation rate of about 3×107 m−2 s−1 and for a water molecule mixing ratio at the bottom of the ice cloud of 0.1 to 1.0 ppmv, we find excellent agreement between the height range within which the ice particles may exist and the height range from which mesospheric echoes are seen. For example, in 17 out of the 22 individual observations, reasonable values for the water vapor mixing ratio can be found such that the height of the strong bottommost echo corresponds to within the resolution of the radar to the bottom height of our model ice cloud. If our height estimates are correct and the observations are representative, our results yield a somewhat smaller water vapor mixing ratio in the summer mesosphere at high latitudes compared to what has been measured at midlatitudes.

A height‐ and time‐dependent model of polar mesosphere summer echoes

The generation of VHF radar echoes in the polar summer mesosphere is studied with a height- and time-dependent model including three mechanisms: electron scavenging by microscopic ice particles created by heterogeneous nucleation on meteoric dust, formation of steep gradients in the ice particle and electron concentrations by the Kelvin effect, and reduction of the electron diffusivity by the presence of negatively charged particles. Then turbulent mixing produces a viscous convective subrange in the variance spectrum of the electron concentration, which in turn, can give rise to strong radar echoes. Based on a gravity-wave-perturbed model and many sensitivity tests, several conclusions are reached. Taking into account the high variability of relevant parameters, the model can predict the wide range of reflectivities measured on the northern hemisphere and indicates that the virtual absence of detectable radar echoes on the southern hemisphere is probably due to the annual variation of the meteoric mass influx and relatively high mesopause temperatures. The frequently occurring double-layer structure of the reflectivity is characteristic of almost unperturbed rnesospheric conditions. In the presence of weak harmonic gravity waves, the inherently strong nonlinearity of the model yields steepened and breaking reflectivity structures. In agreement with observational results, the model further predicts a positive (negative) correlation between the vertical velocity variance and the reflectivity of spectrally broad (narrow) echoes. It is suggested that the frequently measured aspect sensitivity of spectrally narrow echoes results from anisotropic turbulence in the electron gas occurring if the inertial subrange is small or absent and the viscous-convective subrange virtually adjoins the buoyancy subrange. Even under extremely favorable conditions, the calculated reflectivities at radar frequencies near 1 GHz are much weaker than measured ones, confirming an earlier result that in the UHF range, polar mesosphwre summer echoes originate from modified Thomson scattering.

A two‐ion ice particle model of the polar summer mesopause region

A one-dimensional model is used to compute height dependent concentrations of molecular ions, cluster ions, positively and negatively charged ice particles, and electrons in the high-latitude summer mesopause region. In agreement with experimental results, the computed height profiles show a rapid transition from cluster to molecular ions between 85 and 90 km without ion depletion, whereas deep electron “bite-outs” occur within a particle layer. The model can be forced by turbulent neutral density fluctuations. It is assumed that gradient mixing generates perturbations in the concentration of all particles which are considered as passive conservative constituents so that their energy-wavenumber spectrum contains a viscous-convective subrange due to a high Schmidt number. In particular at the boundaries of the particle layer, small fluctuations in the neutral gas give rise to strong fluctuations in the ionized constituents. Computed radar reflectivity profiles are characterized by a double-peak structure and agree well with reflectivity profiles measured at frequencies of 53.5 and 224 MHz. Without assuming excessive water vapor mixing ratios, the model cannot explain measured reflectivities at frequencies near 1 GHz, supporting observational evidence that radar echoes in the VHF and UHF ranges are generated by distinct scatter mechanisms.

Excitation of acoustic-gravity waves in a realistic thermosphere

Abstract The response of the upper atmosphere due to the excitation of acoustic-gravity waves by various sources is studied under realistic thermospheric conditions. Losses due to viscosity, thermal conduction and ion drag as well as the effects of vertical temperature variations and winds are all taken into account. The general solution is obtained in terms of the eight characteristic wave modes that propagate in the atmosphere. An atmospheric transfer function is defined which determines the response of the upper atmosphere due to general sources. Examples of the transfer function are computed for model atmospheres. It is found that the transfer functions are peaked in the wave-number space indicating strong filtering effects, and that their behaviour is affected by the background winds. The transfer functions are then applied to obtain some transient responses of the atmosphere due to excitations. The results are found to differ substantially from those obtained for a lossless, isothermal atmosphere. Implications of the computed results to the interpretation of observed acoustic-gravity wave data are discussed.

