P. Czechowsky

Absolute reflectivities and aspect sensitivities of VHF radio wave scatterers measured with the SOUSY radar

Abstract By accurately calibrating the SOUSY radar in West Germany it has been made possible to measure absolute values of effective reflection coefficients and turbulence structure constants. Some typical values of these parameters as a function of altitude are presented. Such profiles are presented for both a vertically directed beam, and also for two beams directed 7° off-vertical. Comparisons of powers on the vertical and off-vertical beams show that scatter became more aspect sensitive at the tropopause and in the lower stratosphere, but, unexpectedly, scatter was observed to become considerably more isotropic in the higher regions of the stratosphere (above 15–18 km) on this occasion. An enhancement of signal from the tropopause occurred not only on the vertical beam, but also on the off-vertical beams. Comparisons of signal strengths scattered from the mesosphere and measured with the vertical and off-vertical beams showed that for the present observations mesospheric scatter was close to isotropic. The backscatter cross-sections at VHF have been compared with other measurements at medium and high frequencies at other locations, and these comparisons help set some limits on the scales of turbulent and specular scatterers in the mesosphere.

Complementary Code and Digital Filtering for Detection of Weak VHF Radar Signals from the Mesosphere

Measurements of the structure and the dynamics of the middle atmosphere with a fine height resolution have been carried out using the SOUSY-VHF-Radar (SOUSY = SOUnding SYstem), operated by the Max-Planck-Institute for Aeronomy in the Harz mountains in Germany. Since the echoes from the middle atmosphere are coherent within a time scale of the order of a second, the received signals are over-sampled and added coherently for that time period. This integration, which is equivalent to a digital combfiltering, improves the signal-to-noise ratio. It is carried out by a hardware adder. In order to detect echoes from the mesosphere, an increase in the effective radiated power is necessary. By applying a 32-element complementary code to long transmitted pulses, the average power is increased and radar returns from the height range between about 65 and 90 km have been recorded with a height resolution of 300 m. The special hardware and software used is described. Some observational results from mesospheric measurements are presented.

A comparison of ambipolar diffusion coefficients in meteor trains using VHF radar and UV lidar

In this paper we present the first comparative estimations of ionic diffusion rates for sporadic meteor trains near the mesopause made using VHF radar and UV Rayleigh lidar observations. In both cases we initially assumed that the meteor trains dissipate primarily through ambipolar diffusion. For the radar data, the diffusion coefficient within the meteor train was determined from the decay rate of the backscattered power. From the the lidar data we then calculated profiles of the atmospheric temperature and density in the height range at which the meteor echoes were detected. These data were used to estimate the ambipolar diffusion coefficients that would result assuming different species of ions. Our results appear consistent with the notion that short-lived underdense meteor trains in the height range of 85–95 km decay primarily by ambipolar diffusion. However, the diffusion coefficients obtained from the radar observations were smaller than those found from the lidar data assuming metal meteoric ions. One possible explanation could be that the radar meteor echoes resulted from ionized constituents of the atmosphere.

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.

Small scale structure and turbulence in the mesosphere and lower thermosphere at high latitudes in winter

Abstract The MAP/WINE campaign has yielded information on small scale structure and turbulence in the winter mesosphere and lower thermosphere by a number of very different remote and in situ techniques. We have assimilated the data from the various sources and thus attempted to present a coherent picture of the small scale dynamics of the atmosphere between 60 and 100 km. We review physical mechanisms which could be responsible for the observed effects, such as ion density fluctuations, radar echoes and wind corners. Evidence has been found for the existence of dynamic structures extending over distances of the order of 100 km; these may be turbulent or non-turbulent. The results indicate that gravity wave saturation is a plausible mechanism for the creation of turbulence and that laminar flows, sharply defined in height and widespread horizontally, may exist.