Propagation of turbulence structures detected by VHF radar

Abstract In the high latitude wintertime mesosphere VHF radar measurements usually reveal several turbulence layers at heights between 65 and 85 km which are closely related to strong vertical wind shear. The turbulence layers are superposed by turbulence bursts, which often form sequences with periods similar to those of simultaneously observed velocity oscillations. The horizontal propagation velocity of the resulting turbulence structures can be obtained by cross-correlating the signal power time series measured at three antenna beam positions. A statistical study using a total of 71 events shows that there is a significant correlation between the propagation velocity of turbulence structures and the mean wind, being consistent with the assumption that turbulence is advected by large scale motions. It is suggested that the observed turbulence bursts are due to secondary static instabilities, which for their part are generated by primary Kelvin-Helmholtz instabilities in regions of strong wind shear.

On the diurnal variation of polar mesosphere summer echoes

During quiet solar and geomagnetic conditions, the observed mean diurnal variation of the reflectivity of polar mesosphere summer echoes is characterized by a distinct maximum near noon and a deep minimum near 20 LT, so that it appears to contain a relatively strong semidiurnal component. Theoretical considerations predict a nonlinear relationship between the reflectivity, the tidal temperature variation, and the electron production by solar radiation and energetic-particle precipitation. Least squares fits based on model predictions of atmospheric tides, on a Chapman function, and on cosmic noise absorption data indicate that the observed diurnal variation of the reflectivity is predominantly determined by the electron production. Its semidiurnal component is essentially caused by the fact that the ionization rates due to the solar radiation and the energetic-particle precipitation have maxima at noon and near midnight, respectively.

Observations of Mesospheric Summer Echoes at VHF in the Polar Cap Region

VHF radar measurements have been carried out at high polar latitudes using the SOUSY-Svalbard-Radar in the summer of 1999 and spring of 2000 at Longyearbyen/Norway (78°N, 16°E). The vertical and temporal variabilities of polar mesospheric summer echoes and mean winds in the mesopause region have been studied for the first time at such high latitudes in the polar cap region. The typical height variation of the received signal is characterized by a pronounced double layer structure during June and July, which is much less pronounced at latitudes 10° further south. Within late summer to equinox conditions, however, the two peaks merge and one layer is dominating. It is suggested that these features together with observed changes in the mesospheric wind system are related to the temperature variation in the mesopause region.

An investigation of measured temperature profiles and VHF mesosphere summer echoes at midlatitudes

During the summers of 1992 and 1994, experiments were conducted in Germany (52°N) to investigate the region of the summer mesopause in connection with mesospheric summer echoes (MSE). MSE form in layers and are associated with dramatically enhanced radar scattering cross sections. Furthermore, since the summer mesopause is characterized by very low temperatures, it has been proposed that MSE layers are related to the presence of subvisible ice particles. Parallel measurements were made using the Sounding System (SOUSY) VHF radar and Rayleigh lidar. Radar observations showing MSE signatures are presented together with accompanying temperature profiles obtained from the lidar data. Generally, the ambient temperatures in the summer mesosphere at midlatitudes are above that required to create ice crystals. However, the presence of waves can reduce the temperature. Our observations show this to be the case. In particular, during one of the MSE events, two long-period inertia gravity waves were observed. These waves had estimated local periods of 6 and 8 hours with accompanying vertical wavelengths of 10 and 15 km, respectively. We find a good correlation between the locations of the calculated temperature minima and the observed MSE layers. Furthermore, the temperature minima were generally below the saturation temperature for water vapor, lending support to the supposition that the backscatter is connected with the presence of ice particles.