First low power VHF radar observations of tropospheric, stratospheric and mesospheric winds and turbulence at the Arecibo Observatory

Abstract First VHF radar measurements with height resolution of 300 m and angular resolution of 1.7° were carried out in low latitudes at the Arecibo Observatory, Puerto Rico. A short outline is given of the experimental set-up which consisted of a 160W average power radar-transceiver and a self-contained digital radar control and data acquisition unit. The new VHF feed system of the Arecibo dish is described shortly. Reliable radar echoes were detected from the troposphere, lower stratosphere and from some heights in the mesosphere, indicating that the described VHF radar is capable of proper investigations of dynamical processes in the low latitude middle atmosphere. The angular dependence of aspect sensitive tropospheric and stratospheric turbulence structures was measured to be 1.5–2.5 dB degree −1 . Echoes from the mesosphere indicate a patchy structure of turbulence. The analysis of the signal-to-noise ratio shows considerably high reflectivity in the upper troposphere, which can be caused by high-reaching tropical cumulus convection. Wind profiles measured with the VHF radar between 7.5 and 19.5 km with a height resolution of 300m are very similar to radiosonde wind profiles. Mesospheric VHF radar winds are roughly consistent in amplitude with tidal winds.

Multiple-frequency studies of the high-latitude summer mesosphere : implications for scattering processes

Abstract The characteristics of polar mesosphere summer echoes (PMSE) are studied at 53.5 and 224 MHz. Observations at 2.78 MHz, simultaneous with the ones at the other two frequencies, were carefully compared for indications of PMSE, but no obvious relation was found. Relationships between relative scattering cross-section, spectral width and vertical velocity are studied for the 224 MHz radar, and observations at 53.5 MHz are compared with those at 224 MHz. Results of aspect sensitivity measurements at 53.5 MHz are presented. The implications of these characteristics for several possible scattering mechanisms are discussed. We rule out incoherent scatter and chemically induced fluctuations from the evidence that we have. In view of the extremely low temperatures near the high-latitude mesopause in summer, we discuss several scenarios involving heavy cluster ions and charged aerosol particles.

Intercomparison of density and temperature profiles obtained by lidar, ionization gauges, falling spheres, datasondes and radiosondes during the DYANA campaign

Abstract During the course of the DYANA campaign in early 1990, various techniques to measure densities and temperatures from the ground up to the lower thermosphere were employed. Some of these measurements were performed near simultaneously (maximum allowed time difference: 1 h) and at the same location, and therefore offered the unique chance of intercomparison of different techniques. In this study, we will report on intercomparisons of data from ground-based instruments (Rayleigh- and sodium-lidar), balloon-borne methods (datasondes and radiosondes) and rocket-borne techniques (falling spheres and ionization gauges). The main result is that there is good agreement between the various measurements when considering the error bars. Only occasionally did we notice small but systematic differences (e.g. for the datasondes above 65 km). The most extensive intercomparison was possible between the Rayleigh lidar and the falling sphere technique, both employed in Biscarrosse (44°N,1°W). Concerning densities, excellent agreement was found below 63 km: the mean of the deviations is less than 1 % and the root mean square (RMS) is ~ 3%. Systematic differences of the order of 5% were noticed around 67 km and above 80 km. The former can be accounted for by an instrumental effect of the falling sphere (Ma = 1 transition; Ma = Mach number), whereas the latter is tentatively explained by the presence of Mie scatterers in the upper mesosphere. Concerning temperatures, the agreement is excellent between 35 and 65 km: the mean of the deviations is less than ± 3 K and the variability is ± 5 K. The two systematic density differences mentioned above also affect the temperatures: between 65 and 80 km, the Rayleigh lidar temperatures are systematically lower than the falling sphere values by ~ 5 K.

VHF radar observations of tides at polar latitudes in the summer mesosphere

Abstract Doppler measurements at mesospheric heights were carried out in June 1984 using the mobile SOUSY VHF radar at Andenes/Norway (69°N, 16°E). The temporal wind variations at heights between 80 and 95 km reveal wave motions with periods of about 12 h, 14.5 h, 24 h and 54 h. The propagating semi-diurnal tide has amplitudes up to 20 m s −1 and a vertical wavelength of about 30 km. The diurnal tide is much weaker and the phase shows a more complex variation with height. The 14.5 h period motion reveals a maximum amplitude of 15 m s −1 at about 84 km and a vertical wavelength of 15 km.

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.